What if we consider two genes at one

What if we consider two genes at one time? • Mendel’s law of independent assortment. • What happens with one gene has no effect on the other.

Outcome • • • 9/16 Smooth, Yellow 3/16 Wrinkled, Yellow 3/16 Smooth, Green 1/16 Wrinkled, Green Combining these, you’ll find that – 9/16 + 3/16 = 12/16 = 3/4 are smooth, 1/4 wrinkled – 9/16 + 3/16 = 12/16 = 3/4 are yellow, 1/4 green

Law of independent assortment • Each gene assorts independently, not affected by other genes. • This occurs if each gene is on a different chromosome. • Story is different if 2 genes are on the same chromosome (linkage) but we won’t discuss that in detail.

Dihybrid cross • Aa. Bb x Aa. Bb phenotypic ratios 9: 3: 3: 1 9/16 dominant phenotype for both traits (A-B-) 3/16 are A-bb 3/16 are aa. B 1/16 are aabb (recessive for both traits) • Consider outcome of other crosses.

2 -gene crosses: multiplying probabilities instead of punnett square • Problem: Aa. Bb x Aabb • Consider outcome of each gene separately: Aa x Aa = ¾ A- and ¼ aa Bb x bb = ½ Bb and ½ bb • Fraction of offspring with each phenotypic combination can be derived by multiplying A-Bb = (3/4) (1/2) = 3/8 A-bb = (3/4) (1/2) = 3/8 aa. Bb = (1/4) (1/2) = 1/8 aabb = ((1/4) (1/2) = 1/8

Extensions to Mendelian genetics • • • Multiple alleles Incomplete dominance Polygenic inheritance Pleiotropy Linkage Sex-linkage

Multiple alleles • Across the whole population, there may be many more than just two alleles of a given gene. • (Each diploid individual has at most two alleles, if they are heterozygous. ) • For example, there are 4 alleles for fur color in rabbits. Particular color of a rabbit depends on which allele it possesses. (See fig. 10. 12 but don’t sweat the details unless you want to. )

Incomplete dominance • Up till now, the heterozygote always looked just like one homozygote, but that’s not always the case. • Sometimes the heterozygote is intermediate between the two homozygotes. • However, this is not blending inheritance. The alleles remain distinct and will segregate in the next generation.

Human ABO Blood Groups: an example of both multiple alleles and codominance (a kind of incomplete dominance) • There are four phenotypes: A, B, AB, and O • These are controlled by three alleles: IA, IB, and IO (Io is often written as “i” because it is recessive) • Genotypes responsible for the phenotypes: – IAIA and IAIO both have Type-A blood – IBIB and IBIO both have Type-B blood – IAIB has Type-AB blood (codominance) – IOIO has Type-O blood (homozygous recessive)

ABO Transfusion Compatibility • Both the IA and IB alleles put a protein on the rbc’s that is recognized by the immune system. • The immune system of a Type-A person will attack any rbc’s with B protein (cannot accept B or AB blood). • The immune system of a Type-B person will attack any rbc’s with A protein (cannot accept A or AB blood). • A type-AB person will accept both A and B proteins. • The IO allele does not put any protein on the rbc’s, therefore, type O blood is accepted by anyone. • The immune system of a Type-O person will attack any rbc’s with either A or B protein (can only accept type O).

ABO blood group compatibility Blood group Can donate to: Can receive from: A A or AB A or O B B or AB B or O AB AB only A, B, AB, or O O only (universal recipient) O (universal donor)

Polygenic inheritance • Sometimes many separate genes control the same characteristic. • Each gene may have two or more alleles. • The effects of the genes are often additive: each adds a little to the character in question. • This explains continuous variation in characters like height, weight, etc.

Continuous variation • Continuous characters like height and weight are also affected by the environment (e. g. nutrition during development). • Thus, there are no apparent categories in the offspring. • Another reason that Mendel’s work was ignored at first was that everyone thought that continuous characters were more important than 2 -state characters, but couldn’t see how Mendelian genetics explained them. Polygenic inheritance was not worked out until the 1920’s.

Pleiotropy • One gene may affect unrelated characters. • Gene for coat color in Siamese cats also causes crossed eyes. • Gene that causes sickle-cell anemia (when homozygous) also confers resistance to malaria (when heterozygous).

Sex determination • A variety of mechanisms exist in various species. Many (not all) involve sex chromosomes. • In humans, one pair of chromosomes determines sex. • Individuals with two X chromosomes are female. • Individuals with an X and a Y are male. • The Y chromosome is very small and carries few genes other than those that code for maleness. • The X chromosome is large, and carries many genes that have nothing to do with gender.

Sex-linked inheritance • Applies to genes located on the X chromosome. • Examples include color-blindness, male pattern baldness, and hemophilia. • These traits are recessive, but in males they are always expressed, since the male has only one X. • Sons cannot inherit these from their fathers, because they get a Y, not an X, from dad.

Sex-linked genetics problems • What happens if a normal man marries a normal woman whose father was color-blind. • C = normal vision, c=color-blindness • Dad is CY. Mom is Cc. • Half of their daughters are CC; half are Cc. All have normal vision. • Half of their sons are CY (normal); half are c. Y (color-blind)