In a review of studies of human genome diversity, Guido Barbujani and colleagues note that various cluster studies have identified different numbers of clusters with different boundaries.
The human chromosomes A new era in cytogeneticsthe field of investigation concerned with studies of the chromosomes, began in with the discovery by Jo Hin Tjio and Albert Levan that human somatic cells contain 23 pairs of chromosomes.
Since that time the field has advanced with amazing rapidity and has demonstrated that human chromosome aberrations rank as major causes of fetal death and of tragic human diseases, many of which are accompanied by mental retardation.
Since the chromosomes can be delineated only during mitosis, it is necessary to examine material in which there are many dividing cells.
This can usually be accomplished by culturing cells from the blood or skin, since only the bone marrow cells not readily sampled except during serious bone marrow disease such as leukemia have sufficient mitoses in the absence of artificial culture.
After growth, the cells are fixed on slides and then stained with a variety of DNA-specific stains that permit the delineation and identification of the chromosomes.
The Denver system of chromosome classificationestablished inidentified the chromosomes by their length and the position of the centromeres.
Since then the method has been improved by the use of special staining techniques that impart unique light and dark bands to each chromosome.
These bands permit the identification of chromosomal regions that are duplicated, missing, or transposed to other chromosomes. Micrographs showing the karyotypes i.
In a typical micrograph the 46 human chromosomes the diploid number are arranged in homologous pairs, each consisting of one maternally derived and one paternally derived member.
The chromosomes are all numbered except for the X and the Y chromosomes, which are the sex chromosomes. In humans, as in all mammals, the normal female has two X chromosomes and the normal male has one X chromosome and one Y chromosome. The female is thus the homogametic sex, as all her gametes normally have one X chromosome.
The male is heterogametic, as he produces two types of gametes—one type containing an X chromosome and the other containing a Y chromosome. There is good evidence that the Y chromosome in humans, unlike that in Drosophila, is necessary but not sufficient for maleness.
Fertilizationsex determination, and differentiation A human individual arises through the union of two cells, an egg from the mother and a sperm from the father.
Human egg cells are barely visible to the naked eye. They are shed, usually one at a time, from the ovary into the oviducts fallopian tubesthrough which they pass into the uterus. Fertilization, the penetration of an egg by a sperm, occurs in the oviducts.
This is the main event of sexual reproduction and determines the genetic constitution of the new individual. Human sex determination is a genetic process that depends basically on the presence of the Y chromosome in the fertilized egg.
This chromosome stimulates a change in the undifferentiated gonad into that of the male a testicle. The gonadal action of the Y chromosome is mediated by a gene located near the centromere; this gene codes for the production of a cell surface molecule called the H-Y antigen.
Further development of the anatomic structures, both internal and external, that are associated with maleness is controlled by hormones produced by the testicle. The sex of an individual can be thought of in three different contexts: Discrepancies between these, especially the latter two, result in the development of individuals with ambiguous sex, often called hermaphrodites.
The phenomenon of homosexuality is of uncertain cause and is unrelated to the above sex-determining factors. It is of interest that in the absence of a male gonad testicle the internal and external sex anatomy is always female, even in the absence of a female ovary. A female without ovaries will, of course, be infertile and will not experience any of the female developmental changes normally associated with puberty.
If X-containing and Y-containing sperm are produced in equal numbers, then according to simple chance one would expect the sex ratio at conception fertilization to be half boys and half girls, or 1: Direct observation of sex ratios among newly fertilized human eggs is not yet feasibleand sex-ratio data are usually collected at the time of birth.
In almost all human populations of newborns, there is a slight excess of males; about boys are born for every girls. Throughout life, however, there is a slightly greater mortality of males; this slowly alters the sex ratio until, beyond the age of about 50 years, there is an excess of females.
Studies indicate that male embryos suffer a relatively greater degree of prenatal mortality, so the sex ratio at conception might be expected to favour males even more than the Firm explanations for the apparent excess of male conceptions have not been established; it is possible that Y-containing sperm survive better within the female reproductive tract, or they may be a little more successful in reaching the egg in order to fertilize it.
In any case, the sex differences are small, the statistical expectation for a boy or girl at any single birth still being close to one out of two. During gestation—the period of nine months between fertilization and the birth of the infant—a remarkable series of developmental changes occur.
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Cell Press has been chosen by The American Society of Human Genetics to publish its premier monthly journal from January The American Journal of Human Genetics (AJHG) is an exciting new venture for Cell Press as its first society-owned journal.
The American Society of Human Genetics (ASHG) and Cell Press anticipate tremendous synergies between AJHG content and that of the 12 . Positions Vacant. Job vacancies can be viewed by following the link below.
To advertise a vacancy please email [email protected] View job board. Special Interest Groups. Australasian Association of Clinical Geneticists (AACG).
The mission of the Human Genetics Institute of NJ (HGINJ) is to operate a world-renowned institute of excellence in human and medical genetics, to collaborate with and enhance research and training programs in the life sciences of Rutgers, and to facilitate scientific relationships with the private sector.
Some use “microbiome” to mean all the microbes in a community. We and others use it to mean the full collection of genes of all the microbes in a community.
The human microbiome (all of our microbes’ genes) can be considered a counterpart to the human genome (all of our genes). The genes in. The Genetics Education Center, for educators interested in human genetics and the human genome project.
Resources on the human genome project, curricula, lesson plans, books, videotapes, and other resources. Information on mentors, genetic conditions, genetic careers, and glossaries.
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