Genes, Alleles, Mendel's Laws and Mutations

• A gene is a length of DNA that codes for a product (usually a polypeptide) and sits at a fixed position, its locus, on a chromosome.
• An allele is one of the alternative forms of a gene. Alleles of the same gene sit at the same locus on the two homologous chromosomes.
• The genotype is the pair of alleles an individual has (e.g. $Tt$); the phenotype is the observable trait (e.g. tall), shaped by the genotype and the environment.
• Homozygous: two identical alleles ($TT$ or $tt$). Heterozygous: two different alleles ($Tt$).
• A dominant allele shows in the phenotype even in a heterozygote; a recessive allele shows only when homozygous.
• All the body cells of one person carry the same genes and the same alleles. Cells differ because they switch different genes on, not because their alleles differ.

• Law of segregation: the two alleles of a gene separate when gametes form, so each gamete carries only one allele of each gene, chosen at random.
• Law of independent assortment: alleles of different genes go into gametes independently of each other. This holds for genes on different chromosomes (or far apart on one).
• Law of dominance: in a heterozygote, the dominant allele decides the phenotype.
• Chromosomal theory of inheritance (Sutton and Boveri, about 1902): genes are located on chromosomes, and the behaviour of chromosomes in meiosis explains Mendel's laws.
• Mendel's laws are about inheritance. Ideas about survival of the fittest belong to natural selection (Darwin), not to Mendel.

A mutation is a change in the DNA base sequence (a gene mutation) or in the structure or number of chromosomes (a chromosome mutation).

• Substitution: one base replaced. It can be silent (same amino acid, because the code is degenerate), missense (a different amino acid) or nonsense (a stop codon).
• Insertion or deletion of one or two bases causes a frameshift: every codon after it is read wrongly.
• Only mutations in cells that make gametes (germline) are passed to offspring; somatic mutations are not.
• Sickle-cell anaemia: a missense substitution in the $\beta$-globin gene (codon 6 changes from $\text{GAG}$ to $\text{GTG}$ in the DNA) puts valine in place of glutamic acid. The haemoglobin (HbS) has a different structure; it still carries oxygen, but at low oxygen it forms fibres that distort red cells into sickles. It is autosomal recessive, and carriers are partly protected against malaria.