Extensions of Mendelian GeneticsOutline/Study Guide

Broader course objectiveExplain more complex modes of inheritance and how this influences the

inheritance and expression of genes; use this information in predicting genetic outcomes and the analysis of genetic data

Necessary for Labs--Patterns of Inheritance in Maize, Blood typing.

Lecture outline/study guide• Other factors that can change ideal Mendelian ratios

– How can lethality affect the ratios of the remaining genotypes/phenotypes in Mendelian segregation?

– What is the difference between “penetrance” and “expressivity”?– What is incomplete dominance? Co-dominance? What examples of

each might you be able to give?• What is an “allelic series”?• Epistasis--two (or more) genes can interact to affect one phenotype (e.g.

color of peppers). – When is a gene interaction considered “epistatic”? – What is the difference between the epistatic gene and the hypostatic

gene?• What is the difference between “epistatic” vs. “dominant”?

2:1 ratio from cross between two yellow mice results from a

lethal allele.

Egg1:2 ratioof kittensthat are born

Sperm

x

MM(early

embryonicdeath)

Mm(Manx)

M

M

m

m

Mm(Manx)

Mm(Manx)

Mm(Manx)

mm(non-Manx)

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or displayBrooker, Figure 5.13b

Lethality in Manx cats

causes altered ratio

Phenotype affected by temperature

Biology: Unity and Diversity of Life, Starr and Taggert

Fig from iGenetics 1st ed., P. Russell

Penetrance—in this group of identical genotypes, how many actually show the phenotype?

Fig from iGenetics 1st ed., P. Russell

Expressivity—in this individual of particular genotype, how strongly does he express the phenotype?

Variable expressivity of Neurofibromatosis (NF1)

Weak phenotype: cafe au lait spots

Strong phenotype--cutaneous neurofibromas

P0 generation

x

Pink

Gametes

Gametes

Self-fertilization

CRCW

CWCWCRCR

F1 generation

F2 generation

CR

CR CW

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CW

CRCR CRCW

CRCW CWCW

Brooker Figure 5.5

1:2:1 phenotypic ratio NOT the 3:1 ratio observed in simple Mendelian

inheritance

In this case, 50% of the CR protein is not sufficient to produce the red phenotype

CWCR or

CR

CW

Incomplete dominance as seen

in plants

Allelic Series—being “dominant” or “recessive” depends upon which alleles are being expressed together

Allele Phenotype Enzyme activity

CR Red 100%

C50 Dark pink 50%

C20 Light pink 20%

C0 white 0%

Genotype Phenotype

CR CR Red

C50 CR Red

C50 C50 Dark pink

C50 C20 Pink

C50 C0 Light pink

C20 C20 Light pink

C0 C0 white

e.g. flower color gene: C [enzyme necessary for making pigment]

Figure 5.6

Dominant (functional) allele: R (round)Recessive (defective) allele: r (wrinkled)

Amount of functional(starch-producing)protein

RR Rr rr

100% 50% 0%

Genotype

With unaided eye(simple dominant/recessive relationship)

With microscope(incompletedominance)

Round Round WrinkledPhenotype

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Co-dominance—both alleles equally

expressed

(example: bloodtypes—genotype and surface

antigensA

A

A

A

A

BB B

B

B

B

B

B

A

A

A

Why is this called a phenotype and not a genotype?

• Allele i is recessive to both IA and IB

• Alleles IA and IB are co-dominant (in cells with both alleles the trait is a mixture of both phenotypes seen in the homozygotes)

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Blood type:

Genotype:

Surface antigen:

Serum antibodies:

O

RBC RBC RBC RBC

Antigen O

Antigen A

N-acetyl-galactosa

mine

Antigen B

Antigen B

Antigen A

neither A nor B

A

IAIA or IAiii

A

B

IBIB or IBi

B

AB

IAIB

A and B

Galactose

against A and B against B against A none

Brooker Figure 5.9a

ABO Blood Type

Brooker, fig 5.12

(a) Hen (b) Rooster

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© robert Maier/Animals, Animals © robert Maier/Animals, Animals

Examples of sex limited inheritance?

Duplicate Recessive Epistasis

Purple Pigment

Precursor 1

Gene A

Precursor 2

Gene B

When the recessive allele of either gene masks the expression of the other gene.white

white

White variety #1 (CCpp)

F1

F2

All purple(CcPp)

Self-fertilization

Complementation: Each recessive allele (c and p) is complemented by awild-type allele (C and P).This phenomenon indicates that the recessive alleles are in different genes.

Epistasis: Homozygosity for the recessive allele of either gene results in a white phenotype, thereby masking the purple (wild-type) phenotype.

Both gene products encoded by the wild-type alleles (C and P) are needed for a purple phenotype.

x

White variety #2(ccPP )

CP

Cp

cP

cp

CCPPPurple

CCPpPurple

CcPPPurple

CcPpPurple

CCPpPurple

CCppWhite

CcPpPurple

CcppWhite

CcPPPurple

ccPPWhite

ccPpWhite

CcPpPurple

CcppWhite

ccPpWhite

ccppWhite

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CcPpPurple

Brooker, Figure 5.14

CP Cp cP cp

Duplicate Recessive Epistasis

“dominant” ≠ “epistatic”

“dominant/recessive”• Alleles of same gene

Gene 1—Hair color

Black

Brown

Auburn

Red

Blond

Epistatic—one gene masking another

Gene 1—Hair color Gene 2-Baldness

Black Full hair

Brown partial hair

Auburn bald

Red

Blond

7q21.1

“dominant” ≠ “epistatic”

“dominant/recessive”• Alleles of same gene

Gene 1—Hair color

Black

Brown

Auburn

Red

Blond

Epistatic—one gene masking another

Gene 1—Hair color Gene 2-Baldness

Black Full hair

Brown partial hair

Auburn bald

Red

Blond

7q21.1

14p1.31

Hypostatic gene

Epistatic gene

• A geneticists crossed a red eyed fly with another red eyed fly. In the next generation she observed phenotypic proportions of 263 red-eyed flies : 137 brown-eyed flies. What hypothesis would best explain the parental cross that gave rise to these flies? Use chi-square analysis to support your hypothesis.

• How would you ‘prove’ this without doing the chi-square test?

Extensions of Mendelian Inheritance : practice questions

The following comprehension questions (at end of each chapter section) in Brooker, Concepts of Genetics are recommended:•Comprehension Questions (at end of each section): 5.2, 5.4, 5.6, 5.8 Answers to Comprehension Questions are at the very end of every chapter.

•Solved Problems at end of chapter (answers included):

•Conceptual questions and Experimental/Application Questions at end of chapter (answers found by logging into publisher’s website, or find them in the book):

– Concepts—C1, C2, C3, C4, C5, C6, C8, C9, C10, C11, C15, C20

– A little more challenging—C18, E1, E2, E7, E10