Meiosis and genetic variationEdexcel A-Level Biology A: Revision notes
Section 1
Loci, alleles and homologous chromosomes
A gene is a length of DNA that codes for a polypeptide. The position of a gene on a chromosome is its locus (plural loci). Alternative forms of a gene are alleles, which differ in base sequence.
A diploid cell has pairs of homologous chromosomes, one from each parent. They carry the same genes at the same loci, but may carry different alleles. A person with the genotype HbA HbS has one allele at the beta-globin locus on each chromosome 11.
Do not use locus and allele interchangeably: the locus is the position, the allele is the version of the gene found there.
Section 2
Gene linkage
Genes at different loci on the same chromosome are linked. They tend to be inherited together because the chromosome is passed on as one unit, so they do not show independent assortment.
In a test cross with a double heterozygote, linked genes give mostly parental phenotypes. A few recombinant phenotypes appear because of crossing over between the loci. The further apart the loci, the more likely a crossover is between them, so the higher the percentage of recombinants.
Example: 41 + 39 = 80 recombinants in 1000 offspring is 8%.
If a dihybrid test cross is not 1:1:1:1 and the two parental types are most common, say the genes are linked.
Section 3
Sex linkage
The sex chromosomes are X and Y. In humans females are XX and males are XY. The Y chromosome is small and carries few genes, so many genes on the X chromosome have no matching allele on the Y chromosome. A gene on the X chromosome shows sex linkage.
A male has only one allele for an X-linked gene, so a single recessive allele is expressed. A female needs two recessive alleles to be affected, but can be a heterozygous carrier. This is why conditions such as red-green colour blindness and haemophilia are more common in males.
A father passes his X chromosome to all his daughters, and his Y chromosome to all his sons. A son receives his X chromosome only from his mother.
A male cannot be a carrier of an X-linked recessive allele: if he has it, he shows the condition.
Section 4
Meiosis and genetic variation
Meiosis is the division that makes haploid gametes from a diploid cell. The chromosome number is halved, so that fertilisation restores the diploid number. The details of the stages are not required, but two events cause the gametes to be non-identical.
Section 5
Crossing over
Homologous chromosomes pair up. Chromatids of the two homologous chromosomes can break and rejoin at corresponding points, exchanging sections. This is crossing over.
The result is chromatids carrying new combinations of alleles. Without crossing over, all the alleles on a chromosome would always stay together, so crossing over creates variation within linked groups of genes.
Section 6
Independent assortment
The maternal and paternal chromosomes of each homologous pair line up at the equator at random. Either member of a pair can go to either pole, regardless of how the other pairs line up. This is independent assortment.
The number of combinations of chromosomes in gametes is 2ⁿ, where n is the number of homologous pairs. In humans 2²³ = 8 388 608 even before crossing over.
Together, independent assortment and crossing over mean the gametes from one person are very unlikely to be identical, which is a source of genetic variation in the population.
Show the working 2²³ = 8 388 608 and state that it is a minimum, as crossing over adds more variation.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Meiosis and genetic variation
- The gene for the beta-globin polypeptide of haemoglobin is found at one position on chromosome 11 in humans. Two alleles of this gene are common in some populations: HbA, which codes for normal beta-globin, and HbS, which codes for the beta-globin found in sickle cell haemoglobin.State what is meant by an allele and explain why HbA and HbS are found at the same locus.2 marks
- Red-green colour blindness is caused by a recessive allele on the X chromosome. A woman who is a carrier of the allele and a man with normal colour vision have a family together.Explain why none of the daughters of this couple will be colour blind, although some may be carriers.2 marks
- In fruit flies, the gene for body colour (grey G, dominant, or black g) and the gene for wing length (long L, dominant, or vestigial l) are on the same autosome. A heterozygous grey, long-winged fly, with G and L on one chromosome and g and l on its homologue, was crossed with a black, vestigial-winged fly. Among 1000 offspring, 462 were grey and long-winged, 458 were black and vestigial-winged, 41 were grey and vestigial-winged and 39 were black and long-winged.Explain the results of this cross.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).