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Lecture #6 – Bombay Phenotype & Epistasis

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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

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