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