Genetics Lecture 12, 9/26: Sex determination and Sex Chromosomes

• • The chromosomes that determine sex are different.
• • Heteromorphic chromosome pair – a chromsome pair which is not homologous.
• o Ex. XY chromsome pair in human males
• • Multicellular organisms go through a two step sexual differentiation process.
• o Primary sexual differentiation. Embryonic, intial determination of male or female
• o Secondary sexual differentiation (puberty).
• • In general terms, organisms have one of two systems for sexes.
• o Unisexual organisms – the organism has one type of gonad, male or female
• o Bisexual – one organism with both gonads AND are fertile/functional
• • Life Cycles
• o Chlamydomonas – organism that rarely undergoes sexual reproduction. Spends the majority of the lifecycle as a haploid organism. Reproduce here through asexual reproduction. When the cells are stressed → nitrogen depletion. They will form isogametes, they will have two mating types, + and - . Mating is mating type dependent, plus with minus and vice versa. In turn you produce a diploid zygote. This diploid zygote will help protect the cell from the harsh conditions. When conditions improve the zygote undergoes meiosis and you produce vegetative cells (haploid).
• • C. Elegans (17ish)
• o Two sexes, hermaphrodites or males. Hermaphrodites have testes and ovaries and can make both functional gametes therefore they can self fertilize
• o 99% of worms are hermaphrodites 1% are males
• o hermaphrodites self fertilize – 99% herm. And 1% male
• o mate a male with a hermaphrodite you get 50% male and 50% herm.
• o In C. Elegans you have no Y chromosome so a hermaphrodite has two X chromosomes and the male has one X chromosome. SO when you self fertilize the hermaphrodite. 1% of males, due to non disjunction. Hermaphrodite produce a sperm or egg with no sex chromosomes.
• o Cross male and hermaphrodite with no sex chromosomes you get half herm. And half male.
• • (20ish)X and Y chromosomes were first linked to sex determination in the early 20th century. This was done in 1906 by Edmund Wilson.
• o Observed an insect called the protenor. Females with 12 autosomal chromomse and 2 X chromosomes. Produce gametes with 6 autosomes and 1 x chromosome.
• o Males have 12 autosomes and a sincle x chromosome. Produce two types of gametes. The first has 6 autosomes and x chromsome AND the other has 6 autosomes
• o In the protenor, maleness is defined by the lack of chromosome.
• • (24:30) – lygaeus mode of inheritance
• o females have 12 autromse and 2 X chromosomes
• o males have 12 autosome and Xand Y chromosome. Maleness is defined by the presence of a Y chromosome.
• o Typically the male of the species is the heteromorphic organism. There are some organisms where the female is heteromorphic.
• • Fish and reptiles – males are ZZ. Female is ZW
• • Y chromosome is responsible for maleness in humans (lygaeus mode of inheritance)
• o In 1920’s the Y chromosome was first visualized – small and heterochromatic.
• o Originally thought that humans had 48 chromosomes (we have 46). 44 are autosomes and an XX chromosomes and XY for males.
• • Y chromosome is genetically inert – very few important genes. HOWEVER – there are a number of important regions on the Y chromosomes.
• o PAR – pseudo autosomal region: regions that share homology with portions of the X chromosome. Allow the X and Y chromsome to bond to each orther during mitosis and meiosis. Represents 5% of the Y chromosome. The remaining 95% is called the NRY
• o NRY – non recombining region: this is genetically different from the X chromosome. 95% of the Y chromsome.
• o SRY – sex determining region of the Y chromosome: within it is a gene for the testes determining factor. This is a gene that is believed to be responsible for the development of testes.