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Additional Genetic Patterns . Additional Genetic Patterns . RR x R’R’ Red White. RR’ pink. Incomplete Dominance . Incomplete dominance: neither allele masks the other and both are observed as a blending in the heterozygote. Four o’clock flowers R = red, R’ = white.

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Incomplete dominance

RR x R’R’

Red White

RR’

pink

Incomplete Dominance

Incomplete dominance: neither allele masks the other and both are observed as a blending in the heterozygote

Four o’clock flowers

R = red, R’ = white


Incomplete dominance1

½ R ½ R’

½ R

½ R’

Incomplete Dominance

RR’ x RR’

Pink x Pink

¼ RR

¼ RR’

¼ RR’

¼ R’R’

Genotypic Ratio: ¼ RR + ½ RR’ + ¼ R’R’

Phenotypic Ratio: ¼ red + ½ pink + ¼ white


Multiple alleles
Multiple Alleles

  • Multiple alleles: three or more alleles exist for one trait

    (Note: A diploid individual can only carry two alleles at once.)


Codominance
Codominance

Codominance: Neither allele masks the other so that effects of both alleles are observed in heterozygote without blending

IA =IB > i

IA and IB are codominant.

IA and IB are completely dominant over i.



IAi

IBi

Antigens on Red Blood Cells

IAIB


Inheritance of rh factor
Inheritance of Rh Factor

*There are multiple alleles for the Rhesus protein

(R1, R2, R3, etc.) and all are dominant to the multiple

alleles for the absence of Rhesus protein (r1, r2, r3, etc.) .


Multiple alleles and codominance
Multiple Alleles and Codominance

Type A, Rh positive x Type B, Rh negative

(father is Type O, Rh negative) (mother is Type O)

IB irr

IA iRr x

IAR IAr iR ir

IBr

ir

IAIBrr

IBiRr

IBirr

IAIBRr

IAirr

iiRr

iirr

IAiRr

Phenotypic Ratio of Offspring

1/8 Type AB positive 1/8 Type A positive

1/8 Type AB negative 1/8 Type A negative

1/8 Type B positive 1/8 Type O positive

1/8 Type B negative 1/8 Type O negative


Lethal alleles

½ ML ½ m

X

½ ML

½ m

2/3 tailless + 1/3 tails

Lethal Alleles

Example: Manx cat

ML = tailless, lethal in homozygote

m = tail

Tailless male x Tailless female

MLm x MLm

¼ MLML

¼ MLm

dies

tailless

¼ mm

¼ MLm

tailless

tail


Hierarchy of dominance
Hierarchy of Dominance

Example: hair curling

Sw = wooly Sc= curly Swa= wavy s = straight

Sw>Sc> Swa>s


Hierarchy of dominance1
Hierarchy of Dominance

Sw>Sc> Swa>s

Dad Colavito has wavy hair.

Mom Colavito has curly hair.

Their daughter Jean has straight hair.

What are the expected genotypic and

phenotypic ratios for their offspring?


Hierarchy of dominance2

½ Sc ½ s

½ Swa

½ s

Hierarchy of Dominance

Sw> Sc> Swa>s

Dad C x Mom C

Wavy Curly

Swas

Scs

¼ Swas

¼ ScSwa

curly

wavy

¼ Scs

¼ ss

curly

straight

Bonus: What is Dr. C’s genotype?


Pleiotropic effects
Pleiotropic Effects

One gene affects many phenotypic characteristics


Example of polygenic inheritance
Example of Polygenic Inheritance

Two genes affecting skin coloration

*Based on a study conducted in Jamaica.


Polygenic inheritance
Polygenic Inheritance

Medium Black Woman

(mother is white)

X Darkest Black Man

AaBb

AABB

AB Ab aB ab

AABb

AaBB

AaBb

AB

AABB

Dark

Black

Dark

Black

DarkestBlack

Medium

Black

¼ Darkest Black + ½ Dark Black + ¼ Medium Black


Interacting genes affecting a single characteristic
Interacting Genes Affecting a Single Characteristic

eg. Skin coloration in snakes

One gene

O = orange pigment

o = no orange pigment

Second gene

B= black pigment

b = no black pigment


Interacting genes affecting a single characteristic1
Interacting Genes Affecting a Single Characteristic

eg. Skin coloration in snakes

Oo Bb x Oo Bb

OB

Ob

o b

o B

OB

Ob

o B

o b


Interacting genes affecting a single characteristic2
Interacting Genes Affecting a Single Characteristic

eg. Skin coloration in snakes

OoBb x OoBb

9/16 O_B_ camouflaged

3/16 O_bb orange

3/16 ooB_ black

1/16 oobb albino


Epistasis
Epistasis

  • An allele of one gene masks the expression of alleles of another gene and expresses its own phenotype instead.

