Complex Patterns of Inheritance

I. Incomplete Dominance Characterized by blending of traits producing intermediate phenotypes. o A long watermelon (LL) crossed with a round watermelon (RR) produces all oval watermelons (LW).  Since neither trait is dominant, we use different capital letters to represent the alleles. LL WWLW + =

b. Japanese Four O’Clock Flowers Can be: red (RR) white (WW) or pink (RW) RR x WW Genotype: Phenotype:

RW x RW Genotypic Ratio: % Red Offspring: % White Offspring: % Pink Offspring:

1:2:1 homozygous black to heterozygous to homozygous white

I. Codominance Both alleles are expressed completely. Suppose a plant which is homozygous for red flowers is crossed with a plant that is homozygous for white flowers: Incomplete DominanceCodominance RR X WW RW RR X WW RW

Codominance

c. An example of codominance is the MNO blood group in humans. There are three different blood groups: M, N, and MN o These groups are based on two distinct molecules located on the surface of red blood cells. o A person can be o The MN genotype is not intermediate between M and N.  The M and N traits are both fully expressed on the surface of red blood cells.

IV. Polygenic Inheritance Many traits are controlled by several different genes and are therefore called polygenic. Skin color, hair color, height, etc. Short parents might carry more “short” genes than “tall genes,” but can still pass on their “tall” genes to their offspring, resulting in an individual taller than his/her parents.

b. Polygenic traits result in bell-shaped curves for the entire population. Height

Skin Pigmentation

III. Multiple Alleles Most genes within a population have only two alleles. o Pea plants are either round (R) or wrinkled (r). When there are more than two allelic forms of a gene, we refer to the situation as multiple alleles. o In humans, there are four different blood types: A, B, AB, and O, determined by the presence of different surface molecules on red blood cells.

These blood types are determined by three alleles: A, B, and O. o A and B are codominant.  When both alleles are present, both traits are expressed (AB blood type).  Often, you will see these two alleles written as I A and I B o The O allele is recessive and often written as i

People with specific blood types have specific antibodies that attack “foreign” red blood cells. This immune response is trigged by specific antigens found on the red blood cells.