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Chapter 8: Heredity

Grade 10 Science • Chapter 8

Heredity

How do living things pass traits from one generation to the next? This chapter explores the science of heredity — the biological rules that govern how characteristics travel through families, from Mendel’s landmark pea plant experiments to how your own sex was determined before you were born.

📊 Mendel’s Laws

🧬 Genetics & DNA

⚡ Sex Determination

📚 What You Will Learn

① Accumulation of Variation	⑥ Worked Examples (10+)
② Key Definitions & Terms	⑦ Practice Sets A–D
③ Mendel’s Contributions	⑧ Chapter Summary
④ How Traits Are Expressed	⑨ Exam Quick-Check (8 Points)
⑤ Sex Determination

🧬	Section 1: Accumulation of Variation During Reproduction

Every time a living thing reproduces, the offspring it produces share the same basic body plan as the parent — yet they are never quite identical. This is the foundation of heredity: the biological system by which traits are passed down across generations, carrying forward both similarities and subtle differences.

When reproduction is asexual (one parent only), offspring are very nearly identical to the parent. Tiny errors in DNA copying during cell division introduce minor differences. However, when reproduction is sexual (two parents), the mixing of genetic material from two sources creates far greater diversity among offspring.

Each new generation inherits variations from the previous generation AND gains new variations of its own. Over many generations, this process builds up a wide range of different characteristics within a species. Not every variation has an equal chance of survival — the environment selects for advantageous traits. This process forms the basis of evolution.

📈	Diagram 1 — How Variation Builds Up Over Generations

Each generation inherits differences from the previous one AND introduces new ones. Read top to bottom.

GENERATION P (Parent)

↓

GENERATION F1 (First offspring)

F1a

F1b

↓

GENERATION F2 (Second offspring — maximum diversity)

F2a

F2b

F2c

F2d

Each coloured box = a unique individual. Different colours show variation in traits. All four F2 individuals differ from each other and from their grandparent.

📚	Section 2: Key Definitions

Heredity

The biological process by which characteristics and traits are transmitted from parents to offspring through genetic material (DNA).

Gene

A specific segment of DNA that contains the instructions for producing a particular protein, which in turn controls a specific trait or characteristic in an organism.

Allele

One of two or more alternative forms of the same gene. For example, the gene for plant height has two alleles: the tall allele (T) and the short allele (t).

Dominant Trait

A trait that is expressed (visible) in the offspring even when only one copy of its allele is present. Written as a capital letter (e.g., T for tallness). A single copy is enough to show the trait.

Recessive Trait

A trait that is only expressed when both copies of the allele are the same (homozygous recessive). Written as a lowercase letter (e.g., t for shortness). It is hidden whenever the dominant allele is present.

Genotype

The actual genetic combination of alleles an organism has for a particular trait. For example: TT (homozygous dominant), Tt (heterozygous), or tt (homozygous recessive).

Phenotype

The physical appearance or observable characteristic that results from the genotype. Both TT and Tt plants look tall — they have the same phenotype (tall) but different genotypes.

Chromosome

A thread-like structure made of DNA and protein, found in the nucleus of every cell. Humans have 46 chromosomes (23 pairs). Genes are arranged along chromosomes like beads on a string.

F1 & F2 Generations

F1 (first filial generation) refers to the direct offspring of the parent cross. F2 (second filial generation) refers to the offspring produced when F1 individuals breed with each other.

Sex Chromosomes

The pair of chromosomes that determines the biological sex of an individual. In humans, females carry two X chromosomes (XX) while males carry one X and one Y chromosome (XY).

🐴	Section 3: Mendel’s Contributions & Laws of Inheritance

🧑‍🔬

Gregor Johann Mendel (1822–1884)

An Augustinian monk and scientist who studied inheritance in garden pea plants at his monastery in Brno. He was the first researcher to systematically count offspring and record precise ratios, which allowed him to discover mathematical patterns in inheritance. His work was largely ignored during his lifetime but later became the foundation of modern genetics.

