SEX AND SEX-LINKED TRAITS
I.
SEX DETERMINATION
A. Sex chromosomes in humans are the X and Y chromosomes
1. Females are XX and males are XY
a) Females are said to be homogametic (as all their gametes will have an X
chromosome) and males heterogametic (producing gametes with either an X
or Y chromosome)
2. All factors being equal, there is a 50% chance of having a boy or a girl
a) Females must donate a X chromosome to their eggs, males may donate
either the X or Y to their sperm
3. Sex is determined by the Y chromosome, not the two X chromosomes
a) XO, XX, XXX, XXXX are all females
b) XY, XXY, XXXY and all males
c) The Y chromosome contains a gene called the testes determining factor
(1) The gene product regulates at least two other genes
(a) Inhibits p450 aromatase gene
(i) The gene product converts testosterone to estradiol
(b) Activates the Mullerian-inhibiting substance
(i) The gene product is needed for testicular development and causes
digression of female reproductive ducts
4. Dosage compensation
a) About twelve days after conception, each female has one of her X
chromosome in each cell inactivated
(1) Forms heterochromatin
(2) Females are mosaic as far as the X-chromosome is concerned
B. ZZ - ZW system
1. Found in moths, butterflies, most birds, some fish, reptiles, amphibians, and at
least one plant
2. Males are homogametic (ZZ) and females are heterogametic (ZW)
a) Z and W are used so not confused with X and Y of other system
C. XX - XO system
1. A type of parthenogenesis found in Hymenoptera
2. Males are haploid and arise from unfertilized eggs
3. Females are diploid and are produced sexually
D. Temperature dependent sex determination
1. Found in crocodiles, most turtles, and some lizards
2. Increased temperatures may cause offspring to be females, males, or extreme
temperatures may result in one sex while the middle temperature results in the
opposite sex
a) Different patterns seen in different species
II.SEX
LINKED INHERITANCE
A. Genes on the sex chromosome are termed sex linked
1. Most of the genes are found on the larger X chromosome and are called X-linked
genes
a) Males are more likely to show recessive X-linked traits, since they will only
get one allele for that trait
b) A female can only show recessive X-linked traits if her father showed the
trait and her mother was a carrier (or had the trait)
2. X-linked Recessive Disorders
a) Types of color blindness
b) Types of hemophilia
c) Muscular dystrophy