1. Sex Chromosome Abnormalities
Occur during non-disjunction of X or Y chromosomes at meiosis I or meiosis II
Normally in meiosis to produce eggs, the two X’s pair at meiosis I
Egg X
1st polar body X
2nd polar body X X
1st division nd division

2. Sex Chromosome Abnormalities
non-disjunction of X chromosomes at meiosis I in female
Egg XX
polar body XX X
There are cases of gametes with more than 2 X’s As the number gets over 3, there are significant costs in IQ.
1st division nd division
XX eggs fertilized by X sperm give Triple X female;
XX eggs fertilized by Y sperm give XXY male

3. Sex Chromosome Abnormalities
Occur during non-disjunction of X or Y chromosomes at meiosis I or meiosis II
Normally in meiosis to produce sperm, a small part of the X and Y pair X Y X Y
1st division nd division

4. Sex Chromosome Abnormalities
Occur during non-disjunction of X or Y chromosomes at meiosis I or meiosis II
Non disjunction in meiosis to produce sperm, can cause: XX, XY, YY or O sperm XY X Y
YY sperm only arise with nondisjunction in Meiosis II. (Can get XX sperm too)
XY sperm meets X egg to produce XXY male

5. Triple X females- (47 XXX)
Fertile; tend to have XX or XY offspring
Normal IQ range
75% slow in learning reading, math
tendency for anxiety
• frequency in population is about 1/ 4,000 live births
also called triplo X

6. Turners syndrome females (45 XO)
sterile
Turners syndrome females are under 5’ and usually normal IQ
difficulty in 3D rotation tests
heart and kidney problems; frequency 1/2,000 live births
Estrogen helps for secondary sex character development
HGH allows increase in stature
about 1 in 2,500 female births

7. Klinefelter Males (47 XXY)
Taller than average; infertile; small testes
At puberty some breast development is easily treated with testosterone but does not restore fertility
Occurs 1 in 500 to 1000 male births

8. XYY males Taller than average; acne worse than average; normal IQ
XYY males are fertile; have XX or XY children
Frequency 1 in 300 to 1 in 1000 births
Slight increase of XYY individuals in prison population per capita

9. Sex Determination in humans
Presence or absence of Y at fertilization determines sex of fetus; Y chromosome will produce male; no Y means female phenotype will result
same special mass of cells in very early embryo develops into ovary (XX) or testes (XY)
By 7 weeks embryo is ready to develop into a male or a female individual

10. Y chromosome genes
The Hy gene on the Y chromosome codes for the Hy antigen protruding from the surface of all male cells.
After 7 weeks Tdf (Sry) gene on the Y chromosome is expressed. The product of this gene directs some of the ovotestes cells to develop into the testes and to start making testosterone

11. Mullerian and Wolffian ducts
Ovaries in XX embryos begin to develop in first few weeks;
In XY embryos, the starting at 7 weeks, ovotestes cells will develop into Wolffian ducts, a system of ducts connecting the testes and urinary system in males
Ovotestes cells form Mullerian ducts in XX embryos; ducts develop around 11 weeks, eventually forming oviducts and uterus, connecting the ovaries to the vagina

12. Primordial gametes form outside embryo about 14 d.
The future sperm and egg cells are never a part of the embryonic tissue.
These primordial cells arise separately and do not undergo any differentiation as the other embryonic cells do.
Primordial gametes migrate into developing ovaries and testes.

13. At puberty secondary sex changes occur
Hormones prepare body for reproduction.
Gonadotropic hormones, FSH and LH start menstrual cycle each month, causing maturation of egg
FSH and LH present in trace amounts in males cause beard to form, Adam’s apple to enlarge, facial bones to grow, etc.
Adrenal glands secrete

14. Guevedoces syndrome in XY
XY infants lack enyme 5H reductase
Fail to develop external male genetalia
Phenotype is female at birth
About age 12, the enzyme appears and the penis and testes develop and by age 20 these individuals are fully developed males

15. Many hormones produced by biosynthetic pathways
Testoterone and estrogen are formed from the cholesterol biosynthetic pathway
Many enzymes (proteins) are required to catalyze the reactions in pathway; genes for these enzymes on autosomes not on X or Y
Many mutations affect sexual development by disrupting hormone formation

16. Tfm, an X-linked gene
Tfm gene codes for protein that sticks out of cells and binds testosterone
In XtfmY males normal development of sex characteristics is not possible because cells are not affected by testosterone
Phenotype of Tfm XY is sterile female; no penis; testes form but remain up in the body; may be surgically removed to lower risk of cancer
Also called Complete Androgen Insensitivity

17. Brain-sex Perception of gender
Tfm individuals perceive themselves as female from earliest memories; Guevedoces individuals perceive themselves as male from earliest memories.
Tfm individuals are treated with estrogen to stimulate female characteristics

18. Sex-related inheritance
Barr bodies are condensed (inactive) X chromosomes found in interphase nuclei
In nuclei with two X chromosomes, one X becomes inactivated and appears as a blob in the nucleus when stained

19. Lyon Hypothesis: the condensed X is inactive.
X-inactivation occurs early in development
Inactivation of one of the X’s is random
The same X remains inactive in all daughter cells throughout cell divisions
On 50th anniversary, called “Lyons Law”

20. Lyon hypothesis explains:
Dosage compensation; XCfXCf and XCfY genotypes will show same amount of clotting factor protein in blood as males.
Why females heterozygous for X-linked trait can vary greatly in gene expression
e.g. tortoise shell cats and calico cats are always XBXb females; orange patches are where Xb is active, black patches where XB is active

21. Tortoise shell cat

22. Calico cat
Calico cats also express dominant gene for white spotting.

23. Genomic Imprinting
Small regions of AUTOSOMES are inactivacted during formation of egg and sperm (gametogenesis)
Inactivated regions not expressed in fetus but gene donated from other parent is normally active
If other parent donates a defective gene, aberrant phenotype results

24. Prader-Willi and Angelman
Both syndromes are related to failure of gene expression on chromo. 15
Prader-Willi: mental retardation, hunger uncontrolled causes obesity
Angelman syndrome: severe speech impairment, mental retardation, happy demeanor, sleep disorders

25. Sex-limited traits
Genes are in both sexes but only expressed in one of the sexes
Genes usually not on X or Y chromosomes; usually on autosomes
Examples: lactation in female mammals

26. Sex-influenced traits
Dominant in one sex, recessive in the other
Examples: horns in male sheep, pattern baldness in human males
Must use chart with genotype and sex to determine outcome or phenotype

27. Horns in sheep Genotype male female H’H’ Horns Horns
H’H heterozygote gives different phenotype depending on sex; sex hormones involved
Genotype male female
H’H’ Horns Horns
H’H Horns No horns
HH No horns No horns