Sex determination

Question 1

Sex chromosomes in Drosophila

Answer 1

Drosophila has polytenic chromosomes, the sex ones are heteromorphic. Morgan (1933) found the white-eye mutation to be X-linked.

Question 2

Hemizygous

Answer 2

What males, with XY chromosomes, are in respect of X-linked traits.

Question 3

Aneuploidies in humans

Answer 3

Due to non-disjunction in male meiosis I or II.
- Turner syndrome: X0
- Klinefelter’s syndrome: XXY, XXXY
Sex is determined by X:Autosomes ratio (Bridges)

Question 4

Y chromosome

Answer 4

• 50 genes
• Homologous to X only in pseudoautosomal regions (PARs) that recombine and undergo synapses in meiosis
• Contains the SRY gene that encodes for the testis determining factor TDF

Question 5

X chromosome

Answer 5

• 1000 genes
• Homologous to Y only in pseudoautosomal regions (PARs) that recombine and undergo synapses in meiosis
• If more than one, gene-compensation system inactivates all but one Xs into Barr bodies

Question 6

X inactivation system

Answer 6

Random epigenetic event happening in embrionic developent, can cause mosaic (Mary Lyon, 1961), status memory erased during oogenesis
- Initiation (reversible, Xist dependent): number of Xics counted, N-1 Xs targeted for inactivation, Xist gene starts inactivation.
- Spreading: from Xic, the RNA coat spreads and inactivates the chromosome (15% of genes still expressed)
- Maintainance (irreversible, Xist independent).