Human Fertilization Porritt R The symmetry-breaking of egg and sperm see also p emphasizes the polarization between the sexes Although we perceive sex as a simple division into male and female rooted in fertilization, sexuality is a complex interwoven relationship with many forms of interaction more subtle and pervasive than the raw sexual urge. To fully understand sex we thus need to appreciate this deep relationship between sex and the warp and weft of entanglement.
During meiosis It was the behaviour of chromosomes during meiosis, however, that provided the strongest evidence for their being the carriers of genes. In American scientist Walter S. Sutton reported on his observations of the action of chromosomes during sperm formation in grasshoppers.
Sutton had observed that, during meiosiseach chromosome consisting of two chromatids becomes paired with a physically similar chromosome. These homologous chromosomes separate during meiosis, with one member of each pair going to a different Sex linked traits through meiosis and how.
It is now known that the number of chromosomes within the nucleus is usually constant in all individuals of a given species—for example, 46 in the human40 in the house mouse8 in the vinegar fly Drosophila melanogaster; sometimes called fruit fly20 in corn maize24 in the tomatoand 48 in the potato.
In sexually reproducing organisms, this number is called the diploid number of chromosomes, as it represents the double dose of chromosomes received from two parents. The nucleus of a gamete, however, contains half this number of chromosomes, or the haploid number. Thus, a human gamete contains 23 chromosomes, while a Drosophila gamete contains four.
Meiosis produces the haploid gametes. The essential features of meiosis are shown in the diagram. For the sake of simplicity, the diploid parent cell is shown to contain a single pair of homologous chromosomes, one member of which is represented in blue from the father and the other in red from the mother.
At the leptotene stage the chromosomes appear as long, thin threads. At pachytene they pair, the corresponding portions of the two chromosomes lying side by side.
The chromosomes then duplicate and contract into paired chromatids. At this stage the pair of chromosomes is known as a tetrad, as it consists of four chromatids. Also at this stage an extremely important event occurs: This exchange process, called crossing overresults in chromatids that include both paternal and maternal genes and consequently introduces new genetic combinations.
The first meiotic division separates the chromosomal tetrads, with the paternal chromosome whose chromatids now contain some maternal genes going to one cell and the maternal chromosome containing some paternal genes going to another cell. During the second meiotic division the chromatids separate.
The original diploid cell has thus given rise to four haploid gametes only two of which are shown in the diagram. Not only has a reduction in chromosome number occurred, but the resulting single member of each homologous chromosome pair may be a new combination through crossing over of genes present in the original diploid cell.
Behaviour of chromosomes at meiosis. Suppose that the red chromosome shown in the diagram carries the gene for albinismand the blue chromosome carries the gene for dark pigmentation.
It is evident that the two gene alleles will undergo segregation at meiosis and that one-half of the gametes formed will contain the albino gene and the other half the pigmentation gene. Following the scheme in the diagram, random combination of the gametes with the albino gene and the pigmentation gene will give two kinds of homozygotes and one kind of heterozygote in a ratio of 1: The same is true of the second law, that of independent assortment.
Since maternal and paternal members of different chromosome pairs are assorted independently, so are the genes they contain.
This explains, in part, the genetic variety seen among the progeny of the same pair of parents.
As stated above, humans have 46 chromosomes in the body cells and in the cells oogonia and spermatogonia from which the sex cells arise. At meiosis these 46 chromosomes form 23 pairs, one of the chromosomes of each pair being of maternal and the other of paternal origin. Independent assortment is, then, capable of producingor 8,, kinds of sex cells with different combinations of the grandmaternal and grandpaternal chromosomes.
The population of the world is now more than 6 billion persons, or approximately persons. It is therefore certain that only a tiny fraction of the potentially possible chromosome and gene combinations can ever be realized.
Yet even is an underestimate of the variety potentially possible. The grandmaternal and grandpaternal members of the chromosome pairs are not indivisible units.
Each chromosome carries many genes, and the chromosome pairs exchange segments at meiosis through the process of crossing over. This is evidence that the genes rather than the chromosomes are the units of Mendelian segregation.
Linkage of traits Simple linkage As pointed out above, the random assortment of the maternal and paternal chromosomes at meiosis is the physical basis of the independent assortment of genes and of the traits they control.
This is the basis of the second law of Mendel see the section Mendelian genetics above. The number of the genes in a sex cell is, however, much greater than that of the chromosomes. When two or more genes are borne on the same chromosome, these genes may not be assorted independently; such genes are said to be linked.
When a Drosophila fly homozygous for a normal gray body and long wings is crossed with one having a black body and vestigial wings, the F1 consists of hybrid gray, long-winged flies see the figure.Prophase I is the first step in meiosis I.
And a lot happens during this step, some of which is a departure from what we saw in mitosis. During prophase I, the chromatin condenses. Humans have 22 chromosome pairs and two sex chromosomes. Females have two X chromosomes; males have an X chromosome and a Y chromosome.
The inheritance of sex-linked traits through meiosis is where chromosomes are replicated. This relates to genetics in that the traits are passed on to divided cells from the parent cell.
One of the basic properties of life is reproduction, the capacity to generate new individuals, and sex is an aspect of this process. Life has evolved from simple stages to more complex ones, and so have the reproduction mechanisms. acquired trait: A phenotypic characteristic, acquired during growth and development, that is not genetically based and therefore cannot be passed on to the next generation (for example, the large.
Pearson, as an active contributor to the biology learning community, is pleased to provide free access to the Classic edition of The Biology Place to all educators and their students.