SBI 3U

• Dihybrid Cross
• Two traits Therefore TWO pairs of alleles
• Alleles can be either Dominant or Recessive

Questions Involving More Than One Trait
In mice, grey coat colour, G, is dominant to white coat, g, and long tail, T, is dominant to short, t. What fraction of F1 mice would you predict to have grey coats and short tails when a male ggTt parent is crossed with a female GGtt parent?

Given: G is dominant to g. T is dominant to t. Therefore:
GG or Gg = grey coat gg = white coat TT or Tt =long tail tt = short tail
Required:
• The fraction of F1 mice expected to have grey coats and short tails;
• that is, to be of genotypes GGtt or Ggtt
Analysis:
• The cross is ggTt X GGtt.
• Produce a Punnett square.
Solution:
The ggTt parent produces two types of gametes gT and gt
The GGtt parent produces only one type of gamete – Gt
The dimensions of the Punnett square are 2 X 1.

Statement:
In the F1 mice that are produced, half of the mice will be expected to have the genotype Ggtt and as a result, half of the F1 mice will exhibit the grey coat and short tail phenotype combination.

Under dominance the dominant allele can hide the expression of a recessive allele in the heterozygous condition.
However, in the determination of some traits, the different alleles of a gene may be expressed in the heterozygous condition to produce an intermediate phenotype. When neither gene is completely dominant over the other, we say that there is incomplete
dominance. One of the clearest examples of incomplete dominance is found in the colours of snapdragon flowers.

A slightly different form of inheritance is observed in horses and shorthorn cattle where two alleles are expressed at the same time.
“Roan” coat Colour - a blend of red and white.

The two alleles have been expressed at the same time, a type of inheritance termed co-dominance. In this condition, neither allele dominates the expression of the other. Capital letters with superscripts or simply capitals are often used to represent genotypes when co-dominance is in effect.
A roan calf (RW or C R C W ) results from crossing a white parent (WW or C W C W ) with a red parent (RR or C R C R ).

Gametes R
RR X WW
W

• Blood groups can be used to disprove the paternity of a baby.
• Example Mum Blood Group A and Dad Blood
Group B have a child Blood Group A. “The man says this is not my child.” Is he correct?

• Mums Blood group could be AA or AO
• Dad could be BB or BO.
• Given that A and B are codominant the child could be:

A
O
B
AB
BO
O
AO
OO
Baby could have any of the four blood groups so using this method it would be impossible to prove or disprove the paternity of the father.

• Mum is blood group A and Dad is blood group
O. Baby is blood group B. Dad says: “not my child.” Is he correct?

O
O