MORE PATTERNS THAN MENDEL THOUGHT
Chapter 11.4
AP BiologyFall 2010
Codominance in ABO Blood Types
Codominance: a pair of non-identical alleles affecting two phenotypes are both expressed at the same time in heterozygotes
In codominance, both alleles are expressed in heterozygotes
Codominance in ABO Blood Types
Blood type is determined by markers produced by three genes
The occurrence of 3+ alleles for a single gene locus available to individuals in the population is called a multiple allele system IA and IB are each dominant to i, but are codominant to
each other Therefore, some persons can express both genes and
have AB blood
Incomplete Dominance
Incomplete dominance: a dominant allele cannot completely mask the expression of another
Phenotype will be intermediate between the two traits
Incomplete Dominance
Example: True breeding red flowered snapdragon crossed with
a white flowered snapdragon Produce pink flowers because there is not enough red
pigment (produced by the dominant allele) to completely mask the effects of the white alleles
Epistasis
One gene pair can influence other gene pairs Their combined activities producing some
effect on phenotype
Epistasis
Example: Labrador retrievers One gene pair codes for the quantity of melanin
produced while another codes for melanin deposition Another gene locus determines whether melanin will
be produced at all- absence of melanin produces an albino (recessive)
Two interacting gene pairs can give rise to a phenotype that neither produces by itself
Epistasis
Example: Comb shape in chickens Is of at least four types, depending on the interaction
of two gene pairs (R and P)
Single Genes With A Wide Reach
Sometimes the expression of alleles at one location can have effects on two or more traits Termed pleiotrophy Cystic fibrosis, sickle cell anemia
Marfan syndrome is caused by autosomal dominant mutation of the gene for fibrillin, a protein found throughout the body in connective tissue
Marfan syndrome is characterized by these effects: Lanky skeleton, leaky heart valves, weakened blood vessels,
deformed air sacs in lungs, pain, and lens displacement in the eyes
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