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SEX Determination Maleness or femaleness is determined at conception

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Presentation on theme: "SEX Determination Maleness or femaleness is determined at conception"— Presentation transcript:


2 SEX Determination Maleness or femaleness is determined at conception
Humans are monoecious: meaning that each person can make one type of gamete either eggs or sperm Dioecious organisms can make both types of gametes in the same body

3 Sex Chromosomes Determine Gender
Human males are the heterogametic sex with different sex chromosomes, (XY) Human females are the homogametic sex (XX) In other species sex can be determined in many ways - For example, in birds and snakes, males are homogametic (ZZ), while females are heterogametic (ZW) 3

4 In grasshoppers, the female contains 24 chromosomes and is XX while the male contains 23 chromosomes and is XO

5 X and Y Chromosomes X chromosome - Contains > 1,500 genes
- Larger than the Y chromosome - Acts as a homolog to Y in males Y chromosome - Contains 231 genes - Many DNA segments are palindromes and may destabilize DNA Figure 6.2 5

6 Anatomy of the Y Chromosome
Pseudoautosomal regions (PAR1 and PAR2) - 5% of the chromosome - Contains genes shared with X chromosome Male specific region (MSY) - 95% of the chromosome - Contains majority of genes including SRY and AZF (needed for sperm production) Figure 6.3 6

7 SRY Gene Encodes a transcription factor protein
Controls the expression of other genes Stimulates male development Developing testes secrete anti-Mullerian hormone and destroy female structures Testosterone and dihydrotesterone (DHT) are secreted and stimulate male structures 7

8 Abnormalities in Sexual Development
Pseudohermaphroditism = Presence of male and female structures but at different stages of life - Androgen insensitivity syndrome = Lack of androgen receptors - 5-alpha reductase deficiency = Absence of DHT - Congenital adrenal hyperplasia = High levels of androgens 8

9 Abnormalities in Sexual Development
The degree to which pseudohermaphroditism disturbs the individual depends as much on society as it does on genetics. In the Dominican Republic in the 1970s, 22 young girls reached the age of puberty and began to transform into boys.

10 Abnormalities in Sexual Development
They had a form of 5-alpha reductase deficiency that was fairly common due to consanguinity ( relatives having children with relatives). The parents were happy that they had had sons after all, and so these special adolescents were given their own gender nave at age 12.

11 Figure 6.4 Figure 6.4

12 Table 6.1

13 Sex Ratios The proportion of males to females in a human population
Calculated by # of males / # of females multiplied by 1,000 Primary sex ratio – At conception Secondary sex ratio – At birth Tertiary sex ratio – At maturity Sex ratios can change markedly with age 13

14 Sex Ratios Sex ratios can be altered intentionally by a society
- Example: China’s one-child policy has led to a scarcity of females Figure 6.5 14

15 In China by the year 2020, 20 million men will find themselves without female partners as a result of the nations “ one-child” policy The average number of births per woman fell from 5.4 in 1971 to 1.8 in 2001. There is about 117 boys born for every 100 girls. The Chinese government is now promoting a “Care for Girls” program which awards housing subsidies and scholarships to families with children

16 At the other end of the human life cycle, sex ratio favors females in most populations.
For people over the age of 65 in the United States there is a ratio of 75 men for every 100 women

17 Sex Determination in Humans
Figure 6.6 Figure 6.6

18 Inheritance related to sex
Genes located exclusively on the x chromosome are called sex-linked Genes that occur only on the Y chromosome can produce their effects only in males and are called Holandric genes The mechanisms where a given trait is limited to one sex is called sex-limited and fi dominance of a given allele depends on the sex of the bearer is called sex-influenced.

19 Y-linked Traits Genes on the Y chromosome are said to be Y-linked
Y-linked traits are very rare Transmitted from male to male No affected females Currently, identified Y-linked traits involve infertility and are not transmitted 19

20 X-linked Traits ( we will do some examples on the board
Possible genotypes X+X+  Homozyogus wild-type female X+Xm  Heterozygous female carrier XmXm  Homozygous mutant female X+Y  Hemizygous wild-type male XmY Hemizygous mutant male 20

21 X-linked Recessive Inheritance

22 X-linked Recessive Traits
Examples: - Ichthyosis = Deficiency of an enzyme that removes cholesterol from skin - Color-blindness = Inability to see red and green colors ( only about 0.4 % are girls) - Hemophilia = Disorder of blood-clotting 22

23 Figure 6.7 Figure 6.7

24 Figure 6.8 Figure 6.8

25 X-linked Dominant Inheritance

26 X-linked Dominant Traits
Incontinentia pigmenti A newborn girl with IP has yellow, pus filled vesicles on her limbs, the lesions become warty and eventually give way to brown splotches that remain for life Figure 6.9 26

27 X-linked Dominant Traits
Congenital generalized hypertrichosis Produces many extra hair follicles, and more denser more abundant upper body hair. An affected man would pass the trait to all his daughter, but non of his sons. Why? Figure 6.10 27

