Monohybrid Complete Dominance Sample Problem
In a certain plant, long stems (T) are dominant to short stems (t). A farmer
crosses a short-stemmed plant with a heterozygous long-stemmed plant.
What are the expected genotypic and phenotypic ratios?
Incomplete Dominance
 For some traits—one allele of a pair is not dominant to the other
 Example—some plants—red flowers and white flowers—but neither
is dominant, so heterozygous produce pink flowers
 Incomplete dominance of two alleles results in the possibility of three
different phenotypes
 Sample problem:
In four-o-clocks, the alleles for red flowers (R) and white flowers (R’) show
incomplete dominance. The heterozygous condition results in pink flowers.
A gardener crosses a red four-o-clock with a white one. What are the
expected genotypes and phenotypes of the offspring?
Two Traits (Dihybrid Cross)
 After Mendel’s first experiment—he studied the inheritance of two
traits at once.
Example problem:
What types of plants would develop from a cross between a tall, redflowered plant (TTRR) and a short, white-flowered plant (ttR’R’)?
Remember—each gamete must contain one allele for each trait—Law of
Segregation
Law of Independent Assortment—states that genes for different traits
segregate independently during gamete formation
Sample Problem:
In guinea pigs, rough coat (R) is dominant to smooth coat (r). Black color
(B) is dominant to albino (b). A heterozygous black, smooth male is bred to
a heterozygous black, rough coat female. What are the probable genotypic
and phenotypic ratios among their offspring?
Multiple Alleles:
 Although each organism normally has just two alleles for a trait, there
may be more than two alleles possible for that trait in a population.
 These are multiple alleles—a set of three or more different alleles
controlling a trait
 Example in humans—blood type—IA, IB, and i
o IA and IB are codominant to one another. Codominance results
in both alleles being expressed equally
o IA and IB are dominant to i
o Possible human blood types—A, B, AB, O
Sample problem:
A woman who is known to have type B blood marries a man who has type A
blood. They have five children, all with type AB blood. What are the most
probable parental genotypes?