Heredity
Heredity is the transmission of characters (traits) from parents to their offspring. The rules of heredity determine how these traits are inherited. Offspring resemble their parents but also show variations. Sexual reproduction generates the greatest diversity of characters because the offspring receive genes from two parents.
Inherited Traits
Traits are controlled by genes, which are units of DNA on chromosomes. Every trait is influenced by both paternal and maternal DNA. Each gene may exist in different forms called alleles. An organism has two copies (one from each parent) of the gene for a trait.
Mendel and His Experiments
Gregor Johann Mendel, using the garden pea plant (Pisum sativum), studied the inheritance of contrasting characters such as tall/short plants, round/wrinkled seeds and green/yellow seeds. He worked with pure-breeding plants and carefully counted offspring over generations, which allowed him to propose the rules of inheritance.
Monohybrid Cross (One trait)
Mendel crossed a pure tall (TT) pea plant with a pure short (tt) plant.
- F1 generation: All plants were tall (Tt). The short character disappeared. Tall (T) is the dominant trait; short (t) is the recessive trait.
- F2 generation: Self-pollinating F1 gave tall and short plants in the ratio 3 : 1 (phenotype). The short trait reappeared, showing it was not lost, only masked.
The genotype ratio in F2 is 1 TT : 2 Tt : 1 tt. Both TT and Tt are tall (phenotype); only tt is short.
Dihybrid Cross (Two traits)
Mendel crossed plants differing in two traits — round yellow seeds (RRYY) with wrinkled green seeds (rryy).
- F1: All round, yellow seeds (RrYy).
- F2: Four types appeared in the ratio 9 : 3 : 3 : 1 — 9 round-yellow, 3 round-green, 3 wrinkled-yellow, 1 wrinkled-green. New combinations (round-green, wrinkled-yellow) appeared, proving that the two traits are inherited independently of each other.
Mendel's Laws of Inheritance
- Law of Dominance: In a pair of contrasting alleles, one (dominant) expresses itself and masks the other (recessive) in the F1 generation.
- Law of Segregation: The two alleles of a trait separate during gamete formation, so each gamete carries only one allele. They rejoin at fertilisation.
- Law of Independent Assortment: The alleles of two different traits are inherited independently of one another (basis of the 9:3:3:1 ratio).
Genotype and Phenotype
| Term | Meaning |
|---|---|
| Genotype | The genetic (allele) make-up of an organism, e.g. TT, Tt, tt. |
| Phenotype | The observable/physical appearance, e.g. tall or short. |
| Homozygous | Both alleles same (TT or tt) — pure/true-breeding. |
| Heterozygous | Both alleles different (Tt) — hybrid. |
How Traits Are Expressed
Genes control the production of proteins/enzymes. For a trait like plant height, an efficient enzyme (gene T) makes enough plant hormone for a tall plant; a less efficient version (gene t) makes a short plant. If one T gene is enough, the plant is tall — so T is dominant over t.
Sex Determination in Humans
Humans have 23 pairs of chromosomes; one pair is the sex chromosomes. Females are XX; males are XY.
- All eggs from the mother carry an X chromosome.
- Sperms from the father are of two types: half carry X, half carry Y.
- If a sperm carrying X fertilises the egg → child is a girl (XX).
- If a sperm carrying Y fertilises the egg → child is a boy (XY).
Therefore, the sex of the child is determined by the father (whether the fertilising sperm carries X or Y), not the mother. The chance of a boy or a girl is 50 : 50.