1. Introduction to Heredity
Heredity is the transmission of characteristics (traits) from parents to their offspring through genes.
When a child is born, the child shows some features similar to the parents – a similar face, hair colour, height or skin colour. This passing on of features from one generation to the next is called heredity.
Parents → Heredity → Offspring (children inherit features of parents)
Why Do Offspring Resemble Their Parents?
Every cell of the body contains a set of instructions called hereditary information. This information is written in a molecule called DNA (deoxyribonucleic acid). During reproduction, a copy of this information is passed from parents to the offspring. That is why children resemble their parents.
Why Are Offspring Not Exactly Identical to Their Parents?
- DNA copying during cell division is not perfectly accurate – small changes (mistakes) creep in.
- In sexual reproduction, the genetic information of two parents combines, so the offspring get a mixture of traits.
- Because of these small differences, every individual is slightly different from the others. These differences are called variation.
Variation is the difference in traits shown by individuals of the same species, or between parents and their offspring.
The Role of Genes
- A gene is a segment of DNA that carries information for a particular trait (for example, tallness, eye colour or seed shape).
- Genes are present on thread-like structures called chromosomes, which are found inside the nucleus of every cell.
- Chromosomes pass from parents to offspring through the gametes (reproductive cells: sperm and ovum).
- The complete set of hereditary instructions of an individual is its genetic information.
Relationship Between DNA, Genes and Chromosomes
DNA is the molecule of heredity. A gene is a segment of DNA that codes for a trait. Chromosomes are structures inside the nucleus that contain DNA tightly packed with proteins. So the flow of hereditary information is:
DNA → segments called Genes → carried on Chromosomes → hereditary information of the cell
When a cell divides, the DNA first makes a copy of itself (DNA copying), and each new cell receives a copy of the chromosomes, and therefore a copy of the genes.
2. Accumulation of Variation During Reproduction
Why DNA Copying Happens
Reproduction produces new individuals. For the new individual to have the features of the species, the hereditary information (DNA) must be copied and passed to it. So DNA copying is the basic event of reproduction, whether the reproduction is asexual or sexual.
Why DNA Copying Is Not Perfectly Accurate
- DNA is a very long molecule, and copying it is a complex job.
- Sometimes small errors creep in during copying.
- These errors make the copy slightly different from the original.
- Each change may show up as a new version of a trait – this is the origin of variation.
Variation Arising During Reproduction
- In asexual reproduction: offspring get copies of the parent’s DNA, so variation is small (most offspring are nearly identical).
- In sexual reproduction: DNA from two parents combines, so there is a lot of variation – every offspring is different from both parents and from every sibling.
Importance of Variation
- Survival: If the environment changes (a new disease, a change in climate), some variants may survive better than others.
- Adaptation: Better-suited individuals survive and reproduce, passing on their useful traits.
- Evolution: Over many generations, accumulated variation becomes the material on which natural selection acts.
Variation in a Population
Think of the students in a school: all are human beings, but each has a slightly different height, hair colour and face. This is variation in a population. Not all of it is inherited – some differences are due to food, exercise or other life experiences.
Inherited vs Acquired Characteristics (Basic Idea)
If a player trains hard and grows strong muscles, the muscles are an acquired feature – they developed during the player’s lifetime. The player’s children will not automatically be born with such muscles. Height, on the other hand, is largely an inherited feature controlled by genes.
A full comparison of inherited and acquired traits is given in section 8.
3. Mendel and His Experiments
An Austrian monk who carried out breeding experiments on garden pea plants and discovered the basic rules of inheritance. He is known as the Father of Genetics.
Why Mendel Chose Pea Plants
- Pea plants can be self-pollinated easily, so pure (true-breeding) lines could be obtained.
- They have many contrasting traits that are easy to observe (tall vs dwarf, round vs wrinkled seeds, etc.).
- They have a short life span and produce many offspring, so results could be counted in large numbers.
- Their flowers could be cross-pollinated artificially, so Mendel could control which plants mated with which.
Contrasting Traits Studied by Mendel
| Character | Dominant Form | Recessive Form |
|---|---|---|
| Stem height | Tall | Dwarf (short) |
| Seed shape | Round | Wrinkled |
| Seed colour | Yellow | Green |
| Flower colour | Violet | White |
| Pod shape | Inflated/full | Constricted |
| Pod colour | Green | Yellow |
| Flower position | Axial (on stem) | Terminal (top) |
Generations in Mendel’s Experiment
- Parental generation (P): the two pure (true-breeding) plants used to begin the cross – for example, a pure tall plant (TT) and a pure dwarf plant (tt).
- F1 generation (first filial): the offspring of the P cross. When Mendel crossed a pure tall (TT) with a pure dwarf (tt) pea plant, all F1 plants were tall. So tallness is the dominant trait and dwarfness the recessive trait.
- F2 generation (second filial): produced by allowing the F1 plants to self-pollinate (Tt × Tt). In F2, both traits reappeared in the ratio of about 3 tall : 1 dwarf.
When two contrasting traits are crossed, only one (the dominant) appears in F1. But the other (the recessive) reappears in F2 in the ratio 3 : 1. This means the recessive trait was not lost – it remained hidden in F1.
A recessive trait that disappears in one generation can reappear in later generations. It is never permanently lost.
4. Dominant and Recessive Traits
Key Terms of Inheritance
| Term | Meaning |
|---|---|
| Gene | A segment of DNA that carries information for one trait (e.g. tallness). |
| Allele | An alternative form of a gene. A gene for stem height has two forms: T (tall) and t (dwarf). Each form is an allele. |
| Dominant trait | The trait that shows (is expressed) even when only one copy of its allele is present. |
| Recessive trait | The trait that is expressed only when both copies of its allele are present (no dominant allele present). |
| Genotype | The genetic make-up (combination of alleles) of an individual, e.g. TT, Tt or tt. |
| Phenotype | The physical appearance or observable trait produced by the genotype, e.g. tall or dwarf. |
| Homozygous | Having two identical alleles for a trait, e.g. TT (pure tall) or tt (pure dwarf). |
| Heterozygous | Having two different alleles for a trait, e.g. Tt (hybrid tall). |
Which Traits Are Expressed?
| Genotype | Type | Phenotype shown | Reason |
|---|---|---|---|
| TT | Homozygous dominant | Tall | Both alleles are dominant (tall). |
| Tt | Heterozygous | Tall | Dominant allele T hides the recessive allele t. |
| tt | Homozygous recessive | Dwarf | Both alleles are recessive; no dominant allele to hide them. |
A dominant allele is written as a capital letter (T) and a recessive allele as the same small letter (t). Different letters are used for different characters.
