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Active Lecture PowerPoint® Presentation for Essentials of Genetics Seventh Edition Klug, Cummings, Spencer, Palladino Chapter 5 Sex Determination and Sex Chromosomes Copyright © 2010 Pearson Education, Inc. Outline • Life Cycles & Sexual Differentiation • Chlamydomonas • Maize (Zea mays) • Caenorhabditis elegans • Humans • Drosophila • Reptiles Asexual reproduction vs. Sexual reproduction Life Cycles Depend on Sexual Differentiation • In multicellular organisms, it is important to distinguish between: 1. primary sexual differentiation - involves only gonads where gametes are produced and 2. secondary sexual differentiation - involves overall appearance of organism Sexual Differentiation • Unisexual (dioecious or gonochoric): - individuals contain only male or female reproductive organs • Bisexual (monoecious or hermaphroditic): - individuals contain both male and female reproductive organs - can produce both male and female gametes Life Cycle of Chlamydomonas • spend most of life cycle in haploid phase • asexually produce daughter cells by mitotic divisions • under unfavorable nutrient conditions, certain daughter cells function as gametes Life Cycle of Chlamydomonas • two gametes that fuse together during mating are not morphologically distinguishable • such gametes are called isogametes Figure 5-1 Copyright © 2006 Pearson Prentice Hall, Inc. Chlamydomonas Mating • 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, though no visible difference exits Copyright © 2006 Pearson Prentice Hall, Inc. Maize (Zea mays) Life Cycle • The diploid sporophyte stage predominates • Both male and female structures are present on adult plant (monoecious) • Thus, sex determination occurs differently in different tissues of same plant Figure 5-2 Copyright © 2006 Pearson Prentice Hall, Inc. Maize (Zea mays) Life Cycle Figure 5-2 Copyright © 2006 Pearson Prentice Hall, Inc. Caenorhabditis elegans • The nematode worm C. elegans has two sexual phenotypes: 1. Males - have only testes 2. Hermaphrodites - have both testes and ovaries • Males are X; hermaphrodites are XX • C. elegens lacks a Y chromosome. Maleness determined by genes on both the X chromosome and autosomes • Ratio of X chromosomes:autosomes determines the sex:Hermaphrodites have a ratio of 1.0 and males 0.5 Caenorhabditis elegans • Self-fertilization occurs in hermaphrodites and produces primarily hermaphrodite offspring, with < 1% male offspring • As adults, males can mate with hermaphrodites, producing ~½ male and ½ hermaphrodite offspring Figure 5-3 Copyright © 2006 Pearson Prentice Hall, Inc. X and Y Chromosomes in Sex Determination • X and Y chromosomes were first linked to sex determination early in the twentieth century • XX/XO (Protenor )mode of sex determination • XX/XY (Lygaeus) mode of sex determination XX/XO (Protenor) Mode of Sex Determination • Depends on random distribution of X chromosome into ½ of male gametes • The presence of two X chromosomes in zygote results in female offspring • The presence of only one X chromosome in zygote results in male offspring http://natureproducts.net/Animals/Insects/Butterflies/Papilio _protenor2.jpg Figure 5-4a Copyright © 2006 Pearson Prentice Hall, Inc. XX/XY (Lygaeus) Mode of Sex Determination • Female gametes all have an X chromosome • Male gametes have either an X or a Y chromosome • Zygotes with two X chromosomes (homogametous) result in female offspring • Zygotes with one X and one Y chromosome (heterogametous) result in male offspring http://perso.orange.fr/groisynature/images/punaiseimage/Lygaeus_saxatil is_050815DSCN4482groisyweb.jpg Figure 5-4b Copyright © 2006 Pearson Prentice Hall, Inc. ZZ/ZW Sex Determination • Females are heterogametic (ZW) sex and males are homogametic (ZZ) sex • Examples: some birds, fish, reptiles Sex Determination in Humans • The human karyotype revealed that one pair of chromosomes differs in males and females – females have two X chromosomes – males have one X and one Y chromosome • Y chromosome determines maleness in humans Sex Determination in Humans Figure 5-5 Copyright © 2006 Pearson Prentice Hall, Inc. Klinefelter Syndrome • Persons with Klinefelter syndrome have more than one X chromosome (usually XXY or a 47,XXY karyotype) • Occurs 1 in 1000 male births • Have male genitalia; feminine sexual development not entirely suppressed Klinefelter Syndrome Figure 5-6 Copyright © 2006 Pearson Prentice Hall, Inc. Turner Syndrome • Persons with Turner syndrome usually have a single X chromosome and no Y chromosome (45, X karyotype) • Have female genitalia; ovaries are rudimentary • They are short, have a low hairline, relatively broad chest and folds of skin on the neck. • Occurs 1 in 3000 female births. Turner Syndrome Figure 5-6 Copyright © 2006 Pearson Prentice Hall, Inc. XXX Condition (Poly X females) • The presence of three X chromosomes along with a normal set of autosomes (47,XXX) results in female differentiation • Frequently, 47,XXX women have no distinctive features other than a tendency to be tall and thin. • 1 in 1000 female births • Although a few are sterile, many menstruate regularly and are