Download Extensions and Modifications of Basic Principles Lecture 6

Lecture 6
Chapter 5: Extensions and
Modifications of Basic Principles
Not negation of Mendel’s principals, but additions to them.
Codominance: Phenotype of the heterozygote includes the
phenotype of both homozygotes
The ABO Blood Group: codominance, dominance, and multiple alleles
Penetrance: Percentage of individuals with genotype that express the
expected phenotype.
Polydactyly: normally from a dominant allele, but not always.
Cancer, Alzheimer’s
Expressivity: The degree to which a trait is expressed.
Polydatyly
Cancer, Alzheimer’s
Which alleles are codominant?
Which alleles show dominance/recessiveness?
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Segregation of lethal alleles can alter Mendelian ratios.
So far today: all of these situations involve interactions of
different alleles at single loci.
Previously, dihybrid or trihybrid crosses have involved genes
(loci) that independently assort, AND
The genes are independent in their phenotypic expression:
9/16
3/16
3/16
1/16
R_Y_
R_yy
rrYy
rryy
round, yellow
round, green
wrinkled, yellow
wrinkled, green
In gene interaction, genes at different loci contribute to
determination of a single phenotypic characteristic.
Gene interactions can lead to modified Mendelian ratios.
What’s odd about this figure?
The Y allele is recessive with respect to lethality!
Gene Interaction for Fruit Color in Peppers
Genotype
R_C_
R_cc
rr C_
rr cc
Phenotype
red
brown
yellow
green
Molecular Reason
red pig., decomp. chlor
red pig., chlor. persists
no red, decomp. chlor.
no red, chlor. persisits
Gene Interaction with Epistasis
Epistasis: the masking of the expression of one gene by another gene
at a different locus.
The gene that does the masking is the epistatic gene.
1. Recessive epistasis
2. Dominant epistasis
3. Duplicate recessive epistasis
Hint: Ratios involving 1/16s suggest that two loci are
interacting.
A test cross doesn’t give the same frequencies.
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Recessive epistasis in Labrador retrievers
Black
Chocolate
Yeller
B: black pigment
b: brown pigment
E: deposition of the pigment into hair shaft
e: no deposition into shaft
Genotype
B_E_
bbE_
B_ee
bbee
Phenotype
Black
Brown
Yellow
Yellow
P: BB EE x bb ee
Black x yellow
F1: BbEe
Black
e is epistatic to B or b
BbEe x BbEe
F2: B_E_
bbE_
B_ee
bbee
B: black pigment
b: brown pigment
E: deposition of the pigment into hair shaft
e: no deposition into shaft
9/16 Black
3/16 Brown
3/16 Yellow
1/16 Yellow
9/16 Black, 3/16 Brown, 4/16 Yellow
= recessive epistasis
Dominant Epistasis
Dominant Epistasis
Genotype
W_Y_
W_yy
wwY_
wwyy
Color in winter squash
P: white squash x green squash
F1:
Phenotype
white
white
yellow
green
white squash
white squash x white squash
F2: 12/16 with white squash
3/16 with yellow squash
1/16 with green squash
How can gene interaction explain this?
3/4 produce white squash, 1/4 produce squash with color.
A dominant allele at one locus can block color?
W_ blocks pigment production. ww allows pigment.
¼
Then among plants with pigment: 3/4 and
Thus a second locus:
Yellow and Green
Y_
yy
12/16 blocked
by W_
1/16 have ww
to allow green AND
yy that blocks
production of yellow
3/16 have ww
to allow green
production
AND Y_ to
convert green
compound to
yellow.
Many cases of epistasis: a gene having an effect on an early step in a
biochemical pathway is epistatic to genes affecting later steps.
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Duplicate recessive epistasis
Albinism in a freshwater snail
P
Another way to look at this duplicate recessive epistasis
aaBB
AAbb
albino x albino
7/16 albino
F1
AaBb x AaBb
F2
9/16 pigmented
AaBb
pigmented
9/16 A_B_
3/16 aaB_
3/16 A_bb
1/16 aabb
pigmented
albino
albino
albino
7/16 albino
Look at A locus:
¾ are A_
And thus make
Enzyme 1
Look at B locus
¾ are B_
And thus make
enzyme II
Both traits, when recessive, are epistatic: need presence of aa or of bb to block
production of colored pigment.
¾ x ¾ Make both
A enzyme and B
enzyme = 9/16
Make pigmented
compound C
Epistatic genes can be dominant or recessive.
One gene effects the other genes, but they are still assorting
independently.
Before lab next week: Read Chapter 6, pp 132-139.
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