Sex Linked Inheritance
Chromosome pairs that are the same for males and females are called autosomal chromosomes. Chromosomes that differ between sexes are called sex chromosomes. In most animals, with the exception of birds, females have two X chromosomes, and males have an X and a Y chromosome.
Some genes are sex-linked genes, meaning that they are inherited with the sex chromosomes. Genes carried on the X chromosome are called X-linked genes, and are generally denoted by an X with a superscript gene symbol. These genes are not present on the male Y chromosome.
Red-green colorblindness is caused
by a recessive, X-linked allele, which is denoted as
X- in this animation, while the
dominant allele for normal vision is denoted as X. Note
that, since a male has only one X
chromosome, his phenotype will be determined by the allele
on that chromosome.
Thus, an
X-Y male is colorblind.
The first mating seen here is between a homozygous normal female (XX) and a colorblind male (X-Y). As you can see from the resulting Punnett Square, all of the females will inherit a normal X from their mother and a colorblind X- from their father, and thus will be heterozygous carriers of the trait. Since the sons inherit a normal X from their mother, and a Y from their father, they will also all have normal vision.
The last two parts of this animation show the results from crosses between carrier females and normal males (2), or between carrier females and colorblind males (3). Note that the phenotypic ratios of male progeny can differ from those seen in their female siblings when we are dealing with an X-linked trait.