Sex Chromosome
In female pigeons it was discovered that all chromosomes were paired except one. This very fact - a single (not paired) chromosome - gives rise to a female. (Figure 1.)
When the same chromosome is paired the offspring results in a male. This chromosome has been labeled as the "sex chromosome". (Figure 2.)
Because this chromosome is linked to the sex of the individual (single produces a female; paired produces a male) its genes are called "sex-linked genes". The sex of some individuals can often be determined by the expression of its sex-linked gene(s). (Keep in mind that the characteristics of an individual are the end result of the action of a gene, either acting alone or in combination with other genes.)
Figure 1.
Female with un-paired sex chromosome.
Showing a single chromosome which gives rise to a female; and the
eventually
discarded polar body.
Figure 2.
Male with paired sex chromosomes.
Showing
the same chromosome in Figure_1; but paired; therefore, giving
rise
to a male offspring.
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