Inheritance

• Haploid and diploid, homologous pairs, and why gametes need a reduction division.
• Chromosome behaviour in the eight stages of meiosis; crossing over, independent assortment and random fertilisation produce variation.
• Genetic terms; monohybrid and dihybrid crosses with dominance, codominance, multiple alleles, sex linkage, autosomal linkage, epistasis and test crosses.
• The chi-squared test.
• Genes → proteins → phenotype: TYR, HBB, F8, HTT, and the Le/le gibberellin alleles.
• Gene control: structural and regulatory genes, the lac operon, transcription factors and DELLA proteins.

Haploid, diploid and homologous chromosomes

• A diploid (2n) cell contains two complete sets of chromosomes, one set from each parent. Human body cells: 2n = 46.
• A haploid (n) cell contains one complete set of chromosomes. Human gametes: n = 23.
• Homologous chromosomes are a pair of chromosomes, one maternal and one paternal, that have the same genes at the same loci (but may carry different alleles), are the same size and shape, and have the centromere in the same position. They pair up during meiosis.

Why a reduction division is needed

In sexual reproduction two gametes fuse. If gametes were diploid, the zygote would have double the chromosome number, and the number would double every generation. Meiosis halves the number so that fertilisation restores the diploid number and it stays constant from generation to generation.

Meiosis: what the chromosomes do

Before meiosis the DNA replicates in interphase, so each chromosome has two sister chromatids. Meiosis has two divisions and produces four genetically different haploid cells.

How meiosis and fertilisation create variation

• Crossing over
• sister chromatids