Inheritance of Sex-linked characteristics
- Sex linkage refers to an allele that code for a characteristic which is located on a sex chromosome
- As the Y chromosome is smaller than the X, fewer genes are carried on it
- Therefore, most sex-linked genes are found on the X chromosome (x-linked genes)
- As males only have one X chromosome they therefore often only have one allele for sex-linked genes, and so there is a higher chance of a recessive gene being expressed
- Ultimately, males are more likely to express a recessive gene relative to females
- Genetic disorders caused by a faulty allele
located on a sex chromosome include:
- Colour blindness
- Haemophilia
e.g. As haemophilia is sex linked, it is therefore on the X chromosome. Therefore, females would require 2 recessive alleles for expression, whereas males would only require one.
As the Y chromosome does not have either gene it is just represented as Y
XH | Xh | |
XH | XH XH | XH Xh |
Y | XH Y | Xh Y |
If a female carrier and a male non-carrier have offspring there is a 2:1:1 ratio (female without colour blindness : male without colour blindness : male with colour blindness). Overall there is a 50% chance of a male offspring to be colour blind.
Linkage of autosomal Genes
- Autosome is any chromosome that is not a sex chromosome
- Autosomal genes are genes found on the autosomes
- Genes found on the same autosome are linked (as they stay together during independent segregation of chromosomes in meiosis 1 and the allele will be passed to the daughter cell)
- The will not occur is crossing over splits them linked genes first
- The closer the two genes are on the autosome (the closer they are linked due to the decreased chance of crossing over)