1. Complex Patterns of Inheritance INCOMPLETE DOMINANCE, CODOMINANCE, MULTIPLE ALLELES, EPISTASIS AND POLYGENIC INHERITANCE

2. Rules and Rulebreakers  Mendel’s experiments led to the following conclusions about inheritance  Traits are determined by a pair of alleles  If the alleles are different, one is expressed (“dominant”) and the other is not (“recessive”)  The alleles segregate with equal probability during gamete formation  While many traits are inherited by these simple rules, some are not  Some traits are controlled not by one pair of alleles, but my several pairs (“polygenic”)  Some alleles are not clearly dominant or recessive.  Both dominant = “codominance  Neither dominant = “incomplete dominance”  Sometimes more than 2 alternative alleles exist (“multiple alleles”)

3. Insight into the nature of Alleles  Our modern understanding of genetics includes some things Mendel never knew  Genes are located on Chromosomes  Genes are instructions for the production of proteins  There is a distinct protein code that consists of variations in the sequence of nucleotides in the DNA molecules  Occasionally mutations occur which create variations in the instructions that a particular gene carries. These may result in the production of a new protein, unlike the original, which may function in a very different way  The result may also make only minor variations, which manifest themselves more subtly  They may even result in the inability of a cell to produce the protein at all

4. Multiple Alleles  As the name suggests, multiple alleles describes a trait for which more than 2 possible alleles exist for a particular gene  For example, Mendel’s peas expressed 2 possible seed colors, yellow and green. Yellow was dominant and green was recessive. Simple.  If alleles existed for yellow, green and red – That would be a multiple alleles scenario  Several more possible combinations would exist for allele pairs  If yellow was dominant over green, would it also be dominant over red?  If green was recessive to yellow, would red be dominant to green as well?

5. Multiple Alleles – ABO Blood Type  ABO blood type is a classic multiple alleles situation. Three alleles exist for blood type at the same “locus” (locus = position on a chromosome)  The three alleles are for type A, type B, and type O  The A allele is dominant. The B allele is dominant. The O allele is recessive  The term for a genetic system with 2 dominant alleles is “codominance”  The A allele and the B allele code for the production of proteins on the surface of red blood cells. The type A protein and the type B protein are both “antigens” – proteins used to allow the immune system to distinguish between cells that belong and cells that do not, a necessary part of identifying and fighting infections

6. ABO Blood type Genotypes and Phenotypes  Because there are 2 dominant alleles, we use a neutral letter to represent dominance and a notation to identify the type  For blood type, we use the letter I  I a is the dominant A allele  I b is the dominant B allele  i is the recessive type O allele

7. ABO Blood type Genotypes and Phenotypes  More alleles results in more genotypes  More genotypes results in more phenotypes  Note that there are now 3 different homozygous conditions and 2 different heterozygous conditions  Note also that the dominant blood types can be homozygous or heterozygous, but that type O must be homozygous recessive

8. Sample blood type problems  Given the following parents, predict the probabilities of each blood type in the offspring:  heterozygous type A x heterozygous type B  type O x homozygous type B  type O x heterozygous type B  type O x type O  homozygous type A x homozygous type B  Type AB x type O

9. More blood type problems  For each of these families determine the genotypes of the parents and the children  A child with type O blood has a type A father and a type B mother.  A man with type AB blood marries a woman with type B blood. Their daughter has type A blood.  Father = type A, Mother = type B, 2 kids: type B and type A  Same family: Predict the probability of possible blood types for the next child  A man with type AB blood marries a woman with type B blood. Their child has type O blood. How angry should the man be?

10. Codominance Problems  A roan horse has a combination of chestnut colored hairs and white hairs  No individual hair has both chestnut and white  Predict the offspring  White x White  Chestnut x White  Roan x White  Roan x Roan

11. Incomplete Dominance  In these flowers there are alleles for red and for white, neither is dominant. The trait is controlled by a single pair of alleles. Heterozygous flowers are pink.  Predict the probable outcomes from these crosses:  Red x White  White x Pink  Pink x Pink  Pink x Red