Dominant vs Recessive Traits
| Dominant Trait | Recessive Trait |
|---|---|
| Expressed even when only one copy of the allele is present (TT or Tt). | Expressed only when both alleles are present (tt). |
| Visible in all F1 offspring of a cross between pure dominant and pure recessive plants. | Hidden in F1; reappears in F2. |
| Example: tallness (T) in pea plants. | Example: dwarfness (t) in pea plants. |
Genotype vs Phenotype
| Genotype | Phenotype |
|---|---|
| Genetic make-up (alleles), e.g. TT, Tt, tt. | Observed physical trait, e.g. tall or dwarf. |
| Cannot be seen directly; found by crossing/analysis. | Can be seen directly. |
| Two different genotypes (TT and Tt) can give the same phenotype (tall). | One phenotype may come from more than one genotype. |
Homozygous vs Heterozygous
| Homozygous | Heterozygous |
|---|---|
| Both alleles identical, e.g. TT or tt. | Alleles different, e.g. Tt. |
| Also called pure/homozygous (pure tall TT or pure dwarf tt). | Also called hybrid. |
| Produces only one type of gamete (T or t). | Produces two types of gametes (T and t). |
5. Monohybrid Cross and the Punnett Square
A cross between two pure parents that differ in only one character (for example, tallness only) is called a monohybrid cross.
Step-by-Step: Tall × Dwarf
Punnett Square for Tt × Tt
| Genotype | Genotype ratio | Phenotype | Phenotype ratio |
|---|---|---|---|
| TT | 1 | Tall (homozygous dominant) | 3 tall |
| Tt | 2 | Tall (heterozygous, T hides t) | |
| tt | 1 | Dwarf (homozygous recessive) | 1 dwarf |
Phenotypic ratio: 3 : 1 (3 tall : 1 dwarf) | Genotypic ratio: 1 : 2 : 1 (1 TT : 2 Tt : 1 tt)
In the F2 generation, about 3 out of every 4 plants are tall and about 1 out of 4 is dwarf. This 3 : 1 ratio was repeatedly obtained by Mendel for each character he studied, which proved the basic rules of inheritance.
6. Sex Determination in Humans
Chromosomes in Humans
- A human cell has 46 chromosomes arranged in 23 pairs.
- 22 pairs are autosomes – they are the same in males and females.
- The last pair are the sex chromosomes – they decide sex.
| Category | Males (XY) | Females (XX) |
|---|---|---|
| Autosomes | 22 pairs | 22 pairs |
| Sex chromosomes | X and Y | X and X |
| Total | 46 (44 + XY) | 46 (44 + XX) |
Autosomes vs Sex Chromosomes
| Autosomes | Sex Chromosomes |
|---|---|
| Chromosomes other than the sex chromosomes. | A pair of chromosomes that determines the sex of an individual. |
| Similar in males and females. | In humans: XX in females, XY in males. |
| Carry most of the body’s genes for various traits. | In humans the X carries many genes; the Y is small and carries few genes, including those that trigger male development. |
Gamete Formation
- Female (XX): every ovum carries one X chromosome. So all ova are the same as far as sex chromosomes are concerned.
- Male (XY): about half the sperms carry an X chromosome and about half carry a Y chromosome. So sperms are of two kinds: X-sperms and Y-sperms.
How the Sex of a Child Is Decided
At fertilisation, the ovum (X) fuses with a sperm. If the sperm carries an X chromosome, the child has XX (female). If the sperm carries a Y chromosome, the child has XY (male). Because it is the father’s sperm that decides which sex chromosome combination the child gets, the father’s gamete determines the sex of the child.
Probability, Not a Guarantee
- Because about half the sperms carry X and about half carry Y, the chance is approximately 50% for either a female (XX) or a male (XY) child.
- This is a probability for a population of births – it is not a guarantee for any one particular pregnancy.
In humans the ovum always carries an X chromosome; it never carries a Y. The sex of a child does not depend on the mother at all – it depends only on which type of sperm fertilises the ovum.
Sex determination is a biological process. It carries no social value – male and female babies are equally valuable.
7. Inheritance of Traits
How Traits Are Passed On
- Traits are carried by genes, which are segments of DNA present on chromosomes.
- Chromosomes are present in pairs. When gametes are formed, each gamete receives one chromosome from each pair.
- At fertilisation, the ovum and sperm join, so the offspring gets one set from the mother and one set from the father.
- That is why a child shows a mix of both parents’ traits, and why siblings can differ from each other.
DNA → Gene → Chromosome → Hereditary Information
Scientifically Accurate Distinction
| DNA | Gene | Chromosome |
|---|---|---|
| The molecule of heredity; carries genetic instructions. | A segment of DNA that codes for a particular trait or function. | A thread-like structure in the nucleus made of DNA coiled around proteins. |
| Is the material that makes up genes and chromosomes. | Is a functional unit of DNA. | Carries many genes arranged along its length. |
| Copied during reproduction (DNA copying). | Determines a specific trait such as height or seed shape. | Segregates and recombines at gamete formation. |
Genes are segments of DNA. Chromosomes are structures containing DNA and associated proteins. Hereditary information flows as: one gene = one trait instruction carried on a chromosome.
8. Inherited and Acquired Traits
A trait that is passed from parents to offspring through genetic information (DNA), for example height or seed colour in pea plants.
A trait that develops during an individual’s own lifetime due to experience, use or damage, and is generally not inherited simply because it was acquired.
