Meiosis, mutation and inheritance

Genes, alleles and chromosome pairs

• A gene is a length of DNA coding for one characteristic, and its position on a chromosome is called its locus.
• Alleles are different versions of the same gene, such as the allele for tall and the allele for short stems in pea plants.
• Body cells are diploid: human body cells hold 46 chromosomes in 23 pairs, one of each pair from each parent. The two chromosomes in a pair are homologous: same genes, same loci, but not always the same alleles.
• Gametes (sperm and eggs) are haploid, with one chromosome from each pair, so a human gamete has 23; fertilisation restores 46.
• Homozygous means both alleles for a gene are the same (TT or tt); heterozygous means they differ (Tt). The genotype is the alleles; the phenotype is the trait you can see or measure.

Meiosis: one cell, four different gametes

• Before meiosis the DNA is copied, so each chromosome becomes two identical sister chromatids joined at a centromere.
• In meiosis I the homologous chromosomes pair up side by side, and non-sister chromatids can swap matching sections (crossing over).
• Still in meiosis I, the homologous pairs are pulled apart into two cells, so the two alleles of every gene separate (segregation), and each new cell is haploid.
• In meiosis II the sister chromatids separate, giving four haploid cells.
• Because of crossing over, the four gametes in the figure are all different: two keep the parent's combinations (AB and ab) and two are recombinant (Ab and aB).

Where the variation comes from

• Crossing over makes new combinations of alleles on a single chromosome, which did not exist in either of the parent's own chromosomes.
• Independent assortment: each homologous pair lines up on the middle of the cell in meiosis I in a random orientation, independently of every other pair, so the chromosomes from mum and dad are shuffled into the gametes.
• With 23 pairs, independent assortment alone gives 2²³ = 8 388 608 possible combinations of chromosomes in a human gamete.
• Random fertilisation then joins any one sperm with any one egg: about 8.4 million × 8.4 million ≈ 70 trillion possible zygotes, before crossing over adds even more.
• None of this makes new alleles; it reshuffles the alleles that are already in the gene pool into new combinations.

Mutation: the source of new alleles

• A mutation is any lasting change to the base sequence of DNA. It is the only way a brand new allele appears.
• Gene mutations include a substitution (one base swapped for another), an insertion (an extra base) and a deletion (a base lost). Insertions and deletions shift the reading frame, so every codon after them changes.
• Mutations happen at random when DNA is copied, and more often with mutagens such as UV light, X-rays and some chemicals.
• Only mutations in gametes, or in the cells that make gametes, are passed on to offspring. A mutation in a skin cell can harm the person but is never inherited.
• Most mutations are neutral or harmful, but a few give an advantage in a particular environment, and those are the raw material natural selection works on.

Codominance, incomplete dominance, multiple and lethal alleles

• Incomplete dominance: neither allele fully masks the other, so the heterozygote is in between. Red snapdragons (CᴿCᴿ) crossed with white (CᵂCᵂ) give all pink (CᴿCᵂ); pink × pink gives 1 red : 2 pink : 1 white.
• Codominance: both alleles are fully expressed side by side in the heterozygote. Roan Shorthorn cattle (RW) have a mix of red hairs and white hairs, not pink hairs.
• Multiple alleles: a gene has three or more alleles in the population, though each person still carries only two. The ABO blood group gene has Iᴬ, Iᴮ and i; Iᴬ and Iᴮ are codominant and both are dominant to i.
• Lethal alleles: a genotype that kills the embryo. In mice, the yellow allele Aʸ is dominant for coat colour but AʸAʸ embryos die, so yellow × yellow gives 2 yellow : 1 non-yellow instead of 3 : 1.
• In every case, show your symbols in a key: capital letters with superscripts for codominant or incompletely dominant alleles.

Two genes at once: dihybrid crosses and linkage

• If two genes are on different chromosomes, independent assortment means AaBb makes four kinds of gamete (AB, Ab, aB, ab) in equal numbers, so AaBb × AaBb gives 9 : 3 : 3 : 1 and a test cross (AaBb × aabb) gives 1 : 1 : 1 : 1.
• Linked genes sit close together on the same chromosome, so they tend to be inherited together and the parental combinations turn up far more often than 1 : 1 : 1 : 1 predicts.
• Crossing over between the two loci makes a few recombinant gametes. The further apart the genes are, the more often a crossover lands between them and the more recombinants there are.
• Genes far apart on the same chromosome cross over so often that they behave as if unlinked.

1. test cross AaBb × aabb (A and B on the same chromosome), 200 offspringa made-up practice cross
2. 82 AB, 80 ab, 19 Ab, 19 aBobserved phenotypes
3. unlinked would give 50 of each1 : 1 : 1 : 1
4. recombinants = (19 + 19) ÷ 200 × 100% = 19%
5. mostly parental types, so the genes are linked; 19% recombinants comes from crossing over between them

What Achieved, Merit and Excellence look like

• Achieved defines and describes; Merit explains how or why; Excellence links the ideas together, often comparing two processes or two crosses, and uses the numbers in the question.
• The examiners mark each question as a whole, so one strong linked discussion beats a list of definitions.