9a&b - Sex Determination & Linkage, Human Genetics Flashcards
appreciation for the experiments that led to the discovery of the sex chromosomes
L.I
understanding of how transmission of loci on sex chromosomes differs from those on autosomes by undertaking problems that involve sex linkage
L.I
basic understanding of sex determination in mammals
L.I
Exceptions to reciprocal crosses lead to speculation about the existence of sex chromosomes…
give 2 examples of exceptions
- 1906: Magpie moth wing colour (dark & light wings)
- chicken feathers (barred & non-barred)
how do we explain chicken feather & moth wing colour results in light of what we know about Mendelian genetics?
Thomas Hunt Morgan (1909)
adv. of Drosophila as an experimental organism for genetics:
- Drosophila are small & grow easily in lab
- Drosophila have short life cycle (~12 days), so possible to look at many generations
- Mutants can be identified -> eg. normal eye colour: red (W), but Morgan isolated mutant (w) with white eyes
ZW system
in moths, butterflies, etc
males: ZZ
females: ZW
Mammals have an XY sex determination system.
what does this mean?
- males are XY (heterogametic), females are XX
- presence of Y determines maleness
- Y chromosome (usually) carries sex determining region (SRY) that determines male phenotype
X-linked inheritance involves genes located on X chromosome…
but not necessarily involved in sex determination / sex function
- lots of genes on X unrelated to sex determination / sex function …
- eg. some X-linked traits in humans:
-> red-green colour blindness
-> haemophilia
-> Duchenne’s muscular dystrophy - males are hemizygous for genes on X chromosome (effectively dom. as single copy)
X-linked recessive traits can be deduced from certain clues.
what are these clues?
- more males than females express trait
- for female to express trait, male parent must express it & female must either express it or be carrier
- characteristic often skips a generation
- if female expresses characteristic, all of her male offspring will express trait
In female mammals 1 X chromosome is inactivated early in development
- inactivated X can be seen as highly condensed ‘Barr’ body
- inactivation is random - maternal / paternal X inactivated
- so female body is mosaic for genes on X
chromosome - eg. tortoiseshell / calico cats (tissue ‘mosaics’)
Y-linked inheritance results from genes on Y chromosome being passed from father to son
why do we need to consider human genetics separately?
- can’t do controlled crosses
- limited numbers of offspring
- human genetics is of medical importance
- we are inherently interested in ourselves
one way to study human genetics is through pedigree analysis.
what are the factors to consider in pedigrees?
-
is trait located on sex chromosome / autosome?
-> Autosomal - not sex chromosome
-> Sex Linkage - located on 1 of sex chromosomes… - … Y-linked - only males carry trait
- … X-linked (recessive) - sons inherit disease from normal parents
-
how is trait expressed?
-> Dominant - trait expressed in every gen.
-> Recessive - expression of trait may skip gens.
Marfan’s Syndrome: Abraham Lincoln
- Marfan’s syndrome: inherited disorder of connective tissue
- caused by mutations in gene fibrillin-1
- mutation results in overgrowth of long bones of body => long limbs & tall stature
- other symptoms are seen in eyes & heart