1. Sex Determination and Sex Chromosomes
Wheeler High School The Center for Advanced Studies in Science, Math & Technology
Sex Determination and Sex Chromosomes
Post-AP DNA/Genetics – Ms. Hager

2. Sexual Differentiation and Life Cycles
Chlamydomonas haploid gametes are of two mating types, mt– and mt+. mt– cells can mate only with mt+ cells, and vice versa. There are chemical differences between these mating types.
Isogametes
Post-AP DNA/Genetics – Ms. Hager

3. Sexual Differentiation and Life Cycles
In maize (Zea mays), the diploid sporophyte stage predominates and both male and female structures are present on the adult plant, indicating that sex determination must occur differently in different tissues of the same plant.
Post-AP DNA/Genetics – Ms. Hager

4. Sexual Differentiation and Life Cycles
The nematode worm Caenorhabditis elegans has two sexual phenotypes: males, which have only testes, and hermaphrodites, which have both testes and ovaries. Self-fertilization occurs in the hermaphrodites and produces primarily hermaphrodite offspring, with less than 1% male offspring.

5. Male or Hermaphrodite? How can you tell?
Figure 7-4a (a) Photomicrograph of a hermaphroditic nematode, C. elegans; (b) The outcomes of self-fertilization in a hermaphrodite and a mating of a hermaphrodite and a male worm.
Sex determination in C. elegans results from the presence of only one X chromosome in the males and two in the hermaphrodites.
Figure 7- 4a Copyright © 2006 Pearson Prentice Hall, Inc.

6. The Y Chromosome Determines Maleness in Humans
The human karyotype shows that one pair of chromosomes differs in males and females: females have two X chromosomes; males have one X and one Y chromosome.
Post-AP DNA/Genetics – Ms. Hager

7. Figure 7-6 Copyright © 2006 Pearson Prentice Hall, Inc.
The traditional human karyotypes derived from a normal female and a normal male. Each contains 22 pairs of autosomes and two sex chromosomes. The female (a) contains two X chromosomes, while the male (b) contains one X and one Y chromosome (see arrows).
Figure 7-6 The traditional human karyotypes derived from a normal female and a normal male. Each contains 22 pairs of autosomes and two sex chromosomes. The female (a) contains two X chromosomes, while the male (b) contains one X and one Y chromosome (see arrows).
Figure Copyright © 2006 Pearson Prentice Hall, Inc.

8. Figure 7-6a Copyright © 2006 Pearson Prentice Hall, Inc.
Figure 7-6a The traditional human karyotypes derived from a normal female and a normal male. Each contains 22 pairs of autosomes and two sex chromosomes. The female (a) contains two X chromosomes, while the male (b) contains one X and one Y chromosome (see arrows).
Figure 7-6a Copyright © 2006 Pearson Prentice Hall, Inc.

9. Figure 7-6b Copyright © 2006 Pearson Prentice Hall, Inc.
Figure 7-6b The traditional human karyotypes derived from a normal female and a normal male. Each contains 22 pairs of autosomes and two sex chromosomes. The female (a) contains two X chromosomes, while the male (b) contains one X and one Y chromosome (see arrows).
Figure 7-6b Copyright © 2006 Pearson Prentice Hall, Inc.

10. Klinefelter and Turner Syndrome
Persons with Klinefelter syndrome have male genitalia but have more than one X chromosome (usually XXY, or a 47,XXY karyotype).
Persons with Turner syndrome usually have a single X chromosome and no Y chromosome (45,X karyotype) and have female genitalia. Such syndromes provide evidence that the Y chromosome determines maleness.
Post-AP DNA/Genetics – Ms. Hager

11. Klinefelter and Turner Syndrome
Figure 7-7 The karyotypes and phenotypic depictions of individuals with (a) Klinefelter syndrome (47,XXY) and (b) Turner syndrome (45,X).
The karyotypes and phenotypic depictions of individuals with (a) Klinefelter syndrome (47,XXY) and (b) Turner syndrome (45,X).
Figure Copyright © 2006 Pearson Prentice Hall, Inc.