Mendel’s Experimental Setup

Mendel chose garden peas (Pisum sativum) for his experiments because they grow quickly, produce large numbers of offspring, and naturally self-pollinate. He identified seven pairs of contrasting traits:

Trait	Dominant Form	Recessive Form
Plant Height	Tall (T)	Short (t)
Seed Shape	Round (R)	Wrinkled (r)
Seed Colour	Yellow (Y)	Green (y)
Pod Colour	Green	Yellow
Flower Colour	Violet	White
Flower Position	Axial (side)	Terminal (tip)
Pod Shape	Inflated	Constricted

Mendel’s Two Laws

① Law of Segregation

Every organism carries two alleles for each trait. During the formation of gametes (sperm and egg cells), these two alleles separate (segregate) so that each gamete carries only one allele. When fertilisation occurs, the offspring receives one allele from each parent, restoring the pair.

② Law of Independent Assortment

When two or more traits are inherited together, the alleles for each trait separate independently of one another during gamete formation. This means that the inheritance of one trait does not affect the inheritance of another. This is possible because different genes sit on different chromosomes.

📈	Diagram 2 — Monohybrid Cross: Tall (TT) × Short (tt)

P Generation (Parents)

Tall Plant

Short Plant

↓

Gametes

T T

t t

↓

F1 Generation — All Tall

Tall

💡 Key Observation: All F1 plants are tall even though they carry the ‘t’ allele. The dominant ‘T’ allele masks the recessive ‘t’. The shortness trait has not disappeared — it is hidden!

↓ F1 × F1 (Self-pollination)

F2 Generation — Punnett Square (Tt × Tt)

	T	t
T	TT TALL	Tt TALL
t	Tt TALL	tt SHORT

3 Tall : 1 Short

75% Tall : 25% Short

Ratio = 3:1

🧬	Section 4: How Are Traits Expressed? (Genes & Proteins)

The connection between genes and visible traits involves a chain of biological steps:

Step 1: DNA Instruction

A gene is a section of DNA that carries the code (instructions) for building a specific protein.

Step 2: Protein Synthesis

The gene’s code is used to produce a specific enzyme or structural protein in the cell.

Step 3: Biochemical Effect

The protein carries out a biological job. For example, an enzyme may produce a plant growth hormone (like auxin or gibberellin).

Step 4: Visible Trait

The amount and type of protein produced determines the physical trait. More growth hormone = tall plant. Less growth hormone = short plant.

Chromosome Mechanism: Each gene is not a single long thread of DNA but sits on a separate piece of DNA called a chromosome. Humans have 23 pairs of chromosomes (46 total). Because each gene sits on a separate chromosome, genes for different traits can sort independently during gamete formation — this is why Mendel’s Law of Independent Assortment works.

📈	Diagram 3 — Dihybrid Cross: Round Yellow (RRYY) × Wrinkled Green (rryy)

RRYY

Round + Yellow

rryy

Wrinkled + Green

↓ All F1 = RrYy (Round + Yellow)

F2 Generation Ratio (9:3:3:1)

9
Round Yellow

3
Round Green

3
Wrinkled Yellow

1
Wrinkled Green

Notice that new combinations appear in F2 (Round Green and Wrinkled Yellow) that did not exist in either parent. This proves independent assortment.

⚡	Section 5: Sex Determination in Human Beings

In humans, biological sex is determined genetically, not by the environment. Our cells contain 46 chromosomes arranged in 23 pairs. The first 22 pairs are called autosomes — identical in both males and females. The 23rd pair consists of the sex chromosomes.

Female

Two identical X chromosomes. All eggs (gametes) carry one X chromosome.

Male

One X and one smaller Y chromosome. Sperm carry either X or Y — this determines the sex of the child.

