1. Biology 331: Chapter 4
Gene Interactions

2. 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)

3. 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

4. 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?

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

6. Harebells

7. How does this work?

8. 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)

9. Interactions between alleles of one gene:
Review!

10. Lethal alleles:
Some mutant alleles are lethal recessives

11. 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

12. Mice

13. Manx cats

14. 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

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

16. 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!

17. Wild type

18. Black

19. Orange

20. Albino

21. Snake probabilities

22. 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

23. Genes interacting in the same pathway:

24. 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

25. Harebell ratios

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

27. Mutations with different phenotypes:

28. 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

29. Ratios

30. 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

31. Biochemical Pathway

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

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

34. Supressors:
An allele that reverses the affect of another mutation

35. 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

36. Supressor Probabilities

37. 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

38. Compensatory protein mutations

39. Duplicate genes:
Genes present more than once in the genome

40. 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

41. Duplicate Gene Ratios

42. 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

43. Agouti

44. ch (Himalayan) gene

45. Color Patterns

46. 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

47. Penetrance and expressivity:

48. Expressivity

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