12. Figure 7-7a Copyright © 2006 Pearson Prentice Hall, Inc.
Figure 7-7a The karyotypes and phenotypic depictions of individuals with (a) Klinefelter syndrome (47,XXY) and (b) Turner syndrome (45,X).
Figure 7-7a Copyright © 2006 Pearson Prentice Hall, Inc.

13. Klinefelter Syndrome

14. Figure 7-7b Copyright © 2006 Pearson Prentice Hall, Inc.
Figure 7-7b The karyotypes and phenotypic depictions of individuals with (a) Klinefelter syndrome (47,XXY) and (b) Turner syndrome (45,X).
Figure 7-7b Copyright © 2006 Pearson Prentice Hall, Inc.

15. Turner syndrome

16. Frequency of XXY Individuals
Table 7.1 Frequency of XYY Individuals in Various Settings
Table Copyright © 2006 Pearson Prentice Hall, Inc.

17. Why does ‘Y’ make the Guy? Let’s ask SRY……………..
Sex-determining region (SRY) codes for testis determining factor (TDF).
Around 6-8 weeks TDF becomes present which causes the undifferentiated gonadal tissue of the embryo to form testes.
Y chromosome contains the male-specific region of the Y (MSY) and a sex-determining region of the Y (SRY)
Post-AP DNA/Genetics – Ms. Hager

18. Any Questions?

19. X Inactivation and Dosage Compensation
Wheeler High School The Center for Advanced Studies in Science, Math & Technology
X Inactivation and Dosage Compensation
Post-AP DNA/Genetics – Ms. Hager

20. Dosage Compensation
The term ‘dosage compensation’ describes the genetic mechanisms that balance the dose of X chromosome gene expression in females and males.
Post-AP DNA/Genetics – Ms. Hager

21. Figure 7-9 Copyright © 2006 Pearson Prentice Hall, Inc.
Barr Bodies
The Barr body is an inactive X chromosome
No Barr bodies in males
If one of the 2 X chromosomes is inactive in the female, then the dosage of genetic information expressed in males is the same as females.
Figure 7-9 Photomicrographs comparing cheek epithelial cell nuclei from a male that fails to reveal Barr bodies (bottom) with a female that demonstrates Barr bodies (indicated by an arrow in the top image). This structure, also called a sex chromatin body, represents an inactivated X chromosome.
Figure Copyright © 2006 Pearson Prentice Hall, Inc.

22. Dosage Compensation Prevents Excessive Expression of X-Linked Genes in Humans and Other Mammals
Occurrence of Barr bodies in various human karyotpyes where all X chromosomes except 1 are inactivated.
Post-AP DNA/Genetics – Ms. Hager

23. The Lyon Hypothesis
Mary Lyon hypothesized that X chromosome inactivation occurs randomly in somatic cells at some point during early embryonic development.
Possibly during the blastocyst stage
Once inactivation occurs, all decedent cells have the same X inactivated as the parent cell

24. Figure 7-11 Copyright © 2006 Pearson Prentice Hall, Inc.
Figure 7-11 (a) A calico cat, where the random distribution of orange and black patches illustrates the Lyon hypothesis. The white patches are due to another gene; (b) A tortoiseshell cat, which lacks the white patches characterizing calicos.
(a) A calico cat, where the random distribution of orange and black patches illustrates the Lyon hypothesis. The white patches are due to another gene; (b) A tortoiseshell cat, which lacks the white patches characterizing calicos.
Figure Copyright © 2006 Pearson Prentice Hall, Inc.

25. Figure 7-12 Copyright © 2006 Pearson Prentice Hall, Inc.
Depiction of the absence of sweat glands (shaded regions) in a female heterozygous for the X-linked condition anhidrotic ectodermal dysplasia. The locations vary from female to female, based on the random pattern of X chromosome inactivation during early development, resulting in unique mosaic distributions of sweat glands in heterozygotes.
Figure 7-12 Depiction of the absence of sweat glands (shaded regions) in a female heterozygous for the X-linked condition anhidrotic ectodermal dysplasia. The locations vary from female to female, based on the random pattern of X chromosome inactivation during early development, resulting in unique mosaic distributions of sweat glands in heterozygotes.
Figure Copyright © 2006 Pearson Prentice Hall, Inc.

