non mendelian inheritance · non mendelian inheritance part iii. lethal genes • french geneticist...
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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