Klug et al. 2006, 2009 Concepts of Genetics Chapter 7 STUDY UNIT 5 SEX DETERMINATION AND SEX CHROMOSOMES Some species reproduce asexually Most diploid eukaryotes reproduce sexually Parent (2n) Parent (2n) meiosis segregation & independent assortment genetic variation Gamete (n) Gamete (n) fertilisation Offspring (2n) Sexual differentiation and life cycles Chlamydomonas: sexual differentiation: development of gonads where gametes are produces Secondary sexual differentiation: overall appearance of organism Unisexual / dioecious / gonochoric: An individual containing only male or female reproductive organs: Bisexual / monoecious / hermaphrodite: An individual containing both male and female organs: Intersex: Individuals of intermediate sexual differentiation: Isogametes = morphologically indistinguishable, but of two different mating types Fig 7-1 & 7-2 Fig 2-12 Zea mays (fig 7-3): Monoecious seed plant, sporophyte phase predominant Caenorhabditis elegans 2 sexual phenotypes Male only testes Hermaphrodite testes and ovaries : Stamen (tassel) microspore mother cell (2n) meiosis microspores mitosis microgametophyte (pollen grain) : Pistil megaspore mother cell (2n) meiosis megaspores mitosis megagametophyte (embryo sac) Double fertilisation occurs Sexual differentiation determined by Larval development of hermaphrodites: Testes form produce sperm stored Ovaries form oogenesis occurs in adult stage Eggs fertilized by stored sperm Fig 7-4 1
Male vs hermaphrodite Triggered by genes on X chromosomes and autosomes Sex chromosomes and sex determination XX / XO or Protenor mode: Hermaphrodites: Males: Believed that the ratio of X chromosomes to number of sets of autosomes determines sexual phenotype. X / A = 1 = hermaphrodite Certain insect species, eg grasshoppers Contain only one type of sex chromosome = X chromosome Females: XX all gametes X Males: X ½ gametes X : ½ gametes Fig 7-5a X / A = 0.5 = male XX / XY or Lygaeus mode: Some insects, all mammals, some other species Contain two types of sex chromosome = X chromosome & Y chromosome (usually smaller) Females: XX all gametes X Males: XY ½ gametes X : ½ gametes Y Fig 7-5b sex that produces different gametes with respect to chromosome numbers and types male in XX / XO system male in XX / XY system sex that produces uniform gametes with respect to chromosome numbers and types female in XX / XO system female in XX / XY system QUESTIONS: ZZ / ZW mode: Similar to XX / XY mode, only female is heterogametic and male homogametic sex. Most birds, some moths, butterflies, fish. Contain two types of sex chromosome = Z chromosome & W chromosome (usually smaller) ZW ½ gametes Z : ½ gametes W ZZ all gametes Z 1. An insect species is discovered in which the heterogametic sex is unknown. An X-linked recessive mutation for reduced wing (rw) is discovered. Contrast the F 1 and F 2 from a cross between a female with reduced wings and a male with normal sized wings when: a. the female is the heterogametic sex b. the male is the heterogametic sex 2
2. Based on your answer in Q1, is it possible to distinguish between the Protenor and Lygaeus mode of sex determination based on the outcome of the crosses? 3. In the wasp Bracon hebetor, a form of parthenogenesis (where unfertilized eggs initiate development) resulting in haploid organisms is not uncommon. All haploids are male. When offspring arise from fertilization, females almost invariably result. It has been shown that an X-linked gene with nine multiple alleles (X a, X b, etc) controls sex determination. Any homozygous or hemizygous condition results in males and any heterozygous condition results in females. If an X a X b female mates with an X a male and lays 50% fertilized and 50% unfertilized eggs, what proportion of male and female offspring will result? Sex determination in humans XX / XY system : : Fig 7-6 Turner syndrome 1/2000 female births. Normal intelligence. Female external genitalia. Fig 7-7b Rudimentary ovaries. Short stature. Results from: Gamete where nondisjunction of X occurred during meiosis. Somatic mosaics due to mitotic error in early embryo. (45,X / 46,XY or 45,X / 46,XX) Does Y chromosome determine maleness? (Stine, 1989) Klinefelter syndrome 2/1000 male births. Below normal intelligence. Fig 7-7a Male external genitalia. Rudimentary testes. Tall. Results from: Gamete where nondisjunction of X occurred during meiosis. Other karyotypes: 48,XXXY, 48,XXYY, 49,XXXY, 49,XXXYY. From Turner & Klinefelter syndromes: Y-chromosome determines maleness. 47,XXX Syndrome 48,XXXX 49,XXXXX 1/1200 female births. Variable expression. Delicate balance of gene expression disrupted. 47,XYY Condition Additional Y chromosome Table 7-1 (Stine, 1989) 3