26. Mechanism of X Inactivation
Critical gene for X-inactivation
Xist lacks open reading frames (ORF) which are necessary for translation. Since there are no ORF’s in this region you only have transcription and NOT translation.
Scientists speculate that RNA products of Xist produce a ‘cage’ that entraps and inactivates the chromosome

27. Sex Determination in Drosophila
The Ratio of X Chromosomes to Sets of Autosomes Determines Sex in Drosophila
The Y chromosome is not involved in D. melanogaster sex determination
Post-AP DNA/Genetics – Ms. Hager

28. X Chromosomes and Autosomes
In 1916, Calvin Bridges realized that the critical factor in determining the sex is the ratio of X chromosomes to the number of haploid sets of autosomes.
In this case it is not the ‘Y that makes the guy’…in fact, it’s the expression of genes on the X chromosome that cause a fly to be male
Females = XX
Males = X (may have the Y but don’t need to)
Post-AP DNA/Genetics – Ms. Hager

29. Figure 7-13 Copyright © 2006 Pearson Prentice Hall, Inc.
Figure 7-13 Chromosome compositions, the ratios of X chromosomes to sets of autosomes, and the resultant sexual morphology in Drosophila melanogaster. The normal diploid male chromosome composition is shown as a reference on the left (XY2A).
Figure Copyright © 2006 Pearson Prentice Hall, Inc.

30. Figure 7-14 Copyright © 2006 Pearson Prentice Hall, Inc.
Bilateral gynandromorph formed following the loss of one X chromosome in one of the 2 cells during the 1st mitotic division. The left side of the fly is male and the right is female.
Figure 7-14 A bilateral gynandromorph of Drosophila melanogaster formed following the loss of one X chromosome in one of the two cells during the first mitotic division. The left side of the fly, composed of male cells containing a single X, expresses the mutant white-eye and miniature-wing alleles. The right side is composed of female cells containing two X chromosomes heterozygous for the two recessive alleles.
Figure Copyright © 2006 Pearson Prentice Hall, Inc.

31. Temperature Variation Determines the Sex of Reptiles
Three different patterns of temperature-dependent sex determination (TSD) in reptiles. The relative pivotal temperature is crucial to sex determination during a critical point during embryonic development (FTfemale-determining temperature; MTmale-determining temperature).

32. Any Questions?

33. Figure 7-11a Copyright © 2006 Pearson Prentice Hall, Inc.
Figure 7-11a (a) A calico cat, where the random distribution of orange and black patches illustrates the Lyon hypothesis. The white patches are due to another gene; (b) A tortoiseshell cat, which lacks the white patches characterizing calicos.
Figure 7-11a Copyright © 2006 Pearson Prentice Hall, Inc.

34. Figure 7-11b Copyright © 2006 Pearson Prentice Hall, Inc.
Figure 7-11b (a) A calico cat, where the random distribution of orange and black patches illustrates the Lyon hypothesis. The white patches are due to another gene; (b) A tortoiseshell cat, which lacks the white patches characterizing calicos.
Figure 7-11b Copyright © 2006 Pearson Prentice Hall, Inc.

35. 7.5 Dosage Compensation Prevents Excessive Expression of X-Linked Genes in Humans and Other Mammals
7.5.3 The Mechanism of Inactivation

36. 7.6 The Ratio of X Chromosomes to Sets of Autosomes Determines Sex in Drosophila

37. Figure 7-15 Copyright © 2006 Pearson Prentice Hall, Inc.
Figure 7-15 Three different patterns of temperature-dependent sex determination (TSD) in reptiles, as described in the text. The relative pivotal temperature is crucial to sex determination during a critical point during embryonic development (FTfemale-determining temperature; MTmale-determining temperature).
Figure Copyright © 2006 Pearson Prentice Hall, Inc.