cytogenetics and sex determination

Question 1

3 types of chromosome variation

Answer 1

metacentric - middle
acrocentric - close to end
telocentric - at end

Question 2

Cytogenetics

Answer 2

the study of inheritance by visualising the structure

and function of chromosomes

Question 3

what determines sex

Answer 3

genes (not just those on sex chromosomes) rather than chromosomes are the
basis of sex determination.

Question 4

Genetic sex determining systems

Answer 4

normally have populations of ½ females and ½
males.
- one sex is heterogametic, the other is homogametic.

Question 5

Non-genetic systems

Answer 5

can have unequal numbers of the two sexes.

Question 6

Sex chromosomes in different animal species

Answer 6

X-0 system in certain insects 
- X = male, XX = female
Z-W system in birds
- ZZ = male, ZW = female
haplo-diploid system in bees
16 haploid = male, 32 diploid = female

Question 7

Why isn’t the ratio of males to females 1:1

Answer 7

• Do males produce more Y-bearing than X-bearing sperm?
• Are Y-bearing sperm more viable and motile?
• Is the egg surface more receptive to Y-bearing sperm?

Question 8

temperature-dependant sex determination

Answer 8

female to male as temp increases
male to female as temp increases
female are at extreme low and high temp, male at middle

Question 9

do x and y form a homologous pair

Answer 9

yes in meiosis

but only in the PAR regions 1 and 2

Question 10

where is SRY located and what does it do

Answer 10

PAR1
encodes the protein (testis-determining factor
or TDF

Question 11

How was the SRY discovered?

Answer 11

Crossing-over occurs within PAR
• Typically within PAR1 (because it’s larger)
• Discovered in abnormal cases where
karyotype didn’t match sex phenotype
- cross-over below SRY, making them 46,XY, female that are sterile

Question 12

Is the same system used in Drosophila?

Answer 12

Sex determined by ratio of X chromosomes to autosomes
• 1 X : 3 pairs autosomes = male
• 2 X : 3 pairs autosomes = female
2 or more x = female
1x =male
e.g. X = sterile male
XXY = female

Question 13

Dosage compensation

Answer 13

There is compensation for the different gene dosages between males
and females – called Lyonisation

Question 14

maternal and paternal X-inactivation

Answer 14

In females (at about 2 weeks ~ 500
cells) a random X chromosome is
inactived in EACH somatic cell
• The inactivation state is maintained
in all progeny from that cell
• Tissues composed of ~50% maternally
inactivated & 50% paternally inactivated

Question 15

Barr bodies

Answer 15

Inactivated X chromosomes called Barr bodies

• But silenced X chromosome is reactivated at oogenesis

Question 16

number of barr bodies

Answer 16

Number of Barr bodies per nucleus
= number of X chromosomes – 1
• XX = 1 Barr body
• XXX = 2 Barr bodies
• XXY = 1 Barr body

Question 17

Example: Tortoiseshell cats (& Calico cats)

Answer 17

X-linked orange gene, two alleles:
X
o+
- black coat colour (dominant)
X
o
- orange coat colour
So, tortoiseshell cats are always heterozygotes for the pigment genes. They are moisaics, with some cells expressing
orange allele and other expressing black allele

Question 18

Human example: Red-green colour blindness

Answer 18

X-linked recessive trait
• In males
• If recessive (mutant) allele inherited = individual colourblind
• If dominant (WT) allele inherited = individual have normal colour perception
• In females
• If homozygous dominant for wildtype allele = normal colour perception
• If homozygous recessive for mutant allele = individual colourblind
• If heterozygous = retinas are mosaics (colour perception usually good enough)

Question 19

X-inactivation

Answer 19

• Active X chromosome is euchromatic
• Silenced X chromosome is mostly heterochromatic
• Most genes are not expressed
• However some regions escape silencing on inactive X chromosome
• These regions are the PAR and the X Inactivation Centre (XIC)
• Within XIC encodes a lncRNA (Xist)
• X - Inactive specific transcript

Question 20

Mechanism of X-inactivation

Answer 20

1. initiation
2. spreading
3. maintenance

Question 21

initiation

Answer 21

Xist gene is ONLY expressed on the
silenced X chromosome (or
chromosomes if XXX , etc)
Xist is a long non-coding RNA

Question 22

spreading

Answer 22

Xist RNA spreads out along the silenced
X chromosome(s) and coats them
Xist coating attracts histone modifying
enzymes inactivating the chromosome
Note: Xist and PAR escape silencing

Question 23

maintenance

Answer 23

Xist expression is maintained on silenced
X chromosome
X-inactivation can be reversed (for
instance meiosis)