Accumulation of Variation and Mendel’s Laws of Inheritance

Introduction — Why Do Children Resemble Their Parents?

Look around any family — a child may have her mother's eyes, her father's dimple, and her grandmother's curly hair. Yet no two siblings (except identical twins) are exactly alike. Some traits clearly pass from parent to child; others seem to skip a generation and reappear later. How does nature decide which features get passed on, and in what combination?

That is the story of heredity. In this chapter we will see how Gregor Mendel, a monk working with humble pea plants, uncovered the rules of inheritance more than 150 years ago — rules that still guide modern genetics.

8.1 Accumulation of Variation During Reproduction

Reproduction transfers the DNA "design" of a parent to its offspring. But the copying is never perfect — small differences called variations always creep in.

• Asexual reproduction: only one parent, one DNA source. Errors during DNA copying are few, so offspring show only tiny variations — almost identical to the parent.
• Sexual reproduction: two parents contribute DNA. Offspring inherit a new mixture of the two parents' genes. The variations produced are large.

Variations, once produced, are not lost. They get accumulated generation after generation. Over long periods, the accumulated variations lead to the diversity of life and form the raw material for evolution. Whenever the environment changes, a few individuals who happen to carry a useful variation survive and reproduce. That is how a species adapts without any individual being "trying" to change.

8.2 Heredity

Heredity is the passing of traits from parents to offspring. The information that decides a trait is carried by genes, which are short segments of DNA (deoxyribonucleic acid). The DNA is tightly coiled inside thread-like structures called chromosomes, which sit in the nucleus of every body cell.

A human body cell contains 23 pairs of chromosomes = 46 chromosomes in total:

• 22 pairs are autosomes — they control the ordinary body features.
• 1 pair is of sex chromosomes — they decide whether the person is male or female (discussed in Part 2).

Each chromosome of a pair comes from one parent — one from the mother (through the egg) and one from the father (through the sperm). So, for every gene you carry two copies, one inherited from each parent.

8.2.1 Inherited Traits

Because every gene comes in two copies, one copy may be slightly different from the other. These alternative forms of a gene are called alleles.

• A dominant allele expresses its trait whenever even one copy is present. It is written with a capital letter — for example, T for tall.
• A recessive allele expresses its trait only when both copies are of the recessive type. It is written in lowercase — for example, t for short/dwarf.

So a plant with genes TT or Tt will look tall, while only a tt plant will look dwarf. TT and tt are called homozygous (both alleles same); Tt is heterozygous (alleles different).

8.2.2 Rules for the Inheritance of Traits — Mendel's Contributions

Gregor Johann Mendel (1822–1884), an Austrian monk who taught physics and natural history, is remembered as the "Father of Genetics". In 1866 he published the results of eight years of patient experiments in the garden of his monastery at Brno.

Mendel chose the garden pea (Pisum sativum) because it had:

• short life cycle and produced many offspring;
• clear contrasting traits — tall/dwarf, round/wrinkled seeds, yellow/green seeds, violet/white flowers, etc. (7 traits in all);
• flowers that normally self-pollinate but can easily be cross-pollinated by the experimenter.

(a) Monohybrid Cross — One Trait at a Time

Mendel took pure-breeding tall plants (genotype TT) and pure-breeding dwarf plants (genotype tt). He cross-pollinated them to produce the first filial (F₁) generation.

What Mendel saw:

• Every F₁ plant was tall — the dwarf trait seemed to disappear.
• When F₁ plants were allowed to self-pollinate, the F₂ generation showed both tall and dwarf plants in a 3 : 1 ratio.
• The dwarf trait had not disappeared — it was only hidden in F₁. Mendel called the visible trait dominant (T) and the hidden one recessive (t).

(b) Dihybrid Cross — Two Traits Together

Mendel next asked: do two traits inherit together or independently? He crossed plants with round yellow seeds (RRYY) with plants bearing wrinkled green seeds (rryy).

• All F₁ plants had round, yellow seeds (RrYy) — confirming dominance of round over wrinkled and yellow over green.
• When F₁ was self-pollinated, F₂ showed four phenotypes in the ratio 9 : 3 : 3 : 1.