Examples
- Inherited: tallness/shortness of pea plants, human eye colour, skin colour, blood group.
- Acquired: a scar from an injury, muscles built by exercise, losing a finger in an accident, a style of speaking learned from surroundings.
Inherited vs Acquired Traits – Comparison
| Feature | Inherited Trait | Acquired Trait |
|---|---|---|
| How it arises | Present in the genetic information (DNA) from the start. | Develops during the individual’s lifetime. |
| How it is passed | Passed through genes from parents to offspring. | Generally not passed to offspring. |
| Basis | Changes in genes/DNA of gametes. | Changes in body cells by use, damage or environment. |
| Example | Tallness, eye colour. | A scar, muscles from exercise. |
If a dancer dances for years and becomes extremely skilful, the skill is an acquired (learned) feature. The dancer’s baby will need to learn dancing from scratch – the skill is not inherited.
9. Variation and Its Importance
Sources of Variation (Class 10 Level)
- Errors in DNA copying during reproduction.
- Combination of genetic material from two parents in sexual reproduction (new mixes of alleles).
Why Variation Is Important
- Survival in change: if the environment changes (a new disease, a temperature change, loss of food source), some variants may survive while others may not.
- Adaptation: individuals with a trait that fits the environment survive and reproduce, passing the trait on.
- Basis of evolution: over many generations, accumulated variation enables species to change and new species to arise.
- No single type: a population with greater variation is less likely to be wiped out by a single event.
Suppose a bacterial infection targets one particular type of bacteria. If a population of bacteria has variation, some bacteria may be naturally less affected and survive, keeping the population going.
At Class 10 level we only need the basic idea: reproduction produces variation, and variation helps populations cope with a changing environment. Detailed evolutionary mechanisms are studied in higher classes.
10. Important Definitions
The transmission of characteristics (traits) from parents to offspring through genes.
Differences in traits between individuals of the same species, or between parents and offspring.
A segment of DNA that carries the information for a particular trait or function.
Deoxyribonucleic acid – the molecule of heredity that carries genetic instructions in living cells.
A thread-like structure in the nucleus formed of DNA coiled around proteins; carries genes.
A characteristic feature of an organism, such as tallness, eye colour or seed shape, that can be inherited.
An alternative form of a gene; for example, T (tall) and t (dwarf) are alleles of the height gene.
A trait that is expressed even when only one copy of its allele is present (e.g. tall, genotype TT or Tt).
A trait that is expressed only when both alleles are present and no dominant allele exists (e.g. dwarf, genotype tt).
The genetic make-up of an individual for a trait, shown by allele symbols (e.g. TT, Tt, tt).
The observable appearance of a trait produced by the genotype (e.g. tall or dwarf).
Having two identical alleles for a trait (e.g. TT or tt).
Having two different alleles for a trait (e.g. Tt).
A trait transmitted from parents to offspring through genetic information (DNA).
A trait developed during an individual’s lifetime; generally not inherited just because it was acquired.
Any chromosome other than the sex chromosomes; similar in males and females.
The pair of chromosomes that determines sex – X and X in human females, X and Y in human males.
11. Important Diagrams
The important diagrams of this chapter are drawn above in the relevant sections. The table below lists each diagram and the key labels you should be able to reproduce in an exam.
| Diagram | Where it appears | Labels you should write |
|---|---|---|
| Contrasting traits of pea plants (Fig. 1) | Section 3 | Tall/Dwarf, Round/Wrinkled seeds, Yellow/Green seeds, Violet/White flowers. |
| Punnett square Tt × Tt (Fig. 2) | Section 5 | Gametes of male (T, t), gametes of female (T, t); offspring TT, Tt, Tt, tt; genotypic ratio 1:2:1, phenotypic ratio 3:1. |
| Human sex determination (Fig. 3) | Section 6 | Mother XX, Father XY, ova (X), sperms (X, Y), offspring XX and XY (about 50% each). |
| DNA → gene → chromosome (Fig. 4) | Section 7 | Chromosome, coiled DNA, genes as segments of DNA, hereditary information. |
| Inherited vs acquired traits (Fig. 5) | Section 8 | Inherited → passes to children; acquired → not passed to children. |
How to Present Diagrams Neatly
- Use a pencil and ruler for straight lines; keep the square of a Punnett square equal in size.
- Write allele symbols (T/t or X/Y) clearly inside the boxes.
- Label the generations (P, F1, F2), the parents, the gametes and the ratios.
- Headings should be meaningful, e.g. “Punnett square of a monohybrid cross (Tt × Tt)”.
All diagrams on this page are original, hand-drawn style SVG illustrations created for Vyasa Academy. They are not textbook scans.
12. Solved Examples
All questions below are original, CBSE exam-style problems written for Vyasa Academy. None is claimed as a CBSE previous-year question unless marked.
A. Basic Conceptual Questions
What is heredity? Name the molecule that carries hereditary information.
Show Solution
Approach/Concept: Heredity is the passing of traits from parents to offspring. Such traits are carried in the genetic material.
Solution: During reproduction, parents pass copies of their hereditary information to offspring, which is why children resemble parents.
Final Answer: Heredity is the transmission of characteristics from parents to offspring. The molecule that carries hereditary information is DNA.
Why do offspring resemble their parents, yet are not exactly identical to them?
Show Solution
Approach/Concept: Two ideas: (1) DNA is copied and passed on, so resemblance; (2) copying is imperfect and two parents contribute DNA, so variation.
Solution: Offspring receive a copy of the parents’ DNA (resemblance). DNA copying has small inaccuracies and, in sexual reproduction, DNA from two parents combines (difference/variation).
Final Answer: Offspring resemble parents because they inherit a copy of their parents’ DNA; they differ because DNA copying is not perfectly accurate and sexual reproduction mixes two parents’ genetic material.
Distinguish between a gene and a chromosome.
Show Solution
Approach/Concept: Recall that a gene is a segment of DNA and a chromosome is a structure carrying DNA and proteins.
Solution: A gene is a functional unit of DNA (codes for a trait). A chromosome is a thread-like structure made of DNA coiled around proteins that carries many genes.
