Biology 331: Chapter 4 Gene Interactions

Introduction: Genes do not typically work in isolation Many genes can contribute to a particular biochemical pathway Pleiotropy: one gene affecting many phenotypes Epistasis: many genes affecting one phenotype (book defines differently)

Is it one gene or many? Complementation test: Complementation: the production of wild type phenotypes when two different haploid mutations unite Requires that the mutation be recessive

Harebell example: Normally blue Create three white mutants Are they really different? Cross wild type with mutants F1 are all blue F2 have 3:1 blue to white So they are recessive....but are they the same? Cross all combinations of mutants with each other $$ x && ----> F1 white $$ x @@ ------> F1 blue @@ x && -----> F1 blue Conclusion?

Conclusion: $ and & are the same @ is a novel mutation Thus there are two genes involved Why?

Harebells

How does this work?

Biochemical pathways & complementation: The proteins are what actually complement Genes code for proteins functioning at different parts of the pathway In this case the pathway leads to blue pigment (anthocyanin)

Interactions between alleles of one gene: Review!

Lethal alleles: Some mutant alleles are lethal recessives

Yellow mouse example: Yellow AY is dominant to brown A However if heterozygous yellow mice are crossed we get 2/3 yellow and 1/3 brown Does this make sense? In this case AY AY homozygotes all die before birth Lethal mutation Thus there are two phenotypes for this genotype...one dominant and one recessive Thus we get the degenerate 2:1 ratio instead of 1:2:1

Mice

Manx cats

Lethal recessives: It is more common to have lethal recessives with no other phenotypic affect All of us are heterozygous for a number of lethal recessives Some are uncommon and some common Implications for marriages between relatives? Implications for fertility problems? lethal dominants? Can act in a wide variety of ways: Structural or biochemical pathways

Gene interactions and modified dihybrid ratios: Genes interacting in the different pathways

Corn snake example: o+ = orange pigment; o = absence of orange b+ = black pigment; b = absence of black pigment So o+ /- ; b+ /- are normal o/o ; b+ /- individuals are black and gray b/b ; o+ / - are orange & "peach" o/o ; b/b are albino But note there is still a pattern!

Wild type

Black

Orange

Albino

Snake probabilities

Snake Ratios Thus we get our normal 9:3:3:1 ratio Four different genotypes Mutations act on different biochemical pathways However, if mutations are in the same pathway different ratios can be seen

Genes interacting in the same pathway:

Mutations with the same phenotype: Our harebell example Cross two pure breeding white lines and get a 9:7 ratio of blue to white in the F1 This is clearly a modification of our 9:3:3:1 ratio...but how? Homozygosity for either (or both) of the recessive white alleles yields a white flower

Harebell ratios

Causes Could occur due to two loci One functions to produce a regulatory protein Another acts to produce the required protein

Mutations with different phenotypes:

Two colors produced in one pathway white ---> magenta -----> blue An enzyme catalyzes each step in this pathway Cross homozygous white and magenta plants yields a 9:3:4 blue:magenta:white ratio A modification of our expected 9:3:3:1 ratio

Ratios

Gene interactions In this case white is a sort of "trump" card If you are homozygous for the white gene you will be white no matter what Explain? Blue is dominant to magenta

Biochemical Pathway

Other examples Also occurs in yellow, black and brown labradors In this case yellow is the "trump"

Epistasis?? The book defines epistasis as an interaction where one gene masks the expression of another This definition is too narrow

Supressors: An allele that reverses the affect of another mutation

Drosophila example: Purple eye (pd) is recessive to red eye (pd+) The gene su is a recessive supressor of pd We end up with a 13:3 red:purple ratio Again a modified 9:3:3:1 ratio In this case the only way to be purple is to be homozygous pd and NOT homozygous su

Supressor Probabilities

How do they function? Can be dominant or recessive Nonsense supressors (an example) Nonsense mutations form premature stop codons A nonsense supressor might cause a change at a tRNA allowing the insertion of an amino acid Not terminal because tRNA genes are typically found in many copies Compensatory changes in proteins are possible

Compensatory protein mutations

Duplicate genes: Genes present more than once in the genome

Shepherds Purse example: Two phenotypes heart shaped and narrow are crossed Pure breeding Yields a 15:1 heart shaped to narrow ratio Modified 9:3:3:1 ratio Only double heterozygotes yield narrow phenotypes

Duplicate Gene Ratios

Gene interactions in mouse hair color: A agouti; a non agouti; AY yellow shaft etc. B black; b brown C color expression, c non color expression ch heat activated albinism D full color; d dilute color (milky) S no spots; s spots

Agouti

ch (Himalayan) gene

Color Patterns

Penetrance and expressivity: The percentage of individuals with a given genotype that express the phenotype Why? Modifiers, epistatic genes, supressors, environment etc. Expressivity: The degree to which a given genotype is expressed in the phenotype Variation is due to the same factors

Penetrance and expressivity:

Expressivity

Due to expressivity and penetrance it is often hard to do analysis of phenotypic ratios