Sex-Linked & Multiple Alleles

Sex-linked – when phenotype is located on sex chromosome

example → White eyes are sex-linked in fruit flies. Predict the possible phenotypes and genotypes of the offspring of a heterzygous red-eyed female with a red-eyed male.


mom\ -\ X^{R}X^{r}

dad\ -\ X^{R}Y


                • 50% red-eyed female
                • 25% red-eyed male
                • 25% white-eyed male (X^{r}Y)

Multiple alleles – a gene with more than 2 alleles.

example → blood type – 3 alleles that determine blood type:

            • I^{A}\ A-type
            • I^{B}\ B-type
            • i\ O-type\rightarrow recessive\ -\ needs\ to\ be\ homozygous\ to\ show


What are the chances of a woman with AB blood and a man with A blood having a child with O blood?


mom\ -\ I^{A}I^{B}

dad\ -\ I^{A}I^{A}\ or\ I^{A}




Whether the dad is homo or heterozygous, there is no chance of the child having O-type blood.



Polygenic traits – traits where 2 or more genus interact to produce the phenotype.