28 N SexLinked Traits SexLinked Traits Traits controlled

28 N Sex-Linked Traits

Sex-Linked Traits • Traits controlled by a gene found on the sex chromosome • The Y chromosome is shorter than X; therefore, some genes on the X chromosome may be missing from Y • Sex-linked disease is less likely to affect females because they have 2 X chromosomes

Sex-Linked Traits • Carrier = an individual who is heterozygous for a recessive, disease-causing allele. The carrier of the allele usually shows no symptoms of the disease but can pass the defective allele on to his or her children. – Females can be carriers of sex-linked traits because they can be heterozygous for the recessive trait

Sex-Linked Traits • Example 1: Colorblindness = can’t tell the difference between red and green • Caused by a defective allele C = not colorblind c = colorblind

Sex-Linked Traits Cross a normal vision female with a colorblind male c C X XC X Y c C X X C X Y Phenotype Ratio: 50% female carrier of colorblindness 50% normal vision male

Sex-Linked Traits • Example 2: Hemophilia = the person’s blood is unable to clot and bleeding will not stop H = does not have hemophilia h = has hemophilia

Sex-Linked Traits Cross a female carrier of hemophilia with a hemophiliac man Xh Y XH XHXh X HY Xh X h. Y X h Xh Phenotype Ratio: 25% female carrier of hemophilia 25% hemophiliac female 25% normal male 25% hemophiliac male

Sex-Linked Traits • Example 3: Male Baldness Cross a normal female with a bald male Xb Y XB XB X b XB Y B = not bald b = bald Phenotype Ratio: 50% female carrier of baldness 50% normal male