Sex Determination and Sex Chromosomes

Wheeler High SchoolThe Center for Advanced Studies in Science, Math & Technology

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.

Post-AP DNA/Genetics – Ms. Hager

Isogametes

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

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.

Sexual Differentiation and Life Cycles

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 sex-determining region of the Y (SRY)

Post-AP DNA/Genetics – Ms. Hager

Any Questions?

X Inactivation and Dosage Compensation

Wheeler High SchoolThe Center for Advanced Studies in Science, Math & Technology

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 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 Copyright © 2006 Pearson Prentice Hall, Inc.

Mechanism of 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

Critical gene for X-inactivation

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 (FTfemale-determining 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.