Topic 3 - Lesson 9 Flashcards
Linked Genes
The position of a gene on a chromosome is called its locus. If the loci of two different genes are on the same chromosome, they are likely to be inherited together and are said to be linked. The only way that the genes will not be inherited together is if crossing over separates them during meiosis (the chiasmata would have to form between the two genes). The closer the loci of the two genes, the less likely this is to happen and the higher the probability that the genes will be inherited together. This means that any offspring will probably express both phenotypes together than either phenotype separately.
Sex Linkage
Genes which are located on one of the sex chromosomes (X or Y) are said to be sex-linked and their expression will depend on whether the offspring is male (XY) or female (XX). The Y chromosome is much smaller than the X chromosome, so most alleles are carried on the X chromosome (they are X-linked). Men only have one X chromosome which means that they will only inherit one allele for these genes, compared to women who will inherit two. This means that if men inherit a recessive allele (which causes disease) for a gene found on the X-chromosome, they will have the disease. Women who inherit the recessive allele will just be a carrier, since they have another X chromosome with the dominant, functioning allele. For women to have X-linked diseases, they must inherit two disease alleles (they will have a homozygous recessive phenotype). Examples of sex linked disorders include haemophilia and red-green colour blindness.