Final Answer: Gene = segment of DNA that codes for a trait; chromosome = thread-like structure of DNA + proteins that carries many genes.
What is variation? Give one example from humans.
Show Solution
Approach/Concept: Variation is the difference in traits among individuals of the same species.
Solution: Differences such as height, eye colour or hair colour between different people are examples of variation within a population.
Final Answer: Variation is the difference in traits between individuals of the same species, e.g. difference in human eye colour.
Why is variation important for a population?
Show Solution
Approach/Concept: Variation gives some individuals features that may suit a changed environment.
Solution: If conditions change (disease, climate), a few individuals with a suitable variation survive and reproduce, keeping the species alive.
Final Answer: Variation helps populations survive changing environments and is the raw material for evolution by natural selection.
B. Inheritance and Genetics Questions
State the three important observations Mendel made from his monohybrid crosses with pea plants.
Show Solution
Approach/Concept: Recall the P, F1 and F2 pattern of a monohybrid cross.
Solution: (1) Crossing two pure contrasting parents (e.g. TT and tt) gives an all-dominant F1 (all tall Tt). (2) In F1 both dominant and recessive alleles are present but only the dominant shows. (3) On selfing F1, both traits reappear in F2 in a 3:1 ratio.
Final Answer: (1) F1 shows only the dominant trait; (2) the recessive trait is hidden, not lost; (3) F2 shows the traits in 3:1 (dominant:recessive) ratio.
In the cross TT × tt, why are all F1 plants tall even though they carry the t allele?
Show Solution
Approach/Concept: Dominance: T is dominant over t.
Solution: Every F1 plant receives a T allele from the tall parent and a t allele from the dwarf parent, so all are Tt (heterozygous). The dominant allele T masks the recessive allele t.
Final Answer: All F1 plants are Tt and tall because T is dominant over t.
What is meant by genotype and phenotype? Give the genotype and phenotype of a pure tall pea plant.
Show Solution
Approach/Concept: Genotype = alleles, phenotype = appearance.
Solution: A pure tall plant has two identical dominant alleles, TT. Its genotype is TT and its phenotype is tall.
Final Answer: Genotype = genetic make-up (TT); phenotype = observable trait (tall). A pure tall plant has genotype TT and phenotype tall.
Why is a dwarf pea plant always tt, but a tall pea plant can be TT or Tt?
Show Solution
Approach/Concept: Recessive trait shows only in homozygous recessive condition.
Solution: Dwarfness is recessive (t). It is expressed only when no dominant allele is present, i.e. tt. Tallness is dominant, so it appears in both TT and Tt.
Final Answer: A recessive trait appears only in homozygous recessive form (tt); a dominant trait appears in both homozygous (TT) and heterozygous (Tt) forms.
Two pea plants of genotype Tt are crossed. Write the genotypic and phenotypic ratios of F1.
Show Solution
Approach/Concept: Monohybrid cross Tt × Tt with a Punnett square.
Solution: Gametes of each parent: T and t. Combine gametes: TT, Tt, Tt, tt.
Final Answer: Genotypic ratio = 1 TT : 2 Tt : 1 tt; phenotypic ratio = 3 tall : 1 dwarf.
Why are peas a good choice for Mendel’s experiments? Give two reasons.
Show Solution
Approach/Concept: Recall the suitable features of the garden pea plant.
Solution: Pea plants can be self-pollinated to maintain pure lines, show many easily observable contrasting traits, and produce many offspring in a short time.
Final Answer: (1) Several clear contrasting traits; (2) short generation time and large number of offspring; (3) easy control of self- and cross-pollination.
C. Punnett-Square / Monohybrid Cross Questions
Construct a Punnett square for Tt × tt and give the phenotypic ratio.
Show Solution
Approach/Concept: Gametes of Tt are T and t; gametes of tt are only t.
Solution: Offspring: Tt, Tt, tt, tt. So half tall (Tt), half dwarf (tt).
Final Answer: Phenotypic ratio = 1 tall : 1 dwarf (genotypes 1 Tt : 1 tt).
Out of 160 plants of the F2 of a monohybrid tall × dwarf cross, how many would you expect to be dwarf?
Show Solution
Approach/Concept: F2 phenotypic ratio is 3:1 (3 tall : 1 dwarf).
Solution: Dwarf fraction = 1/4. Number dwarf = 160 × 1/4 = 40.
Final Answer: 40 plants are expected to be dwarf.
A cross between two plants gives offspring in the ratio 1 TT : 2 Tt : 1 tt. What were the genotypes of the parents?
Show Solution
Approach/Concept: The 1:2:1 genotypic ratio is the signature of a monohybrid cross between two heterozygotes.
Solution: Both parents must be heterozygous (Tt), each producing T and t gametes.
Final Answer: Both parents were Tt (heterozygous).
When Mendel crossed a pure tall with a pure dwarf plant, all F1 were tall. What would the F2 phenotypic ratio be if F1 plants self-pollinate?
Show Solution
Approach/Concept: Selfing F1 (Tt × Tt) gives the standard monohybrid outcome.
Solution: F1 genotype Tt; gametes T and t. F2: TT, Tt, Tt, tt → 3 tall, 1 dwarf.
Final Answer: F2 phenotypic ratio = 3 tall : 1 dwarf.
A plant of genotype TT is crossed with a plant of genotype TT. What are the genotypes of all offspring?
Show Solution
Approach/Concept: A homozygous parent can only make one type of gamete.
Solution: Each TT parent makes only T gametes. Every fertilisation gives TT.
Final Answer: All offspring are TT (tall, homozygous dominant).
D. Sex-Determination Questions
How is the sex of a child determined at fertilisation?
Show Solution
Approach/Concept: The ovum always carries X; the sperm carries X or Y.
Solution: Ovum (X) + sperm (X) → XX (female). Ovum (X) + sperm (Y) → XY (male). The father’s sperm decides the combination.
Final Answer: If an X-sperm fertilises the ovum the child is female (XX); if a Y-sperm fertilises it the child is male (XY).
