In the last article we looked at meiosis – the type of cell division that produces haploid gametes in the reproductive organs. Today we’re going to look at how meiosis can produce genetic variation in the gametes, and therefore genetic variation in offspring after the gametes fuse at fertilisation. We will also finish off by looking at chromosome mutations that can occur during meiosis.
Crossing over
Crossing over happens during prophase I of meiosis when the homologous chromosomes are associated (next to each other). The chromatids from each one of the pair wrap and twist around each other. The crossing point is called the chiasma. Parts of the non-sister chromatids literally swap places, so now part of the maternal chromosome is attached to the paternal chromosome and vice versa. This creates new combinations of alleles and increases genetic variation.
Independent assortment (independent segregation)
In meiosis I, the homologous pairs attach to the spindle fibres in metaphase I and are then separated to opposite poles of the cell in anaphase I (check out the meiosis article for a diagram). It is completely random which of each pair ends up in the two daughter cells. For example in the diagram below with only two pairs of chromosomes, either both the maternal or both the paternal chromosomes could end up in the daughter cells, or one maternal and one paternal chromosome. But don’t forget that humans have 23 pairs, so the possible combinations are huge. The shuffling is completely random, and don’t forget that crossing over happens before this as well, so a large amount of genetic variation is possible.
Don’t forget that fertilisation is also a source of genetic variation in offspring because it is random which gametes combine with each other to form the zygote.
Chromosome mutations (non-disjunction)
Sometimes meiosis doesn’t quite go to plan and the gametes end up with the wrong number of chromosomes, or parts of chromosomes are missing. When this happens, it is called a chromosome mutation. Because these occur during production of gametes, the mutations are inherited.
A specific example is the chromosome mutation found in Down’s Syndrome. Non-disjunction occurs in meiosis I; this is where the homologous pairs (or chromatids if in meiosis II) don’t separate properly. In Down’s Syndrome specifically it means that at the end of meiosis I, one daughter cell has two copies of chromosome 21 and one daughter cell has no chromosome 21. At the end of meiosis II, two gametes have two copies of chromosome 21 and two gametes no chromosome 21. Therefore at fertilisation, it is possible to end up with a zygote containing three copies of chromosome 21 – two from the gamete formed with non-disjunction, and one from a normal gamete. So people with Down’s Syndrome have an extra copy of chromosome 21.
Summary
- Crossing over and independent assortment create genetic variation in gametes and therefore genetic variation in offspring.
- Crossing over occurs in prophase I, and is where parts of non-sister chromatids swap over within a pair of homologous chromosomes.
- Independent assortment occurs in metaphase I and anaphase I, and is where the homologous chromosomes are randomly shuffled into the two daughter cells.
- Chromosome mutations are inherited. An example is non-disjunction of chromosome 21 resulting in Down’s Syndrome.
