NON MENDELIAN

INHERITANCE

PART III

Lethal Genes

• French geneticist Lucien Cuenot,experimentaly crosses on coat colour inmice, found a gene that was not consistentwith mendelian predictions.

• Observations,

• Two yellow mice yielded approximately a 2:1ratio of yellow to wild type, rather than theexpected 3:1.

• Crosses yellow individuals to the recessivewild type, all yellow mice produced wild typeprogeny.

Conclusions

• Yellow mice were heterozygotes

• No yellow homozygotes were produced.

• Homozygosity for yellow is lethal, and that

individuals died in utero

• When a genetic defect causes 100%

mortality, it is termed a lethal gene.

The lack of a tail in the Manx cat is another trait

caused by an allele that has a dominant effect in

heterozygotes and is a lethal in homozygotes

Manx allele – M

Normal allele –m

Cross between two Manx cat ??

PLEIOTROPY

• Many of these genes may also have secondary or related effects.

• Ex: 01; Yellow coat colour in mice is an allele that affects more than one character;

Body colour –Heterozygotes

Lethal – Homozygotes

• Ex : 02; Mendel – Flower colours violet and white also influenced seed colour

• The phenomenon in which a single gene affects two or more characteristics is called pleiotropy

Pleiotropic effects in humans

• Phenylketonuria is a

genetic disorder where the

body’s enzyme,

phenylalanine hydroxylase

(PAH), is missing or

malfunctioning so that it

cannot properly break down

the amino acid,

phenylalanine (phe).

• Phe is found in food

containing proteins, but

without PAH,

consumption of such

foods would lead up to a

build-up of phe.

• The high level of phe

affects the nervous

system and other

physical features.

• Mental Retardation

• Microcephaly (small head size)

• Lower IQ

• Stunted growth

• “Musty” body odor from the excess phenylalanline

• Fair skin, hair, and eyes (phenylalanine is linked to melanin production)

Symptoms

Penetrance and Expressivity

• All of the genes we have considered to this point have a definite genotype-phenotype relationship.

• However, some genes, a given genotype may or may not show a given phenotype.

• It is described as penetrance of a gene

• The level of penetrance can be calculated as the proportion of individuals with a given genotype who exhibit a particular phenotype

• Eg: When all individuals of a particular genotype have the same phenotype, the gene shows complete penetrance and the level of penetrance is 1.0.

Expressivity

• A particular genotype exhibits the

expected phenotype, the level of

expression or expressivity may vary.

• Ex:- A gene causes a detectable disease

in most individuals with a given genotype

some may be much more severely

affected than others.

Fig. 13.18, Penetrance and expressivity

Causes of incomplete penetrance and variable

expressivity?

• Environmental factors and other genes are known to

influence the penetrance or expressivity of a gene.

• Environmental factors

Eg: primrose have red flowers when grown at 24oC but

white flowers when grown above 32oC.

Temperature can affect coat coloration.

Siamese cat fur in the extremities is darker due to cooler temperatures. The enzyme making darker pigment doesn’t work well at the higher temperatures in the rest of the body.

Sex Linked Genes

The Xs and Ys of Genetics

Chromosomes

• We have 2 types of Chromosomes:

• 1. Autosomal chromosomes – pairs 1

through 22 in humans (non-sex

chromosomes)

• 2. Sex chromosomes – xx or xy

• What about a fruit fly with a 2n = 8?

Sex Linked Genes

• These chromosomes

not only carry the

genes that determine

male and female

traits, but also those

for some other

characteristics as

well.

• Genes that are

carried by either

sex chromosome

(X or Y) are said

to be SEX

LINKED

Males

• Men normally have an

X and a Y

combination of sex

chromosomes.

• Since only men inherit

the Y chromosome,

they are the only ones

that inherit Y-linked

traits.

Females

• Females have 2 X

chromosomes

• Designated as XX

X Linked Traits

• Men and Women

can get the X-

linked traits since

both inherit X

chromosomes.

SEX DETERMINATION

XX =

Xy =

female

male

Who decides?

Mom can give X

Dad can give X or y

Dads determine sex of babies.

If dad gives X with mom’s X = girlIf dad give y with mom’s X = boy

X X

X

y

X X X X

X y X y

SEX CHROMOSOMES CAN CARRY OTHER GENES TOO= ___________________SEX LINKED TRAITS

Sex chromosomes can carry other genes

X-LINKED GENES: Genes carried on the X chromosome

EX: __________________________

_________________________

HemophiliaColorblindness Muscular Dystrophy

Hemophilia

• An X-linked

genetically inherited

recessive disease

• Can not clot their

blood

• Severe bleeding from

minor cuts

• Mostly in males

Make a cross with an X-linked gene

Hemophilia is an ________________________________ disease

X-linkedrecessive

Use ______ for normal dominant blood clotting gene.

Use ______ for recessive hemophilia gene.

On X chromosome so write it as ________

H

h

Xh

Mother

Without hemophilia = X X

Without hemophilia = X X

With hemophilia = X X

Father

Without hemophilia = X y

With hemophilia = X y

H H

H h

h h

CARRIER

H

h

XH

y

XH

Xh

Make a cross with an X-linked geneCarrier Mom XNormal dad

XHXH XHy

XHXh Xh y

GIRLS_______ = normal_______ = look normal

but areCARRIERS

BOYS_______ = normal_______ = hemophilia

1/2

1/2

1/2

1/2

Red-Green Color Blindness

• X-linked

• Cannot perceive red and green in

the same

• Most often affect by males

Are you colorblind?

Normal Color Vision:

A: 29, B: 45, C: --, D:

26

Red-Green Color-

Blind:

A: 70, B: --, C: 5, D: --

3.

Red Color-blind:

A: 70, B: --, C: 5, D: 6

4.

Green Color-Blind:

A: 70, B: --, C: 5, D: 2

Make a cross with an X-linked gene

Colorblindness is an ________________________________ disease

X-linkedrecessive

Use ______ for normal dominant color gene

Use ______ for recessive colorblind gene.

On X chromosome so write it as ________

B

b

Xb

Mother

normal vision = X X

normal vision = X X

colorblind = X X

Father

normal vision = X y

colorblind = X y

B B

B b

b b

CARRIER

B

b

Xb y

XB

XB

Color blindness is sex linked

HOMOZYGOUSNormal Mom Xcolorblind dad

100% carriers

XBXb XBy

XBXb XBy

GIRLS = _______________BOYS = ________________100% normal

X linked recessive genes appear __________ in males than females.

Females can be __________ for X linkedrecessive traits.

Males can _________ carriers for X linkedrecessive genes. The either have trait OR are normal.

more often

CARRIERS

NEVER BE

Muscular Dystrophy (MD)

• Characterized by a gradual irreversible

wasting away of skeletal muscle.

• An X-linked trait most often passed on to

sons from their mothers.

• Begins to weaken the legs of boys by age

3 and gets worse every year.

• No cure and inflicted usually die by age

30.

Y-linked Traits

• The Y chromosome is small and therefore

does not contain many genes

• Y linked diseases are very rare

• Only passed from farther to son

Sex chromosomes can carry other genes

Y-LINKED GENES:Genes carried on Y chromosome

EX:Hairy ears

_________genes only show up in _______Y linked males.

Make a cross with a y-linked gene

Hairy ears is a ________________________________ trait

Y linkeddominant

Use ______ for hairy ears.

Use ______ for recessive normal ears.

On y chromosome so write it as ________

H

h

yH

Make a cross with a y-linked gene

X X

X

yH

X X X X

X yH X yH

ALL GIRLS =____________Normal ears

ALL BOYS =____________Hairy ears