Why do we say that a male human produces two kinds of gametes?
Show Solution
Approach/Concept: Sex chromosomes of a male are X and Y.
Solution: During gamete formation, half the sperms receive an X chromosome and half receive a Y chromosome. So a male produces X-sperms and Y-sperms.
Final Answer: Males produce two kinds of sperms – X-bearing and Y-bearing – in about equal numbers.
About what percentage of births are expected to be boys and girls in a large population, and why?
Show Solution
Approach/Concept: Roughly equal numbers of X- and Y-sperms fertilise ova.
Solution: X-ova fusing with X-sperms give XX (girls) and with Y-sperms give XY (boys), each about 50% in a large population.
Final Answer: About 50% boys and 50% girls, on average, across a large population – a probability, not a guarantee for any single pregnancy.
E. Competency / Application Questions
Ravi’s mother is a trained swimmer who swims very well. His father is a musician. Ravi inherited all of his music teacher’s training-free ear for rhythm? No. Explain which of these traits can and cannot be inherited.
Show Solution
Approach/Concept: Distinguish inherited vs acquired traits.
Solution: Swimming skill and musical training are acquired during life – they do not change the genes in gametes, so they are not inherited. What can be inherited are genetically controlled traits such as body build or natural hearing ability.
Final Answer: Skills learned in life (swimming, trained musicianship) are acquired traits and are not inherited; only gene-controlled traits are inherited.
In a garden, a tall plant of unknown genotype is crossed with a dwarf plant. Half the offspring are tall and half dwarf. What is the genotype of the tall parent?
Show Solution
Approach/Concept: 1:1 ratio is produced by a Tt × tt cross.
Solution: Dwarf parent is tt (only t gametes). For half offspring to be tall and half dwarf, the tall parent must give both T and t gametes, so it is Tt.
Final Answer: The tall parent is heterozygous Tt.
A farmer has two pea lines: one pure tall (TT) and one unknown. He crosses the unknown line with pure dwarf (tt) and gets all tall plants in one case but a 1:1 ratio in another. What do these two results tell him about the unknown line?
Show Solution
Approach/Concept: Test-cross reasoning: cross with a homozygous recessive.
Solution: If all offspring are tall, the unknown parent produced only T gametes → genotype TT. If the ratio is 1:1, the unknown parent produced T and t gametes → genotype Tt. The unknown line must therefore be a mixture of TT and Tt plants.
Final Answer: The all-tall result indicates TT plants; the 1:1 result indicates Tt plants; so the unknown line contains both genotypes.
F. Reasoning / Case-Based Questions
Two same-height parents (one TT, one Tt) are crossed. Predict the heights of their children, giving the phenotypic ratio.
Show Solution
Approach/Concept: Cross TT × Tt; gametes T (from first) and T, t (from second).
Solution: Offspring: TT, TT, Tt, Tt → all tall. The dwarf allele t is present in half the children (as Tt) but is not expressed.
Final Answer: All children will be tall; genotypes 1 TT : 1 Tt; can a dwarf trait appear later? Only if two t alleles come together in a future cross.
Study this family: a colour-blind father and a mother with normal colour vision have children. Which parent’s gamete would decide whether a child is a son or daughter? (Colour blindness is carried on the X chromosome.)
Show Solution
Approach/Concept: Sex is decided by the father’s sperm (X or Y). The X-linked condition adds that sons get their X from the mother.
Solution: The father gives a Y for a son and an X for a daughter. The mother always gives an X. So the father’s gamete decides the sex. (For an X-linked trait, a son inherits his X from his mother.)
Final Answer: The father’s sperm decides sex. Sons receive their single X chromosome from the mother; daughters get one X from each parent.
Assertion-style reasoning: In a monohybrid cross, the F2 generation always shows the recessive trait again even though it disappeared in F1. Reason: recessive alleles do not get destroyed; they stay hidden in heterozygous individuals. Choose the correct option.
Show Solution
Approach/Concept: Check if the reason correctly and fully explains the assertion.
Solution: F1 plants are Tt; recessive t is present but masked by T. Selfing Tt × Tt produces tt individuals in F2, which show the recessive trait. The reason is true and is the correct explanation.
Final Answer: Both the assertion and the reason are true, and the reason correctly explains the assertion.
13. Application-Based Examples
Original – CBSE Exam-Style
A gardener wants only tall pea plants forever. Which genotype should he breed together to get only tall plants generation after generation? Explain.
Show Solution
Concept: A cross that never produces dwarf offspring.
Solution: Breeding TT × TT produces only TT (tall) offspring. Breeding TT × Tt also gives all tall plants but introduces the t allele. To guarantee 100% tall, use only TT (pure tall) plants.
Final Answer: Breed TT (homozygous tall) plants together; every offspring remains tall (TT).
A puppy born to a trained retriever and a house pet is expected to be a natural retriever. Is this claim scientifically correct?
Show Solution
Concept: Inherited genetics vs acquired skill.
Solution: The tendency (instinct) to retrieve may have a genetic basis and can be inherited. But the actual skill depends on training during life. So the puppy may have an inclination, not a guaranteed trained skill.
Final Answer: Partly correct – instincts can be inherited, but trained behaviour is acquired and learned during the puppy’s lifetime.
In a certain plant, red flowers (R) are dominant and white flowers (r) are recessive. A red-flowered plant was crossed with a white-flowered plant and half the offspring were red and half white. What is the genotype of the red parent?
Show Solution
Concept: 1:1 ratio results from a heterozygous × homozygous-recessive (test) cross.
Solution: White parent is rr (only r gametes). For half Rr and half rr, the red parent must produce R and r gametes, hence Rr.
Final Answer: The red parent is Rr (heterozygous).
Cat fur: a black cat (BB) and a white cat (bb) have kittens, all black. What colour are the kittens’ genotypes, and what would happen if two of these kittens were crossed?
Show Solution
Concept: Complete dominance in a monohybrid cross.
Solution: All kittens are Bb (black). Crossing Bb × Bb gives BB, Bb, Bb, bb → 3 black : 1 white.
