Heredity

What this chapter is about

Heredity is the biological process by which parents pass traits to their offspring. This chapter explains how characteristics such as eye colour, height and blood group are transmitted from one generation to the next through genes located on chromosomes. You will learn the basic rules of inheritance first discovered by Gregor Mendel through his experiments on pea plants.

At the Class 10 level, you study heredity because it connects earlier work on cell division (mitosis and meiosis) with the idea that DNA carries genetic information. Understanding heredity helps you see why children resemble their parents yet are not identical to them, and why certain disorders appear in families.

After studying this chapter, you should be able to distinguish between dominant and recessive traits, predict the outcome of a monohybrid cross, explain the role of chromosomes in sex determination, and describe how variations arise in a population.

Key ideas

• A gene is a unit of heredity: a segment of DNA on a chromosome that codes for a specific protein and hence a specific trait.
• Alleles are different forms of the same gene; an organism inherits one allele from each parent, giving it a pair for every gene.
• A dominant allele expresses itself even when only one copy is present; a recessive allele expresses itself only when both copies are the same.
• Genotype refers to the genetic makeup (e.g., Tt), while phenotype refers to the observable trait (e.g., tall plant).
• Mendel's Law of Segregation: the two alleles of a gene separate during gamete formation, so each gamete carries only one allele.
• In humans, sex is determined by sex chromosomes: females have XX and males have XY; the father's gamete decides the sex of the child.
• Variations arise from the shuffling of genes during sexual reproduction and from occasional mutations in DNA.

Formulas and facts to remember

1. Homozygous: both alleles identical (TT or tt). Heterozygous: alleles different (Tt).
2. Monohybrid cross phenotypic ratio (F₂ generation): 3 dominant : 1 recessive.
3. Monohybrid cross genotypic ratio (F₂ generation): 1 homozygous dominant : 2 heterozygous : 1 homozygous recessive.
4. Probability of a child being male = 1/2; probability of being female = 1/2, because the father contributes either X or Y with equal chance.
5. Mendel chose pea plants because they have clearly contrasting traits, short life cycle, and allow both self-pollination and cross-pollination.
6. Acquired traits (e.g., muscles built by exercise) are not inherited because they do not change the DNA in reproductive cells.
7. A Punnett square is a grid used to predict genotype and phenotype ratios of offspring from a genetic cross.

Worked examples

Example 1: Monohybrid cross in pea plants

A pea plant with round seeds (RR) is crossed with a pea plant with wrinkled seeds (rr). What are the genotypes and phenotypes of the F₁ and F₂ generations? (Round is dominant over wrinkled.)

Solution

Step 1 – Identify parental gametes. Parent 1 (RR) produces gametes: all R. Parent 2 (rr) produces gametes: all r.

Step 2 – F₁ generation. All offspring receive one R and one r, so genotype = Rr. Because R is dominant, phenotype = round seeds.

Step 3 – Cross two F₁ plants (Rr × Rr) to get F₂.

Punnett square:

• R · R: RR · r: Rr
• r · R: Rr · r: rr

Genotypic ratio: 1 RR : 2 Rr : 1 rr. Phenotypic ratio: 3 round : 1 wrinkled.

---

Example 2: Sex determination in humans

A couple has four children. What is the probability that all four are girls?

Solution

Step 1 – The mother (XX) can contribute only X. The father (XY) contributes X or Y with probability 1/2 each.

Step 2 – Probability of one child being a girl (XX) = 1/2.

Step 3 – For four independent births, probability that all are girls = (1/2) × (1/2) × (1/2) × (1/2) = 1/16.

---

Example 3: Identifying genotype from offspring data

In a cross between two tall pea plants, a farmer finds that about one-quarter of the offspring are dwarf. What can you conclude about the genotypes of the parent plants?

Solution

Step 1 – Dwarf appears despite both parents being tall; therefore dwarf must be recessive (let t = dwarf, T = tall).

Step 2 – For one-quarter (1/4) dwarf offspring, parents must both be heterozygous: Tt × Tt.

Step 3 – Punnett square confirms: 1 TT : 2 Tt : 1 tt → 3 tall : 1 dwarf.

Conclusion: Both parent plants have the genotype Tt.

Common mistakes

• Thinking that a dominant trait is more common in a population → Dominant only means it masks the recessive allele in heterozygotes; frequency depends on allele distribution.
• Believing the mother determines the sex of a child → The father's sperm (X or Y) determines sex; the mother always contributes X.
• Confusing genotype with phenotype → Genotype is the allele combination (e.g., Tt); phenotype is the visible trait (e.g., tall).
• Assuming acquired characteristics are inherited → Only changes in DNA of germ cells pass to offspring; traits gained during life do not.
• Adding ratios instead of multiplying probabilities → For independent events (like successive births), multiply the individual probabilities.

Quick revision

• Gene = unit of heredity on a chromosome; alleles = different versions of a gene.
• Dominant allele masks recessive; recessive shows only when homozygous.
• Monohybrid cross F₂ ratio: 3 dominant phenotype : 1 recessive phenotype.
• Sex in humans: XX = female, XY = male; father's gamete decides sex.
• Variations come from genetic recombination in sexual reproduction and from mutations.
• Acquired traits are not heritable because they do not alter germ-cell DNA.