How Sex Is Determined

MOTHER (XX)

X X

All eggs carry X

FATHER (XY)

X Y

Sperm = X or Y

↓

Egg (X) + Sperm (X)

= GIRL ♀

Egg (X) + Sperm (Y)

= BOY ♂

💡 Important conclusion: The mother always contributes an X chromosome to every child. It is the father’s sperm — either X-bearing or Y-bearing — that determines whether a child is a girl or a boy. The probability is 50% for either sex.

🔬	Worked Examples

Example 1 Basic Dominant/Recessive Identification

Question: When tall pea plants (TT) are crossed with short pea plants (tt), all F1 offspring are tall. What does this tell us about tallness and shortness?

👁 Reveal Solution

Since all F1 plants show tallness and none show shortness, tallness must be the dominant trait. The shortness trait did not disappear — it is masked by the dominant tall allele. Every F1 plant has genotype Tt: they are tall in appearance (phenotype) but still carry the short allele (genotype).

Example 2 F2 Ratio Prediction

Question: Two F1 tall plants (Tt) are crossed. What phenotype ratio do you expect in F2?

👁 Reveal Solution

Cross: Tt × Tt. Possible combinations: TT, Tt, Tt, tt. Phenotypes: TT = tall, Tt = tall, Tt = tall, tt = short. Result: 3 tall : 1 short (3:1 ratio). Three out of four plants carry at least one T allele (tall phenotype). Only tt is short.

Example 3 Genotype Ratios in F2

Question: In the cross Tt × Tt, what are the genotype ratios in the F2 generation?

👁 Reveal Solution

The four possible offspring genotypes are: TT, Tt, Tt, tt. So the genotype ratio is 1 TT : 2 Tt : 1 tt. Even though TT and Tt look the same (both tall), they have different genetic compositions. Only the tt plants are short.

Example 4 Dihybrid Cross — F1 Offspring

Question: A pure round yellow pea plant (RRYY) is crossed with a pure wrinkled green plant (rryy). What will the F1 offspring look like?

👁 Reveal Solution

RRYY produces only RY gametes. rryy produces only ry gametes. All F1 offspring receive one gamete from each parent: genotype = RrYy. Since R (round) is dominant over r, and Y (yellow) is dominant over y, all F1 plants appear round and yellow.

Example 5 Sex Determination Probability

Question: A couple has three daughters. What is the probability that their next child will be a son?

👁 Reveal Solution

Each pregnancy is an independent event. The father’s sperm are 50% X-bearing and 50% Y-bearing. The sex of previous children does NOT affect future pregnancies. Therefore, the probability of the next child being a boy is exactly 50% (1 in 2), regardless of how many daughters they already have.

Example 6 Recessive Trait Identification from Family Data

Question: Two parents with free earlobes have one child with attached earlobes. Which earlobe type is dominant?

👁 Reveal Solution

The parents both show free earlobes, yet produced a child with attached earlobes. This means attached earlobes is the recessive trait, and free earlobes is the dominant trait. Both parents must be carriers (heterozygous: Ff), and the child inherited the recessive allele from each parent (ff).

Example 7 Flower Colour Cross

Question: Violet flower (VV) is crossed with white flower (vv). All F1 plants have violet flowers. Two F1 plants are crossed. What ratio of flower colours appears in F2?

👁 Reveal Solution

F1 genotype = Vv. Cross Vv × Vv gives: VV (violet), Vv (violet), Vv (violet), vv (white). Ratio = 3 violet : 1 white. The white flower trait (vv) reappears in F2 after being hidden in F1, proving it was recessive all along.

Example 8 Blood Group Genetics

Question: A man with blood group A (genotype I^Ai) and a woman with blood group B (genotype I^Bi) have children. What blood groups could their children have?

👁 Reveal Solution

Cross: I^Ai × I^Bi. Possible offspring: I^AI^B (blood group AB), I^Ai (blood group A), I^Bi (blood group B), ii (blood group O). All four blood groups (A, B, AB, and O) are possible in equal probability, making this an excellent example of multiple alleles and codominance.

Example 9 Chromosome Number in Gametes

Question: Human body cells have 46 chromosomes. How many chromosomes will be found in a human egg cell, and why?

