Worked example: Dihybrid genetic diagram

• Horses have a single gene for coat colour that has two alleles:
  • B, a dominant allele produces a black coat
  • b, a recessive allele produces a chestnut coat

   

• Horses also have single gene for eye colour
  • E, a dominant allele produces brown eyes
  • e, a recessive allele produces blue eyes

   

• In this example a horse which is?heterozygous?for?both genes?has been crossed with a horse that is?homozygous?for?one gene?and?heterozygous?for the?other

Parental phenotypes: black coat, brown eyes x chestnut coat, brown eyes

Parental genotypes:???BbEe??????????????????????????????????????bbEe

Parental gametes: ?????BE?or?Be?or?bE?or?be??????????bE?or?be

Worked example: Explaining autosomal linkage

• The genes for tail length and scale colour in a species of newt have displayed autosomal linkage
• The gene for tail length has two alleles :
  • Dominant allele?T?produces a normal length tail
  • Recessive allele?t?produces a shorter length tail

   

• The gene for scale colour has two alleles:
  • Dominant allele?G?produces green scales
  • Recessive allele?g?produces white scales

   

• A newt heterozygous for a normal tail and green scales is crossed with a newt that has a shorter tail and white scales

Parental phenotypes: normal tail, green scales x short tail, white scales

Parental genotypes: ???(TG)(tg)?????????????????(tg)(tg)

Parental gametes: ???????(TG)?or?(tg)???????????(tg)

Worked example: Explaining epistasis

• There is a gene that dictates the feather colour of pigeons
• The gene has two alleles (R / r) :
  • Allele?R?codes for a pigment that produces grey feathers
  • Allele?r?doesn’t produce a pigment, resulting in white feathers

   

• Another gene has also been found to have an effect on feather colour
• This gene has two alleles (F / f)?:
  • The dominant allele?F?stops grey feathers being produced even if the allele?R?is present

   

• These are the possible phenotypes:
  • RRFF? ?white feathers
  • RrFF? ?white feathers
  • rrFF? ?white feathers
  • RRFf? ?white feathers
  • RrFf??white feathers
  • rrFf? ?white feathers
  • rrff? ?white feathers
  • RRff? ?grey feathers
  • Rrff? ?grey feathers

   

When you are working through different genetics questions you may notice that test crosses involving autosomal linkage predict solely?parental type?offspring (offspring that have the same combination of characteristics as their parents).However in reality?recombinant?offspring (offspring that have a different combination of characteristics to their parents) are often produced.? This is due to the?crossing over?that occurs during meiosis. The crossing over and exchanging of genetic material?breaks the linkage?between the genes and recombines the characteristics of the parents.So if a question comes along that asks you why recombinant offspring are present you now know why!