How is sex determined in placental mammals? What is the function of a Barr body
ID: 151029 • Letter: H
Question
How is sex determined in placental mammals? What is the function of a Barr body in female placental mammals; why is it necessary? 6. How is sex determined in birds? In which gender would you expect to see a Barr body in birds? 7. How is sex determined in insects, and how do they balance gene dosage differences between the two genders for the X chromosome? 8. What is the cause of Down Syndrome? Why is trisomy 21 more common than other types of trisomies? How is sex determined in placental mammals? What is the function of a Barr body in female placental mammals; why is it necessary? 6. How is sex determined in birds? In which gender would you expect to see a Barr body in birds? 7. How is sex determined in insects, and how do they balance gene dosage differences between the two genders for the X chromosome? 8. What is the cause of Down Syndrome? Why is trisomy 21 more common than other types of trisomies? How is sex determined in placental mammals? What is the function of a Barr body in female placental mammals; why is it necessary? 6. How is sex determined in birds? In which gender would you expect to see a Barr body in birds? 7. How is sex determined in insects, and how do they balance gene dosage differences between the two genders for the X chromosome? 8. What is the cause of Down Syndrome? Why is trisomy 21 more common than other types of trisomies?Explanation / Answer
5.
Primary sex determination is the determination of the gonads. In mammals, primary sex determination is strictly chromosomal and is not usually influenced by the environment. In most cases, the female is XX and the male is XY. Every individual must have at least one X chromosome. Since the female is XX, each of her eggs has a single X chromosome. The male, being XY, can generate two types of sperm: half bear the X chromosome, half the Y. If the egg receives another 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 that encodes a testis-determining factor. This factor organizes the gonad into a testis rather than an ovary. Unlike the situation in Drosophila (discussed below), the mammalian Y chromosome is a crucial factor for determining sex in mammals. A person with five X chromosomes and one Y chromosome (XXXXXY) would be male. Furthermore, an individual with only a single X chromosome and no second X or Y (i.e., XO) develops as a female and begins making ovaries, although the ovarian follicles cannot be maintained. For a complete ovary, a second X chromosome is needed.
In mammalian primary sex determination, there is no “default state.” The formation of ovaries and testes are both active, gene-directed processes. Moreover, as we shall see, both diverge from a common precursor, the bipotential gonad.
A Barr body (named after discoverer Murray Barr) is the inactive X chromosome in a female somatic cell, rendered inactive in a process called lyonization, in those species in which sex is determined by the presence of the Y (including humans) or W chromosomerather than the diploidy of the X. The Lyon hypothesis states that in cells with multiple X chromosomes, all but one are inactivated during mammalian embryogenesis. This happens early in embryonic development at random in mammals, except in marsupialsand in some extra-embryonic tissues of some placental mammals, in which the father's X chromosome is always deactivated.
6.The sex chromosomes in birds are designated Z and W, and the male is the homomorphic sex (ZZ) and the female heteromorphic (ZW). In most avian species the Z chromosome is a large chromosome, usually the fourth or fifth largest, and it contains almost all the known sex-linked genes. The W chromosome is generally a much smaller microchromosome, containing a high proportion of repeat sequence DNA. Recently a gene encoding a protein involved in transcriptional activation of chromatin has been detected on the W chromosome. The weight of evidence suggests that sex determination in birds is by a genic balance mechanism, in which the ratio of autosomes to Z chromosomes is the crucial factor. DNA sequences homologous to the testis determining factor in humans have been detected in both male and female birds, but it is not clear that they have a sex-related function in birds. A number of different practical methods have been developed to distinguish the sex of birds, based on sex-linked genes, the amount of DNA per cell and using DNA probes for sex-linked sequences.
Sex chromosomes and sex determining mechanisms in birds.
In birds the Z-linked genes (Z=X) occur in hemizygous condition
7.The XX/XY sex-determination system is the most familiar, as it is found in humans. The XX/XY system is found in most other mammals, as well as some insects. In this system, most females have two of the same kind of sex chromosome (XX), while most males have two distinct sex chromosomes (XY).
8.
Down syndrome happens when there is an extra copy of genetic material on all or part of the 21st chromosome.
Every cell in the body contains genes that are grouped along chromosomes in the cell's nucleus. There are normally 46 chromosomes in each cell, 23 inherited from the mother and 23 from the father.
When some or all of a person's cells have an extra full, or partial, copy of chromosome 21, the result is Down syndrome.
The reason we see trisomy 21 as “common” is that it has the smallest effect of duplication on embryonic development, so we see more cases of such children carried to term. Trisomies of the other autosomes happen, we just rarely see the results since those embryos abort within the first trimester.