Download INCOMPLETE DOMINANCE

INCOMPLETE DOMINANCE
INTRODUCTION
For the pea plants that Mendel studied, one allele was always dominant over the other.
However, traits with two distinct alleles are rare in nature. Many genes do not have purely
dominant and recessive alleles. When two different alleles for the same trait combine, but
neither allele “wins” expression over the other, the offspring may show incomplete
dominance. In this case, heterozygous offspring demonstrate a phenotype in between the
two purebred parents.
EXAMPLE
What happens when you cross a purebred red (RR) and a purebred white
(rr) snapdragon?
R
R
r
Rr
Rr
r
Rr
Rr
genotype ratio: 4 / 4 Rr
phenotype ratio: 4 / 4 pink
PROBLEMS
1. What happens when you cross a red and pink snapdragon?
genotype ratio:
___________________________
phenotype ratio: ___________________________
2. What happens when you cross two pink snapdragons?
genotype ratio:
___________________________
phenotype ratio: ___________________________
3. Incomplete dominance occurs in people as well as plants. For example, people
heterozygous for hypercholesterolemia (Hh) have 2X the amount of blood cholesterol as
normal people (HH). About one in a million people are homozygous recessive (hh) for
the trait. Their cholesterol levels are about 5X greater than normal. This condition is
very serious - it has even led to heart attacks in children two years old.
If a homozygous recessive parent and a parent heterozygous for hypercholesterolemia
have children, what cholesterol levels will their children likely have?
genotype ratio:
___________________________
phenotype ratio: ___________________________
4. If a homozygous recessive parent and a parent homozygous dominant for
hypercholesterolemia have children, what cholesterol levels will their children likely
have?
genotype ratio:
___________________________
phenotype ratio: ___________________________