Alleles are different versions of a gene encoding similar, but distinct, versions of a gene product.
An allele is considered dominant if the trait it encodes is expressed when only one copy of the allele is present. A recessive allele will only be expressed if there are two copies of that allele.
Animal cells, such as humans and flies, have autosomes and sex chromosomes. Most of the chromosomes are autosomes, while the single pair of sex chromosomes determine the sex of the individual.
A sex-linked gene is a gene that encodes a product that has nothing to do with sex determination, but is found on one of the sex chromosomes. This is most frequently the X chromosome.
Males have one X chromosome, while females have two. If a mutation is present in a gene contained on the single X chromosome a male has, then he will express the mutant trait. A female is less likely to inherit the two copies of the allele necessary to express the trait. This is not impossible, just less frequent in a population.
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