Sex Determination Flashcards
What are the biological advantages of sexual reproduction?
It speeds evolution and contributes to survival of species by increasing genetic variabilityIt also protects and preserves species
What is X chromosomal inactivation, and what are the implications?
Cells from normal females inactivate one X chromosome.All diploid somatic cells in both males and females have a single active X chromosome regardless of the total number of X and Y chromosomes present.X inactivation is normally random so that females are mosaic for expression of their X chromosomes. On average, 50% of cells in any given tissue will express the paternally inherited X and 50% will express the maternally inherited X.
What is the mechanism for X chromosome inactivation?
X inactivation results from DNA methylation and modifications of histone proteins that depend on expression of the XIST gene on the inactive X. The XIST gene encodes noncoding RNA, which is expressed in cell nuclei where it associates in cis (i.e. with the inactive X) as part of an XIST RNA/Barr body complex (See Figure 6-13 in Thompson and Thompson).
What are the two cases when X inactivation is not random?
Nonrandom X inactivation occurs when an X chromosome is abnormal or there is an X; autosome translocation.Cytogenetic analysis looking for an X; autosomal translocation is indicated when a female presents with an X-linked recessive phenotype.In the absence of cytogenetic abnormalities, nonrandom X inactivation can be diagnosed by analyzing the methylation pattern of highly polymorphic X chromosomal loci (e.g. gene for the androgen receptor, AR, Xq11-12).Selection will favor normal X’s, and will preferentially deactivate abnormal X’s.
How is sexual differentiation genetically regulated?
- Genetic sex (XX, XY)2. Gonadal sex: Is SRY expressed?SRY is the principal determinant of testicular differentiation. In the absence of SRY expression, undifferentiated gonadal tissue develops into ovaries.
What 2 stages do germ cells go through with regards to gonads?
Sexually independent pregonadal stage, during which they migrate from yolk sac to developing gonads.Gonadal dependent stage: when they mature.
How is male/female genital sex regulated?
Specific hormones and growth factors from differentiated testes determine male gonads.In the absence of these factors or receptors, female genitalia develop (internal and external).
What 2 non-germ cells are within normal testes?What do they produce?
Sertolli: Mullerian Inhibitory Factor (MIF)Leydig cells: Testosterone
What does MIF do?
Prevents formation of mullerian (paramesonephric) duct derivatives (fallopian tubes, uterus, upper vagina).
What does testosterone do?
Stimulates the development of Wolffian ducts (mesonephric) into the epididymal ducts and ductus deferens.Also stimulates descent of testes into the scrotum.
What are the clinical characteristics of 45X?
45 X occurs most frequently (possibly as frequently as 50% of conceptions, but in about 1 in 2000 to 5000 liveborns) and produces Turner syndrome.In this condition early female gonadal and genital development are normal. However, gonadal dysgenesis (involution and degeneration of developing ovaries through apoptotic mechanisms) develops during late fetal life.Short stature, perceptual disorders and anatomic maldevelopment of the heart (coarctation of the aorta) and kidneys (fused) are also associated with this condition.
What are the clinical characteristics of 47 XXY?
47 XXY individuals develop as anatomic males but also have gonadal dysgenesis. This results in infertility due to failed maturation of spermatogonia and low levels of testosterone production. Tall stature (eunichoid appearance), gynecomastea and increased impulsive behavior are also associated with this condition (Klinefelter syndrome). The frequency is about 1 in 1000.
What are the clinical characteristics of 47XYY?
47 XYY individuals are normal males that are generally taller than expected in their families. The frequency is approximately 1 in 900-1000 in the general population but higher among men in penal institutions. The reason(s) for these differences in distribution are incompletely understood.
Describe the clinical approach to disorders of sexual differentiation.
Determine the karyotype, guides both surgical and psychosocial management and genetic counselling.
HOMEWORK: Sex in turtles is determined by the incubation temperature of their eggs. How does this happen?
Above a pivotal temperature, the turtle becomes a female. Below, it becomes a male. This is due to synergy between particular hormones and temperature.
