•Modifier Genes •Gene Interactions –Novel Phenotypes –Recessive Epistasis –Dominant Epistasis –Complementary Genes –Duplicate Genes –Additive Effect

1. BIO 106
lec 3
part 2

2. • Modifier Genes
• Gene Interactions
– Novel Phenotypes
– Recessive Epistasis
– Dominant Epistasis
– Complementary Genes
– Duplicate Genes
– Additive Effect
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3. Modifier genes
genes that
have small
quantitative
effects on
the level of
expression of
another gene
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4. Modifier genes
BbDd BbDd
X
9 B_D_ 3 B_dd 3 bbD_ 1 bbdd
The D gene
modifies the
effect of the
B gene.
B: coat color
D: color intensity
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5. • Modifier genes have a subtle, secondary
effect which alters the phenotypes produced
by the primary genes.
Ex:
Tail length in mice. The mutant allele t
causes a shortening of the tail. Not all
short tails are of the same length:
another gene affects the actual length.
(Variable expressivity).
Modifier effects
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6. Same genotype may produce
different phenotypes
Penetrance: the extent to which a particular gene or
set of genes is expressed in the phenotypes of
individuals carrying it, measured by the proportion
of carriers showing the characteristic phenotype.
ex: retinoblastoma = 75% individuals affected
Expressivity: the degree or intensity with which a
particular genotype is expressed in a phenotype in
a given individual
ex: retinoblastoma: some both eyes are affected
some only one eye
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7. Modifying environment:
The environmental influence of a genotype on the
phenotype = phenocopy
Modifier genes
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8. Modifying environment:
The environmental influence
of a genotype on the
phenotype = phenocopy
Modifier genes
Himalayan rabbitcces2014

9. REVISITING the MENDELIAN DIHYBRID CROSS

10. Gene interactions
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11. Gene Interactions
Two types of interactions:
a. Non-allelic genes control the same
general trait, collectively producing a
new phenotype.
b. One gene masks the expression of
others (epistasis) and alters the
phenotype.
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12. Cucurbita pepo
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13. cces2014

14. AAbb aaBBX
AaBb
9 A-B- 6 A-bb or aaB-
1 aabb
P
F1
F2
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16. Various comb shapes in chicken
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17. buttercupV-shaped comb
Various comb
shapes in chicken
PEA comb
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18. Rose (RRpp) X single (rrpp)
Rose (Rrpp)
3 Rose (R-pp) 1 single (rrpp)
P
F1
F2
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19. P
F1
F2 cces2014

20. 9 walnut : 3 pea : 3 rose : 1 singleF2
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21. 3.
wild
scarlet
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22. Biochemical explanation:
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23. Biochemical
explanation:
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In the conversion of substance a to c (drosopterin), the
dominant allele of gene A is needed while dominant allele
for gene B is needed for the conversion of substance
d to f (xanthommatin)

24. one gene influences the
expression of another gene
The gene that masks another is called the
EPISTATIC GENE.
A gene that gets masked is
HYPOSTATIC.
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25. - when the recessive
allele of 1 gene
masks the effects of
either allele of the
second gene.
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26. Melanin production:
B = more melanin
b = less melanin
Melanin deposition::
E = deposit melanin in
fur
e = don't deposit in fur
EeBb X EeBb
9 E-B- : 3 E-bb : 3 eeB- : 1 eebb
9 BLACK: 3 CHOCOLATE : 4 YELLOW cces2014

27. eeB- and eebb have the same phenotype.
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Take note that in this figure, E gene codes for enzyme needed in synthesis of
eumelanin while B gene allows eumelanin deposition in fur. (in contrast with the E
and B as used in the previous slide)

28. 5 genes controlling mice coat color:
1. A gene – pigment distribution
(A = agouti; a = solid dark)
2. B gene – pigment color (B = black;
b=brown)
3. C gene – C (permits color expression)
and c (prevents)
4. D gene – intensity of pigment
(D = full; d = dilute)
5. S gene – S = absence; s = presence of
spots cces2014

