General Genetics: Gene Segregation and Integration (Part 2)

BIO 106 lec 3

part 2

• Modifier Genes

• Gene Interactions

– Novel Phenotypes

– Recessive Epistasis

– Dominant Epistasis

– Complementary Genes

– Duplicate Genes

– Additive Effect

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Modifier genes

genes that have small

quantitative effects on

the level of expression of another gene

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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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• 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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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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Modifying environment: The environmental influence of a genotype on the

phenotype = phenocopy

Modifier genes

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Modifying environment: The environmental influence

of a genotype on the phenotype = phenocopy

Modifier genes

Himalayan rabbit cces2014

REVISITING the MENDELIAN DIHYBRID CROSS

Gene interactions

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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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Cucurbita pepo

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AAbb aaBB X

AaBb

9 A-B- 6 A-bb or aaB-

1 aabb

P

F1

F2

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Various comb shapes in chicken

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buttercup V-shaped comb

Various comb shapes in chicken

PEA comb cces2014

Rose (RRpp) X single (rrpp)

Rose (Rrpp)

3 Rose (R-pp) 1 single (rrpp)

P

F1

F2

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P

F1

F2 cces2014

9 walnut : 3 pea : 3 rose : 1 single F2

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3.

wild

scarlet

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Biochemical explanation:

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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)

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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- when the recessive allele of 1 gene

masks the effects of either allele of the

second gene.

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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

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)

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

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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

What are the coat colors in this cross? P aacc X AaCc

F1 aaCc

P AACC x aacc F1 AaCc

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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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• 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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A

s

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12 WHITE : 3 YELLOW : 1 GREEN

W is epistatic to Y and y

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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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sweet peas Lathyrus odoratus

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Whenever a dominant gene is present, the trait is expressed. One allele is sufficient to produce the pigment.

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Petal color in snapdragons F2 15 A-B- ; 1 aabb

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ADDITIVE EFFECTS

Dominance

Relationships: – Tan > green – Gray > green – Brown > gray, green and tan. – Tan ^ Gray giving rise to brown.

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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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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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• 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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• 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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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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