BIO132 Lecture 7 - Chromosomal Sex Determination PDF

Dr. Ahmed Deghidy PhD Molecular Biology Alex U, Alexandria, Egypt Field of Basic Sciences ‫الدولية‬ ‫جامعة العلمين‬ 1  “BIO132” Lecture 7 Dr. Ahmed Deghidy PhD Molecular Biology Alex U, Alexandria, Eg...

Dr. Ahmed Deghidy PhD Molecular Biology Alex U, Alexandria, Egypt Field of Basic Sciences ‫الدولية‬ ‫جامعة العلمين‬ 1  “BIO132” Lecture 7 Dr. Ahmed Deghidy PhD Molecular Biology Alex U, Alexandria, Egypt  Chromosomal Sex Determination There are several ways chromosomes can determine the sex of an embryo: In mammals, the presence of either a second X chromosome or a Y chromosome determines whether the embryo will be female (XX) or male (XY). In birds, the situation is reversed: the male has the two similar sex chromosomes (ZZ) and the female has the unmatched pair (ZW). In flies, the Y chromosome plays no role in sex determination, but the number of X chromosomes appears to determine the sexual phenotype. In other insects (such as bees and ants), fertilized, diploid eggs develop into females, while unfertilized, haploid eggs become males.  Sex Determination The Mammalian Pattern of Sex Determination Mammalian sex determination is governed by: 1- the gonad-forming genes 2- the hormones elaborated by the gonads. Primary sex determination is the determination of the gonads “the egg-forming ovaries or sperm-forming testes”. Secondary sex determination is the determination of the male or female phenotype by the hormones produced by the gonads.  Both the male and female gonads diverge from a common precursor, the bipotential gonad (indifferent gonad) Originally, a bipotential (indifferent) gonad develops, with undifferentiated Müllerian ducts (female) and Wolffian ducts (male) ducts both present. If XY, the gonads become testes and the Wolffian duct persists. If XX, the gonads become ovaries and the Müllerian duct persists. Hormones from the gonads will cause the external genitalia to develop either in the male direction or the female direction Female’s karyotype is XX and the male’s is XY. Since the diploid female is XX, each of her haploid eggs has a single X chromosome. The male, being XY, generates two populations of haploid sperm: half will bear an X chromosome, half a Y. If at fertilization the egg receives a second X chromosome from the sperm, the resulting individual is XX, forms ovaries, and is female If the egg receives a Y chromosome from the sperm, the individual is XY, forms testes, and is male  The Y chromosome carries a gene (Sry gene) that encodes a testis-determining factor that organizes the bipotential gonad into a testis XXY individuals (a condition known as Klinefelter syndrome) are male, have functioning testes. individuals having only one X chromosome (XO, sometimes called Turner syndrome) are female, begin making ovaries, but the ovarian follicles cannot be maintained without the second X chromosome. a second X chromosome completes the ovaries, whereas the presence of a Y chromosome (even when multiple X chromosomes are present) initiates the development of testes. If the Sry gene is present, it promote testis formation and to inhibit ovary formation. If the Sry gene is not present, the ovary-forming genes are the ones that will function