Meiosis, linkage and genetic variation

What you’ll learn

• Explain reduction division and the origin of genetically varied gametes.
• Distinguish homologous chromosomes from sister chromatids.
• Relate loci, linkage and crossing over to allele combinations.

Understand the idea

Homologous chromosomes carry the same genes at corresponding loci but may carry different alleles. In a diploid nucleus there are two copies of each chromosome type, one inherited from each parent. DNA replicates once before meiosis, followed by two divisions. Homologous chromosomes separate in meiosis I; sister chromatids separate in meiosis II. The resulting nuclei are haploid, so fertilisation can restore the diploid complement without doubling chromosome number each generation.

During prophase I, homologous chromosomes pair and non-sister chromatids can exchange corresponding DNA segments through crossing over. This produces new combinations of alleles on chromatids. At metaphase I, each homologous pair can orient independently of the others, producing independent assortment of maternal and paternal homologues. Random fertilisation adds another source of variation when different gametes unite.

Genes on the same chromosome are linked and tend to be inherited together. Crossing over can separate linked alleles, so linkage is not an absolute guarantee that a parental combination is preserved. In a simplified calculation excluding crossing over, a cell with n chromosome pairs can produce 2 to the power n combinations by independent assortment. This counts chromosome-origin combinations, not every possible gamete genotype once crossing over and allele differences are considered.

Key formulae

$$2n\xrightarrow{\text{meiosis}}n$$

$$\text{independent-assortment combinations}=2^n$$

Worked example

A species has a diploid chromosome number of 10. Ignoring crossing over, calculate the number of chromosome-origin combinations possible in its gametes.

1. A diploid count of 10 means there are 5 homologous pairs.
2. Each pair has two alternative orientations relative to a gamete’s chromosome set.
3. The independent choices give 2⁵ = 32 combinations.

Answer: 32 combinations, with 5 chromosomes in each gamete.