Post on 10-Apr-2020
SEX DETERMINATION
• How is the sex of an organism determined?
• Is this process the same for all organisms?
• What is the benefit of sexual reproduction
anyway?
Terms
• Primary vs. secondary sexual differentiation
– Development of gonads vs. physical appearances
• Individuals containing only male or female
reproductive organs
– Unisexual, dioecious, gonochoric
• Individuals containing both male and female
reproductive organs
– Bisexual, monoecious, hermaphroditic
Examples of Sexual
Differentiation
• Chlamydomonas
– Sexual differentiation
induced by environmental
conditions
– + & - strains
– chemically different
Fig. 5-1
Examples of Sexual Differentiation
• Zea mays
– Monoecious
– Many alleles involved in sex determination
Fig. 5-2
Examples of Sexual Differentiation
• Homo- and heterogametic sexes
– Homogametic sex
• Like sex chromosomes (uniform gametes)
• E.g., XX
– Heterogametic sex
• Unlike sex chromosomes (unlike gametes)
• Hemizygous
– Gene present in single dose in otherwise diploid cell
– E.g., XY
Examples of Sexual Differentiation
Homo- & Heterogametic Sexes
• Heterogametic not necessarily the same as
male
– E.g., many insects, most birds, some fish,
reptiles, amphibians
– Birds
• Female (ZW)
• Male (ZZ)
An example of an organism which
has different mating strains induced
environmentally is…
• A) Chlamydymonas
• B) Zea mays
• C) Protenor
• D) Lygaeus
• E) Homo sapiens
In organisms with an X and a Y
chromosome, XY always means the
organism is male.
• A) True
• B) False
Homo- & Heterogametic Sexes
• Humans
– Male = heterogametic sex (XY)
– Female = homogametic sex (XX)
Human Karyotype
Fig. 5-5
Sex-Determination in Humans
• What actually determines “maleness” or “femaleness” in
humans?
– Male = presence of Y or lack of 2nd X?
– Female = lack of Y or presence of 2nd X?
• Examined cases of aneuploidy
– Aneuploidy = presence or absence of individual chromosomes
(2n +/- 1)
– Results from nondisjunction
Klinefelter & Turner Syndrome
• Klinefelter = 47, XXY
– Typically male with testicular atrophy & some feminine development
• Turner = 45, X
– Female with diminished ovaries
• Suggests Y determines “maleness”
– So what gene/genes are responsible?
The Y Chromosome
• MSY region
– Male specific region
• PAR region
– Involved in synapsis &
crossover with X
• SRY region
– Sex-determining region
– Encodes gene product that triggers embryonic
gonadal tissue to form testes
• Testis-determining factor (TDF)
Fig. 5-7
Human Embryo and Sex Organ
Development
• XY Embryo
– 7 Weeks old
– Wolffian vs Müllerian
ducts
– TDF on Y
chromosome governs
development of testes
The Y Chromosome
• Further evidence from cases of SRY
translocation
– XX but male phenotype
• SRY region attached to one X chromosome
– XY but female phenotype
• SRY region missing from Y chromosome
In humans, the “default” sex is…
• A) male
• B) female
In humans, the key locus on the Y
chromosome involved in
determining “maleness” is the...
• A) MSY
• B) PAR
• C) SRY
The X Chromosome
• If females have 2 X chromosomes,
shouldn’t they produce twice the genetic
products of X-linked genes?
Dosage Compensation
• One X chromosome is “inactivated”
– Barr body (sex chromatin body)
Fig. 5-9
Dosage Compensation
• If “extra” X chromosomes are inactivated,
why isn’t a 45, X (Turner) a 47, XXY
(Klinefelter) or any other poly-X individual
normal?
– Gene products may be important prior to
inactivation
– Inactivation may not be complete
Dosage Compensation
• Which X chromosome is “inactivated” (paternal or maternal)?
• Lyon Hypothesis
– X inactivation is random in somatic stem cells
– Occurs early in embryonic development
– Once inactivated, all progeny cells have the same X chromosome inactivated
• E.g., calico & tortoiseshell cats
– All female
– Alleles for black or orange coat color on X chromosome
– Patches of skin/fur develop from a single progenitor; each patch represents a different progenitor with a different inactivated X
Dosage Compensation
• How is the X chromosome “inactivated”?
– Region of X chromosome (X-inactivation
center; Xic)
• X-inactive specific transcript (XIST)
• RNA produced, but not translated
• May form molecular “cage” around chromosome
In humans, an individual whose
chromosome complement was
48, XXXY would have how many
Barr bodies?
• Just type the number you want to submit!
Sex-Determination in Drosophila
• Males are XY Females are XX
• Same as humans?
• XXY are normal females
• XO are sterile males
– Presence of Y does not cause maleness
– Absence of Y does not cause femaleness
– Genes on autosomes and X chromosome
contribute to sex determination
Sex-Determination in Drosophila
• Sex is determined by a ratio
– # of X chromosomes : # of haploid sets of
chromosomes (n)
• n = 4 (3 autosomes, 1 sex chromosome)
– Normal female is 2X:2A (1:1, or 1.0)
– Normal male is XY:2A (1:2, or 0.5)
Sex-Determination in Drosophila
• Triploid females
– 3X:3A (1.0) – normal female
• Metafemales
– 3X:2A (1.5) – fertile female
• Males w/o Y
– 1X:2A (0.5) – fertile male
• Metamales
– XY:3A (0.33) – infertile male
• Genic balance theory
Sex Determination & Temperature
Variation
• Observed in certain reptiles
• Temperature may influence synthesis of
steroids involved in differentiation of
ovaries & testes
Fig. 5-12
In Drosophila, a triploid (3n) mutant
with 12 autosomes, 2 X and 1 Y
chromosomes would be most similar
to a …
• A) Male
• B) Female
• C) Metafemale
• D) Metamale