Chapter 10, Lesson 2 Notes
Different Ways Alleles Cooperate
Simple Dominance
 Many human genes have simple dominance.
 In simple dominance, one allele is dominant
to a recessive allele (e.g., brown eyes are
dominant over blue eyes).
 When two dominant alleles come together in
a homozygous pair, the dominant trait shows
(e.g., brown eyes).
 When two recessive alleles come together in
a homozygous pair, the recessive form of the
trait shows (e.g., blue eyes).
 When a dominant allele and a recessive
allele come together in a heterozygous pair,
the dominant form of the trait shows (e.g.,
brown eyes).
Genotypes and Phenotypes
 Organisms have both a genotype and a
phenotype for all traits.
 A genotype is a combination of genes for a
particular trait to indicate dominant and
recessive alleles. (e.g., TT, Tt, tt).
 A phenotype is what an organism looks like
as a result of its genotype (e.g., tall, short).
Determining Unknown Genotypes
 Testcrosses can determine an unknown
genotype of an organism.
 To do a testcross, an individual of unknown
genotype, but dominant phenotype (e.g.,
tall), is bred with a homozygous recessive
individual (short). The appearance of the
offspring from the testcross will indicate the
genotype of the unknown parent.
Other Allele Relationships
 Multiple alleles are genetic traits with more
than two alleles.
 Human blood is an example of multiple
alleles. Blood types are created by three
different alleles: A, B, and O.
 Type A and B are dominant, O is recessive.
 Type AB blood is codominant—both alleles
show their trait.
 In incomplete dominance the dominant and
recessive alleles work together. They
create a trait form that is between the
dominant and recessive trait forms (e.g., a
tall allele mixed with a short allele creates a
medium height phenotype instead of a tall
phenotype).