Heredity — Notes

Each generation inherits the basic body design of the previous one, together with small changes arising from DNA copying. These variations are passed on and accumulate, so a population becomes more diverse over generations.

• Asexual reproduction: offspring differ only by the small changes from DNA copying, so variation is minimal.
• Sexual reproduction: DNA from two parents is combined in new ways in every offspring, so it produces much greater diversity.

Not all variations are equally useful. Depending on the environment, some variations help individuals survive and reproduce better (for example, heat-tolerant bacteria in water that warms up). The environment selects among variations that already exist.

Offspring share the basic features of their parents but differ in details. Earlobes, for example, may be free or attached; both forms are found in human populations.

Rules of inheritance:

• Both parents contribute equal amounts of genetic material to the child.
• So for each trait, a child has two versions of the gene: one from the father and one from the mother.
• When the two versions differ, the one that shows is dominant; the one that is hidden is recessive.

The genotype is the combination of gene versions (TT, Tt or tt); the phenotype is the trait seen (tall or short).

Gregor Mendel worked with garden peas, which show clear contrasting characters (tall/short plants, round/wrinkled seeds, yellow/green seeds, violet/white flowers and others).

Cross: pure tall (TT) × pure short (tt) → $F_1$: all tall (Tt), no medium plants. $F_1$ self-pollinated: germ cells T and t in equal numbers → $F_2$: TT : Tt : tt = 1 : 2 : 1 (genotype), 3 tall : 1 short (phenotype).

Conclusions: a single copy of T is enough to make a plant tall (T is dominant); the short trait was hidden in $F_1$, not lost, and reappeared in $F_2$.

Cross: tall plants with round seeds (TTRR) × short plants with wrinkled seeds (ttrr) → $F_1$: all tall, round (TtRr).

$F_1$ plants form four kinds of germ cell — TR, Tr, tR, tr — in equal numbers. In $F_2$: 9 tall round : 3 tall wrinkled : 3 short round : 1 short wrinkled.

Two new combinations (tall wrinkled, short round) appear, so the traits for height and seed shape are inherited independently. Looking at one character alone still gives 3 : 1 (for example 12 tall : 4 short).

Cellular DNA is the information source for making proteins. A section of DNA that provides the information for one protein is a gene.

Plant height: height depends on the amount of a particular plant hormone. The amount of hormone depends on how efficient the enzyme that makes it is. An efficient enzyme (from gene version T) makes plenty of hormone → tall plant. If the gene is altered so that the enzyme is less efficient (version t), less hormone is made → short plant. One efficient copy (Tt) is enough for a tall plant, which is why tall is dominant.

So genes control traits through the proteins they make.

Genes are carried on chromosomes, which are separate, independent pieces of DNA. Each body cell has two copies of each chromosome, one from each parent. When germ cells are made, each receives one chromosome from each pair, which may be the one from the father or the one from the mother. When two germ cells fuse, the zygote again has two copies of each chromosome.

This explains Mendel’s rules: each parent passes on one version of each gene, and chromosome pairs sort independently, so traits on different chromosomes are inherited independently.

Different species use different strategies. In some animals, such as many reptiles, the temperature at which fertilised eggs develop decides whether the young are male or female. In some animals, such as snails, individuals can change sex. In humans, sex is determined genetically.

Human body cells have 23 pairs of chromosomes: 22 pairs of autosomes (alike in both sexes) and one pair of sex chromosomes — XX in women and XY in men (Y is shorter). Every egg carries an X. Half the sperms carry X and half carry Y. Egg + X sperm → XX, girl; egg + Y sperm → XY, boy. So the father’s sperm decides the sex, and the chance of a boy or a girl is one half at each birth.