  • Gene that masks = epistatic gene

  • Gene that is masked = hypostatic gene

  • Genes that code for enzymes that are upstream in a biochemical pathway usually exert epistasis (“standing on”).


Recessive epistasis

W

M

enzyme 1

enzyme 2

Precursor 1 Precursor 2blue anthocyanin

colorless magenta

Recessive Epistasis

Epistatic gene exerts its affect with homozygous recessive genotype.

eg. Petal color in blue-eyed Mary plants

mm= magenta, ww =white, W__M__= blue


Recessive epistasis1
Recessive Epistasis

eg. Petal color in blue-eyed Mary plants

Ww Mm x Ww Mm

9/16 W__ M__

3/16 W __mm

3/16 w w M__

1/16 w w mm

Blue

Magenta

White

White

Phenotypic ratio: 9/16 blue: 3/16 magenta: 4/16 white


Duplicate recessive epistasis

W

B

enzyme 1

enzyme 2

Duplicate Recessive Epistasis

Defective products of recessive alleles of two different genes interfere with separate steps in a biochemical pathway.

eg. Petal color in harebell flowers

ww = white, bb = white, W_ B_ = blue

Precursor 1 Precursor 2blue anthocyanin

colorless colorless


Duplicate recessive epistasis1
Duplicate Recessive Epistasis

eg. Petal color in harebell flowers

Ww Bb x Ww Bb

9/16 W__B__

3/16 W __b b

3/16 w w B__

1/16 w w bb

Blue

White

White

White

Phenotypic ratio: 9/16 blue: 7/16 white


Dominant epistasis
Dominant Epistasis

Epistatic gene exerts its affect with the presence of a dominant allele. eg. Fruit color in summer squash

Y = yellow, yy = green;

W inhibits either color = white; w has no effect on color


Dominant epistasis1
Dominant Epistasis

eg. Fruit color in summer squash

Ww Y y x Ww Y y

9/16 W__ Y__

3/16 W __yy

3/16 w w Y__

1/16 w w yy

White

White

Yellow

Green

Phenotypic ratio: 12/16 white: 3/16 yellow: 1/16 green


Duplicate dominant epistasis
Duplicate Dominant Epistasis

eg. Fruit shape in Shepherd’s purse

A_ or B_ = heart shape

aa and bb = narrow shape


Duplicate dominant epistasis1
Duplicate Dominant Epistasis

eg. Fruit shape in Shepherd’s purse

A_ or B_ = heart aa and bb = narrow

A a Bb x A a Bb

9/16 A__B__

3/16 A__b b

3/16 aa B__

1/16 aa b b

heart

heart

heart

narrow

Phenotypic ratio: 15/16 heart: 1/16 narrow


Interaction between sex and heredity
Interaction between Sex and Heredity

John Adams

John Quincy Adams

Male pattern baldness

Dominant in males, recessive in females


Interaction between sex and heredity1
Interaction between Sex and Heredity

Cock-feathered male

Hen-feathered female

Hen-feathered male

Cock feathering, autosomal recessive

Expressed only in males


Interaction between sex and heredity2
Interaction betweenSex and Heredity

Leaf variegation caused by inheritance of variable chloroplast genotypes


Interaction between sex and heredity3
Interaction betweenSex and Heredity

Direction of snail shell coiling is determined by genotype of female parent


Interaction between sex and heredity4
Interaction Between Sex and Heredity

Angelman SyndromeDeletion on chromosome 15 inherited from mother

Prader-Willi SyndromeDeletion on chromosome 15 inherited from father


Anticipation
Anticipation

Trait is more strongly expressed or expressed earlier in succeeding generations


Expansion of the Trinucleotide Repeat for Huntington’s Disease

Linda

(6,22)

Allen

(46,13)

age 50

Jama

(7,18)

Andrew

(69,6)

age 37

Kristen

(64,22)

age 40

Ann

(64,22)

age 39

Bill

(8,12)

Greg

(11,19)

Debbie

(13,6)

Christina

(93,7)

age 26

Joseph

(7,6)

Nathaniel

(72,19)

age 35

Paula

(13,12)

Evan

(not tested)


Environmental effects
Environmental Effects Disease

Phenotype is dependent upon the presence of a specific environment.

The temperature-sensitive product of the himalayan allele is inactivated at high temperatures.


Penetrance and expressivity
Penetrance and Expressivity Disease

  • Penetrance = percentage of individuals with a given genotype who exhibit the phenotype

  • Expressivity = extent to which genotype is expressed at the phenotypic level (may be due to allelic variation or environmental factors)


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