29. Recessive
Epistasis
The wild-type coat
color, agouti (AA) is
dominant to black
fur (aa).
A separate gene C, when
present as the recessive
homozygote (cc),
negates any expression
of pigment from the A
gene and results in an
albino mouse.
C gene is epistatic to
the A gene.cces2014

30. What are the coat colors in this cross?
P aacc X AaCc
F1 aaCc
P AACC x aacc
F1 AaCc
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31. BbDd BbDd
X
9 B_D_ 3 B_dd 3 bbD_ 1 bbdd
The D gene modifies the effect of the B gene.
B: coat color
D: color intensity
Albino is epistatic to
brown and dilute (the
recessive albino allele
masks the phenotype of
mice with respect to
brown and dilute).
Brown and dilute are
hypostatic to albino.
Assumption: both
parents are aaCC.
F1
P BBdd X bbDD or BBDD X bbdd
F2
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32. • caused by the
dominant allele of
one gene, masking
the action of
either allele of
the other gene.
• B is epistatic to A
and a
Ex: solid fruit color in summer squash
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35. A
s
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36. 12 WHITE : 3 YELLOW : 1 GREEN
W is epistatic to Y and y
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40. Each genotypic class may not always
dictate a unique phenotype.
A pair of genes can often work
together to create a specific
phenotype.
2 different phenotypes are produced
instead of the 4 seen in 2 genes 1
phenotype
Two or more genotypic classes may
display an identical phenotype.
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41. sweet peas
Lathyrus odoratus
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43. Whenever a dominant
gene is present, the
trait is expressed.
One allele is sufficient
to produce the pigment.
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44. Petal color in snapdragons
F2 15 A-B- ; 1 aabb
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46. ADDITIVE EFFECTS
Dominance
Relationships:
– Tan > green
– Gray > green
– Brown > gray,
green and tan.
– Tan ^ Gray
giving rise
to brown.
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47. Tips for recognizing gene interactions
Look at the F2 phenotypic ratios!!!
• If one gene is involved in the trait,
then the monohybrid phenotypic
ratio is: 3:1 or 1:2:1 or 2:1.
• If two genes are involved in the
trait, then the dihybrid phenotypic
ratio is: 9:3:3:1 or some
permutation (9:4:3 or 9:7 or 12:3:1).
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48. The 1/16 class is always the double
homozygous recessive.
Look for internal 3:1 ratios, which
will indicate dominance/recessive
relationships for alleles within a
gene.
Tips for recognizing gene interactions
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49. • Epistasis: one gene can mask the
effect of another gene
• – 9:3:4 ratio for recessive epistasis
• – 12:3:1 ratio for dominant epistasis
• Complementary Gene Action: one
good copy of each gene is needed for
expression of the final phenotype
(9:7 ratio)
Tips for recognizing gene interactions
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50. • Duplicate genes: only double mutant has
mutant phenotype (15:1 ratio)
2 Genes 1 Phenotype (Additive Gene
Action): You can tell this genotype is
caused by more than one gene because
there are 4 phenotypes (not 3) in F2
(9:3:3:1)
– 1 gene F2 would have 3 phenotypes 1:2:1
ratio
Tips for recognizing gene interactions
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51. Variations on Mendelian Inheritance
Gene
interaction
Inheritance pattern A-B- A-bb aaB- aabb ratio
Recessive
epistasis
Homozyous recessive
genotype at one locus
masks expression at
second locus
9 3 3 1 9:3:4
Dominant
epistasis
Dominant allele at one
locus masks expression at
second locus
9 3 3 1 12:3:1
Complementary
Gene
At least one dominant
allele from each of two
genes needed for
phenotype
9 3 3 1 9:7
Duplicate
Genes
One dominant allele from
either of two genes
needed
for phenotype
9 3 3 1 15:1
Additive
Effects
Each genotype results
in a unique phenotype 9 3 3 1 9:3:3:1
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