28 Solving Genetic Problems
Steps to follow: 1) Look at the inheritance pattern 2) Draw a pedigree 3) List genotypes and phenotypes and their probabilities 4) Assign genotypes and phenotypes 5) Determine how alleles separate into gametes 6) Use Punnett square to determine ratios 7) Repeat for next generation 28

29 Sex-Limited Traits Traits that affect a structure or function occurring only in one sex The gene may be autosomal or X-linked Examples: - Beard growth - Milk production - Preeclampsia in pregnancy 29

30 Sex-limited characteristics do some examples on board
Genotype male chickens female HH Hen feathered Hen feathered Hh Hen feathered Hen feathered hh cock-feathered Hen feathered

31 Sex-Influenced Traits
Traits in which the phenotype expressed by a heterozygote is influenced by sex Allele is dominant in one sex but recessive in the other Example: - Pattern baldness in humans - A heterozygous male is bald, but a heterozygous female is not 31

32 Sex influenced traits Baldness: do some problems on the board
Genotype Men women BB bald Bald Bb bald not bb not not

33 X Inactivation Females have two alleles for X chromosome genes but males have only one In mammals, X inactivation balances this inequality and one X chromosome is randomly inactivated in each cell The inactivated X chromosome is called a Barr body 33

34 X Inactivation X inactivation occurs early in prenatal development
It is an example of an epigenetic change - An inherited change that does not alter the DNA base sequence The XIST gene encodes an RNA that binds to and inactivates the X chromosome 34

35 Barr Bodies Were discovered in 1940s in female cats but not male
Barr body is the inactive X chromosome in a female somatic cell,


37 Figure 6.11 Figure 6.12

38 X Inactivation Animation
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39 X Inactivation A female that expresses the phenotype corresponding to an X-linked gene is a manifesting heterozygote X inactivation is obvious in calico cats LOOK AT KEY ON HOW TO SOLVE THESE PROBLEMS Figure 6.12 39

40 X inactivation Theoretically, X inactivation evens out the sexes for expression of X-linked genes. In actuality, a female may not be equivalent, in gene expression, to a male because she has two cell populations, where the male has only one

41 X inactivation Heterozygous females have much more sever phenotypes with very abnormal faces resulting from abnormal fusing of skull bones A situation in which being heterozygote for an X-linked gene is harmful is craniofrontonasal syndrome. Males and homzygous females have asymmetrical facial features

42 Rett Syndrome: Curious inheritance pattern
Nearly always affects females One symptom is that the girls do strange things with their hands__ uncontrollably taping objects, clapping , repeatedly put hands to mouth and wring their hands This is dominant and in 99% of cases is not passed from parent to child but arises anew

43 Genomic Imprinting The phenotype of an individual differs depending on the gene’s parental origin Genes are imprinted by an epigenetic event: DNA methylation - Methyl (CH3) groups bind to DNA and suppress gene expression in a pattern determined by the individual’s sex 43

44 Imprints are erased during meiosis
Figure 6.13 Imprints are erased during meiosis - Then reinstituted according to the sex of the individual 44

45 Importance of Genomic Imprinting
Function of imprinting isn’t well understood, but it may play a role in development Research suggests that it takes two opposite sex parents to produce a healthy embryo - Male genome controls placenta development - Female genome controls embryo development Genomic imprinting may also explain incomplete penetrance 45

46 Imprinting and Human Disease
Two distinct syndromes result from a small deletion in chromosome 15 - Prader-Willi syndrome - Deletion inherited from father - Angelman syndrome - Deletion inherited from mother The two syndromes may also result from uniparental disomy 46

47 Imprinting and Human Disease
Deletion on chromosome 15 reveals imprinting Figure 6.16

48 Human Sex Anomalies

49 Turner Syndrome Called the XO syndrome 1 in 2,500 female births
99% of affected fetuses die in utero Features include short stature, webbing at back of neck, incomplete sexual development (infertile), impaired hearing Individuals who are mosaics may have children No bar bodies in cells 49

50 XYY Syndrome Also known as Jacobs syndrome 1 in 1,000 male births
96% are phenotypically normal Modest phenotypes may include great height, acne, speech and reading disabilities Studies suggesting increase in aggressive behaviors are not supported 50

51 Triplo-X Called the XXX syndrome 1 in 1,000 female births
Few modest effects on phenotype include tallness, menstrual irregularities, and slight impact on intelligence X-inactivation of two X chromosomes occurs and cells have two Barr bodies May compensate for presence of extra X 51

52 Klinefelter Syndrome Called the XXY syndrome 1 in 500 male births
Phenotypes include: - Incomplete sexual development - Rudimentary testes and prostate - Long limbs, large hands and feet - Some breast tissue development Most common cause of male infertility 52

53 XXYY Syndrome Likely arises due to unusual oocyte and sperm
Associated with more severe behavioral problems than Klinefelter syndrome - AAD, obsessive compulsive disorder, learning disabilities Individuals are infertile Treated with testosterone 53

54 XYY Syndrome Also known as Jacobs syndrome 1 in 1,000 male births
96% are phenotypically normal Modest phenotypes may include great height, acne, speech and reading disabilities Studies suggesting increase in aggressive behaviors are not supported 54

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