The X Chromosome & triplet repeat disorders

Question 1

heterogametic

Answer 1

two types of gametes e.g. XY

homogametic - one type e.g. XX

Question 2

what is the key gene determining male sex

Answer 2

SRY

Question 3

SRY gene is responsible for development of testis at what week

Answer 3

7

androgens developed
embryos that lack Y develop female sex organs

Question 4

46 XX males symptoms

Answer 4

small testes and are sterile ambiguous or external gentile

Question 5

X and Y chromosomes share genes in pseudo autosomal regions called

Answer 5

PARs

PAR1 and PAR2 at top and bottom respectively. Few genes but important as have double dose whereas in middle different
This is to help the two chromones align during meiosis – have to have some sort of homology to pair properly and undergo recombination

Question 6

kleinefelter syndrome 47XXY

symtpoms

Answer 6

extra X 
tall and minor breast development 
testes are small and fail to produce normal level of testosterone leading to gynaecomastia and no spermatogenesis 
taller and thinner and reduced IQ 
50% miscarriages

Question 7

tuner syndrome - most common aneuploidy in females

45, X

Answer 7

loss of X

webbed neck
very short, infertile, broad chest, widely spaced nipples and life span

Question 8

Mosaicism

Answer 8

different cells become different chromosome compliment – cells missing an X chromosome like a pattern over the body

Question 9

The x chromosome possessed a unique regulatory region called the X inactivation centre that is essential for inactivation
what is inactivation

all body cells individual

Answer 9

Early in embryonic development in females, one of the two X chromosomes is randomly and permanently inactivated in cells other than egg cells. This phenomenon is called X-inactivation or lyonization. X-inactivation ensures that females, like males, have one functional copy of the X chromosome in each body cell.
Because it happens in each body cell then like a cat could be both have orange and black fur

Question 10

in autosomal dominant - 50% affected both males and females - is someones affected every gen

Answer 10

yes

Question 11

autosomal recessive what is risk to offspring if both carriers

Answer 11

1/4

Question 12

mticohondial who gets infected

Answer 12

only maternal genes and delivered to all kids

Question 13

in x linked recessive there is no

Answer 13

male to male transmission

Question 14

example of x linked recessive

Answer 14

DMD- childhood, wheelchair by 12, cardiomyopathy by 18 and dead by 30 - Gower sign
haemophilia A and B

Question 15

x linked dominant no male to male but females less severely affected example

Answer 15

rett syndrome 
MECP2 gene 
affects girls 
delayed development from 1 year 
lose ability to walks and speak

Question 16

fragile x syndrome trinucleotide repeat in FMR1 gene what repeat

Answer 16

CGG over 200 normal like 55

methylation of the gene resulting inactivating the gene so no protein or RNA

Question 17

anticipation

Answer 17

increased clinical severity and or earlier stage of onset in successive generations

Question 18

myotonic dystrophy tri-nucelotide repeat

Answer 18

CTG