Heredity

Heredity is the passing on of features or characters from one generation to the next. The rules of heredity decide how a plant or animal inherits its traits from its parents. Variation means the differences seen among individuals of the same species.

When organisms reproduce, offspring resemble their parents because they inherit a set of instructions in the form of genes. However, offspring are never exactly identical to their parents or to one another. For example, in a group of human children of the same parents, we see differences in height, complexion, ear-lobe shape and blood group. These differences are variations.

Variations arise mainly during sexual reproduction, because the offspring gets one set of genes from the mother and one from the father, and these combine in new ways. Small errors while copying DNA also create variation. In asexual reproduction (like in bacteria), offspring are nearly identical, so only tiny variations from copying errors occur.

Variation is useful for a species. If the environment changes suddenly — say the temperature of water rises for bacteria — individuals with a variation that lets them survive the heat will live and reproduce, while others die. So variation gives a species a better chance of survival over time. Variations that are inherited by offspring accumulate generation after generation.

Gregor Mendel used the garden pea plant (Pisum sativum) for his experiments because it has clear contrasting traits, such as tall/dwarf plants and round/wrinkled seeds. A cross that follows the inheritance of a single pair of contrasting characters is called a monohybrid cross.

Mendel crossed a pure tall plant (TT) with a pure dwarf plant (tt). All the plants in the first generation (F₁) were tall (Tt). The dwarf character seemed to disappear. This showed that the tall factor (T) is dominant and the dwarf factor (t) is recessive. Both parents contribute one factor each, so the F₁ plant carries both T and t but looks tall.

Mendel then self-pollinated the F₁ tall plants (Tt × Tt). In the second generation (F₂), the dwarf trait reappeared. He obtained tall and dwarf plants in the ratio 3 tall : 1 dwarf. Working out the cross: the gametes T and t combine to give TT, Tt, Tt and tt.

• 1 TT (tall)
• 2 Tt (tall)
• 1 tt (dwarf)

So the phenotypic ratio is 3 : 1 (tall : dwarf) and the genotypic ratio is 1 : 2 : 1 (TT : Tt : tt). This proves that a trait is controlled by a pair of factors (alleles) that separate during gamete formation, and that a recessive trait is not lost — it is only hidden in the presence of a dominant factor.

A dihybrid cross follows the inheritance of two pairs of contrasting characters at the same time. Mendel crossed pea plants having round, yellow seeds (RRYY) with plants having wrinkled, green seeds (rryy). Here round (R) is dominant over wrinkled (r), and yellow (Y) is dominant over green (y).

All F₁ plants had round, yellow seeds (RrYy), showing round and yellow are the dominant traits. Mendel then self-pollinated the F₁ plants (RrYy × RrYy). In the F₂ generation he obtained four types of seeds in the ratio 9 : 3 : 3 : 1:

• 9 round, yellow
• 3 round, green
• 3 wrinkled, yellow
• 1 wrinkled, green

The important point is that new combinations appeared in F₂ — round-green and wrinkled-yellow — which were not present in the original parents. This proves that the two pairs of traits (seed shape and seed colour) are inherited independently of each other. The gene for seed shape assorts separately from the gene for seed colour during gamete formation.

This is why a child can, for example, inherit the mother's hair type together with the father's eye colour — different traits are shuffled independently.

Each trait of an organism is controlled by a pair of genes (alleles), one inherited from the mother and one from the father, located on chromosomes. A gene works by giving instructions to make a specific protein, often an enzyme, and this protein controls how the trait appears.

Consider plant height. Let us say a gene codes for an enzyme that helps make a plant hormone which makes the plant grow tall. The dominant allele T makes an efficient enzyme, so enough hormone is produced and the plant is tall. The recessive allele t makes a less efficient enzyme, so less hormone is made. A plant with even one T allele (Tt) makes enough hormone and is tall — this is why T is dominant. Only a plant with both recessive alleles (tt) is dwarf.

This explains the rules of inheritance:

1. Traits are controlled by pairs of alleles.
2. During gamete (sex-cell) formation, the two alleles separate, so each gamete carries only one allele of a pair.
3. At fertilisation, the offspring again gets one allele from each parent, restoring the pair.
4. A dominant allele is expressed even in a single dose (Tt); a recessive allele is expressed only when present in a double dose (tt).

Because each parent contributes one allele of every gene, both parents make an equal genetic contribution to the offspring, even though the offspring's appearance may resemble one parent more for a given trait.

In human beings the sex of an individual is decided genetically by a special pair of sex chromosomes. A human cell has 23 pairs (46) of chromosomes. Of these, 22 pairs are autosomes and one pair is the sex chromosomes.

A female has two X chromosomes, written as XX. A male has one X and one Y chromosome, written as XY. During gamete formation the pair separates:

• All eggs (ova) produced by the mother carry one X chromosome.
• Half of the sperms produced by the father carry an X chromosome and the other half carry a Y chromosome.

When a sperm carrying an X chromosome fertilises the egg (X), the child is XX — a girl. When a sperm carrying a Y chromosome fertilises the egg (X), the child is XY — a boy.

Since the egg always contributes an X chromosome, the sex of the child depends entirely on whether the fertilising sperm carries an X or a Y chromosome. Therefore, the sex of the child is determined by the father, not the mother. Because X- and Y-bearing sperms are produced in roughly equal numbers, the chance of having a boy or a girl is about 50 : 50.