Mendel and his experiments
Gregor Mendel carried out hybridisation experiments on garden peas (*Pisum sativum*) for seven years (1856–1863). Peas were a good choice: they have contrasting traits that are easy to see, a short life cycle, many offspring, and flowers that can be self- or cross-pollinated.
He studied seven pairs of contrasting traits, such as tall/dwarf stem, round/wrinkled seed and yellow/green seed.
Key terms:
Gene: a unit of inheritance. Alleles are alternative forms of a gene (T and t).
Homozygous: two identical alleles (TT or tt). Heterozygous: two different alleles (Tt).
Genotype: the genetic make-up (Tt). Phenotype: the observable trait (tall).
Dominant allele: expressed even in the heterozygote (T). Recessive allele: expressed only when homozygous (tt).
Monohybrid cross: one trait
Cross a pure tall plant (TT) with a pure dwarf plant (tt):
F₁: all Tt, all tall.
F₁ selfed (Tt × Tt), giving F₂: TT : Tt : tt = 1 : 2 : 1 (genotypic ratio), and tall : dwarf = 3 : 1 (phenotypic ratio).
A Punnett square shows the possible combinations of gametes.
Mendel's laws
Law of dominance: in a heterozygote, one allele (the dominant one) expresses itself and masks the other.
Law of segregation: the two alleles of a gene separate during gamete formation, so each gamete carries only one allele. Alleles do not blend.
Law of independent assortment (from dihybrid crosses): alleles of different genes assort independently of one another during gamete formation.
Test cross
To find whether a tall plant is TT or Tt, cross it with a homozygous recessive (tt) plant:
all offspring tall → the plant was TT,
tall : dwarf = 1 : 1 → the plant was Tt.
Dihybrid cross: two traits
Cross round yellow (RRYY) with wrinkled green (rryy). The F₁ is all RrYy, round and yellow. Selfing the F₁ gives the F₂ phenotypic ratio
9 : 3 : 3 : 1: round yellow : round green : wrinkled yellow : wrinkled green
Deviations from Mendel's ratios
Pattern | What happens | Example | F₂ phenotypic ratio |
|---|---|---|---|
Incomplete dominance | the heterozygote is intermediate | snapdragon flower colour (red × white gives pink) | 1 : 2 : 1 |
Co-dominance | both alleles are fully expressed | ABO blood groups (Iᴬ and Iᴮ together give AB) | — |
Multiple alleles | more than two alleles exist in the population | ABO blood groups (Iᴬ, Iᴮ, i) | — |
Pleiotropy | one gene affects several traits | phenylketonuria | — |
Polygenic inheritance | several genes add up their effects | human skin colour, height | continuous variation |
Chromosomal theory of inheritance
Sutton and Boveri pointed out that chromosomes behave the way Mendel's "factors" do: they come in pairs, separate during meiosis, and assort independently. So genes are located on chromosomes. T. H. Morgan, working with fruit flies (Drosophila), confirmed this experimentally.
Linkage: genes close together on the same chromosome tend to be inherited together, which breaks Mendel's law of independent assortment. Recombination (from crossing over) produces new combinations. The farther apart two genes are, the more often they recombine, which allows genes to be mapped.
Sex determination
Organism | System | Who decides the sex of the offspring |
|---|---|---|
Humans, Drosophila | XX female, XY male | male (heterogametic) |
Grasshopper | XX female, XO male | male |
Birds | ZW female, ZZ male | female (heterogametic) |
Honeybee | haplodiploidy | fertilised eggs (2n) become females; unfertilised eggs (n) become males |
In humans, the father's sperm carries either X or Y, so the father determines the sex of the child.
Genetic disorders
Mendelian disorders (caused by a single gene):
Haemophilia: X-linked recessive; blood does not clot properly. Carrier females usually pass it to sons.
Colour blindness: X-linked recessive; far more common in males.
Sickle-cell anaemia: autosomal recessive; a single base change makes haemoglobin cause red blood cells to become sickle-shaped.
Phenylketonuria: autosomal recessive; the body cannot break down the amino acid phenylalanine.
Thalassaemia: autosomal recessive; reduced synthesis of haemoglobin chains.
Chromosomal disorders (caused by an abnormal number of chromosomes):
Down's syndrome: trisomy of chromosome 21 (47 chromosomes).
Klinefelter's syndrome: XXY male (47 chromosomes).
Turner's syndrome: XO female (45 chromosomes).