11.3
Other Patterns of Inheritance
Beyond Simple Dominance
• Not all genes show simple
patterns simple patterns of
inheritance
• Most genes have more than
two alleles or are controlled
by more than one gene
Incomplete Dominance
• Some alleles are neither
dominant nor recessive
• In complete
dominance, neither
allele is dominant
• The heterozygous
phenotype lies
somewhere between the
two homozygous
phenotypes
• The heterozygous
genotype yields a mixed
phenotype
Codominance
• Phenotypes
produced by both
alleles are clearly
expressed
• Many human genes
are codominant,
including blood
types
• The heterozygous
genotype yields
BOTH phenotypes
Multiple Alleles
• Many genes exist in several different
forms and are therefore said to have
multiples alleles
• A genes that have more than two
alleles is said to have multiple alleles
• An individual has only two
copies of each gene, but more than
two exist in a population
• EX: Rabbit fur color, human blood
types
Polygenic Traits
• Many traits are produced by the
interaction of several genes
• Traits controlled by two or more
genes are said to be polygenic (many
genes) traits
• Often show a wide range of
phenotypes
• Four+ genes control human skin color
Genes and the Environment
• Environmental conditions
can effect gene expression
and influence genetically
determined traits
• Phenotype is determined by
genes + environment
Genes and the Environment
• Examples: Gene or Environment?
–A blue eyed girl is born to two blue
eyed parents
–A pair of twins is separated at birth.
They grow up in different countries
and speak different languages.
–A kitten is born with six toes.