Sex Determination and Sex-linked Characteristics

Sex Determination and Sex- linked Characteristics  Subtopics Sex Determination systems Sex-linked inheritance Dosage compensation: X-inactivation (Lyon hypothesis)  Sexual reproduction Sexual reproduction in most eukaryotes: Meiosis + Fertilization (both con...

Sex Determination and Sex- linked Characteristics  Subtopics Sex Determination systems Sex-linked inheritance Dosage compensation: X-inactivation (Lyon hypothesis)  Sexual reproduction Sexual reproduction in most eukaryotes: Meiosis + Fertilization (both contribute to genetic variation) Alternation between haploid and diploid state  Sex determination systems Two sexual phenotypes (sexes) in most organisms: male and female (biological/genetic sex) Monoecious species: an individual organism has both male and female reproductive structures Dioecious species: an individual organism has either male or female reproductive structures  Sex-determination systems in dioecious species Chromosomal sex-determining systems Genic sex-determining systems Environmental sex-determining systems  Chromosomal Sex-Determining Systems Sexual phenotype determined by sex chromosomes which differ (in number and/or type) between males and females Non-sex chromosomes = autosomes Eg: XX-XO, XX-XY and ZZ-ZW  Chromosomal Sex-Determining Systems 1. XX-XO Grasshoppers  Females: two X chromosomes (XX)  Males: one X chromosome (XO; O is not a chromosome but indicates the absence of a second X chromosome!)  Gametes: Females: all eggs carry X chromosome = Homogametic sex Males: Half of the sperm carry X; other half no sex chromosome = Heterogametic sex  Chromosomal Sex-Determining Systems 2. XX-XY Mammals, some insects, some reptiles  Females: two X chromosomes (XX)  Males: one X chromosome + one Y chromosome (XY)  Gametes: Females: all eggs carry X chromosome = Homogametic sex Males: Half of the sperm carry X chromosome; other half carry Y chromosome = Heterogametic sex  Chromosomal Sex-Determining Systems X and Y are homologous at only their extremities (= pseudoautosomal regions) and can pair during meiosis  Chromosomal Sex-Determining Systems 3. ZZ-ZW Birds, snakes, butterflies…  Females: ZW= heterogametic sex  Males: ZZ = homogametic sex  Genic Sex-Determination systems No sex chromosomes or any obvious difference in chromosomes between males and females; only sex-determining genes (genotype at particular gene loci) Occurs in some plants, fungi, protozoans, and fish NOTE: In both chromosomal and genic sex determination systems, sexual phenotype is controlled by specific genes but with the former, the sex chromosomes also differ in number or type between males and females  Environmental Sex-Determination systems Environmental factors  Temperature of incubation of turtles during embryonic development (cooler temp.  more males; warmer temp.  more females) - aromatase enzyme (testosterone  estrogen): more active at female-producing temperatures  Environmental Sex-Determination systems Slipper Limpet’s position in a stack Sequential hermaphroditism: An individual organism can be both male and female but not at the same time Please see Table 4.1 for summary of sex-determining systems  Sex Determination in Drosophila melanogaster  Diploid; 8 chromosomes: 3 pairs of autosomes and two sex chromosomes  Normal flies: 2 haploid sets of autosomes (1 haploid set = 1 copy of every type of autosome) and two sex chromosomes Sex chromosomes: Females: XX; males: XY (Y not male determining!!)  Sex Determination in Drosophila melanogaster  Sex determined by X:A ratio = number of X chromosomes number of haploid sets of autosomes TABLE Chromosome complements and sexual phenotypes in 4.2 Drosophila Sex- Haploid Sets X : A Ratio Sexual Chromosome of Phenotype “A” in table = Complement Autosomes one haploid YOU NEED XX AA 1.0 Female set of TO KNOW autosomes XY AA 0.5 Male what X:A ratio XO AA 0.5 Male produces Metafemales XXY AA 1.0 Female male, and female, XXX AA 1.5 Metafemale metamales intersex, usually die as XXXY AA 1.5 Metafemale metamale embryos and XX AAA 0.67 Intersex metafemale XO AAA 0.33 Metamale Influence ofXXXX number ofAAA sets of autosomes 1.3 on sex Metafemale determination is complex  Sex Determination in Humans XX-XY sex determination; Y chromosome determines maleness Role of sex chromosomes in human sex determination inferred from sexual phenotypes associated with abnormal numbers of sex chromosomes (you do not need to remember the sex chromosome complement corresponding to each syndrome!): o Turner syndrome: XO; 1/3000 female births o Klinefelter syndrome: XXY, or XXXY, or XXXXY, or XXYY; 1/1000 male births o Poly-X females: ex. XXX; 1/1000 female births o  Sex Determination in Humans The Y chromosome is the primary determinant of sex and determines maleness. A single Y, even in the presence of several X’s, can still produce a male phenotype. No Y and only X  female phenotype.  Sex Determination in Humans X chromosome contains genes required for both sexes; at least 1 copy of X required for human development. Extra X chromosomes upsets normal development in both males and females. Fertility genes are present on the X and Y chromosome (females need at least 2 copies of the X chromosome for fertility).  Sex Determination in Humans Sex determining region Y (SRY) gene on Y chromosome = primary determinant of maleness in humans SRY gene product causes differentiation of neutral gonads into testes early in embryonic development – SRY switches on genes that cause testes development  Sex Determination in Humans Androgen-insensitivity syndrome in females with XY sex chromosome constitution  genes other than SRY and their functional expression influence sexual development. - testes in abdomen; but ovaries, uterus etc. absent - produce testosterone - female external sexual characteristics BUT, testosterone (normally produces male characteristics in embryonic tissues) cannot bind to its androgen receptor in XY females – because this receptor is defective