Multiple Alleles and Genes

Multiple Alleles Traits usually involve only two alleles per gene But for most genes, more than two alleles exist in a population Existence of multiple alleles means that there may be many possible genotypes for a particular gene and many possible phenotypes

Multiple Genes More commonly a trait is influenced by two or more genes Polygenic trait – has a range of phenotypes due to the additive effects of multiple genes. Example: skin colour and height

Example: Blood Types Blood Types in Humans involves 3 alleles A person can have one of four blood types: A, B, AB or O A person’s red blood cells may be coated with either carbohydrate A (blood type A) or carbohydrate B (blood type B), both carbohydrates (blood type AB) or neither (blood type O)

Blood Type (Phenotype) There are 6 possible genotypes There are 4 possible phenotypes The alleles IA and IB are codominant meaning that and IA IB heterozygous individual expresses both the IA and IB traits and thus the blood type AB Blood Type (Phenotype) Genotype O ii A IA IA IAi B IB IB IBi AB IA IB

Blood Types If someone requires a blood transfusion it is critical that they match the recipient’s blood type with a compatible donor blood type. The immune system detects cells that appear foreign to the body

Example: Someone with blood Type A has an immune system that recognizes red blood cells with carbohydrate B as foreign and will try to destroy them Immune system proteins called antibodies will bind to the foreign carbohydrates causing the donor red blood cells to clump together

Someone who has blood type B has antibodies against carbohydrate A Someone with blood type O has antibodies against both A and B carbohydrates Refer to Figure 5.4 on Page 141 Universal Donor – “O” They can give blood to everyone Universal Reciever – “AB” They can receive from anyone

Example Problem If a man has type AB blood and a woman has type A blood, what are the possible blood types could their children have?

IA IB x IA IA IA IB IA IA IA IB Genotypic Ratio: 50% IA IA: 50% IA IB Phenotypic Ratio: 50% Type A Blood: 50% Type AB Blood IA IB IA IA IA IB

IS THIS THE ONLY POSSIBLE ANSWER???? BUT WAIT!!!!! IS THIS THE ONLY POSSIBLE ANSWER????

IA IB X IAi IA IB IAIA IA IB i IA i IBi Genotypic Ratio: 25% IAIA : 25% IA i: 25% IA IB : 25% IBi Phenotypic Ratio: 50% Type A Blood: 25% Type B Blood: 25% Type AB Blood IA IB IAIA IA IB i IA i IBi

In Drosphila (fruit flies) there are 4 possible alleles that can be inherited for eye colour – red, apricot, honey and white. In the case of multiple alleles, capital letters and superscripts are shown to illustrate this and the order of dominance is indicated using “>” signs. Example: Fruit Flies

E1 > E2 > E3 > E4 Phenotype Possible Genotypes E1 – red (wild) allele E1E1, E1E2, E1E3, E1E4 E2 – apricot allele E2E2, E2E3, E2E4 E3 – honey allele E3E3, E3E4 E4 – white allele E4E4 E1 > E2 > E3 > E4

Problem If a wild type (red eyed) fly with E1E4 genotype is crossed with an apricot eyed fly with a E2E3 genotype, what are the probable genotypic and phenotypic ratios in the offspring?

E1E4 x E2E3 E1 E4 E2 E1E2 E2E4 E3 E1E3 E3E4 Genotypic ratio Phenotypic ratio 2 red eyed: 1 apricot: 1 honey

Homework Multiple Alleles: Additional Problems Using Table 1 from your note, what is the genotypic and phenotypic ratios of the following crosses? E2E3 x E3E4 E2E4 x E3E4 E1E3 x E3E3 E2E2 x E1E4 E1E2 x E3E3 E2E2 x E3E4

Textbook Work Read pages 140 to 143 Page 141 # 2 & 3 Page 145 # 3, 5