Lecture 7 Flashcards

1
Q

What is the primary sex ratio (conceived)/ secondary sex ratio (born)? and why?

A

120-160 males/ 100 females 106 males/ 100 females why? Maybe Y chromsome lighter as it’s smaller than X so sperm with Y swims faster- not proven! Males survive less often, around 19 yrs ratio is 1:1 and at 70 62 males/ 100 females

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2
Q

What is a reciprocal cross?

A

-to discover if it’s X linked disorder - both of the parents should be purebreeding, in one get the male affected in the other female affected and observe the phenotypic ratios, if different= sex linked because males only get Y from dad! -if the disorder recessive and the mother has it, all the male offspring will have it - if it were autosomal the phenotypic ratios are the same in both male affected x normal female and vice versa

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3
Q

Can X-linked disorders be passed from father to son?

A

-never any transmission of X linked disorder from father to son since son only gets Y from the father

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4
Q

When can females get X-linked disorders?

A
  • females can only be affected if father affected
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5
Q

How is colourblindness inherited?

A

X linked dominant, no homologous gene on Y
C-normal vision
c-colour blind

females can be XCXC(normal) XCXc (carrier female) XcXc (colourblind)
males can be XCY (normal) XcY (colourblind) - males are hemizygous
if parents XCXc and XCY- ¼ chance of it being colournlind for a male, no chance for a daughter
if parentXCXc XcY- ¼ chance for both son and daughter

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6
Q
A
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7
Q

What is a major worry for haemoophiliacs?

A
  • cutting yourself and bleeding out, not a major issue, spontaneous bleeding in the joins (painful) is
  • factor VIII manufactured today so you can inject it at home, either keep levels up or inject after injury
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8
Q

Describe the process of blood clotting.

A

-if you don’t have factor VIII can’t convert fibrinogen to fibrin

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9
Q

What is Haemophilia A?

A

= deficiency of Factor VIII
= severe impairment of clotting – the worse haemophilia 7x as common than B

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10
Q

What is Haemophilia B?

A

= Christmas disease
= deficiency of Factor IX
= mild impairment of clotting (better haemophilia, not as bad)

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11
Q

How do we do a reciprocal cross if the female is hetrogametic?

A

-reciprocal crosses are reversed, the transmission is father to daughter (e.g. parrots Budgerigars, albino)

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12
Q

How was X inactivation discovered?

A
  • for an X linked trait females have twice the gene product of a male (have two Xs)
  • 1949 dark staining body on the perimeter of interphase cells in females discovered by Murray Barr
  • not seen in males
  • 1961 Mary Lyon proposes it’s an inactive X chromosome
  • tested on females with XXX- then two bodies, XXXX- three bodies, always only one X switched on
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13
Q

When is on eof the Xs in a female switched off?

A
  • in early embryonic development of a female
  • happens after 15-16 days of gestation (blastocysts stage) till then both switched on- so women not colourblind etc
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14
Q

Which of the Xs switches off, from mother or from father?

A

-random in humans, can be the one from father or from the mother

-1 switches off randomly and the same one is deactivated in all the cells emerging from that particular
cell

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15
Q

Why is one of the Xs switches off?

A

dosage compensation to equalize gene product between males and females, then both have the same number of protein product coming from the X chromosome

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16
Q

Is the X also switched off in oocytes?

A

the X inactivation reverses in the oocytes so mother can pass on both X and X

17
Q

How do we get female mosaics?

A

if female heterozygous for something and different Xs switch off in different parts= mosai

18
Q

Give two example s of moasaics.

A

e.g. Anihidrotic ectodermal dysplasia- sweat glands, hair follicle undeveloped in parts of the body

e. g. Heterozygous (XOXB O-orange, B-black) female cat is a mosaic- that’s why they have different colour is different parts of the body, it depends on where the X’s have switched off, the earlier the switching the bigger the colour patches
- the only way for a male tortoiseshell(the color patches) is Klinfelter XXY (have two Xs so can get it)

19
Q

What is XIST?

A
  • XIST (X inactivation specific transcript)= a gene locus on the X chromosome which is only transcribed from the inactivated X)
  • expression of XIST on the active X is suppressed by methylation
20
Q

What is the process of X inactivation (XIST) 4 steps:

A
  1. The XIST gene is on the X chromosome
  2. Transcription of the XIST gene makes an interference RNA(interferes with gene expression)
  3. The RNA binds to the X chromosome from which it was transcribed, coats it
  4. Recruits other proteins that condense the chromatin by methylation and histone deacetylation to heterochromatin
21
Q

Are all of the genes on the inactivated X switched off?

A

-some genes in the pseudoautosomal region (homologous sequences of nucleotides on the X and Y chromosomes, crossing over mostly there, allows for segregation in meiosis in males and so the X and Y are properly aligned) remain active, that allows explanation for Turner’s etc.

22
Q

What is lyonisation?

A

X inactivation

23
Q

What is atypical lyonisation?

A
  • manifesting heterozygote
  • when a heterozygous mammal female she is a mosaic

e.g. female XHXh (h-haemophilia A)
so if 50% of XH and 50% of Xh switch off enough normal factor VIII to have normal clotting
if 20% XH remain and 80% Xh remain= mild haemophilia
so if 50% and more of the disorder cells= showing some signs!

24
Q

What is Duchenne Muscular Dystrophy?

A
  • heterogenous (lot of genes involved) X linked condition
  • very severe, patients are losing dystrophin in their muscles
  • death in late teens to early twenties
  • never female with full blown condition but can have mild forms if heterozygous, die before reproducing