Inheritance, variation and selection
Chromosomes, genes and alleles, cell division, genetic crosses, variation, mutation, and natural and artificial selection.
📌 Key points
- A chromosome is made of DNA, which contains genes. A gene is a length of DNA that codes for a protein; alleles are alternative forms of a gene.
- Human body cells are diploid (46 chromosomes in 23 pairs) and gametes are haploid (23). Females are XX and males are XY.
- Mitosis produces two genetically identical cells, for growth, repair and asexual reproduction. Meiosis is a reduction division that produces four genetically different haploid cells, the gametes.
- The genotype is the alleles an organism has; the phenotype is its observable features. Homozygous means two identical alleles; heterozygous means two different alleles.
- A dominant allele is expressed if present; a recessive allele is only expressed when there is no dominant allele. A cross of Aa × Aa gives a 3 : 1 ratio of dominant to recessive phenotypes.
- A test cross with a homozygous recessive organism shows whether an organism with the dominant phenotype is homozygous or heterozygous.
- Codominance: both alleles are expressed in the heterozygote, e.g. ABO blood groups, where IA IB gives group AB. Red-green colour blindness is a sex-linked characteristic carried on the X chromosome.
- Continuous variation shows a range of phenotypes (e.g. height) and is caused by genes and environment; discontinuous variation has a few distinct groups (e.g. ABO blood group) and is caused by genes only.
- A mutation is a genetic change. Mutations are the source of new alleles; ionising radiation and some chemicals increase the rate.
- Natural selection: variation, overproduction, competition, survival of better-adapted individuals, which reproduce and pass on their alleles (e.g. antibiotic resistance in bacteria). In selective breeding, humans choose individuals with useful features to breed.
📖 Key terms
- Gene
- A length of DNA that codes for a protein.
- Allele
- An alternative form of a gene.
- Genotype
- The genetic make-up of an organism in terms of its alleles.
- Phenotype
- The observable features of an organism.
- Homozygous
- Having two identical alleles of a gene.
- Heterozygous
- Having two different alleles of a gene.
- Dominant
- An allele that is expressed if it is present.
- Recessive
- An allele that is only expressed when there is no dominant allele.
✏️ Worked example
In pea plants, tall (T) is dominant to short (t). Two heterozygous tall plants are crossed. What proportion of the offspring will be short?
- Parent genotypes: Tt × Tt.
- Gametes from each parent: T or t.
- Punnett square offspring: TT, Tt, Tt, tt.
- Phenotypes: 3 tall (TT, Tt, Tt) : 1 short (tt).
- Proportion short = 14, or 25%.
Set out every genetic cross fully: parent genotypes, gametes, Punnett square, offspring genotypes and phenotypes with the ratio. Each step can earn a mark.
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