Genetics (week 7): Sex Chromosome Anomalies
Y XSM
has XRY region
Xs
spares packaged away as Barr bodies
SEX XSM ABN
relatively common
defects in 1/400 males, 1/650 females
DEFECTIVE TESTIC DEVEL
holandric condition dt SRT deletaion or mutation
XY is female
HAIRY EARS--hypertrichosis
holandric trait
seen in families in India, Aus, Japan
only males have it
p83
X-LINKED GENES
the number of X chromosomes matters because there are homo & heterozygous females but only hemizygous males
strike last sentence in 2nd box on page 83, it's wrong
females can be carriers Aa (or v rare affected aa) or unaffected AA
males can be aY affected or AY unaffected
hemophilia, colorblindness
lyonization in cat coat color: mosaic adult dt heterozygous female-->calico
classic patterns of inheritance
most affecteds are male, black square
obligate carrier female indicated with a dark dot in circle
HEMOPHILIA A
p86-87
X linked recessive disorder of blood clotting factor VIII (intrinsic pathway has quick amplifying action)
OMIM 306700
not same as Hemophilia B aka Christmas dz
mutation at Xq28
Queen Victoria was a carrier
Russian, English, another nation royal families all have relatives of Victoria
1/8000 males (uncommon), virtually unheard of in females
not much factor VIII in regular blood
hard to get enough to injx for tx with factor VIII but can give blood transfusions, this is usu tx
prog fair with tx, can now make factor VIII
prior to detection of HIV almost all hemophiliacs have HIV
affected males are hY, obligate females are Hh
COMMON COLORBLINDNESS
OMIM 303800
x-linked recessive trait of altered color vision
Xq28 mutation
3 genes for eye color vision, any one of these mb affected
each affects vision of a specific wavelength: one blue, one green, one red
mc form is red-green confusion, also green blue and blue red mixers
dx via Ishihara charts, books are expensive, exact right wavelength in dot illus
peeps with this disorder not easy fooled by camoflage because their pattern recognition skills incr developed
there are sometimes affected females because it is so common: cc
males are CY or cY
MUSCULAR DYSTROPHY
Duchenne type MD OMIM 310200
weakness, wasting, legs-->body, starts in males in chidlhood
classic form Xq21.2 in dystropin gene, can't anchor contractile part of mm so it atrophies
1/3 of cases dt new mutation
Becker type is diff spot on same xsome
prog: poor: life expectancy 20-25 yrs
1/4000 males
FRAGILE X SYNDROME
OMIM 309550
x-linked recessive mental retardation
Xq27.3 FMR1 gene, triplicate expansion mutation, variable expansion
variable expression
females 1/2500?? 1/1000 males?
*most common heritable mental impairment (more common than Down's)
sx: hypotonia, macroorchidism, pronathism, autism, elongated face, large ears, ghih arched palate, dental malocclusion, mental impairment sim to Down's, strabismus (crossed eyes), hypertextensible joints, pectus excavatum
prog: fair wtih habvioral, educational therapy
MALE PATTERN BALDNESS--ANDROGENIC ALOPECIA
OMIM 109200
X-linked recessive trait
debate: DOMINANT in males and recessive in females?
many forms of hair loss, this one most in males, females just get thinning
begins as follicular miniaturization, progresses to total loss
gorillas get it too
Xq11.22-12 mutation suspected, only recently shown to be x-linked
prob an androgenic hormone receptor
Norwood-Hamilton scale shows patterns
drugs: monoxidil, propecia,
G6PD DEF
glucose-6-phosphate dehydrogenase
omim305900
x-linked recessive
sensitive to uncooked fava beans, anti-malairal primaquine drugs, sulphonamide abx
sx: hemolytic anemia,
more in Med descent
LESCH-NYHAN SYNDROME
OMIM 3008000
spasticity and severe mental retardation
MANY OTHER CONDITIONS RELATED TO SEX BUT NOT ON SEX XSOME
autosomal genes expressed differently depending on influence of sex
called SEX INFLUENCE
HEREDITARY HEMOCHROMATOSIS
OMIM 235200
6p21.3
progressive dep of Fe in Li, CV, skin, jts
appears to have high new mutation rate
multiple forms of dz found on diff xsms
SX: malaise, cirr, IR, arthritis, skin, cardiomyopathy
1/400, mb mc heritable dz in caucasians of Eur descent, 1/10 are carriers
tx: trepanning is blood letting
SAMPLE QUESTIONS
p92
1) why no imp genes on Y xsm? not everybody gets one
2) how many xsms in Klinefelters? 47
Turner's? 45 or 46 with a major deleted portion of an X
3) LN synd, sex of child? male. Chance of 2nd child? 1/4 for 2nd child
leschnyhan - retard c superuricemia, crystals in diaper
4) female carries deadly x-linked recessive (spont abort if expressed in fetus), what proportion of her offspring would be expected to be male? Aa and AY mate--> AA, AY, Aa, aY dies so 1/3 male
5) How many Barr bodies in XXXY? 2, in XXXX? 3
6) male hemophiliac is reproductively normal, if he mates at random in pop what is prob of affected while? assume carrier females are 1/5000.
hY and HH-->hH, hH, HY, HY so near zero is intuitive answer
but 1/5000 females are carriers gives more detail so
hY = 1/2 x 1/5000 = 1/10,000 so still really low but not exactly near zero