Sex-linked traits are traits determined by genes on the X or Y chromosomes. For example, colorblindness and baldness are found on the X chromosome. In humans, a Y chromosome determines male sex, so traits on this chromosome only affect men. Traits on the X chromosome can be expressed in men, but women need two copies of the gene to show traits as the second X chromosome provides backup. If the mother is bald, both sons and daughters will always be carriers and sons will be bald, while if the father is bald, only daughters can be carriers and sons will not be bald.

1. Sex-Linked Traits X chromosome Y chromosome Boy or Girl? The Y Chromosome “Decides”

5. Punnett Square: What sex will the offspring be? 50% chance of a male or a female child.

6. Baldness is carried by the mother Phenotype: 25% bald males 25% bald carrier females 25% not bald males 25% non-carrier females

7. If Dad is bald, will you be bald? Phenotype: 0% bald males 100% bald carrier females

8. What if Mom is bald? Phenotype: 100% carrier females 100% bald males