👁 Reveal Solution

An egg cell (gamete) contains 23 chromosomes — exactly half the normal number. During gamete formation (meiosis), the chromosome number is halved so that when sperm (23 chromosomes) and egg (23 chromosomes) combine at fertilisation, the resulting zygote has the full 46 chromosomes — restoring the correct number for the species.

Example 10 Variation and Survival

Question: A population of bacteria faces a sudden rise in temperature. Most bacteria die. A few survive and reproduce. How does heredity explain this outcome?

👁 Reveal Solution

Variation exists in every population due to minor DNA copying errors during reproduction. Some bacteria carried a variation that made them more heat-tolerant. When the environment changed, this variation became advantageous. These heat-resistant bacteria survived (natural selection), reproduced, and passed the heat-tolerance trait to their offspring through heredity — illustrating how inherited variation drives evolutionary change.

✏	Practice Sets A – D

📚 Set A — Multiple Choice

A1. In a monohybrid cross between Tt × Tt, what fraction of offspring will be homozygous dominant (TT)?
(a) 1/4 (b) 1/2 (c) 3/4 (d) All of them

A2. Which of the following genotypes would show the recessive phenotype?
(a) TT (b) Tt (c) tT (d) tt

A3. A Mendelian experiment produced progeny in 3:1 ratio. This ratio refers to:
(a) Genotype ratio (b) Phenotype ratio (c) Sex ratio (d) Allele ratio

A4. In humans, sex is determined by:
(a) The mother’s chromosomes (b) The father’s chromosomes (c) Temperature (d) Nutrition

A5. A section of DNA that codes for one protein is called a:
(a) Chromosome (b) Nucleus (c) Gene (d) Gamete

✓ Show Answers

A1: (a) 1/4 | A2: (d) tt | A3: (b) Phenotype ratio | A4: (b) The father’s chromosomes | A5: (c) Gene

📚 Set B — Short Answer

B1. What is the difference between a dominant trait and a recessive trait? Give one example of each from Mendel’s experiments.

B2. Why did Mendel choose pea plants for his inheritance experiments? Give two reasons.

B3. Explain why two parents who both appear tall (phenotype) can have a short child.

B4. State Mendel’s Law of Segregation in your own words.

✓ Show Answers

B1: A dominant trait is visible even with one copy of its allele (e.g., Tallness — T). A recessive trait is only visible when both alleles are recessive (e.g., Shortness — tt).

B2: Pea plants grow quickly producing many offspring for counting; they have easily observable contrasting traits; they naturally self-pollinate making controlled crosses simple.

B3: Both parents are heterozygous (Tt — tall phenotype but carry ‘t’). Each can pass the ‘t’ allele to a child. If a child inherits ‘t’ from both parents, its genotype becomes ‘tt’ — the short phenotype.

B4: Each organism has two alleles for every trait. When gametes (sex cells) are formed, the two alleles separate so each gamete carries only one allele. The allele pair is restored when two gametes join at fertilisation.

📚 Set C — Punnett Square Problems

C1. Draw a Punnett square for the cross: Tt × tt. What is the phenotype ratio of offspring?

C2. In pea plants, round seed (R) is dominant over wrinkled seed (r). Cross Rr × Rr. What percentage of offspring will have wrinkled seeds?

C3. A man (XY) and a woman (XX) have children. Using a Punnett square, show the sex chromosome combinations possible in their children.

✓ Show Answers

C1: Tt × tt gives: Tt, Tt, tt, tt. Phenotype ratio = 1 tall : 1 short (50% each). This is called a testcross.

C2: Rr × Rr gives: RR, Rr, Rr, rr. Only ‘rr’ is wrinkled. 25% of offspring will have wrinkled seeds.

C3: Mother gives X to all children. Father gives X or Y. Results: XX (girl), XY (boy), XX (girl), XY (boy). 50% girls (XX) and 50% boys (XY).

