explain what is meant by homologous pairs of chromosomes

Published by Patrick Mutisya · 14 days ago

Cambridge A-Level Biology – Passage of Information from Parents to Offspring

Passage of Information from Parents to Offspring

Objective

Explain what is meant by homologous pairs of chromosomes.

Key Concepts

  • Every species has a characteristic number of chromosomes.

  • In diploid organisms (e.g., humans, most plants, and animals studied at A‑Level), chromosomes occur in pairs.

  • Each pair consists of one chromosome inherited from the mother and one from the father.

  • These paired chromosomes are called homologous chromosomes or a homologous pair.

Definition of Homologous Pairs

A homologous pair of chromosomes is a set of two chromosomes that are similar in:

  1. Length

  2. Centromere position

  3. Pattern of banding (when stained)

  4. Gene loci – they carry the same types of genes in the same order, although the specific alleles at each locus may differ.

One chromosome of the pair is derived from the sperm (paternal) and the other from the ovum (maternal).

Together they ensure that each somatic cell contains a complete set of genetic information.

Why Homology Matters

  • Meiosis – During prophase I, homologous chromosomes pair up (synapsis) and may exchange genetic material by crossing‑over, increasing genetic variation.

  • Genetic disorders – Errors in pairing or segregation can lead to aneuploidy (e.g., Down syndrome, Turner syndrome).

  • Inheritance patterns – Traits are expressed according to the combination of alleles present on each chromosome of the pair.

Human Example

Humans have 46 chromosomes arranged in 23 homologous pairs:

Pair No.Chromosome(s)Sex Determination
11 (maternal) + 1 (paternal)Autosomes – not involved in sex determination
22 (maternal) + 2 (paternal)
……
2222 (maternal) + 22 (paternal)
23X (maternal) + X (paternal) or X (maternal) + Y (paternal)Sex chromosomes (XX = female, XY = male)

Illustrative Diagram (Suggested)

Suggested diagram: A schematic showing a diploid cell with 23 homologous chromosome pairs, highlighting one pair during synapsis in meiosis I.

Summary

Homologous pairs are fundamental to the transmission of genetic information from parents to offspring.

Their structural similarity allows accurate pairing during meiosis, facilitates genetic recombination,

and ensures that each new cell receives a complete set of genes.