Predicting Inheritance: Dihybrid Crosses (AQA A Level Biology): Revision Note

• 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 a 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

• Predicted ratio of phenotypes in offspring:

  • 3 black coat, brown eyes :  3 chestnut coat, brown eyes: 1 black coat, blue eyes: 1 chestnut coat, blue eyes

• 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)

• Predicted ratio of phenotypes in offspring:

  • 1 normal tail, green scales: 1 short tail, white scales

• Predicted ratio of genotypes in offspring :

  • 1 (TG)(tg) : 1 (tg)(tg)

If phenotypic ratios deviate from standard dihybrid predictions (e.g. not 9:3:3:1), consider autosomal linkage as a possible reason

• 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 from 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

In genetics questions, you may notice that crosses involving autosomal linkage predict offspring that all have the same combination of characteristics as their parents. In reality, recombinant offspring are often produced; this is due to crossing over during meiosis.