Download Sex Determination and Sex Chromosomes

Sex Determination and Sex Chromosomes
Sex determination
 In mammals, females have two similar sex-determining chromosomes; males
have two dissimilar ones
 In some other organisms, this general scheme is reversed
 Some oddities
 In some organisms, an individual can start out life as one sex and change to
the other during their lifetime
 The sex change is due to some environmental condition
o For example, in Armadillidium vulgare, females have a W and a Z
chromosome; males have two Z chromosomes
 However, when an Armadillidium vulgare male is infected by
Wolbachia bacteria, he develops into a she
 This allows transfer of the bacterial infection to the next
generation because transfer occurs through the egg
o Wrasses also can change sex
 When the male in a group dies or leaves, one of the females
changes to a male
o In clown fish, just the opposite occurs
 In C. elegans, things are even stranger
Sex determination
 Sexual reproduction usually consists of the alternation of haploid and diploid
cells
Chromosomal basis for sex determination
 In most cases, there are obvious differences between the sex chromosomes
 Some animals (grasshoppers, C. elegans) exhibit XX-XO sex determination
 Some plants, insects, reptiles, and all mammals exhibit XX-XY sex
determination
Human chromosomes
 The Y chromosome is pretty small when we compare it to the X
 It is also usually acrocentric
Birds, snakes, fish, butterflies, and moths exhibit ZZ-ZW sex determination
Some social insects (bees, wasps, ants in the order Hymenoptera) exhibit
haplodiploidy
 Males develop from unfertilized eggs; females from fertilized eggs
Alternative means of sex determination
 Genetic basis for sex determination
o In some plants and protozoans, there are no specific sex
chromosomes
o Sex determination occurs based on the genotypes of one or more
genes
o In reality, this is the case for the chromosomal basis as well - those
genes just happen to be on the sex chromosomes
 Environmental sex determination
o Slipper limpets exhibit sequential hermaphroditism
o In crocodilians and turtles, the temperature of the nest during a
crucial incubation period determines the sex
 Crocodilians - cooler leads to females
 Turtles - cooler leads to males
 Genic balance system
o In Drosophila, the picture is much more complicated
o It actually depends on the X:A ratio
o The X chromosome has the female-producing genes; autosomes have
the male-producing genes
Human sex determination
 The SRY gene resides on the Y chromosome
 Therefore, the Y chromosome determines maleness
What happens when an incorrect number of sex chromosomes is present?
 Klinefelter syndrome
o XXY males (can also be XXXY, XXXXY, or XXYY)
o Usually have small testes and reduced facial and pubic hair
o Often taller than normal
o ~1 in 1000
 Turner syndrome
o XO females
o Usually have underdeveloped sex characteristics
o ~1 in 3000
 Triplo-X syndrome
o XXX females (also XXXX, XXXXX, XXXXXX)
o Usually tall and thin
o XXXX and XXXXX often have developmental problems
The maleness gene
o SRY (sex-determining region Y) gene is located on the Y chromosome
o The presence of the gene triggers the development of maleness
o There are other genes that play a role in fertility and the development of
sexual characteristics
o Androgen-insensitivity syndrome
o Some women have female external sexual characteristics but fail to
menstruate
o Examination reveals testes in the abdominal cavity producing male levels
of testosterone
o The androgen receptor is defective in these individuals
o Unfortunately, some athletes with this condition have suffered
discrimination
Gene dosage compensation
o In the XX-XY system, females have two copies of all the genes on the X
chromosome; males have only one copy
o Dosage compensation equalizes the amount of protein produced from these
genes in the two sexes
o In fruit flies, the activity of genes on the X chromosome in males is doubled
o In C. elegans, the activity of genes on both of the X chromosomes in females is
halved
o In placental mammals, we have X chromosome inactivation
o X chromosome inactivation
o In mammals, X chromosome inactivation leads to the production of Barr
bodies
o In fact, knowing the sex of the individual and counting the number of
Barr bodies can give a quick idea of the chromosomal abnormality
o Which X chromosome is inactivated is a random process
o Therefore, females are effectively hemizygous at the cellular level for Xlinked genes
o Therefore, females heterozygous at an X-linked locus have ~50% of their
cells expressing one allele and ~50% of their cells expressing the other allele
o That makes mammalian females mosaics for the expression of X-linked genes