Exceptions to Mendel’s Principles: Incomplete Dominance Codominance Multiple Alleles Sex -Linked Disorders

Review: Dominant/Recessive One allele is dominant over the other (capable of masking the recessive allele) PP = purple pp = white Pp = purple

Review Problem: Dominant/Recessive In pea plants, purple flowers (P) are dominant over white flowers (p) show the cross between two heterozygous plants. PpPp P p pp Pp PP -25% Homozygous Dominant (PP) -50% Heterozygous (Pp) -25% Homozygous Recessive (pp) -75% Purple -25% White GENOTYPES: PHENOTYPES:

Exceptions to Mendel’s principles Sometimes, there is no dominant or recessive gene, or the trait is controlled by many alleles or genes.

Incomplete Dominance A third (new) phenotype appears in the heterozygous condition. Flower Color in 4 O’clocks RR = redWW = whiteRW = pink

Problem: Incomplete Dominance Show the cross between a pink and a white flower. -50% Heterozygous (RW) -50% Homozygous (WW) R W WWWW WWRW WW RW GENOTYPES: PHENOTYPES: -50% Pink -50% White

Incomplete Dominance straight hair, wavy, curly Straight Hair: SS Curly Hair: CC Wavy Hair: SC

Codominance The heterozygous condition, both alleles are expressed equally Sickle Cell Anemia in Humans NN = normal cells SS = sickle cells NS = some of each

Problem: Codominance Show the cross between an individual with sickle-cell anemia and another who is a carrier but not sick. N S SSSS SS -50 Heterozygous (NS) -50% Homozygous (SS) -50% Carrier -50% Sickle-Cell GENOTYPES: PHENOTYPES:

Codominance In codominance, neither allele are dominant; both are expressed. A cross between organisms with two different phenotypes produces offspring with has both phenotypes of the parental traits shown.

Multiple Alleles There are more than two alleles for a trait Blood type in humans Phenotypes: –Type A, Type B, Type AB, Type O Blood Alleles: –A, B, O

Rules for Blood Type A and B are codominant –AA = Type A –BB = Type B –AB = Type AB A and B are dominant over O –AO = type A –BO = type B –OO = type O

Problem: Multiple Alleles Show the cross between a mother who has type O blood and a father who has type AB blood. -50% AO -50% BO -50% A -50% B GENOTYPES: PHENOTYPES: O O ABAB AO BO AO BO

Problem: Multiple Alleles Show the cross between a mother who is heterozygous for type B blood and a father who is heterozygous for type A blood. -25% AB-25% BO -25%AO-25% OO -25% AB -25% B -25% A -25% O GENOTYPES: PHENOTYPES: A O BOBO AB OO BO AO

Sex Linked Disorders Sex-linked genetic diseases are those where the defective genes are carried on either the X or Y chromosomes. Example: Color Blindness is carried on the X chromosome. A male only needs the one X to carry the trait to be affected, females need both of their X’s.