Chapter 8: Heredity

(c) TtWW. All progeny are violet, so the tall (violet) parent must give W to every offspring → it is WW (white parent is ww). About half are short → for height the tall parent is Tt (Tt × tt gives 1 tall : 1 short). So its make-up is Tt WW.

No, we cannot decide from this alone. Both dominant and recessive traits can show this pattern: if light eye is recessive, two light-eyed (recessive) parents will always have light-eyed children; if it is dominant, light-eyed parents may also have light-eyed children. We need more data — for example, whether dark-eyed parents can produce a light-eyed child — to determine dominance.

Cross dogs of two pure contrasting coat colours (e.g., black × brown) and observe the coat colour of the F₁ puppies. The colour that appears in all/most F₁ offspring is the dominant trait; the hidden colour is recessive. Repeat with several litters and (self-)cross F₁ to confirm using the 3:1 ratio in the next generation.

During gamete formation, the germ cells receive only half the chromosomes (one of each pair). The male gamete (sperm) and female gamete (egg) each carry one set of chromosomes. At fertilisation these combine, so the offspring receives equal genetic material from both parents (23 chromosomes from each in humans → 46).

3:1.

Recessive.

Male (boy).

1 TT : 2 Tt : 1 tt.

A dominant trait appears in the offspring even when only one allele for it is present; a recessive trait appears only when both alleles for it are present.

Because they do not change the DNA of the germ cells (gametes), so they are not passed to the offspring.

A gene is a segment of DNA that controls a particular trait and is passed from parents to offspring.

Because they inherit genes (DNA) from both parents that carry the parental traits.

Variation helps species survive in changing environments and drives evolution.

Mendel crossed pure tall (TT) with pure short (tt) pea plants. All F₁ plants were tall (Tt), showing tall is dominant. When F₁ were self-pollinated, the F₂ generation had tall and short plants in the ratio 3:1 (genotypes 1 TT : 2 Tt : 1 tt). This proved that recessive traits reappear in F₂.

Human females have two X chromosomes (XX) and males have XY. The egg always carries an X. Sperms carry either X or Y. If an X-sperm fertilises the egg, the child is a girl (XX); if a Y-sperm does, the child is a boy (XY). Thus the father's chromosome determines the sex.

A cross considering two pairs of contrasting traits together (e.g., round-yellow × wrinkled-green seeds). Its F₂ generation shows a 9:3:3:1 ratio, proving the two traits are inherited independently.

Inherited traits are present in the genes and passed on (e.g., eye colour, blood group). Acquired traits are gained during life and are not passed on (e.g., a scar, muscles built by exercise, a learned language).

Genes direct the making of proteins/enzymes that control a characteristic. A change in the gene can change the protein and hence the trait.

So that when the male and female gametes fuse at fertilisation, the normal chromosome number is restored in the offspring (and does not double each generation).