H-Biology Unit 3
Part 2 –Heredity
Name: __________________________
Date ______________Pd: 1 2 6 7 8
HUMAN GENETICS – Solving Incomplete Dominance and Co-dominance Problems
Incomplete dominance is a case of inheritance where the heterozygous phenotype shows a
blend of the dominant and recessive alleles. This type of inheritance can best be determined
when the offspring display three phenotype options. Record the genotypes using upper and
lower case letters.
Sample Problem: A red snapdragon is crossed with a white snapdragon, and all the offspring
have pink flowers. Two of the pink snapdragon offspring are crossed producing all three colors.
r
R
Rr
R
Rr
r
Rr
Rr
All Rr
100% pink
R
r
R
RR
Rr
r
Rr
rr
25% RR = red flowers
50% Rr = pink flowers
25% rr = white flowers
Practice Problems:
1. Two short tailed cats have eight kittens; two have no tail, four have short tails and two
have long tails. Record the cross for the two short tail parents and complete a Punnett
Square. If these same parents have four kittens what is the chance of producing
offspring with each type of tail?
2. If a short tailed cat from the litter above and a long tailed cat are crossed what are the
chances of producing a no tailed cat? A short tailed cat? A long tailed cat?
Co-dominance occurs when both alleles for a trait are expressed equally in the heterozygous
genotype. In co-dominance, neither allele is dominant so each form is written as a superscript
using an upper case letter. Human blood types are a good example of co-dominance
inheritance as well as multiple alleles for a gene. There are four blood types – see the chart below.
BB BO
AB
There are six possible crosses between couples of various blood group types. The first example
has been completed for you. Complete the genotype and phenotype for the other five
crosses shown.
Cross 2:
Both mom and dad are blood group O
Cross 3:
Mom is blood type AB and dad is A
(heterozygous)
H-Biology Unit 3
Part 2 –Heredity
Cross 4:
Mom is blood type A (homozygous) and
dad is B (heterozygous)
Name: __________________________
Date ______________Pd: 1 2 6 7 8
Cross 5:
Mom is blood type A (heterozygous) and
dad is O
Cross 6:
Mom is blood type B (heterozygous) and dad is O
Practice Problem: In a court case involving a paternity dispute (i.e. to determine the father of a
child) a man claims that a male child (phenotype is blood group B) born to a woman is his son
and wants custody. The woman claims that he is not the father. If the man has a blood type of O
and the woman is blood type A, could the child be his son? Use a Punnett square to help you
settle this claim. State with reasons whether the man can be correct in his claim.