Passage of information from parents to offspring

Biology A2 · Inheritance. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

Test this topic free All Biology A2 topics

The idea

A diploid (2n) cell has two complete sets of chromosomes, one from each parent. A haploid (n) cell has one set. Gametes must be haploid, so that fertilisation restores the diploid number and it does not double in every generation.

Meiosis makes haploid cells in two divisions. Meiosis I is the reduction division: homologous chromosomes pair up and are then separated. Meiosis II separates the sister chromatids, as mitosis does.

Meiosis also makes gametes genetically different from one another, by crossing over and by random orientation of chromosomes. Random fusion of gametes at fertilisation adds more variation.

Rules to remember

Homologous chromosomes: same size, same centromere position and the same genes at the same loci, but they may carry different alleles
Prophase I: homologous chromosomes pair to form bivalents and crossing over occurs. Metaphase I: bivalents line up on the equator. Anaphase I: whole chromosomes (each with two chromatids) move to opposite poles
Metaphase II: single chromosomes line up on the equator. Anaphase II: centromeres divide and sister chromatids separate. Result: four haploid cells
Crossing over: non-sister chromatids of a bivalent exchange sections at chiasmata, giving new combinations of alleles
Random orientation (independent assortment) of bivalents at metaphase I gives 2ⁿ chromosome combinations, where n is the haploid number
Animal cells have centrioles and divide by the cell surface membrane pinching in; flowering plant cells have no centrioles and form a new cell wall

Common mistake

Students say that chromatids separate in anaphase I. In anaphase I it is the homologous chromosomes that separate; sister chromatids stay joined until anaphase II.

Worked example

An onion cell with 2n = 16 enters meiosis. How many bivalents are seen at metaphase I, and how many chromosome combinations can random orientation alone give in the gametes?

Haploid number n = 16 ÷ 2 = 8, so there are 8 homologous pairs.
Each pair forms one bivalent, so there are 8 bivalents at metaphase I.
Each bivalent can face either way, so combinations = 2ⁿ = 2⁸ = 256.

Answer: 8 bivalents; 256 combinations

Ask about this

Type what you are stuck on. The helper finds the explanation, worked examples and the notes to read, free and instant.

Read the full notes

Full-course notes

Keep going

← Control and coordination in plants The roles of genes in determining the phenotype →Take a Biology A2 test