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Chapter 4 – Sex Determination and Sex Linked Characteristics

Chapter 4 – Sex Determination and Sex Linked Characteristics. Sex Determination. In sexually reproducing organisms, there is usually two distinct sexes – males and females Sex chromosomes – different between sexes Autosomes – any non-sex chromosome. Sex Determination. XX-XO sex determination

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Chapter 4 – Sex Determination and Sex Linked Characteristics

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  1. Chapter 4 – Sex Determination and Sex Linked Characteristics

  2. Sex Determination • In sexually reproducing organisms, there is usually two distinct sexes – males and females • Sex chromosomes – different between sexes • Autosomes – any non-sex chromosome

  3. Sex Determination • XX-XO sex determination • Grasshoppers • XX – female • Homogametic – all gametes contain same type of sex chromosome • XO – male (one less chromosome) • Heterogametic – 2 different types of gametes in regard to sex chromosomes (X or none) • XX-XY sex determination • All mammals; some plants, insects, reptiles • XX – female • XY – male

  4. XX - XY • Pseudoautosomal region • Small region of both X and Y chromosomes carry same genes • Allow for homologous pairing during meiosis • Not considered sex-linked in terms of heredity

  5. Sex Determination • ZZ – ZW sex determination • Birds; some fish and amphibians • ZW – female • Heterogametic • Female determines sex of offspring • ZZ – male • homogametic

  6. Sex Determination • Haplodiploidy • Hymenoptera (ants, bees, wasps) • Females are diploid; produce eggs by meiosis • Males are haploid; produce sperm by mitosis • Develop from unfertilized eggs (parthenogenesis) • Brothers have 50% relatedness; sisters have 75% relatedness (50% from mother and 100% from father)

  7. Sex Determination • Genic sex determination • No difference in chromosomes between sexes • Sex is determined by genotype at one or more gene loci • Chromosomal sex determination also determined by genes – happen to be on sex chromosomes • SRY gene on Y chromosome determines maleness

  8. Environmental Sex Determination • Credidula fornicata • Larva is free-living • 1st to land is female, and attracts more larva, which are male • As more stack on, the covered males become female • Sequential hermaphroditism • Can be both female and male, but not at the same time • Temperature • Turtles, crocodiles, alligators • Incubation temperature of eggs determine sex of offspring

  9. Sex determination in Drosophila melanogaster • 8 chromosomes • 3 pairs of autosomes; 1 pair of sex chromosomes • Females are XX; males are XY • Y alone doesn’t determine maleness • Genic balance system

  10. Genic balance system in Drosophila • Ratio of X chromosomes:haploid sets of autosomes (X:A) • X chromosome = female producing effects; autosomes = male producing effects • X:A ratio • Female = 1.0 (2X:2n) • Male = 0.5 (1x:2n) • 0.5 < X:A < 1.0 = intersex • Mix of male and female characteristics • 1.0 < X:A = metafemale • Increased developmental problems; rarely emerge from pupal case • 0.5 > X:A = metemale • Weak and sterile

  11. Sex Chromosomal Abnormalities • Nondisjunction – abnormal segregation of chromosomes during meiosis I and II • Turner syndrome • 45, XO • 1 in 3,000 female births • Clinical features • Short stature, broad chest and wide – spaced nipples, low hairline on neck, webbed neck, sterile, usually normal intelligence • Mosaics may be fertile

  12. Sex Chromosomal Abnormalities • Klinefelter syndrome • 47, XXY • 1 in 1,000 male births • Clinical features • Above average height, reduced facial and pubic hair, breast development, underdeveloped testes – sterile, usually normal intelligence • Additional sex chromosomes • XXXY, XXYY • Lower IQ more common • Increases severity as #s increase

  13. Sex Chromosomal Abnormalities • Poly X females • 47, XXX • 1 in 1,000 female births • Clinical features • Tall and thin, most are fertile, increased risk of mental retardation • Increases with additional Xs (XXXX) • Jacob syndrome • 47, XYY • Originally thought to be associated with increased violence, deviant/antisocial behavior • Original study showed higher % in mental and penal institutions • Not supported by other studies • Fertile - #s in population not known since usually asymptomatic

  14. Male Determining Gene in Humans • Early development • Gonads are undifferentiated and have both male and female reproductive ducts • SRY gene • Sex-determining region on Y chromosome • Begins expression approx 6 weeks after fertilization • Develops neutral gonads into testes • Testes produce: • Testosterone – male characteristics • Mullerian-inhibiting substance – degeneration of female reproductive ducts

  15. SRY gene • In the absence of SRY: • Gonads develop into ovaries and male ducts degenerate • Androgen-insensitivity syndrome • Phenotype is female • Menstruation does not occur • No internal female reproductive organs; has testes in abdominal cavity, which produces testosterone • Have testes due to SRY gene • Karyotype is male – XY • Receptor for testosterone is defective – can’t affect appropriate cells • Default to female

  16. Eye color in Drosophila • D. melanogaster • Wildtype = red eyes • P • red-eyed female X white-eyed male • F1 • All red eyes • Looks like Mendelian inheritance

  17. Eye Color in Drosophila cont • F2 • 100% females have red eyes • 50%males have red; 50% have white • Males are hemizygous for X-linked genes • Reciprocal cross at P generation • F1 = all females have red; all males have white • F2 = 50% of both females and males have red; 50% white

  18. X-linked Colorblindness in humans • X-linked recessive trait • More common in males than females • Affected males can only pass mutant allele to daughters (100%) • If heterozygous, females can pass mutant allele to both daughters and sons (50%); if homozygous, female will pass mutant allele to ALL offspring

  19. X linked disorders • Symbols • X = X chromosome • Y or / = Y chromosome • X-linked dominant disorders • More common in females • Have two chances to receive mutant allele; males have only one chance

  20. Dosage Compensation • Females have two copies of each X-linked gene while males have one 1 copy, yet females do not have twice the gene product • D. melanogaster • Male gene expression is doubled • C. elegans • Each female X gene’s expression is halved • Placental mammals • Only one X remains active; other is inactivated

  21. Barr body • Darkly staining body at edge of nucleus • Mary Lyon (Lyon hypothesis) • Barr body is an inactive X • Early in embryonic development, a random X is inactivated in each cell • Once marked for inactivation, all descendant cells will have the same X inactivated • Only 1 X is kept active

  22. Xist (X inactivation specific transcript) • Is only expressed on inactive X • Produces RNA molecule that coats chromosome • Some genes on “inactive” X are kept active (mechanism unknown) • Why “normal” females are different from Turner syndrome females

  23. Holandric inheritance • Y linked gene • All males are affected within family; no affected females • Used to trace ancestry through paternal lines • Y chromosome have few genes, 2/3 is heterochromatin • No trait has been definitely associated with Y chromosome in humans

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