IGCSE Biology / Chromosomes, genes and DNA
Genes, alleles and the genome revision notes
Edexcel International GCSE Biology (4BI1), chapter 16: Chromosomes, genes and DNA, specification point 3.14, 3.32. Free lesson.
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Human body cells contain 46 chromosomes in 23 pairs, while gametes contain 23. Knowing how homologous chromosomes, alleles and the genome relate helps you describe how genetic information is organised.
How to read a human karyotype
A karyotype is a photograph of all the chromosomes in one cell, arranged in pairs from largest to smallest. It is made from a cell during mitosis, when the chromosomes are condensed enough to see. A human body cell has 46 chromosomes arranged in 23 pairs. Pairs 1 to 22 are called autosomes. Each pair is homologous. Pair 23 contains the sex chromosomes: XX in a female and XY in a male.
What homologous chromosomes have in common
A homologous pair contains one chromosome from each parent. The two chromosomes are the same size and carry the same genes in the same order. Each gene is at the same position, called its locus, on both chromosomes. The versions of a gene are called alleles. The alleles may be the same or different. On the autosomes, you have two alleles of every gene, one from each parent. Gametes carry only one chromosome from each pair. This happens through meiosis.
How diploid and haploid differ
Diploid means having two sets of chromosomes, one from each parent. Human body cells are diploid, so they have 46 chromosomes. Haploid means having one set. Human gametes, such as sperm, are haploid and have 23 chromosomes. A liver cell is a body cell, so it is diploid and has 46 chromosomes. A sperm cell is a gamete, so it is haploid and has 23 chromosomes. Halving the chromosome number in gametes means fertilisation restores 46 chromosomes, rather than the number doubling in every generation.
What the genome means
The genome is all the genetic material of an organism: every gene on every chromosome. It describes the whole collection, rather than one chromosome or one gene. The Human Genome Project sequenced all the DNA in a human genome. Knowing the sequence does not mean knowing what each gene does, so work continues.