Human sexual differentiation Embryo week 5 gonadal ridges on kidney 2 sets of undifferentiated tubes Week 7: XY Ridges Testes Fig 7-8 Human Y chromosome Fig 7-9 Absence of Y Ridges Ovaries MSY region of Y chromosome: 23 Mb X-transposed region 15% of MSY, 99% homology to Xq21 X-degenerative region 20% of MSY, some homology to X chromosome Includes SRY gene SRY encodes TDF determines male development Evidence: Humans Mice Ampliconic region 30% of MSY, no X chromosome homology testes development, male fertility Sex ratio: Proportion of male to female offspring. Expect ratio of 1.0 with XX / XY sex chromosome system. Dosage compensation prevents excessive expression of X-linked genes Barr body / sex chromatin In humans: Primary 1.06 Secondary 1.2 1.6 proportion at conception. proportion at birth. Fig 7-10 Dark staining body in interphase nucleus of female somatic cells = Number of Barr bodies = number of X chromosomes - 1 Fig 7-11 Lyon hypothesis X chromosome inactivation is random, inactivation of one X occurs randomly in each somatic cell early in embryonic development. After inactivation all progeny cells have the same X chromosome inactivated. Explains: phenotypic mosaics in females heterozygous for X-linked genes, eg tortoiseshell cats. Fig 7-12 Examples in humans: Red-green colour blindness Anhidrotic ectodermal dysplasia. Mechanism of inactivation: X-inactivation center ( ). Fig 7-13 Expressed only in inactivated X chromosome. X-inactive specific transcript ( ) = large RNA transcript without an open reading frame, RNA product coats X chromosome and leads to inactivation. 4
Sex determination in Drosophila (fig 7-15) : 2n = 8 2 sets of autosomes (2A) + XX : 2n = 8 2 sets of autosomes (2A) + XY Y chromosome plays no role in sex determination, only in male fertility. Sex is determined by the ratio of X chromosomes to the number of haploid sets of autosomes (A). (Model formulated after study of abnormal meioses.) Genic balance theory: Interaction between autosomal genes and genes on X chromosome determine sex. 1X : 2A threshold for maleness 2X : 2A alters balance, female differentiation. Dosage compensation in Drosophila: No X chromosome inactivation in females. Male X-linked genes transcribed at twice the level of the same genes in females. Mechanism of dosage compensation in Drosophila Master switch Sxl gene At least 4 autosomal genes involved Master switch gene Sxl induces female differentiation during sex determination MODEL Mle (maleless) autosomal gene protein binds X chr enhances genetic expression. Products of 3 other autosomal genes required for mle binding. Male-specific lethals (MSLs) proteins bind to gene-rich regions of chr X increase gene expression in male flies. Cluster of gene-activating proteins Dosage Compensation Complex (DCC). Fig 7-16. Sxl activated Autosomal genes activated Enhanced X chromosome activity Sxl active Inactivates 1 / more Autosomal genes mle? No doubling of expression of X-linked genes Drosophila mosaics: Recovered from stock where all females were heterozygous for X-linked genes white eye (w) and miniature wing (m) Bilateral gynandromorph Temperature variation controls sex determination in reptiles Temperature-dependent sex determination as opposed to chromosomal or genotypic sex determination in previous examples. Male half, white eyes, miniature wings Fig 7-17 Female half, normal red eyes, normal large wings Fig 7-18 X w,m O X w+,m+ X w,m XX zygote lost one X chromosome during 1 st mitotic division All crocodiles, most turtles, some lizards 5
In all three patterns: Certain temperatures produce males and females. Pivotal temperature (T p ) is fairly narrow. How? Androgens Estrogens (male hormones) Aromatase (female hormones) enzyme 4. Shown below are two graphs that plot the percentage of males occurring against the atmospheric temperature during early development of fertilized eggs in a) snapping turtles and b) most lizards. Interpret these data as they relate to the effect of temperature on sex determination. Thermosensitive factor controls transcription 5. In chickens, the gene for black feathers (B) is dominant over the gene for brown feathers (b) and is carried on the Z chromosome. A black hen is crossed with a heterozygous black rooster. Give the genotypic and phenotypic ratios of the F 1 progeny. 6