Autosomal linkage and sex linkage are two cases where inheritance is very dependent on which chromosome the genes are found on. This is definitely the most difficult concept to get your head around for this topic for A-Level biology – make sure to have a good understanding of meiosis before you start.
Autosomes are just all the chromosomes which are not sex chromosomes. Humans have 22 pairs of autosomes. If two genes are found on the same autosome, they have autosomal linkage. During meiosis, when the homologous chromosomes are separated by independent assortment, alleles on the same autosome will stay together. This means that is is very likely that those combinations of alleles will be inherited together. However, there is one way that they can be spilt up. Remember crossing over? Regions of non-sister chromatids get exchanged, so it is possible to swap over alleles within a homologous pair. The closer the alleles for two genes are, the less likely they will be separated by crossing over. The diagram below shows how crossing over can produce different combinations of alleles in the gametes. However, the original linked combination would be the most common because crossing over would not happen all the time.
One way to find out if two genes are linked is to look at the phenotypic ratio that you get when carrying out a dihybrid cross. If the ratio is not as expected, and there are many offspring with the same phenotypes as the parents, then this suggests that the genes are linked.
In humans, one pair of the 23 pairs of chromosomes are the sex chromosomes. Females have two X chromosomes, whereas males have one X chromosome and one Y chromosome. The X chromosome is longer than the Y chromosome and therefore has more genes. Because males only have one copy of the X chromosome, they only need one copy of a recessive allele located on the X chromosome in order for it to be expressed in the phenotype. This is why some disorders are more common in men than in women. The inheritance of colour blindness is a good example of this. The genetic cross below shows how it is possible for two parents with normal vision to have a colour blind son, but not a colour blind daughter. XN is the dominant normal vision allele, Xn is the recessive colour blindness allele.
- If two genes are on the same autosome they are linked. The combinations of alleles can only be changed if crossing over occurs.
- More offspring will have the same phenotypes as the parents if there is autosomal linkage.
- Sex linkage occurs if a gene is found on the X or Y chromosome. As males only have one X chromosome, it is more likely they will express a recessive allele found on the X chromosome.