Heredity and variation
Heredity is the passing of traits from parents to offspring. Children resemble their parents, yet no two individuals (apart from identical twins) are exactly alike. The differences are variations.
In asexual reproduction, offspring are almost identical to the parent; only small variations arise from minor errors when DNA is copied.
In sexual reproduction, offspring get genes from two parents, so variation is much greater.
Variations help a species survive changing conditions. If temperatures rise, bacteria with a variation that tolerates heat are more likely to survive and reproduce.
Mendel's experiments
Gregor Johann Mendel (1822–1884), working in a monastery garden, studied inheritance in the garden pea (Pisum sativum). Peas were a good choice: they have clear contrasting traits (tall or dwarf, round or wrinkled seeds), a short life cycle, many seeds, and they normally self-pollinate, while cross-pollination can be done by hand.
The monohybrid cross
Mendel crossed a pure tall plant with a pure dwarf plant.
F1 generation: all plants were tall. There were no medium-height plants.
He let F1 plants self-pollinate. F2 generation: tall and dwarf plants appeared in the ratio of about 3 : 1.
Since the dwarf trait reappeared in F2, it had not been lost in F1; it was only hidden. Mendel concluded:
Each trait is controlled by a pair of factors (now called genes), one from each parent.
One form, the dominant trait (tall, T), shows itself even when only one copy is present.
The other, the recessive trait (dwarf, t), shows only when both copies are recessive.
Generation | Genotypes | Phenotypes |
|---|---|---|
Parents | TT × tt | tall × dwarf |
F1 | all Tt | all tall |
F2 | TT : Tt : tt = 1 : 2 : 1 | tall : dwarf = 3 : 1 |
The genotype is the gene combination (TT, Tt, tt). The phenotype is the visible trait (tall or dwarf).
The dihybrid cross
Mendel then followed two traits at once. He crossed plants with round, yellow seeds (RRYY) and wrinkled, green seeds (rryy).
F1: all round and yellow (RrYy). Round and yellow are dominant.
F2: four kinds of seeds, in the ratio 9 : 3 : 3 : 1: round yellow, round green, wrinkled yellow, wrinkled green.
New combinations (round green and wrinkled yellow) appeared, which showed that the two traits are inherited independently of each other.
How do genes control traits?
A gene is a section of DNA that carries the information for making a protein. Proteins control traits.
Example. Plant height depends on a growth hormone. The gene for tallness makes an enzyme that helps produce this hormone efficiently. If the gene is altered so that the enzyme works poorly, less hormone is made and the plant is short.
Each parent contributes one copy of each gene through its germ cells (gametes). Germ cells carry only one set of genes, half the usual number. This is how each child receives one gene of each pair from each parent, and how the number of chromosomes stays constant from generation to generation.
Sex determination
In some animals, environment decides sex. In some reptiles, the temperature at which eggs are incubated decides whether the young are male or female. Snails can change sex.
In humans, sex is determined genetically. Humans have 23 pairs of chromosomes. Twenty-two pairs are the same in both sexes; the 23rd pair are the sex chromosomes.
Women have a matched pair: XX.
Men have a mismatched pair: XY; the Y chromosome is shorter.
All eggs from the mother carry an X chromosome. Half the father's sperm carry X and half carry Y.
X sperm + X egg → XX → girl
Y sperm + X egg → XY → boy
So the sex of a child is determined by the father's sperm, and the chance of a boy or a girl is equal, 50% each.
Summary
Heredity passes traits on; sexual reproduction produces more variation.
Monohybrid F2: phenotypic ratio 3 : 1, genotypic ratio 1 : 2 : 1. Dihybrid F2: 9 : 3 : 3 : 1.
Genes are DNA segments that code for proteins, which control traits.
In humans, XX is female and XY is male; the father's sperm decides the child's sex.