Topic 8, Sex Determination and Sex
Chromosomes
- Some major systems of sex determination
A. XX are females and XY are males
1. Occurs in humans, Drosophila, and most animals
2. Females have 2 X chromosomes and males have an X and and a Y
3. Females are the homogametic sex (produce one type of gamete) and
males are the heterogametic sex (produce two
types of gametes)
B. XX are females and XO are males
1. Occurs in some insects including grasshoppers
2. Females have two X chromosomes and males have one X chromosome
3. Females are the homogametic sex and males are the heterogametic sex
C. ZW are females and ZZ are males
1. Occurs in birds, moths, butterflies, some fish, and amphibians
2. Females have two different sex chromosomes (Z and W) and males have
two identical sex chromosomes (ZZ)
3. Females are the heterogametic sex and males are the homogametic sex
4. The pattern of inheritance of sex-linked genes is different from
that in XX-XY individuals
D. 2n = females and n = males (drones)
1. Occurs in hymenoptera (ants, bees, wasps)
2. 2n eggs fed "royal jelly develop into queens, and 2n eggs not
fed "royal jelly" develop into workers
E. Most plants have both sexes on each plant
F. Some plants have 1 sex per plant
1. May be controlled by sex chromosomes
2. May be controlled by a single locus
G. Numerous other types of sex-determining mechanisms exist
- Sex differentiation
A. Autonomous (in insects)
1. Each cell ijn an individual has a male or female phenotype
2. Gynandromorphis (sexual mosaics)
B. Non-autonomous (hormonal, in mammals)
1. The sexual phenotype is determined by hormones that circulate in the
body
a. estrogens = female hormones
b. androgens = male hormones
2. Hermaphrodite = intersex occur when there is an improper balance of
hormones
a. Hormanal treatment and/or surgery is often
used to direct development in a male or female direction in these cases
3. Example 1, Bonellia, a marine worm
a. Free-swimming larvae become females and
larvae that are attached to the bodies of
females become males
due to masculanizing hormone from the female
4. Example 2, eunuchs in humans
- Sex determination in
A. Drosophila melanogaster
1. Sex phenotypes in individuals with different chromosome
constitutions were determined during 1910-1930
2. It was found that the ratio of the number of X chromosomes to the
number of sets of autosomes determines
the sex of the individual
a. 1X :1A = female and 1X:2A = male
b. The Y chromosome is not involved in
sex determination; however, because XO males are sterile,
we know that
genes necessary for fertility in males are located on the Y chromosome
B. Homo sapiens (humans)
1. A short history of human cytogenetics
a.Painter (1923) concluded that humans had 48
chromosomes
b.Tjio and Levan (1956) determined that humans
had 46 chromosomes
c. The human chromosome complement has
1) 22 pairs of
autosomes
2) 2 sex chromosomes
(XX in females and XY in males)
2. Sex chromosome aneuploids (individuals with abnormal numbers of
chromosomes
a. 47,XXY; Klinefelter syndrome (abnormal male)
b. 45,X; Turner syndrome (abnormal female)
1) Analysis of
Klinefelter and Turner syndromes demonstrates that when a Y chromosome is present, the
individual is a male
c. 47,XXX syndrome (female, varies from normal
to abnormal)
d. 47, XYY syndrome (male with possibly more
aggressive tendencies)
e. The Y chromosome and male development
1). PARs
(psudoautosomal regions on the Y chromosome), homologus with a region on the X chromosome
and necessary for synapsis
2). SRY (sex
determining region of the Y), encodes TDF (testes determining factor)
f. Mosaics, individuals that have a mixture of
cells of 2 or more genotypes
a. Due to mutations or mistakes during
mitosis
b. Example, David Bowie, has one blue eye
and one brown eye
3. Sex reversal genotypes in humans
a. Testicular feminization (androgen
insensitivity) These XY individuals have a female phenotype
because their cells are
insensitive to androgens
b. Chromosome aberrations (structural changes)
that cause sex reversal
4. Sex ratios (the number of males / the number of females) in humans
a. Primary sex ratio (sex ratio at the time of
conception)
b. Secondary sex ratio (sex ratio at the time
of birth) (1.06 in the US)
- Dosage compensation (1 and 2 copies of a gene have the same level of expression)
A. In Homo sapiens and other mammals
1. Barr bodies (sex chromatin bodies) are present in nuclei of XX
females but not in nuclei of XY males
a. Discovered by Barr and Bertram
in 1949 in nerve cells of cats
b. The number of Barr bodies is 1
less than the number of X chromosomes that are present
2. The Lyon hypothesis (Lyon 1961)
a. In females, one X chromosome is
inactive (the one in the form of a Barr body) and the other X chromosome is active
b. In males, the single X chromosome is
active
c. Thus, there is one functional X
chromosome in both sexes
d. The inactivated X in females can be of
maternal or paternal origin in different cells
e. Once an X chromosome is inactivated,
all progeny cells have this same X chromosome inactivated
f. Calico cat
g. Glucose-6-phosphate dehydrogenase
(G-6-PD)
1) Alleles of
this protein are found that migrate fast or slowl through gels during electrophoresis
2) Single skin
cells (fibroblasts) from heterozygous females can be grown into clones in tissue culture
3) Each clone
(that grew from 1 fibroblast cell) displays only the fast or slow allele
4) Thus, one X
chromosome was inactivated, and all cells produced by mitosis from this cell
have the same X chromosome inactivated
3. Evidence that the inactivated X is not completely inactivated
a. XO females and XXY males have abnormal
phenotypes. If the second X chromosome was completely
inactivated in
normal females or XXY males, these would be the same as normal females and males
B. In Drosophila melanogaster
1. In females, both X chromosomes are about half as active as the
single X chromosome in males
2. In this way, X-linked genes in females and males produce the same
amount of gene products
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Updated 10/11/00*