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Human Genetics Concepts and Applications Ninth Edition RICKI LEWIS Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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Presentation on theme: "Human Genetics Concepts and Applications Ninth Edition RICKI LEWIS Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display."— Presentation transcript:

1 Human Genetics Concepts and Applications Ninth Edition RICKI LEWIS Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display PowerPoint ® Lecture Outlines Prepared by Johnny El-Rady, University of South Florida 6 Matters of Sex

2 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 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)

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

5 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

6 6 Anatomy of the Y Chromosome Figure 6.3 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)

7 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

8 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

9 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 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 11 Figure 6.4

12 12 Table 6.1

13 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

14 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

15 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 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 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 17 Figure 6.6 Sex Determination in Humans Figure 6.6

18 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 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

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

21 21 X-linked Recessive Inheritance

22 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

23 23 Figure 6.7

24 24 Figure 6.8

25 25 X-linked Dominant Inheritance

26 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

27 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

28 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

29 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

30 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 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

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

33 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

34 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

35 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,

36 36

37 37 Figure 6.12 Figure 6.11

38 38 Figure 2.3 X Inactivation Animation Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at

39 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

40 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 41 X inactivation A situation in which being heterozygote for an X-linked gene is harmful is craniofrontonasal syndrome. Males and homzygous females have asymmetrical facial features Heterozygous females have much more sever phenotypes with very abnormal faces resulting from abnormal fusing of skull bones

42 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 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 (CH 3 ) groups bind to DNA and suppress gene expression in a pattern determined by the individual’s sex

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

45 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

46 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

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

48 48

49 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

50 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

51 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

52 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

53 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

54 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

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