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LECTURE
CONNECTIONS
4 | Sex Determination and Sex-Linked
Characteristics
© 2009 W. H. Freeman and Company
Sex Determination Mechanisms
• Hermaphroditism: both sexes in the same
organism
• Monoecious: both male and female reproductive
structures in the same organism
• Dioecious: either male or female reproductive
structures in one organism
Chromosomal Sex-Determination Systems:
Sex chromosomes and non-sex chromosomes
(autosomes)
• XX-XO system:
• XX – female
• XO – male
Example: grasshoppers
• XX-XY system:
• XX – female
• XY – male
Example: some plants, insects, reptiles, and all mammals
Concept Check 1
How does the heterogametic sex differ from the
homogametic sex?
a. The heterogametic sex is male; the homogametic sex is
female.
b. Gametes of the heterogametic sex have different sex
chromosomes; gametes of homogametic sex have the
same sex chromosome.
c. Gametes of the heterogametic sex all contain a Y
chromosome.
d. Gametes of the homogametic sex all contain an X
chromosome.
Chromosomal Sex-Determination Systems
• ZZ-ZW system:
• ZZ – male
• ZW – female
Which one is the homogametic?
Examples: Birds, snakes, butterflies, some amphibians,
and fishes
• Haplodiploidy system:
• Haploid set – male
• Diploid set – female
Examples: Bees, wasps,
and ants
Genic Sex-Determining System
• In some plants and protozoans, sex is genetically
determined, but there are no obvious differences in the
chromosomes of males and females: there are no sex
chromosomes.
• Genic sex determination: genotypes at one or more loci
determine the sex of an individual plant or protozoan.
• No sex chromosomes, only the sex-determining genes
Environmental Sex Determination
• Environmental factors :
• Limpet’s position in the stack
• Temperature in turtles
Sex Determination in Drosophila melanogaster
•
D. melanogaster has 8 chromosomes:
3 pairs of autosomes + 1 par of sex chromosomes
•
Genic balance system: in D. melanogaster, the dosage of
X chromosomes determines sex.
For example, both XY and XO flies are phenotypically male.
In contrast, both XX and XXY flies are phenotypically female.
•
X : A ratio (X, number of X chromosomes;
A, number of haploid sets of autosomes)
Concept Check 2
What will be the sexual phenotype of a fruit fly with
XXYYY sex chromosomes and two sets of
autosomes?
a.
b.
c.
d.
male
female
Intersex
metamale
Sex Determination in Humans XX-XY
• SRY gene on the Y chromosome determines maleness
• Turner syndrome: XO; 1/3000 female births
• Klinefelter syndrome: XXY, or XXXY, or XXXXY, or
XXYY; 1/1000 male births
• Poly-X females: 1/1000 female births, XXX
The Role of Sex Chromosomes
• The X chromosome contains genetic information
essential for both sexes; at least one copy of an X is
required.
• The male-determining gene is located on the Y
chromosome. A single Y, even in the presence of several
X, still produces a male phenotype.
• The absence of Y results in a female phenotype.
The male-determining gene in humans
• About 6 weeks after fertilization: testosterone and
Mullerian-inhibiting substance are secreted by a gene in
the Y chromosome.
• Sex-determining region Y (SRY) gene
The male-determining gene in humans
• Androgen-insensitivity syndrome
• Caused by the defective androgen receptor
4.2 Sex-Linked Characteristics Are Determined
by Genes on the Sex Chromosomes
• X-Linked White Eyes in Drosophila
• Nondisjunction and the Chromosome Theory of Inheritance
• X-Linked Color Blindness in Humans
• Symbols for X-Linked Genes
• Dosage Compensation
• Z-Linked Characteristics
• Y-Linked Characteristics
X-Linked Characteristics
• X-linked white eyes in Drosophila