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)