Final Answer: Kittens are all Bb (black); Bb × Bb produces 3 black : 1 white.
Identical twins share all genes. One twin trains hard to become a sprinter, the other stays inactive. Explain whether their children would differ in athletic ability because of this.
Show Solution
Concept: Acquired traits are not inherited.
Solution: Training changed body cells (muscles, fitness), not the DNA of gametes. Since the children come from gametes, the acquired athletic ability is not passed on; genetically both sets of children are similar.
Final Answer: No – acquired athletic ability is not genetic and is not inherited; it only affects the twin who acquired it.
In humans, a daughter gets one X from her mother and one from her father, while a son gets an X from his mother and a Y from his father. Explain which parent determines the daughter’s two X chromosomes and why a son has no blood-trait copy on a second X.
Show Solution
Concept: Gamete formation determines chromosome composition.
Solution: The ovum always gives an X. The egg and sperm each contribute one of each pair. A daughter (XX) receives an X from each parent; a son (XY) receives his only X from the mother and the Y from the father, which is why he can be affected by an X-linked recessive condition carried by the mother.
Final Answer: Daughter = X from mother + X from father; son = X (mother) + Y (father). The father’s Y or X decides the child’s sex type.
Two pea plants, both tall, were crossed and 25% of the seeds grew into dwarf plants. Working out is required: find the genotypes of the parents.
Show Solution
Concept: 25% (or 1/4) dwarf means a standard Tt × Tt cross.
Solution: Only a cross producing tt offspring at 25% can arise from two heterozygotes: Tt × Tt → TT, Tt, Tt, tt (1/4 dwarf).
Final Answer: Both parents are Tt (heterozygous).
A plant breeder crosses yellow-seeded (YY) with green-seeded (yy) peas. All F1 are yellow. She now self-pollinates the F1. Predict the seed colours and their proportions in F2.
Show Solution
Concept: Monohybrid inheritance of seed colour with dominance.
Solution: F1 are Yy (yellow, since Y dominates y). F1 selfed: Yy × Yy → YY, Yy, Yy, yy. Yellow (YY or Yy) = 3 parts; green (yy) = 1 part.
Final Answer: F2 seeds = 3 yellow : 1 green.
14. Competency-Based and Reasoning Questions
Original – CBSE Exam-Style
Read the cross given below and answer. Parents: Tt × tt. (a) Write the gametes of each parent. (b) Show the offspring in a Punnett square. (c) State the phenotypic ratio.
Show Solution
Reasoning: Gametes of Tt = T, t; gametes of tt = t, t.
Solution: Offspring: Tt, Tt, tt, tt → 2 tall : 2 dwarf = 1:1.
Final Answer: (a) T,t and t; (b) Tt, Tt, tt, tt; (c) 1 tall : 1 dwarf.
Statement A: Two dwarf pea plants, when crossed with each other, always produce dwarf plants. Statement B: Two tall plants may produce a dwarf offspring. Are both statements true? Give the genetic reason.
Show Solution
Reasoning: A recessive phenotype can only be tt; a dominant phenotype can hide t.
Solution: Dwarf = tt, so tt × tt always gives tt (dwarf) – A is true. Tall could be TT or Tt; two tall plants that are both Tt can give tt offspring – B is true.
Final Answer: Both are true: tt × tt → all dwarf; Tt × Tt → can give tt.
Given the F2 genotypic ratio 1 TT : 2 Tt : 1 tt. (a) Which group contributes the "1 dwarf" phenotype of the 3:1 ratio? (b) Why do two different genotypes (TT and Tt) give the same phenotype?
Show Solution
Reasoning: Phenotype depends on the presence of a dominant allele.
Solution: (a) tt group (homozygous recessive). (b) Both TT and Tt contain at least one dominant T, which is enough to express tallness.
Final Answer: (a) tt; (b) one dominant allele is sufficient to mask the recessive allele.
In humans, a couple has three daughters. What is the probability that the fourth child will be a son? (Give a scientific answer.)
Show Solution
Reasoning: Each pregnancy is an independent event with its own gamete combination.
Solution: Sperm X or Y decides; about half of the sperms carry Y. Previous births do not change the father’s sperm proportions.
Final Answer: About 50% (approximately) for each pregnancy; previous children do not change this probability.
Diagram-based: In the Punnett square below, the mother contributes gametes (T, t) and the father also contributes (T, t). Match each cell to its genotype and phenotype: [row1: TT, Tt; row2: Tt, tt].
Show Solution
Reasoning: Combine one gamete from each parent in each cell.
Solution: Top-left: T(from mother) + T(from father) = TT (tall). Top-right: T + t = Tt (tall). Bottom-left: t + T = Tt (tall). Bottom-right: t + t = tt (dwarf).
Final Answer: TT (tall), Tt (tall), Tt (tall), tt (dwarf).
A scientist claims that variation in a population can arise without any change in the environment. Do you agree? Give a reasoned answer.
Show Solution
Reasoning: Sources of variation are internal (DNA copying + sexual reproduction).
Solution: Yes – errors in DNA copying during reproduction and the mixing of two parents’ genetic material create variation even when the environment is unchanged.
Final Answer: Agree: reproduction itself introduces variation through DNA-copying inaccuracy and genetic recombination.
15. Case-Based Learning
Original – CBSE Exam-Style
Mendel crossed pure tall pea plants (TT) with pure dwarf pea plants (tt). The F1 generation had all tall plants. He allowed the F1 plants to self-pollinate and obtained F2 offspring.
(a) What is the genotype of all F1 plants? (b) What is the F2 genotypic ratio? (c) What is the F2 phenotypic ratio? (d) Write the genotypes of two F2 plants that look identical in height but have different genotypes.
Show Solution
Concept: Monohybrid cross with complete dominance.
Solution: (a) F1 are Tt. (b) Self-pollinating Tt × Tt gives 1 TT : 2 Tt : 1 tt. (c) 3 tall : 1 dwarf. (d) TT and Tt are both tall but differ in genotype.
