Sex Determination and Sex-Linked Traits

Chapter 4

Outline

• Sex Determination

• Different Karyotypes

• sry gene and maleness

• Sex Linked Traits

• Barr Bodies

1. Chromosomal Sex determination Systems

• XX-XO

• XX-XY

• ZZ-ZW

• Haplodiploidy

A closer look at 2 different systems of sex determination

2. Genetic Sex Determining Systems

• Genic Balance Sex Determination

• Environmental Sex Determination

drosophila

Slipper limpet

Drosophila: genic balance

Sex chromosome Autosome* complement

X:A ratio

Phenotype

XX and AA 1.0 female

Xy and AA 0.5 male

XXY and AA 1.0 female

XXX and AAA 0.67 intersex

* autosome= any non-sex chromosome

Human Karyotypes

NORMAL

Klinefelter Syndrome

Turner Syndrome

Non-disjunction

Maleness

• Determination of maleness is due to sry gene of Y chromosome

• Expression of gender is more complex than just sry gene.

• Example: androgen insensitivity – Xy genotype– Many female characteristics!

Sex Linked (X-linked) Traits (3 examples)

• Human Color Blindness

Sex Linked (X-linked) Traits

• White eye Drosophila

Hemophilia

Pedigree of hemophilia

½ X½ Xhh

½ Y½ Y

½ X½ XHH ½ X ½ Xhh

¼ X¼ XHHXXhh ¼ X¼ XhhXXhh

¼ X¼ XHHYY ¼ X¼ XhhYY

XXhhY x XY x XHHXXhh

Barr Bodies & X-Inactivation

• What are they?

• Why does it occur?

Number of Barr Bodies in Human Cells

X-Inactivation in cats

Tortoiseshell Color

Y-Linked Inheritance

• Few genes on Y chromosomes

• Few examples of known Y linked traits to date

The End