What is the genotype?

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What is the genotype?. Hairless. Lecture 4. Continued. Example of Probabilities Using Sex Linkage:. X H Y × X H X h. X H X H X H X h X H Y 1/4 normal male X h Y 1/4 male with hemophilia. 1/2 normal females. Sex male female normal hemophilia . Sex male female

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Lecture 4

Continued

Example of Probabilities Using Sex Linkage:

XHY × XHXh

XHXH

XHXh

XHY 1/4 normal male

XhY 1/4 male with hemophilia

1/2 normal females

Sex

male female

normal

hemophilia

Sex

male female

normal 1/4 1/2 3/4

hemophilia 1/4 0 1/4

1/2 1/2

Conditional Probabilities

• P(affected child) = 1/4

Conditional Probabilities

• P(affected child) = 1/4
• P(hemophilia / male)?

Sex

male female

normal 1/4 1/2 3/4

hemophilia 1/4 0 1/4

1/2 1/2

Conditional Probabilities

• P(affected child) = 1/4
• P(hemophilia / male) = 1/4 = 1/2 1/2

Conditional Probabilities

• P(affected child) = 1/4
• P(hemophilia / male) = 1/4 = 1/2 1/2
• P(hemophilia / female) = 0 = 0 1/2

Lecture 5

Exceptions to Mendel’s Ratios

Sex Influenced

• Traits for which the expression of the phenotype depends on the sex of the individuals. Can occur with both autosomal or sex-linked loci.

Example of Sex Influence:

Presence of horns in sheep:

Polled -- without horns

Horned -- with horns

Sheep Example:

P = polled

p = horned

Horned male Polled female

pp × PP

All progeny Pp

Horned male Polled female

pp × PP

• All progeny Pp but,
• female -- polled P > p
• male -- horned p > P

male female

PP Polled Polled

male female

PP Polled Polled

pp Horned Horned

male female

PP Polled Polled

PpHorned

pp Horned Horned

male female

PP Polled Polled

Pp Horned Polled

pp Horned Horned

male female

PP Polled Polled

Pp Horned Polled

pp Horned Horned

p > P

Sex Limited

• A phenotype that is expressed in only one sex.

Sex Limited

• A phenotype that is expressed in only one sex.
• Examples:
• Antlers -- deer
• Color -- pheasants and cardinals
• Milk production -- dairy

X Inactivation

X Inactivation

T = black hair T = t

t = orange hair

PhenotypeGenotype

male female

black XTY XTXT

orange XtY XtXt

PhenotypeGenotype

male female

black XTY XTXT

orange XtY XtXt

tortoiseshell XTXt (unique)

-- calico = tortoiseshell with S_ --

ss XTXt

Cat Fanciers’ Association

S_ XTXt

Cat Fanciers’ Association

Occasionally a tortoiseshell male happens:

• Nondisjuncture -- failure of chromosomes to separate properly during meiosis

Parents XTXt × XTY

Gametes XTXt× XT Y

Parents XTXt × XTY

Gametes XTXt× XT Y

Zygotes

• XTXtXT tortoiseshell female
• XT black female
• Y dies
• XTXtY tortoiseshell male

Non-disjuncture can occur in any chromosome.

• trisomic 21 -- Down’s Syndrome -- incidence by age

Maternal age Incidence/1000

 25 0.5

30 1.0

35 6.0

40 12.0

 45 21.0

Not near as dramatic in males!

(?)

Non-disjuncture can occur in any chromosome.

• trisomic 21 -- Down’s Syndrome
• trisomic 13 -- Patau -- 1/5000
• trisomic 18 -- Edwards -- 1/14000

Lethal Conditions

• Gene or genotype that leads to the death of the individual.

Postnatal Lethal Condition (Abnormalities)

Parrot jaw in cattle:

P = normal P > p

p = parrot jaw

Pp × Pp

3 P_ + 1 pp

normal parrot jaw (dies at young age)

Embryonic Lethal

L = normal L > l

l = lethal

Embryonic Lethal

Ll × Ll

3 live + 1 dies

(not observed)

Embryonic Lethal

Ll × Ll

3 live + 1 dies

(not observed)

• Fertility studies needed to determine the presence of a recessive embryonic lethal.

XLXl × XLY

2 XLX- normal female

1 XLY normal male

1 XlY dies (not observed)

XLXl × XLY

2 XLX- normal female

1 XLY normal male

1 XlY dies (not observed)

-- 2:1 sex ratio at birth --

H = hairless dog H > h

h = hair

Hh × Hh

1 HH + 2Hh + 1hh

Hh × Hh

1 HH + 2Hh + 1hh

dies

Embryonic lethal, not observed at birth!

Hh × Hh

1 HH + 2Hh + 1hh

dies

hairless

(all heterozygotes)

Hh × Hh

1 HH + 2Hh + 1hh

dies

hairless

hair

(all heterozygotes)

2:1 Ratio

Other Examples

• White in HORSES
• Manx tails in CATS
• Bluefrost coat color in MINK
• Leg length and size in Dexter CATTLE
• Creeper (leg length) in CHICKENS
• Yellow color in MICE

Multiple Alleles

• There are > 2 allelic forms possible at a particular locus segregating in the population.

Dog Example:

C series in dogs -- C is required for color and cc is albino.

C Series

C -- color series expressed

c -- albino (pink nose and eyes)

C > c

Dog Example:

C series in dogs -- C is required for color and cc is albino.

However, other alleles exist that don’t produce the albino phenotype.

C Series

C -- color series expressed

cch -- chinchilla pattern

cd -- white coat (black nose, dark eyes)

cb -- Cornaz coat (blue eyes), pale gray

c -- albino (pink nose and eyes)

C > cch > cd > cb > c

cchc × cdcb

2 cch_ chinchilla

1 cdc white

1 cbc Cornaz

Anomalies

• Incomplete penetrance
• Phenocopy
• Variable expressivity

Incomplete Penetrance

• Failure of a genotype to be expressed with the phenotype normally associated with it.

Incomplete Penetrance

• Stripe marking on forehead of fox.

Half Stripe

Full Stripe

SS = full stripe

Ss = half stripe

ss = no stripe

SS × ss

SS = full stripe

Ss = half stripe

ss = no stripe

SS × ss

Ss (18% no stripe)

Phenocopy

• An environmental influence causing an effect similar to a phenotype under genetic control.

Phenocopy

• An environmental influence causing an effect similar to a phenotype under genetic control.
• Example -- plant toxins Thalidomide babies

Variable Expressivity

• Variation in phenotypic expression

Variable Expressivity

• Variation in phenotypic expression
• Example -- Mice
• Dwarfism: Still variations in weights of dwarf mice and may overlap with normal weights

GMBT16

BMS2382

OY3, 5, 15

Somewhere in this region

Callipyge

IDVGA30

CSSM18

Ovine Chromosome 18

Alleles:

C= Callipyge

N = Normal

C

C

N

C

Genotypes:

M

F

M

F

C

N

N

N

M

F

M

F

CN

NN

X

Callipyge males

Normal females

½CN : ½ N N

M

F

M

F

Expected: 1 callipyge to 1 normal

Actual: 203 callipyge and 209 normal

NN

CN

X

Normal males

Callipyge females

½NC: ½ N N

M

F

M

F

Expected 1 callipyge : 1 normal

Actual 33 normal

Polar overdominance

at the callipyge locus

Female callipyge chromosome

(C) “turns off” allele expression

F

Maternal imprinting:

Maternally-derived allele is silent,

paternally-derived allele is

expressed

Silent allele is reactivated in

paternal germ line