Class 10 Science Notes Chapter 8 — Heredity#

Variation and heredity#

• Heredity — the passing of traits from parents to offspring.
• Variation — differences between individuals of a species.
• Asexual reproduction gives very little variation (only small copying errors in DNA). Sexual reproduction gives much more variation, because it combines DNA from two parents.
• Variations build up over generations. Some help survival in a changing environment; these individuals are selected by nature.

Inherited traits#

• A child shares basic features with the parents but is not identical to them.
• Each trait is influenced by both the father’s and the mother’s DNA, so every child has two versions of each trait.

Mendel’s experiments#

Gregor Johann Mendel (1822–1884) worked with garden peas (Pisum sativum), using contrasting traits: round/wrinkled seeds, tall/short plants, violet/white flowers, yellow/green seeds. He counted the offspring of each type in every generation.

Monohybrid cross (one trait)#

Tall (TT) × short (tt):

• F1 generation: all tall (Tt). No medium-height plants.
• F1 × F1 (self-pollinated): the F2 generation is 3 tall : 1 short.

• Phenotypic ratio (appearance) = 3 : 1. Genotypic ratio (gene make-up) = 1 : 2 : 1.
• Both tallness and shortness were inherited by the F1 plants, but only tallness was expressed.

• Traits are controlled by factors (now called genes), and each organism has two copies of each gene, one from each parent.
• Dominant trait — shows even with one copy (T). Recessive trait — shows only when both copies are recessive (tt).
• A germ cell gets only one copy of each gene.

Dihybrid cross (two traits)#

Round yellow (RRYY) × wrinkled green (rryy):

• F1: all round, yellow (RrYy). Round and yellow are dominant.
• F2: 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green. Mendel actually got 315 : 108 : 101 : 32 seeds.
• New combinations appear (round green, wrinkled yellow). So the two traits are inherited independently.

How traits are expressed#

• Gene — a section of DNA that carries the information for one protein.
• Proteins (often enzymes) control traits. Example: an enzyme makes a plant growth hormone. If the gene for this enzyme works well, plenty of hormone is made and the plant is tall. If the gene is altered and the enzyme is less efficient, less hormone is made and the plant is short.
• Genes are carried on chromosomes. Each cell has two copies of each chromosome, one from each parent. Germ cells get only one of each pair, so the normal number is restored after fertilisation.

Sex determination#

• In some animals, sex depends on the environment. In some reptiles, the temperature at which the eggs are kept decides the sex. Snails can change sex.
• In humans, sex is decided genetically.
  • Women have a matched pair of sex chromosomes: XX.
  • Men have a mismatched pair: XY (the Y is shorter).
• Every egg carries an X chromosome. Half the sperms carry X and half carry Y.

The sex of a child is decided by the father’s sperm, whether it carries an X or a Y chromosome. The mother always gives an X.

Common mistakes#

• Writing the F2 phenotypic ratio as 1 : 2 : 1. That is the genotypic ratio; the phenotypic ratio is 3 : 1.
• Saying a recessive trait disappears in F1. It is hidden, not lost, and reappears in F2.
• Saying the mother decides the sex of the child. It depends on the father’s X or Y sperm.