1. EPISTASIS AND SEX LINKED

2. Polygenic inheritance
Some phenotypes determined by additive ____________________________________characteristic
phenotypes on a continuum
human traits
skin color
height
weight
intelligence
behaviors

3. In Snarlymonsters, the number of teeth is polygenic
In Snarlymonsters, the number of teeth is polygenic. aabbcc= toothless AABBCC=6 teeth What are some other variations? How many teeth would a AABbCC Snarlymonster have?

4. Sex linked traits 1910 | 1933 Genes are on sex chromosomes
_______________________________
Disorders linked to sex chromosomes
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Examples: hemophilia and
muscular dystrophy

5. Sex Linked Dominant
Dominant inheritance occurs when an abnormal gene from one parent is capable of causing disease, even though a matching gene from the other parent is normal. The abnormal gene dominates the gene pair.
For an X-linked dominant disorder: If the father carries the abnormal X gene, all of his daughters will inherit the disease and none of his sons will have the disease.
This is because daughters always inherit their father's X chromosome. If the mother carries the abnormal X gene, half of all their children (daughters and sons) will inherit the disease tendency.

6. Sex Linked Recessive
Sex-linked recessive disorders are carried on the X chromosome
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 Daughters can be carries

7. Sample Problems
In humans, colorblindness is sex linked and recessive.  If a woman is a carrier for the trait, what is the chance that her sons will be colorblind?   Her daughters?   (Assume the father has normal vision.)
In humans, hemophilia is sex linked.    If a woman is a carrier for the trait, what is the chance that her daughters will also be carriers.  (assume father is normal)

8. Epistasis

9. Definition
Epistasis is a form of gene interaction in which one gene masks the phenotypic expression of another.
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10. Epistatic versus Hypostatic
The alleles that are masking the effect are called _____________________
The alleles whose effect is being masked are called the ____________________.

11. Epistasis
No new phenotype is produced, but one gene (epistatic) masks the phenotypic expression of another gene (hypostatic).
Dominant epistasis, A masks the effect of B.
Recessive epistasis, caused by recessive alleles, aa masks the effect of B at another locus.
Can occur with two genes, requiring A and B to produce a phenotype (duplicate dominant or recessive epistasis).

12. How do we solve epistatic problems?
We are dealing with polygenic traits as in the previous section, however we now have one pair of alleles masking the other.
______________________________________________________________!

13. Recessive or Dominant?
Epistasis can be described as either recessive epistasis or dominant epistasis.
Let’s look at an example of recessive epistasis….

14. Labrador Retrievers
Fur color in Labrador Retrievers is controlled by two separate genes.
Fur color is a polygenic trait!
Gene 1: Represented by B
: Controls color
Gene 2: Represented by E
: Controls expression of B

15. Labrador Retrievers
If a Labrador retriever has a dominant B allele, they will have black fur.
If they have two recessive alleles (bb) they will have brown fur.

16. Labrador Retrievers
If a retriever receives at least one dominant “E” allele, they will remain the color that the “B” allele coded for.
Either black of brown
However, if a dog receives a pair of homozygous recessive “e” alleles, they will be golden regardless of their “B” alleles!

17. Labrador Retrievers BBEE and BbEe --> Black retrievers
bbEE and bbEe --> Brown retrievers
BBee, Bbee, or bbee --> Golden retrievers

18. Try this cross…
You have decided to cross your golden retriever (bbee) with the neighbor’s chocolate retriever (bbEe). What color pups will they have?

19. bbee x bbEe FOIL: be FOIL: bE or be
Genotypes of F1 generation: bbEe and bbee
Pups phenotypes: Brown and golden

20. Dominant Epistasis Let’s have a look at dominant epistasis…
Squash fruit color is controlled by two genes.
Gene 1 is represented by a W
Gene 2 is represented by a G

21. Squash Fruit Color Genotypes and Phenotypes: W-/G- white W-/gg white
ww/G green
ww/gg yellow

22. Squash Fruit Color Which allele is epistatic in squash color?
How do you know?
The dominant W allele is epistasis
Because every time a dominant W allele
shows up in a squash genotype, the squash
fruit color is white.

23. Try this cross….
Cross a green squash (wwGg) with a white squash (Wwgg).
What color are the offspring?

24. Wwgg x wwGg FOIL: Wg or wg FOIL: wG or wg F1 generation genotypes:
Phenotypes: