Transcript Bombay Phenotype

Wheeler High School
The Center for Advanced Studies in Science, Math & Technology
Lecture #6 – Bombay Phenotype & Epistasis
Post-AP DNA/Genetics – Ms. Kelavkar
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
What type of mutation is this?
Bombay Phenotype = Has the genes to make
but has a mutation in the gene that makes
and
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
Copyright © 2006 Pearson Prentice Hall, Inc.
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.
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!
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.
Epistasis in Mice
Agouti = A-
B is the pigment gene!
Black = aa
Pigment = B-
•wt coat color in mice is called agouti (A-)
No pigment = bb
•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
3/16
aaB-
Black
3/16 black
1/16
aabb
albino
Epistasis in Mice
Mouse color is dependent on 2 different genes that
interact (A- and B-)…
Gene Interaction Diagram
Gene B
BSo there are
3
possibilities
bb
Gene A
Black
Pigment
A-
Agouti
No
Pigment
A-
Albino
Mouse
Agouti = A-
B-
Black
Pigment
aa
Black
Black = aa
Pigment = BNo pigment = bb
Agouti = ABlack = aa
Try One Out!!
Pigment = B-
F1: Aabb x AaBb
F2 Aa x Aa
F2 bb x Bb
No pigment = bb
F2 phenotypic proportions
½ B-
3/8 A-B- (Agouti)
½ bb
3/8 A-bb (Albino)
½ B-
1/8 aaB- (Black)
½ bb
1/8 aabb (Albino)
¾ A-
¼ aa
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!
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.
Any Questions?
I am an Agouti
mouse
I’m an
Albino
I’m a
Black
mouse