Genetics as it applies to evolution, molecular biology, and medical aspects.
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From what i have searched,
Hemizygous is having one copy of a gene in a diploid cell.
I heard that women are just as hemizygous as men but i thought that wasn't the case. Because men have XY chromosome making them hemizygous while women have XX making them homozygous. My search for an answer confused me even more. Can someone with knowledge on this shed some light for me.
To clarify some of the terminology: female humans are referred to as homogametic - they have only one type of sex chromosome, the X. Males are heterogametic, having two different types of sex chromosomes, the X and the Y. In birds and some other animals, females are heterogametic, and males are homogametic. An earlier post implied that human females are homogygous for the X chromosome, which is not true barring incest and no recombination or spontaneous mutation through a couple of generations. Homo-, hetero-, and hemizygosity usually refer to genes or some other defined but limited stretch of DNA rather than entire chromosomes, although the term hemizygous is often used to describe the Y chromosome as a whole. Homozygous means two identical copies of a gene. Heterozygous means two different copies of a gene. Hemizygous means that only one copy is present.
Darby, my experience is that many students (and graduates) are not aware of dosage compensation, so statements that because females are XX they are homozygous for the X chromosome is misleading to them. Your statement that females are effectively hemizygous is accurate. Even though one X is inactivated, the other X is still there, so saying that the individual, or the cell, is homozygous for the X chromosome is inaccurate. Hence my attempt at clarification. Also, not all genes on the Y chromosome are hemizygous, so saying that the Y chromosome is hemizygous, in the individual, or the cell, is also not accurate. So, to address PoOr's original question of relative hemizygosity, effectively, women are more hemizygous than men, but genotypically, men are more hemizygous. I hope this makes my posting clearer. Now, considering imprinting...
Ok, so in english, basicly women are more hemizygous then men because even though they have 2 Xs but one is inactive so really its just 1 active chromosome in diploid cell, While men are less hemizygous then women because even though they have 2 different type of Chromosome XnY, they both are genetically active.
Is that the basics of why women are just as or more hemizygous then men?
"While men are less hemizygous then women because even though they have 2 different type of Chromosome XnY, they both are genetically active."
Yes, partly. The entire active X chromosome in women is effectively hemizygous because the other X is inactivated. On the Y chromosome, though, some genes have no homologue on the X, so they are hemizygous. Other genes on the Y, though, do have homologues on the active X, and so are not hemizygous (whether they are homo- or heterozygous in an individual or cell is irrelevant here - they are not hemizygous). So because some genes on the Y have X homologues, women are effectively more hemizygous than men. Does this help?
11 posts • Page 1 of 1
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