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