fertile, and slight tendency towards mental retardation may occur XYY Condition • Only consistently shared characteristic found so far in the 47,XYY karyotype is that such males are over 6 feet tall Table 5-1 Copyright © 2006 Pearson Prentice Hall, Inc. Y Chromosome • Y is a unique chromosome that determines sex in mammals. • Y chromosome contains far fewer genes than X (Y has about 75 genes, X about 900-1400). • Present on both ends of Y chromosome are the socalled pseudoautosomal regions (PARs) • PARs share homology with regions on X chromosome and synapse and recombine with it during meiosis • The presence of such a pairing region is critical to segregation of X and Y chromosomes during male gametogenesis Sexual differentiation in humans • By the fifth week the human embryo is potentially hermaproditic. There is no phenotypic difference in the gonadal primordia, the tissues that will form the gonad. • Gonadal ridge tissue developes to form either the male or female gonads. • Primodial germ cells migrate into these ridges. • The cortex can develop into an ovary while the medula can develop into a testis. • In addition, two sets of undifferntiated ducts are present. Wolffian ducts that will differentiate into male reproductive tract and the Mullarian ducts that will become structure of the female reproductive tract. Y Chromosome • Y chromosome also contains male-specific region of the Y (MSY) • This includes Sexdetermining region of the Y (SRY) Figure 5-7 Copyright © 2006 Pearson Prentice Hall, Inc. Y Chromosome • SRY or a closely related gene determines sex in all mammals. • In humans, the absence of Y chromosome almost always leads to female development. Thus, SRY is absent from X chromosome. • SRY is the gene responsible for male sex determination. • It encodes a gene product that triggers the undifferentiated embryos to produce testes. • At 6-8 weeks the SRY gene becomes active and causes the undifferentiated gonads of the embryo to Y Chromosome • The testis-determining factor (TDF) is a protein encoded by a gene in the SRY that triggers testes formation • The MSY consists of three regions: 1. X-transposed region: originally derived from the X; nearly identical to region in X called Xq21 2. X-degenerative region: originally derived from the X; distantly related to parts in X 3. Ampliconic region: no counterpart in X, genes associated with testes development Q: In which region is the SRY located? We’re Getting More Boys!!! • Primary sex ratio vs. Secondary sex ratio • Ratio of males to females in humans is not 1.0! • Why? Dosage Compensation: Barr Bodies • Darkly staining body in female nuclei that is absent in male cells • Barr body is an inactivated X chromosome • Mechanism for dosage compensation – If one X in females in inactivated, dosage of genetic information that can be expressed in males and females is equivalent Barr Bodies Female Male Figure 5-8 Copyright © 2006 Pearson Prentice Hall, Inc. Occurrence of Barr Bodies None inactivated One inactivated Two inactivated Three inactivated The Lyon Hypothesis • The inactivation of X chromosomes occurs randomly in somatic cells at a point early in embryonic development • Once inactivation has occurred, all progeny cells have same X chromosome inactivated • Makes females mosaics for X-linked traits Calico cat Figure 5-10 Tortoiseshell cat Copyright © 2006 Pearson Prentice Hall, Inc. This figure not in the Sixth edition 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. Q: Are females homozygous for Xlinked traits mosaics also? Copyright © 2006 Pearson Prentice Hall, Inc. The Mechanism of Inactivation: Imprinting • X-inactivation Center (Xic) – Major control unit in humans – Expression occurs only on X chromosome that is inactivated – Contains four genes • X-inactive specific transcript (XIST) – Believed to be critical gene in Xic – Lacks an extended open reading frame (ORF) – RNA product along with other RNA products of Xic genes may coat X chromosome that produced it Sex Determination in Drosophila • Females are XX & males are XY • Unlike in humans, the Y chromosome is not involved in sex determination in Drosophila • Ratio of X chromosomes to sets of autosomes (X:A) determines sex • 1:1 X Chromosome: sets of autosome ratio = female 1:2 X Chromosome: sets of autosome ratio = male • This is known as the Genic Balance theory of Calvin Bridges Drosophila Chromosome compositions and sex Figure 5-11 Copyright © 2006 Pearson Prentice Hall, Inc. Sex Determination in Reptiles • In many reptiles sex is determined by sexchromosome composition • However, in some reptiles, sex determination is achieved according to incubation temperature of eggs during a critical period of embryonic development • Temperature is thought to influence the enzymes involved in production of estrogens and androgens • Examples: crocodiles, some turtles and lizards Three Different Patterns of TemperatureDependent Sex Determination in Reptiles Figure 5-12 Copyright © 2006 Pearson Prentice Hall, Inc. Genetics, Technology & Society (p107) A Question of Gender: Sex selection in humans Problems Q 32-34 (p113) The Problem of Cat Cloning Cloned Cats are not Copy-Cats !