1. Lecture #6 – Bombay Phenotype & Epistasis
Wheeler High School The Center for Advanced Studies in Science, Math & Technology
Lecture #6 – Bombay Phenotype & Epistasis
Post-AP DNA/Genetics – Ms. Kelavkar

2. The Bombay Phenotype Discovered in 1952
Woman from Bombay who was genetically blood type B but phenotypically O
Homozygous for recessive mutation in gene known as FUT1
Having this mutation meant she could not make any H substance

3. What type of mutation is this?
Bombay Phenotype = Has the genes to make
and
but has a mutation in the gene that makes

4. Bombay Woman’s Pedigree What does it mean to lack the H substance?
Must have the H substance to make antigen A or B
Can only receive a blood transfusion from other individuals with the Bombay phenotype
OhOh
Figure 4-3 A partial pedigree of a woman displaying the Bombay phenotype. Functionally, her ABO blood group behaves as type O. Genetically, she is type B.
Figure Copyright © 2006 Pearson Prentice Hall, Inc.

5. Bombay Phenotype & Epistasis
Many traits characterized by a distinct phenotype are affected by more than one gene
Epistasis occurs when one gene masks the effect of another gene or when two gene pairs complement each other such that one dominant allele is required at each locus to express a certain phenotype.

6. Bombay Phenotype & Epistasis
The Bombay phenotype for ABO blood groups is an example of epistasis
homozygous recessive condition at one locus masks the expression of a second locus
A locus is a position on a chromosome
Individuals with the Bombay Phenotype have the genes to make the A or B antigen at one loci but lack the genes that produce the H substance produced at another loci.
No H antigen gives you the O phenotype!

7. A ratio expressed in 16 parts (e. g
A ratio expressed in 16 parts (e.g., 3:6:3:4) suggests that epistasis is occurring which means genes are ‘interacting’ to give a particular phenotype.

8. Epistasis in Mice B is the pigment gene!
Agouti = A-
Black = aa
Pigment = B-
No pigment = bb
Agouti = A-
Black = aa
Pigment = B-
No pigment = bb
wt coat color in mice is called agouti (A-)
Agouti is dominant to black (aa)
When a recessive mutation, bb, at a separate locus is homozygous, all pigmentation is eliminated giving albino mice
F1: AaBb x AaBb
F2 Ratio
Genotype
Phenotype
Phenotypic Ratios
9/16
A-B-
Agouti
9/16 agouti
3/16
A-bb
Albino
4/16 albino
aaB-
Black
3/16 black
1/16
aabb
albino

10. Gene Interaction Diagram
Epistasis in Mice
Mouse color is dependent on 2 different genes that interact (A- and B-)…
Gene Interaction Diagram
Gene B
Gene A B-
Black Pigment A-
Agouti
So there are 3 possibilities
No Pigment bb A-
Albino Mouse
Agouti = A-
Black = aa
Pigment = B-
No pigment = bb B-
Black Pigment aa
Black

11. Try One Out!! F1: Aabb x AaBb Agouti = A- Black = aa Pigment = B-
No pigment = bb
Try One Out!!
F1: Aabb x AaBb
F2 phenotypic proportions
F2 Aa x Aa
F2 bb x Bb
½ B-
3/8 A-B- (Agouti)
¾ A-
½ bb
3/8 A-bb (Albino)
1/8 aaB- (Black)
½ B-
¼ aa
1/8 aabb (Albino)
½ bb

12. Lethal Mutations
A loss of function mutation can sometimes be tolerated in the heterozygous state but may behave as a recessive lethal allele in the homozygous state, in which case homozygous recessive individuals will not survive.
This is a lethal mutation!

13. Inheritance patterns in 3 crosses involving the normal wt agouti allele (A) and the mutant yellow (AY) allele in the mouse. Note that the mutant allele behaves dominantly to the normal allele in controlling coat color, but it also behaves as a homozygous recessive lethal allele. The genotype AYAY does not exist.

14. Any Questions?
I am an Agouti mouse
I’m an Albino
I’m a Black mouse