📚 Set D — Extended / Higher Order Thinking

D1. A trait exists in only 5% of a sexually reproducing population while another trait exists in 80% of the same population. Which trait is likely to have arisen earlier, and why?

D2. A Mendelian experiment produced tall violet flower pea plants when tall plants with violet flowers were crossed with short plants with white flowers. Almost half the F1 plants were short. What does this suggest about the genotype of the tall parent? Explain fully.

D3. Why is it scientifically incorrect to blame the mother for the sex of a child? Use genetic evidence in your answer.

✓ Show Answers

D1: The trait found in 80% of the population likely arose earlier. Traits that appeared earlier have had more generations to spread through a population. Rarer traits (5%) are likely newer mutations or less advantageous for survival.

D2: If almost half the F1 plants are short, the tall parent must be heterozygous (Tt), not homozygous (TT). A TtWW × ttww cross would give half Tt (tall, violet) and half tt (short, violet), matching the observation. The tall parent’s genotype is TtWW.

D3: The mother’s cells are XX — she can only ever contribute an X chromosome to any child. It is the father’s sperm that carries either X or Y. An X-carrying sperm produces a girl (XX); a Y-carrying sperm produces a boy (XY). The mother has no genetic influence over which type of sperm fertilises the egg. Therefore, sex determination is entirely controlled by the father’s genetic contribution.

📋	Chapter Summary

🧬 Variation Variations arise during reproduction. Asexual reproduction creates very little variation; sexual reproduction maximises diversity by combining DNA from two parents.	🐴 Mendel’s Laws Law of Segregation: allele pairs separate during gamete formation. Law of Independent Assortment: different genes are inherited independently of each other.
🎒 Dominant vs Recessive Dominant traits (e.g., tall, round, violet) show in offspring with just one copy. Recessive traits (e.g., short, wrinkled, white) only show when both copies are recessive (homozygous).	⚡ Genes & Proteins Genes are segments of DNA that code for proteins. Proteins determine physical traits. An efficient enzyme = more hormone = taller plant. Altered gene = less efficient enzyme = shorter plant.
♂ ♀ Sex Determination Human females are XX; males are XY. Every child inherits X from the mother. The father’s sperm — carrying either X (girl) or Y (boy) — determines the child’s sex. Chromosomes are the physical structures that carry genes, with 23 pairs (46 total) in human body cells and 23 in gametes.

⚡	8-Point Exam Quick-Check

1	Heredity is the transfer of traits from parents to offspring through genes located on chromosomes made of DNA.
2	In Mendel’s monohybrid cross (Tt × Tt), the F2 phenotype ratio is always 3 dominant : 1 recessive.
3	The F2 genotype ratio in a monohybrid cross is 1 TT : 2 Tt : 1 tt.
4	A dihybrid F2 cross produces a 9:3:3:1 phenotype ratio — demonstrating independent assortment.
5	Dominant traits are shown by capital letters (T); recessive traits by lowercase letters (t). Two lowercase letters (tt) are needed to show a recessive phenotype.
6	Genes work by directing the production of proteins (enzymes) that control biological processes, which in turn produce visible traits.
7	Human females are XX; males are XY. Each gamete carries 23 chromosomes (half of 46). Fertilisation restores 46.
8	The father’s sperm determines the sex of the child: X-sperm → girl (XX); Y-sperm → boy (XY). The probability is 50:50 for every pregnancy.

This Grade 10 Science study guide on Heredity (Chapter 8) covers all key concepts from Mendel’s laws of inheritance, monohybrid and dihybrid crosses, dominant and recessive traits, gene expression through proteins, chromosomes, and sex determination in human beings. Students preparing for their CBSE or equivalent science board exams will find comprehensive explanations, fully solved worked examples, interactive Punnett square diagrams, and graded practice problems across four sets. Understanding heredity is foundational not only for biology but also for topics in genetics, evolution, and biotechnology at higher levels. Bookmark this page for revision before your Chapter 8 science exam.