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genetics > 9a&b - Sex Determination & Linkage, Human Genetics > Flashcards

9a&b - Sex Determination & Linkage, Human Genetics Flashcards

1
Q

appreciation for the experiments that led to the discovery of the sex chromosomes

A

L.I

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1
Q

understanding of how transmission of loci on sex chromosomes differs from those on autosomes by undertaking problems that involve sex linkage

A

L.I

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2
Q

basic understanding of sex determination in mammals

A

L.I

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3
Q

Exceptions to reciprocal crosses lead to speculation about the existence of sex chromosomes…

give 2 examples of exceptions

A
  1. 1906: Magpie moth wing colour (dark & light wings)
  2. chicken feathers (barred & non-barred)
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4
Q

how do we explain chicken feather & moth wing colour results in light of what we know about Mendelian genetics?

A
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5
Q

Thomas Hunt Morgan (1909)

adv. of Drosophila as an experimental organism for genetics:

A
  1. Drosophila are small & grow easily in lab
  2. Drosophila have short life cycle (~12 days), so possible to look at many generations
  3. Mutants can be identified -> eg. normal eye colour: red (W), but Morgan isolated mutant (w) with white eyes
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6
Q

ZW system

A

in moths, butterflies, etc

males: ZZ
females: ZW

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7
Q

Mammals have an XY sex determination system.

what does this mean?

A
  • males are XY (heterogametic), females are XX
  • presence of Y determines maleness
  • Y chromosome (usually) carries sex determining region (SRY) that determines male phenotype
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8
Q

X-linked inheritance involves genes located on X chromosome…

but not necessarily involved in sex determination / sex function

A
  • 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)
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9
Q

X-linked recessive traits can be deduced from certain clues.

what are these clues?

A
  • 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
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10
Q

In female mammals 1 X chromosome is inactivated early in development

A
  • 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’)
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11
Q

Y-linked inheritance results from genes on Y chromosome being passed from father to son

A
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12
Q

why do we need to consider human genetics separately?

A
  • can’t do controlled crosses
  • limited numbers of offspring
  • human genetics is of medical importance
  • we are inherently interested in ourselves
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13
Q

one way to study human genetics is through pedigree analysis.

what are the factors to consider in pedigrees?

A
  • 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.
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14
Q

Marfan’s Syndrome: Abraham Lincoln

A
  • 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
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15
Q

Albanism

A
  • expressed in both sexes at approx. equal freq
    -> so autosomal
  • not expressed in every gen
    -> so recessive
16
Q

Hairy Ears

A
  • only males affected
  • all sons of affected father have hairy ears

-> so hairy ears are Y-linked

17
Q

Why aren’t human genetics as simple as pedigree diagrams?

A
  1. most genetic diseases are not result of single gene mutation
  2. chromosomal disorders - alteration of no. / structure (e.g. deletion) of chromosomes
    -> eg. Trisomy 21: Down Syndrome -> massive variation in phenotype
  3. genetic disorders can be caused by non-nuclear genes (mitochondrial)
  4. complex genetic disorders - resulting from actions of large no. of genes & interactions between genes and env.
18
Q

3 -> genetic disorders can be caused by non-nuclear genes (mitochondrial)

A

maternal (mitochondrial) inheritance:

  • inheritance through maternal lineage
  • sperm do not contribute mitochondria to embryo

Kearns-Sayre Syndrome = rare neuromuscular disorder
-> mainly affects eye (progressive limitation of eye movements until there is complete immobility, accompanied by eyelid droop)

19
Q

Phenylketonuria (PKU)

A
  • classic eg. of single gene mutation in a biochemical pathway => disease
  • 1950s: biochemical defect identified
  • 1960: neonatal screening to allow early diagnosis & treatment
  • treatment: avoid dietary phenylalanine
20
Q

some patients didn’t respond to PKU treatment.

why not?

/ why might there be a variation in phenotype in identical genotypes?

A
  • in 1983: mapped & cloned PAH gene …
  • & confirmed there is allelic heterogeneity in PKU
  • as well as other loci involved
  • so even though we have same allele of PAH, other genes modify the effect -> giving variation of phenotype in identical genotypes…
  • so genetic background is important!!
21
Q

Cystic Fibrosis

A
  • another classic eg. of single gene mutation => disease
  • CF: autosomal recessive inheritance
  • CFTR gene cloned => speculation that mutational analyses would be sufficient to predict clinical outcome (specific alleles associated with severity of disease)
  • despite this, disease phenotype turns out to be difficult to predict & MONOGENIC model of inheritance was limited in its utility i.e. POLYGENIC
22
Q

what is CTFR in Cystic Fibrosis?

A
  • Cystic Fibrosis Transmembrane conductance Regulator (CFTR)
  • ABC transporter-class protein
  • transports Cl⁻ ions across epithelia
  • mutations => defects in Cl conductance, CF & congenital absence of vas deferens
23
Q

what do modifier genes that contribute to CF phenotypes do?

A
  • NOS1: Nitric oxide synthase -> in macrophages nitric oxide (NO) mediates bacteriacidal activity
  • MUC1: mucin 1-> glycoprotein that lines apical surface of lung epithelia & prevents bacterial infection
  • so problems with either modifier promote bacterial infections associated with CF

genetics (7 decks)

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