Final Answer: (a) Tt; (b) 1:2:1; (c) 3:1; (d) TT and Tt.
A family studies ear lobe shape. Free ear lobes (F) are dominant and attached lobes (f) are recessive. The father has free lobes and the mother has attached lobes (ff). Two of their children have attached lobes and one has free lobes.
(a) Deduce the father’s genotype. (b) Write the genotypes of the three children. (c) If the free-lobed child grows up and marries a free-lobed person homozygous FF, what fraction of that couple’s children will have attached lobes?
Show Solution
Concept: Use phenotypes of offspring to deduce parental genotype.
Solution: Mother ff can give only f. Since some children are ff (attached), the father must carry f → father is Ff. Children: Ff (free), Ff or ff... The two attached ones are ff; the free one is Ff. Free-lobed child (Ff) × FF gives FF, Ff only → all free lobes.
Final Answer: (a) Father is Ff; (b) attached children ff, free child Ff; (c) 0% attached lobes.
A breeder has a yellow-seeded plant (Yy). To check whether a separate yellow plant is YY or Yy, he crosses it with a green-seeded plant (yy). The seeds produced are all yellow.
(a) What is the genotype of the separate yellow plant? (b) What cross technique has the breeder used? (c) State the result expected if the separate yellow plant had been Yy.
Show Solution
Concept: Test cross with a homozygous recessive.
Solution: Green plant is yy. If the yellow plant were YY, all seeds are Yy (yellow). If it were Yy, half the seeds would be yy (green). All yellow seeds were obtained, so the plant is YY.
Final Answer: (a) YY; (b) test cross; (c) green seeds (yy) would appear in 1:1 ratio.
In a maternity ward study, 200 gametes from a large group of men were checked. 102 sperms carried X and 98 carried Y. Ova always carried X.
(a) Which chromosome is present in all ova? (b) Predict the proportion of girl and boy newborns from the data. (c) Does the mother’s body or environment influence the sex of the child? Give one reason.
Show Solution
Concept: Sex is fixed at fertilisation by the sperm type.
Solution: (a) X. (b) X-sperms give XX (girls), Y-sperms give XY (boys) → roughly equal, about 50% girls and 50% boys. (c) No – the ovum always gives X; only the sperm’s X or Y decides the sex.
Final Answer: (a) X; (b) about 50% girls and 50% boys; (c) no, the father’s sperm decides.
Two fields of the same wheat variety are grown. In one field a few plants appear slightly resistant to a fungal disease. When the disease spreads, most plants die but these few survive and produce seeds.
(a) What caused the resistant plants to differ? (b) What will happen to the next generation of these survivors? (c) Name the process illustrated here in simple terms.
Show Solution
Concept: Variation + natural selection in a basic form.
Solution: (a) Variation arose during reproduction (DNA-copying error or recombination). (b) The survivors reproduce; their resistant trait spreads in the population over generations. (c) Natural selection acting on variation – survival of the better-suited.
Final Answer: (a) Variation from reproduction; (b) resistance becomes common over generations; (c) natural selection.
16. Common Mistakes Students Make
| Common Mistake | Correction |
|---|---|
| Confusing genotype with phenotype. | Genotype = alleles (TT, Tt, tt); phenotype = appearance (tall, dwarf). |
| Thinking dominant means "more common". | Dominant means expressed even with one copy; frequency is a different idea. A recessive trait may still be common in a population. |
| Assuming dominant means "better". | Dominant/recessive is about expression, not quality. |
| Forgetting recessive traits can be inherited and reappear. | Recessive alleles hide in heterozygotes (Tt) and reappear as tt in later generations. |
| Mixing up genotype and phenotype ratios. | Monohybrid F2: genotype 1:2:1; phenotype 3:1. Write them together. |
| Confusing XX and XY. | Female = XX; male = XY. The ovum is always X; the sperm can be X or Y. |
| Treating acquired traits as automatically inherited. | Only changes in the DNA of gametes are inherited; body changes during life generally are not. |
| Constructing Punnett squares incorrectly (mixing up parent gametes). | List the gametes of each parent along the top and side; combine top gamete with side gamete in each cell. |
| Confusing gene, DNA and chromosome. | DNA is the molecule; a gene is a segment of DNA; a chromosome is DNA coiled with proteins that carries genes. |
| Saying the mother decides the child’s sex. | The mother’s ovum always has X; it is the father’s sperm (X or Y) that decides. |
17. Exam Tips
- Solve a monohybrid cross in four steps: write parents (P) → gametes → F1 → F1 self → F2. Always show genotype and phenotype.
- Construct a Punnett square properly: put gametes of one parent across the top, gametes of the other down the side, and combine each row-column pair in the cells.
- Identify genotype and phenotype: ask "what alleles?" for genotype and "what does it look like?" for phenotype.
- Write ratios clearly: genotype ratio 1:2:1 and phenotype ratio 3:1 for a monohybrid cross, with symbols defined (T = tall, t = dwarf).
- Answer sex-determination questions with the XX/XY scheme: ovum (X) + sperm (X) = girl; ovum (X) + sperm (Y) = boy; state the approximate 50% chance and that the sperm decides.
- Use important keywords: gene, allele, dominant, recessive, homozygous, heterozygous, genotype, phenotype, inherited, acquired.
- Present diagrams neatly: rule-drawn boxes, clear alleles, labelled P, F1, F2 and the ratios; meaningful titles.
- For "distinguish" questions, use a table: put each difference in its own row for easy marking.
- For case/application questions: start from the given phenotype, deduce the genotype, then work out the cross.
18. Quick Revision
- Heredity: passing of traits from parents to offspring through genes.
- Variation: differences in traits among individuals of a species.
- DNA: the molecule of heredity; copied during reproduction (copying is not perfect).
- Gene: a segment of DNA that codes for one trait.
- Chromosome: thread-like DNA + protein structure carrying many genes.
- Mendel: Father of Genetics; worked on pea plants using contrasting traits.
- Dominant trait (T): expressed even with one copy (TT or Tt).
- Recessive trait (t): expressed only in tt.
