HONR 1034 Chapter Notes - Chapter 7: Testis Determining Factor, Sexual Differentiation, Gonochorism
Chapter 7: Sex Determination and Sex Chromosomes
I. Life cycles depend on sexual differentiation
A. Sexual dimorphism: differences between males and females in multicellular animals
B. Primary sexual differentiation → gonads, where gametes are produced
C. Secondary sexual differentiation → overall appearance of the organism
D. Unisexual, dioecious, gonochoric → individual containing only male or only female
reproductive organs
E. Bisexual, monoecious, hermaphroditic → individuals containing both male and female
reproductive organs
F. Intersex: individuals of an intermediate sexual condition (usually sterile)
G. CHROMOSOMES decide whether an organism is male or female
II. An example: zea mays: maize/corn
A. Male and female structures present in the adult plant → sex determination occurs differently in
different tissues of the same organism
B. Tissues that form the male and female gametes are the same
III. An example: Chlamydomonas: green algae
A. Haploid organism that produces bisexually and infrequently → spend most of life in haploid
phase
B. If it encounters environmental factors that it can’t withstand, gametes fuse together through
fertilization to form a diploid zygote to be able to withstand these conditions and will remain this
way
1. After the conditions return to normal, meiosis occurs and haploid cells are produced
2. Gametes that fuse during mating look identical → isogametes
IV. Example: caenorhabditis elegans → nematode worm
A. Two types of sexual phenotypes: males and hermaphrodites
B. Hermaphrodites go through self-fertilization to produce sperm and egg
C. Less than 1% of offspring are males, but when males become adults and mate with
hermaphrodites, they create about a 50/50 split of both
D. 2X chromosomes = female; 1X chromosome = 1X
V. Klinefelter syndrome: (47 chromosomes, XXY)
A. Affects about 1/160 males
B. Tall, long arms and legs, large feet, etc
VI. Turner syndrome (45 chromosomes, X)
A. Affects females → have female external genitalia, but ovaries are rudimentary
B. Short stature, cognitive impairment
C. This disease is caused by the improper pairing of sex chromosomes that results in the passing of
more or less genetic material
D. BOTH TURNER AND KLINEFELTER SYNDROME are caused by nondisjunction, which is
the failure of the X chromosome to segregate properly during meiosis
VII. The Y chromosome determines maleness in humans
A. PAR: Pseudoautosomal regions on both ends of the Y chromosome
that share homology with regions on the X chromosome
B. The distinction between euchromatin is that the genes are active
there and the genes in the heterochromatin are not active
C. SRY: sex determining region within euchromatin → testis
determining factor
D. MSY: male-specific region of the Y
VIII. Dosage compensation: the balancing of X chromosome gene expression in
females and males demonstrated by X-linked gene expression
A. Sex chromatin body/barr body: a darkly staining body in
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