Glaucoma trait

Question

A women who is homozygous normal for vision and a man who is homozygous for glaucoma have children. What is the possibility that any of their children will have glaucoma?

Answer 1

Hi JoAna!

Let's give the Glaucoma gene a letter. I'll choose a little g to demonstrate that the gene is recessive. Normal vision can be written as a big G. So:

Normal vision gene: G

Glaucoma gene: g

Homozygous means that both copies of the gene are the same (either two dominant or two recessive).

Woman who is homozygous normal: GG

Man who is homozygous for glaucoma: gg

Now, if I had a white board over Zoom I could show you this better but we can do a Punnett-square to determine what the genetic makeup of the offspring would be. You draw a square and divide that square into four smaller squares. Put the genes of one parent on top, and the genes of the other parent on one side. Now you just take one gene from each parent and carry it over into each small box (representing a possible offspring).

G G

__________________

| | |

g | G g | Gg |

|________ _|________ |

| | |

g | Gg | Gg |

|________ _|________ |

Recessive traits can only be expressed if the child has two copies of the recessive gene (like gg). You can see in the Punnett Square that there is a 0% possibility of any of the children receiving two copies of the recessive gene for glaucoma. Therefore, there is a 0% possibility that any of their children will inherit the glaucoma trait.