VMBO Year 2 · HAVO/VWO Years 2–3 · Inheritance and evolution · Section 5.2

Chromosomes

Pairs

Most human body-cell nuclei contain 46 chromosomes arranged in 23 pairs. In a typical matching pair, the chromosomes carry the same genes at corresponding positions, but may carry different versions of those genes.

One chromosome in each pair is inherited from each biological parent. The versions may therefore be identical or different. Eye colour is a familiar inherited characteristic, although it involves several genes rather than a single simple pair.

Sex chromosomes

The sex chromosomes are conventionally shown as the 23rd pair. Typical female chromosome patterns are XX and typical male patterns are XY.

Sex development also depends on genes, hormones and how tissues respond. Variations can occur, including people whose anatomy does not follow the most common pattern for their chromosomes. Some variations are described as intersex and may become apparent at birth or later.

The X and Y chromosomes do not carry exactly the same set of genes.

Different information

Two corresponding chromosomes can carry different alleles, or versions of a gene. These differences contribute to variation in characteristics.

Having the same genes does not mean that two chromosomes contain identical DNA sequences.

Sex cells

Sex cells, or gametes, normally contain one set of 23 chromosomes rather than 23 pairs.

Meiosis halves the chromosome number and creates new combinations of genetic information. The allocation of parental chromosomes and exchange of DNA between matching chromosomes contribute to variation.

At fertilisation, the nuclei of an egg and a sperm fuse, normally restoring 46 chromosomes: 23 from each gamete.

Variation

Different chromosome combinations enter gametes, and different gametes combine at fertilisation. As a result, siblings usually have different genotypes even when they have the same parents. Identical twins are an important exception to the usual pattern of different starting genotypes.