Heredity Class 10: Notes and Important Questions

Heredity Class 10 explains how traits pass from parents to children, why children are similar to their parents but not identical, and how a child's sex is decided. This guide covers variation, Mendel's experiments with garden peas, the 3:1 and 9:3:3:1 ratios worked out step by step, how genes control traits, and sex determination, with important questions and model answers at the end.

How variation builds up

Offspring inherit a basic body design from their parents, but each one also shows small differences called variations.

• Asexual reproduction (for example, one bacterium dividing into two): differences are very small, mainly from tiny inaccuracies in DNA copying. With only one parent, diversity is limited.
• Sexual reproduction: two parents combine their genetic material, which creates far more variation.

Over generations these small changes add up. Picture one organism giving two offspring, each with a slightly different DNA copy; those give four, each carrying inherited differences plus new ones.

Whether a variation lasts depends on the environment. During a heatwave, bacteria that can withstand higher temperatures survive better. So variation is the raw material for adaptation.

Thinking question: In an asexually reproducing population, trait A is in 10% of individuals and trait B in 60%. Trait B probably appeared first. Traits pass to all descendants in asexual reproduction, so a trait found in more individuals has had more generations to spread.

Inherited traits

All children have basic human features like two eyes and two ears, but they differ in details such as nose shape, hair colour or earlobe type. Earlobes can be free (hanging loose) or attached (joined to the side of the head).

The key rule: father and mother contribute equal amounts of DNA. So in the simple Mendelian model, a child has two versions of each trait, one from each parent. Which one shows depends on how the two versions interact.

Gregor Johann Mendel, a 19th-century scientist and mathematician, studied these rules with garden peas. By counting plants with each trait in every generation, he turned the study of inheritance into a quantitative science.

Mendel's monohybrid cross

Mendel crossed tall pea plants with short ones.

• F1 generation: all plants were tall. None were medium. So tallness is dominant.
• F2 generation (F1 plants self-pollinated): one-quarter (25%) were short.

The F1 plants must have carried the shortness factor without showing it. Mendel proposed that each trait has two factors, now called genes. If the two are different (Tt), the dominant one (T) decides the appearance and the recessive one (t) stays hidden. Shortness shows only in tt. Any plant with at least one T (TT or Tt) is tall.

Worked example: Tt × Tt

• Genotypes: TT : Tt : tt = 1 : 2 : 1
• Phenotypes: TT and Tt are tall, tt is short, so tall : short = 3 : 1

Genotype is the inherited combination of alleles; phenotype is the characteristic you can see. The recessive allele did not disappear in F1; it was only masked.

Use a capital letter for the dominant form and the same letter in lowercase for the recessive form (T and t, not T and S).

Independent inheritance: the dihybrid cross

Mendel then followed two traits at once: seed shape (round is dominant over wrinkled) and seed colour (yellow is dominant over green).

• Parents: round yellow (RRYY) × wrinkled green (rryy)
• F1: all round yellow, because those are the dominant forms
• F2: round yellow : round green : wrinkled yellow : wrinkled green = 9 : 3 : 3 : 1

Round green and wrinkled yellow were not in either parent. These new combinations show that seed shape and seed colour can be inherited independently in this cross. The two pairs of factors separate when germ cells form and come together in new combinations at fertilisation. This mixing is called independent assortment. It does not mean every pair of human traits is independent.

Worked example: If a dihybrid F2 gives 160 seeds, the expected numbers are 160 × 9/16 = 90 round yellow, 160 × 3/16 = 30 round green, 30 wrinkled yellow and 160 × 1/16 = 10 wrinkled green. Check: 90 + 30 + 30 + 10 = 160.

How genes produce traits

DNA is the information source for making proteins. A gene is a section of DNA that carries the instructions for one particular protein. Many proteins work as enzymes and control characteristics.

From gene to trait (plant height):

1. Gene: a section of DNA codes for an enzyme.
2. Enzyme: the cell makes the enzyme.
3. Process: the enzyme helps produce a growth hormone.
4. Trait: lots of hormone gives a tall plant. If the gene is altered so the enzyme works less efficiently, less hormone is made and the plant stays short.

Keeping the DNA amount right: germ cells get only one set of chromosomes instead of two. When sperm and egg fuse at fertilisation, the double set is restored, one from each parent.

Sex determination in humans

Different species decide sex differently. In some reptiles, the temperature at which eggs are incubated matters. In humans, sex is decided by chromosomes.

Humans have 23 pairs of chromosomes. 22 pairs are the same in males and females and are called autosomes. In the simple XX/XY model, females have XX and males have XY.

• X-bearing sperm + X egg → XX, girl
• Y-bearing sperm + X egg → XY, boy

Each type of sperm is equally likely to fertilise the egg, giving an expected 1 : 1 ratio. So the sex of the child depends on the father's contribution. Neither parent chooses which sperm fertilises the egg, and nobody should be blamed for a child's sex.

Remember this

• Each parent gives equal DNA; each trait has two copies.
• Dominant shows with one copy (T); recessive shows only as tt.
• Monohybrid F2: genotype 1 : 2 : 1, phenotype 3 : 1.
• Dihybrid F2: 9 : 3 : 3 : 1.
• Gene → protein (enzyme) → trait.
• Mother gives X; father gives X or Y.

Important questions with answers

1. Why does every trait in a child have two versions? Each parent contributes an equal amount of DNA through the germ cells, so the child gets one copy of the gene from the father and one from the mother.

2. Distinguish between a dominant and a recessive trait. A dominant trait shows even with one copy of its gene (T in Tt). A recessive trait shows only when both copies are recessive (tt).

3. A pure tall plant (TT) is crossed with a short plant (tt). What will the F1 be? Every offspring gets T from one parent and t from the other, so all are Tt and all are tall.

4. A cross gives 400 F2 plants from Tt × Tt. How many are expected to be short? Short = 1/4 of 400 = 100. Tall = 300.

5. Why does a 1 : 2 : 1 genotype ratio give a 3 : 1 phenotype ratio? TT and Tt are both tall because T is dominant; only tt is short. So three of the four combinations are tall.

6. How did Mendel show that traits are inherited independently? In a cross of round yellow with wrinkled green, the F2 had new combinations (round green and wrinkled yellow) not seen in either parent.

7. What is a gene? A section of DNA that provides the information to make a particular protein.

8. Is it right to blame a mother for the sex of her child? No. The mother always gives an X. The child's sex depends on whether the father's sperm carries X or Y.

Common mistakes to avoid

• Writing 3 : 1 as the genotype ratio. It is the phenotype ratio; the genotype ratio is 1 : 2 : 1.
• Thinking the recessive trait vanished in F1. It was only hidden.
• Using different letters (T and S) for the two forms of one trait.
• Saying the mother decides the child's sex.

When you want more practice with crosses and ratios, study this chapter with Joy on Learnijoy.