Chapter 7 - Sex Determination & Sex Chromosomes
I. Introduction
A. Sexual reproduction requires some sort of differentiation.
Found at all evolutionary levels - not always phenotypic dimorphism.
B. Some terminology
1. Sex chromosomes- heteromorphic chromosomes that differentiate one sex from another.
Some cases - sex determining genes may be located on autosomes.
2. Multicellular organisms
Primary sexual differentiation - differentiation b/w sexes in the gonads and gametes produced.
Secondary sexual differentiation - aspects of physical appearance that differ between the sexes.
3. Unisexual, dioecious, gonochoric/gonochoristic - separate sexes.
Bisexual, monoecious, hermaphroditic - individual with male and female reproductive organs.
Intersex - intermediate sexual differentiation (usually sterile).
II. Sexual differentiation
A. Chlamydomonas - green alga (Figure 7.1)
1. Mostly haploid, reproduces asexually by mitosis.
2. Environmental stress - haploid cells fuse to form a diploid zygote (a resting stage). Later it then
undergoes meiosis (4 new haploid cells).
3. Don't look different, but are (isogamous). Two mating types (+ & -), mates with opposite.
B. Corn - Zea mays
1. Monoecious/bisexual - with both male and female structures
2. Sex determination is complex. Several autosomal genes play a role in sex determination.
C. X and Y chromosomes - sex chromosomes - early studies of insects.
Two modes of sex determination: XX/XO or Protenor; XX/XY or Lygaeus
1. XX/XO – presence/absence of 2nd X chromosome (XX = female, XO = male).
2. XX/XY - female is XX; male determined additional sex chromosome, XY.
3. Males have dissimilar sex chromosomes - produce dissimilar gametes. They determine the sex of
offspring. Male called the heterogametic sex.
Females - similar sex chromosomes, uniform gametes = homogametic sex.
4. Male is not always the heterogametic sex.
Lepidopterans, birds, some fish/herps, the female is heterogametic. ZZ = male & ZW = female
III. The Y chromosome and sex determination
A. New staining techniques - made possible accurate chromosome counts
1. Humans - 23 pairs, but in males a difference between one pair (the X and Y).
2. Not necessarily proof that the Y chromosome causes maleness.
B. Early proof Y chromosome has a role in sex determination:
Studies of aberrant sexual development
1. Klinefelter syndrome - karyotype = XXY or 47, XXY
a. Phenotypically male. Underdeveloped testes, some slight female body characteristics.
b. More severe symptoms: 48, XXXY; 48 XXYY; 49, XXXXY; 49, XXXYY
2. Turner syndrome - karyotype = 45, X
a. Phenotypically female, with external genitalia; but ovaries are rudimentary.
b. Lower frequency compared to Klinefelter
3. 47, XXX
a. Phenotypically female but with variable expression of effects.
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Chapter 7 - Sex Determination & Sex Chromosomes
b. Some are perfectly normal; others sterile and mentally retarded.
c. Extra X chromosome - upsets balance of gene expression in the X?
4. 47, XYY
a. Male but may deviate from norm - over 6' tall.
b. A correlation with criminality? Later studies failed to support this connection.
C. Y chromosome & male development (more than empty DNA) Figure 7.9 (7.8 – 8th)
1. PAR - pseudoautosomal region - shares homology with X - allows for synapsis.
2. NRY - nonrecombining region of the Y (Or Male specific region of the Y – MSY)
a. genetically blank (heterochromatin) and important parts (euchromatin)
b. some genes are homologs of X chromosome genes
c. SRY (sex determining region Y) - produces testis determining factor (TDF) - triggers development of
male gonadal tissue in the embryo.
3. evidence for role of SRY
a. rare cases where sex chromosomes don't equal gender expressed
b. male XX = on one X is attached portion of Y chromosome with SRY
c. transgenic mice - inject DNA from sry gene into XX egg, most will develop into males.
4. Events of sex determination not clear, but SRY appears to be key
D. Sex ratios in humans
1. We've been claiming it's simply 1:1 - but that is not true.
2. Census data - slightly more male than female births.
3. The secondary sex ratio - who is born; sex ratio at conception = the primary sex ratio.
4. Fewer female fetuses go to term, explaining the skew? Actually, more males fail to be born.
5. Why is there this skew from 1:1?
a. Do males produce gametes with X and Y in equal frequencies? Interesting question.
b. Are X and Y bearing sperm equally motile? Is Y sperm quicker?
c. Are X and Y bearing sperm equally good at fertilization? Are Y sperm preferentially used to fertilize
egg or more viable in female's reproductive system?
IV. Dosage compensation and X-linked genes.
A. Female with 2 X and male with 1 X - expect females to produce 2x the amount of X-linked gene
products. Could cause a problem. Some sort of mechanism of dosage compensation.
B. Barr Bodies
1. Darkly staining structure in female cells, but not males = sex chromatin bodies or Barr body.
2. Barr body thought to be an inactivated X chromosome (means of dosage compensation)
3. Indirect evidence - individuals with abnormal karyotypes; # Barr bodies - Figure 7.11 (7.10 - 8th)
Turner (45, X) = 0; Klinefelter (47, XXY) = 1; 47, XXX female = 2
C. Which X chromosome becomes inactivated?
Lyon hypothesis - random inactivation of X in somatic cells early in embryonic development. Once
inactivated, condition is passed on to all future daughter cells.
D. Mechanism of inactivation - how is the X chromosome rendered inactive?
1. Region of p arm of X seems to be responsible - X-inactivation center (XIC)
Contains the gene - XIST, X-inactive specific transcript. The RNA transcribed from this gene is large and
does not seem to code for any particular protein. The RNA spreads over the X chromosome forming a
sort of coating.
2. Still there are important unanswered questions.
- What is the mechanism that "chooses" the X chromosome to be inactivated?
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Chapter 7 - Sex Determination & Sex Chromosomes
- How is this inactivation maintained through mitosis?
V. Temperature and Sex Determination - Figure 7.18 (7.15 – 8th)
- Temperature-Dependent Sex Determination (TSD) in many reptiles - turtles, crocodiles, lizards
- Type III pattern is most common - pivotal temperature sometimes a narrow range (1-5 °C).
Mechanism - probably related to temperature sensitive transcription of enzymes involved in synthesis of
various sex hormones and hence the development of gonads.
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