- Genotype: alleles, e.g. TT/Tt/tt. Phenotype: appearance, e.g. tall/dwarf.
- Monohybrid cross: one character; F1 all dominant; F2 = 3:1 phenotype, 1:2:1 genotype.
- Punnett square (Tt × Tt): cells TT, Tt, Tt, tt.
- Sex determination: female XX (ova always X); male XY (sperms X or Y); offspring XX (girl) or XY (boy) – about 50% each; the father’s sperm decides.
- Inherited vs acquired traits: inherited pass through genes; acquired form in life and are generally not passed on.
- Variation’s role: survival in changing environments and raw material for evolution.
19. Self-Practice Questions
Original – CBSE Exam-Style (answers and hints are in section 20)
MCQs
- Which pair of chromosomes decides sex in humans? (a) Autosomes (b) Sex chromosomes (c) Both pairs 22 and 23 (d) Only the Y chromosome
- A gene is: (a) a thread-like structure (b) a segment of DNA that codes for a trait (c) a protein (d) a gamete
- In the cross TT × tt, the genotype of every F1 plant is: (a) TT (b) tt (c) Tt (d) T
- The F2 phenotypic ratio of a monohybrid cross is: (a) 1:2:1 (b) 3:1 (c) 1:1 (d) 9:3:3:1
- Which of the following is an acquired trait? (a) Eye colour (b) Scar from an injury (c) Flower colour in pea (d) Blood group
- How many types of sperms does a male human produce for sex chromosomes? (a) 1 (b) 2 (c) 22 (d) 46
- A dwarf pea plant can have only the genotype: (a) TT (b) Tt (c) tt (d) any of these
- What does a test cross use as one parent? (a) Homozygous dominant (b) Heterozygous (c) Homozygous recessive (d) Any plant
- An offspring gets its genes from: (a) mother only (b) father only (c) both parents (d) neither
- Variation is important because it: (a) makes all offspring identical (b) helps populations survive changes (c) stops reproduction (d) removes all traits
Very Short Answer Questions
- Define heredity.
- Name the Greek-letter symbol used for a dominant allele in Mendel’s convention (use T/t example).
- What is a chromosome made up of?
- Write the gametes formed by a Tt plant.
- State the genotypic ratio of a monohybrid F2 generation.
Short Answer Questions
- Why did Mendel use pea plants for his experiments? Give two reasons.
- Differentiate between phenotype and genotype with an example.
- Differentiate between homozygous dominant and heterozygous genotypes.
- Why is the ovum always X in humans?
- Give one example each of an inherited and an acquired trait.
Reasoning / Application Questions
- If a cross Tt × tt gives 1:1 tall:dwarf, work out the Punnett square and confirm the ratio.
- A tall plant crossed with a tall plant produced 25% dwarf offspring. Determine the parental genotypes.
- Explain why a recessive trait may disappear for one generation and reappear in the next.
- Among two identical twins, one becomes a bodybuilder. Would his child inherit the muscles? Give the reason.
- In humans a daughter having free earlobes (Ff) married a man with attached lobes (ff). Find the probability that their child will have free earlobes.
Punnett-Square Problems
- Construct a Punnett square for TT × tt and state all offspring genotypes.
- Construct a Punnett square for Tt × Tt, and state both ratios (genotype and phenotype).
- A yellow-seeded plant (Yy) self-pollinates. Make the Punnett square and state the F2 seed-colour ratio.
Case-Based Questions
- Case: In a family, both parents have attached earlobes (ff). Predict the earlobes of all their children with a reason.
- Case: In a maternity study, 40 sperms were examined: 21 carried X and 19 carried Y. Predict the expected percentage of boys and girls, and state whether the mother has any role in deciding sex.
20. Answer Key / Hints
Refer to these only after you have attempted the questions in section 19.
| Q.No. | Answer / Hint |
|---|---|
| 1 | (b) Sex chromosomes |
| 2 | (b) A segment of DNA that codes for a trait |
| 3 | (c) Tt |
| 4 | (b) 3:1 |
| 5 | (b) Scar from an injury |
| 6 | (b) 2 (X and Y sperms) |
| 7 | (c) tt |
| 8 | (c) Homozygous recessive |
| 9 | (c) both parents |
| 10 | (b) helps populations survive changes |
| 11 | Heredity = transmission of traits from parents to offspring through genes. |
| 12 | Dominant allele = capital letter (T), recessive = small letter (t). |
| 13 | DNA coiled around proteins (histones). |
| 14 | T and t (two kinds of gametes). |
| 15 | 1 TT : 2 Tt : 1 tt. |
| 16 | Contrasting traits; short generation; many offspring; easy self/cross-pollination (any two). |
| 17 | Phenotype = appearance (tall); genotype = allele make-up (TT/Tt/tt). |
| 18 | Homozygous dominant (TT) has identical dominant alleles; heterozygous (Tt) has one dominant + one recessive allele; both appear tall. |
| 19 | Female has XX; gametes get one X each; no Y exists in ova. |
| 20 | Inherited: eye/height; acquired: scar or learned skill (examples may vary). |
| 21 | Gametes (T,t) × (t) produces Tt, tt in equal numbers → 1:1. Hint: use a small Punnett square. |
| 22 | Parents are both Tt (Tt × Tt gives 1/4 tt). |
| 23 | The recessive allele is hidden in Tt (heterozygotes); it shows only when two t alleles combine (tt). |
| 24 | No – muscles are acquired (body cells), not in the DNA of gametes, so not inherited. |
| 25 | Ff × ff → Ff, Ff, ff, ff → 50% chance of free earlobes. |
| 26 | TT × tt → all Tt (tall). |
| 27 | Tt × Tt → TT, Tt, Tt, tt; genotype 1:2:1; phenotype 3 tall : 1 dwarf. |
| 28 | Yy × Yy → YY, Yy, Yy, yy; seeds 3 yellow : 1 green. |
| 29 | ff × ff → all ff; all children have attached earlobes. |
| 30 | About 50% boys (Y-sperms) and 50% girls (X-sperms); the mother (always X) does not decide sex. |