Genetics ?

Question

If an autosome is present in only one copy at conception, that individual will not develop to term. Yet human males have only one copy of the X chromosome and they do develop normally. Why the difference? Discuss.

Answer 1

The X chromosome in males is not completely "unpaired" because the Y chromosome is also present.

Because the X chromosome is unpaired in males, any deleterious gene on the X chromosome that a male inherits will be expressed.

In females (in most cells) one of the X chromosomes is inactive. Interestingly, the same X chromosome is not inactivated in all cells (see mosaicism).

The X chromosome carries hundreds of genes but few, if any, of these have anything to do directly with sex determination. Early in embryonic development in females, one of the two X chromosomes is randomly and permanently inactivated in nearly all somatic cells (cells other than egg and sperm cells). This phenomenon is called X-inactivation or Lyonization, and creates a Barr body. X-inactivation ensures that females, like males, have one functional copy of the X chromosome in each body cell. It was previously assumed that only one copy is actively used. However, research suggests that the Barr body may be more biologically active than was previously supposed.

The X-chromosome is a remarkably gene-poor region. It is composed primarily of repeated segments of DNA which do not code for proteins or any known function. Only 1.7% of the chromosome encodes for any functional proteins at all--lowest density of genes to date--and the genes themselves are very short compared to the length of the average human gene. It is estimated that about 10% of the genes encoded by the X-chromosome are associated with a family of "CT" genes, so named because they encode for markers found in both tumor cells (in Cancer patients) as well as in the human Testis (in healthy patients). These CT genes found on the X-chromosome are estimated to account for about 90% of all the CT genes encoded within the human genome. Due to their relative abundance, it is thus hypothesized that these genes (and thus the X-chromosome) confer evolutionary fitness to human males.

Answer 2

Doesn't say much for the "Y" chromosome. It seems that the "X" has so many remarkable genes that it can get by and express itself without it's heterozygous mate. A "Y" chromosome by itself would result in non-development of the zygote.
Perhaps ,that's why the "XX" combination results in an
an organism with a much stronger biological constitution. Human females, for example, can survive the stress of giving birth and live longer than males; Male birds (XX is a male bird!) are larger (and better looking!)than their female counterpart.
One notable comment: In Turner's syndrome (individual has only one X, no homolog, and therefore 45 body chromosomes), the individual develops , is a female, but is not normal.
My answer here is based upon what I recall from past study in biology. Interested in replies from others on this question.

Answer 3

It has to do with the types of genes located on the X and Y chromosomes. Evolution has made sure nothing truly essential, or that needs a double dose, is located on the X chromosome.

Interestingly, calico cat coloring (requires two X chromosomes) is produced from each cell turning off one of the X chromosomes, the gene for color is on the X chromosome so you get variable color.

Answer 4

Good question. I would agree agresivness is a dominant trait , and tempament is inherited from the parents . In saying it is a dominant trait, it also means that this would be the trait inherited from the parent , rather than say the submissive trait from the other parent . As some one else said , sometimes (and only sometimes) genetic traits can be overcome with goood traing etc , other times this is just not the case