Section 18.5
Dihybrid Crosses and Polygenic Traits

• A dihybrid cross is a genetic cross involving two genes, each of which has more than one allele.
• Ex. A pure yellow, round seed is crossed with a pure green, wrinkled seed.
Y R Y R y r y r
YyRr
YyRr
YyRr
YyRr
100% yellow, round seeds.

• After many observations, Mendel noticed that when he crossed pea plants while looking at more than one trait, many more possible combinations were produced than he expected.
• This is because instead of the parents producing only two possible gametes like in a monohybrid cross, they can produce four, from all the different combinations of alleles.

• The reason for this is because the genes for the two different traits are on different chromosomes.
• The chromosomes can separate however they like, they do not have to separate as a pair with a certain allele for the other gene.
• This is known as the idea of independent assortment .
• The alleles for the different genes are separating individually.

• Probability is more difficult to determine for dihybrid crosses, because we must determine what the chances are of certain traits occurring together.
• Example : What is the probability of a child having attached ear lobes and a straight hairline (both recessive), if the parents are both heterozygous for both traits.
• To have attached ear lobes and a straight hairline, the child must have the genotype eeww. Since the two genes are on separate chromosomes, the gene for ear shape and hairline shape will assort independently. The outcome that the child will receive two ‘e’ alleles is, therefore, independent of the outcome that the child will receive two ‘w’ alleles.

• Solution: First determine the probability that the child will receive attached ear lobes separately from a straight hairline.
E e
E
EE
Ee e
Ee ee
¼ or 25% chance of being ‘ee.’
W
W
WW w Ww w
Ww ww
¼ or 25% chance of being ‘ww.’

• Now to determine the probability that the child will have both traits together you must multiply them together.
• ¼ x ¼ = 1/16
• 0.25 x 0.25 = 0.0625
• The child has a 6.25% chance of having both attached ear lobes ‘ee’ and a straight hairline ‘ww.’

• There is another way that probability can be determined, by using a bigger Punnett square.
EeWw x EeWw
EW
EEWW
Ew
EEWw eW
EeWW ew
EeWw
EW
EEWw EEww EeWw Eeww
Ew
EeWW EeWw eeWW eeWw eW ew
EeWw Eeww eeWw eeww
There is a 1/16 chance that the child will have the genotype ‘eeww’ and have both attached ear lobes and a straight hairline.

• Selective breeding – is the crossing of plants or animals with desired traits to produce offspring with desired characteristics.
• Many dogs and horses are breed by using a specific type of selective breeding called inbreeding in order to keep the original traits of the purebred dog.
• Another special type of selective breeding is hybridization, where organisms are bred specifically to change the traits for the better.
• Hybridization is usually a mixture of two different species to create entirely new organisms.

• In dihybrid crosses, two genes determine two separate traits.
• However, sometimes a single trait is determined by more than one gene.
• These are known as polygenic traits – inherited characteristics that are determined by more than one gene.
• In other words, two different genotypes interact to produce a phenotype that neither genotype is capable of producing itself.

• Examples are skin colour, hair colour and eye colour.
• This is why it is hard to track these inherited traits on a pedigree chart because they are influenced by more than one gene.
• Polygenic traits tend to have more variability than those determined by a single gene.
• Each of the genes for polygenic traits are capable of having multiple alleles, incomplete dominance or codominance and can be affected by the environment.

• Epistatic genes – are polygenic trait genes that can mask or interfere with the expression of the other gene.
• Ex. Coat colour of dogs:
• The coat colour of dogs is determined by two different genes, the coat colour gene and another gene that affects pigmentation of the hairs.
• Normally the ‘B’ allele produces a black coat, while a ‘b’ allele produces a brown coat.

• However, dog coat colour is a polygenic trait and is affected by another gene for pigmentation.
• When the ‘W’ allele is present, it prevents pigmentation of the coat, resulting in a white colour.
• When the ‘w’ allele is present, it does not have any affect on the pigmentation of the coat and the dog will be black or brown depending on the genotype of the coat gene.
* ‘B’ (black) is dominant over ‘b’ (brown).
* ‘W’ (prevention of pigmentation) is dominant over ‘w’ (no affect).

• The punnett square to prove epistatic genes of dog coat colour: WwBb x wwBb
WB wB
WwBB wb
WwBb
4/8 are white because of the ‘W’ allele.
Wb WwBb Wwbb wB wb wwBB wwBb wwBb wwbb
3/8 are black because of the ‘w’ and ‘B’ alleles.
1/8 are brown because of the ‘w’ and ‘b’ alleles.