The Post-Mendel Era Exceptions to Mendel

Problem Cross a homozygous red flowered plant with a homozygous white flowered plant. (R and r) What offspring do you expect?

After the time of Mendel we have discovered most genes are more complex then simply dominant and recessive Examples Incomplete dominance Codominance Multiple alleles

Incomplete Dominance The blending of two different traits in a heterozygous genotype Example: red and white crossed making pink WW RR RW

Practice Problem Incomplete Dominance – If color exhibits incomplete dominance in violets, what will be the phenotypes of the offspring of a homozygous red and a homozygous white plant?

Incomplete Dominance

Incomplete Dominance R R RW W

Codominance In a heterozygous genotype both genes are fully expressed. Example yellow begonias crossed with red

Codominance

Practice Problem Codominance – Cross a cat with black fur (B) with a cat that has tan fur (b). If we assume that fur color exhibits codominance, what will be the phenotypes of the offspring. (Hint: black and tan fur together make a color called “tabby” in cats).

Practice Problem Codominance – What if we wanted to get some cats that have tan fur, some with black, and some tabby? What would have to be the phenotypes of the parents?

Multiple Alleles More then two alleles can exist for one trait. Each individual can only have two. This leads to more than three possible genotypes. In the example of blood type where 3 alleles exist there are 6 genotypes and 4 phenotypes possible

IA IB i Above are the three possible alleles for blood type. You have a combination of two of these. Phenotype (blood type) Genotype A IAIA or IA i B IBIB or IB i AB IAIB O i i Bl

IA and IB are codominant i is recessive

Practice Problem Multiple Alleles – What are the chances of a woman with Type AB and a man with Type A having a child with Type O?

Practice Problem Multiple Alleles – Dr. Paul is blood type O. His father was blood type A and his mother was blood type B. What were the genotypes of his parents?

California Court Case 1946 the California supreme court ruled that Charlie Chaplin was the biological father of child he claimed was not his. His defense was that the baby had type B blood. He had type A and the woman who sued him had type O Was the court correct?

i A Ai ii AA, AO = Type A BB, BO = Type B i AB = Type AB OO = Type O i The judge should take a course in genetics!

Polygenic Describes a trait that is controlled by more than one pair of alleles This allows for a variety of intermediates Human height, skin and hair color, body build.

Polygenic traits tend to form a bell curve

Sex linked inheritance A trait that travels on the X chromosome. Results that men can not be carriers Women can be carriers For a woman to be afflicted she has to have an afflicted father and a mother that is either afflicted or a carrier