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Wheeler High School The Center for Advanced Studies in Science, Math & Technology Sex Determination and Sex Chromosomes Post-AP DNA/Genetics – Ms. Hager 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 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 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. Male or Hermaphrodite? How can you tell? 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. 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 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 Copyright © 2006 Pearson Prentice Hall, Inc. Figure 7-6a Copyright © 2006 Pearson Prentice Hall, Inc. Figure 7-6b Copyright © 2006 Pearson Prentice Hall, Inc. 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 Klinefelter and Turner Syndrome The karyotypes and phenotypic depictions of individuals with (a) Klinefelter syndrome (47,XXY) and (b) Turner syndrome (45,X). Figure 7-7 Copyright © 2006 Pearson Prentice Hall, Inc. Figure 7-7a Copyright © 2006 Pearson Prentice Hall, Inc. Klinefelter Syndrome Figure 7-7b Copyright © 2006 Pearson Prentice Hall, Inc. Turner syndrome Frequency of XXY Individuals Table 7-1 Copyright © 2006 Pearson Prentice Hall, Inc. 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 sexdetermining region of the Y (SRY) Post-AP DNA/Genetics – Ms. Hager Any Questions? Wheeler High School The Center for Advanced Studies in Science, Math & Technology X Inactivation and Dosage Compensation Post-AP DNA/Genetics – Ms. Hager 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 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 Copyright © 2006 Pearson Prentice Hall, Inc. 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 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 (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 7-11 Copyright © 2006 Pearson Prentice Hall, Inc. Depiction of the absence of sweat glands (shaded regions) in a female heterozygous for the Xlinked 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 Copyright © 2006 Pearson Prentice Hall, Inc. Mechanism of X Inactivation Critical gene for Xinactivation • 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 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 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 Figure 7-13 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 Copyright © 2006 Pearson Prentice Hall, Inc. 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 (FTfemaledetermining temperature; MTmale-determining temperature). Any Questions? Figure 7-11a Copyright © 2006 Pearson Prentice Hall, Inc. Figure 7-11b Copyright © 2006 Pearson Prentice Hall, Inc. 7.5 Dosage Compensation Prevents Excessive Expression of X-Linked Genes in Humans and Other Mammals 7.5.3 The Mechanism of Inactivation 7.6 The Ratio of X Chromosomes to Sets of Autosomes Determines Sex in Drosophila Figure 7-15 Copyright © 2006 Pearson Prentice Hall, Inc.