Which statement is true about sex-linked genes? * 1 point The male needs two copies of the gene, but will show the trait whether the alleles are dominant or recessive The male needs two copies of the gene, but will show the trait only if both alleles are dominant The male needs only one copy of the gene and will show the trait whether the alleles are dominant or recessive The male needs only one copy of the gene, but will show the trait only if the allele is dominant.
Answers
Sex chromosomes in humans
Human X and Y chromosomes determine the biological sex of a person, with XX specifying female and XY specifying male. Although the Y chromosome contains a small region of similarity to the X chromosome so that they can pair during meiosis, the Y chromosome is much shorter and contains many fewer genes.
To put some numbers to it, the X chromosome has about 800-900800−900 protein-coding genes with a wide variety of functions, while the Y chromosome has just 60-7060−70 protein-coding genes, about half of which are active only in the testes (sperm-producing organs)^{1,2,3,4}
1,2,3,4
.
Diagram of the human X and Y chromosomes. The X is much larger than the Y. The X and Y have small regions of homology at both tips, which allow pairing of the chromosomes during meiosis. The SRY gene is found on the Y chromosome, near the tip, just below the region of homology with the X chromosome.
Diagram of the human X and Y chromosomes. The X is much larger than the Y. The X and Y have small regions of homology at both tips, which allow pairing of the chromosomes during meiosis. The SRY gene is found on the Y chromosome, near the tip, just below the region of homology with the X chromosome.
Image based on ideograms from the Genome Decoration Page, maintained by the U.S. NCBI.
The human Y chromosome plays a key role in determining the sex of a developing embryo. This is mostly due to a gene called SRY (“sex-determining region of Y”). SRY is found on the Y chromosome and encodes a protein that turns on other genes required for male development^{5,6}
5,6
.
XX embryos don't have SRY, so they develop as female.
XY embryos do have SRY, so they develop as male.
In rare cases, errors during meiosis may transfer SRY from the Y chromosome to the X chromosome. If an SRY-bearing X chromosome fertilizes a normal egg, it will produce a chromosomally female (XX) embryo that develops as a male^7
7
. If an SRY-deficient Y chromosome fertilizes a normal egg, it will produce a chromosomally male embryo (XY) that develops as a female^8
8
.
[Do all species with X and Y chromosomes have the SRY gene?]
^9
^{10}
^{11}
1:22:2^{12}
X-linked genes
When a gene being is present on the X chromosome, but not on the Y chromosome, it is said to be X-linked. X-linked genes have different inheritance patterns than genes on non-sex chromosomes (autosomes). That's because these genes are present in different copy numbers in males and females. [What about genes on the Y?]
Since a female has two X chromosomes, she will have two copies of each X-linked gene. For instance, in the fruit fly Drosophila (which, like humans, has XX females and XY males), there is a eye color gene called white that's found on the X chromosome, and a female fly will have two copies of this gene. If the gene comes in two different alleles, such as \text X^WX
W
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