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MM16: Mendelian Genetics 2 (i) (Lyonisation (inactivation = RANDOM (hence…
MM16: Mendelian Genetics 2 (i)
Sex chromosomes
sex determined @ fertilisation (fixed)
males are hemizygous for X chromo
yet males + females don't differ in protein product due to dosage compensation in females
x chromo inactivation / lyonisation
Lyonisation
in female cells, 1 of the 2 X chromos is inactivated
occurs in early embryonic life (embryo approx 100 cells, 1 wk old)
inactivation = RANDOM
hence approx 50% paternal, 50% maternal
permanent
clonal propagated
message to inactive 1 x chromo is passed down every time cell divides
under microscope inactive X becomes barr-body
condensed heterochromatin (dark, no genes)
virtually complete
X-linked diseases
can be dom/rec
most are rec (rarely expressed in females with mutation)
very few are dom (usually expressed in females with mutation)
fully expressed in males as they are x hemizygous
variably expressed in females
intermediate effect
biochemical deficiency but no phenotype; approx 50% of protein present -> usually sufficient
skewed x inactivation
more mutant x chromos inactivated than expected :smiley:
biochemically normal + no phenotype
manifesting heterozygote
more normal x chromos inactivated than expected :cry:
biochemically abnormal + phenotype present
X-linked Rec
vertical pedigree
complete absence of male->male transmission (father never passes on x-chromo to son
affected male: mother must have mutation
female heterozygote (1 mutant X) + normal male...
25% normal daughter
25% normal son
25% heterozygote daughter
25% affected son
normal female + affected male...
all daughters will be heterozygote (must get mutant X from dad)
all boys normal
Haemophilia A
incidence = 1 in 5000-10000
clotting factor 8 deficiency
soft tissue bleeding, prolonged bleeding + bruising
treatment = cf 8 supplements
Duchenne muscular dystrophy
incidence = 1 in 3300
onset @ 5 y/o
progressive muscle weakness until death in 20s due to resp/heart muscle complications
X-linked Dom
usually more severe + constant in males
less severe + variable in females
vertical pedigree
complete absence male->male transmission
affected male will never pass it on to his sons, and always pass it on to his daughters
affected females have a 50% chance of passing it onto offspring