Chapter 8: Heredity (Biology)

In asexual reproduction, a new variation is passed on to all the offspring of the individual in which it appears, and it spreads through the population over generations. The longer a trait has existed, the more individuals it will have reached. So trait B (60%) is likely to have arisen earlier than trait A (10%).

Trait B, because it has had more time to spread through the population.

Individuals with different variations respond differently to changes in the environment. If conditions change (heat, a new disease, shortage of food), individuals with suitable variations survive and reproduce while others may die. For example, bacteria that can tolerate heat survive a heat wave. So variation ensures that at least some members survive, and the species continues.

Some variants are better suited to changed conditions and survive, so the species is not wiped out.

Mendel crossed pure tall pea plants (TT) with pure short (dwarf) plants (tt). All the F1 plants were tall; none was short, even though each had received a "short" gene (Tt). So the tall trait expressed itself and the short trait was hidden: tall is dominant and short is recessive.

When the F1 plants were self-pollinated, the F2 generation had tall and short plants in the ratio 3 : 1. The short trait reappeared, showing that it had been present but hidden in F1.

All F1 plants of a tall × short cross were tall (dominant), while the short trait (recessive) was hidden and reappeared in one-fourth of the F2 plants.

Mendel crossed plants differing in two traits: round, yellow seeds (RRYY) × wrinkled, green seeds (rryy). All F1 plants had round, yellow seeds (RrYy). On self-pollinating F1, the F2 generation had four kinds of seeds:

Two of these, round green and wrinkled yellow, are new combinations not seen in either parent. This could happen only if the seed-shape gene and the seed-colour gene were passed on separately. So traits are inherited independently.

In a dihybrid cross the F2 showed new combinations (round green, wrinkled yellow) in the ratio 9 : 3 : 3 : 1, so the two traits were inherited independently.

No. One child is too small a sample. The daughter's O could arise in two ways:

• If A is dominant, the father could be AO (carrying a hidden O). The daughter got O from each parent, giving OO.
• If O were dominant, the daughter would show O whenever she received an O gene, which she did from her mother.

Both explanations fit this single family, so we cannot decide. We would need data from many families and many children. (In fact the A trait is dominant over O.)

No; with only one child, the result can be explained whether A or O is dominant. Many more offspring would be needed.

Humans have 23 pairs of chromosomes; one pair are the sex chromosomes. Women have a matched pair, XX; men have a mismatched pair, XY.

• All eggs from the mother carry an X chromosome.
• Half the sperms carry an X and half carry a Y.

So the sex of the child depends on which sperm fertilises the egg: an X sperm gives a girl, a Y sperm a boy. There is an equal chance of either.

The child's sex depends on whether the father's sperm brings an X (girl, XX) or a Y (boy, XY) chromosome.

• All progeny are violet, so the tall parent passed W to every offspring: it must be WW (a Ww parent would give about half white).
• Half the progeny are short (tt), so the tall parent gave t to half of them: it must be Tt.

So the tall parent is TtWW. (Cross: TtWW × ttww gives TtWw and ttWw in equal numbers, all violet.)

(c) TtWW

No. The observation only shows that the trait is inherited. A light-eyed child of light-eyed parents is possible whether light colour is dominant (parents and child each have at least one copy) or recessive (all three have two copies). To decide, we would need to study crosses between light-eyed and dark-eyed parents and see which colour appears in their children, and in what ratio, over many families.

No; the data only show that eye colour is inherited. We would need to study children of parents with different eye colours to tell dominance.

• Choose pure-bred parents: for example, a black dog and a white dog, each from a line that has produced only that colour for many generations.
• First cross: mate them and record the coat colour of all the puppies (F1). If all puppies are black, black is dominant over white (and vice versa).
• Second cross: mate F1 dogs with each other. If black is dominant, the F2 puppies should be in the ratio of about 3 black : 1 white.
• Repeat with several pairs of dogs to collect enough data, and record the results in a table.

The colour that appears in all F1 puppies and in three-fourths of the F2 is the dominant coat colour.

Cross pure-bred dogs of two colours: the colour shown by all F1 puppies (and 3 : 1 in F2) is dominant.

Each body cell has chromosomes in pairs. When the germ cells (sperms and eggs) are formed, the number of chromosomes is halved: each germ cell gets only one chromosome from each pair (23 in humans). At fertilisation, a sperm and an egg fuse, so the zygote gets 23 chromosomes from the father and 23 from the mother, restoring the full number of 46. So both parents contribute equally to the DNA of the child.

Germ cells carry half the chromosomes (one of each pair); their fusion gives the zygote one set from each parent.