• Question: I had a question in a science class once that said a small community had a recessive lethal gene that has been in the community for generations. A woman then has offspring that, while she herself does not have the disease, the offspring do. And then it asked "Which woman would it be" or something like that, and the answer was the heterozygous woman. What would be a good way to figure that out besides a Punnett Square?

    Asked by Markimoo to Candace, Jenn, Matthew, Sahra, Sharon on 10 Mar 2017.
    • Photo: Matthew Bainbridge

      Matthew Bainbridge answered on 10 Mar 2017:

      Because the disease is lethal, you know no adult can be homozygous or compound heterozygous. Because you know it’s a small community, it’s much more likely that everyone shares the same mutation (called a founder effect) because everyone is *kind of* related to everyone else. So our first assumption would be that both parents are heterozygous for the same mutation and the affected child is homozygous.

    • Photo: Jennifer Hintzsche

      Jennifer Hintzsche answered on 10 Mar 2017:

      Well Punnett Squares are awesome and I love them. You would be able to tell it was the woman because half of her offspring will have the recessive lethal gene. Depending on who they mate with (the dad) the kids could have two copies of the lethal gene- so they wouldn’t survive. Or they could have one copy of the lethal gene so they will pass that on to their kids depending on if their mate also has that lethal gene. If this stays within a community Matt is right that is the founder effect and means that people in this town usually don’t leave and only mate within the community so everyone ends up being kind of related over many many generations and some will carry this lethal gene as adults that are heterozygous. Any time two heterozygous adults mate they will have a 25% chance of their child getting two lethal copies.

    • Photo: Xueyuan Jiang

      Xueyuan Jiang answered on 10 Mar 2017:

      Besides Punnett Square, you would know that this particular offspring has two lethal alleles because he/she is sick. And we know that he/she gets one allele from dad and the other allele from mom. Then, we know for sure the woman has a lethal allele. Since she is perfectly healthy, she must also have a dominant allele, which makes her heterozygous.

    • Photo: Sahra Uygun

      Sahra Uygun answered on 14 Mar 2017:

      I like the pedigrees in the genetics class! Tracing the genes across family members 🙂
      I loved the answers from other scientists. So you are dealing with a recessive gene, therefore the individual needs to have 2 copies of the disease gene (let’s say bb) to see the disease. So if the offspring has the disease, it means its genotype is bb, one b gene from the mother and another b from the father. If the mother do not have the disease, then it means she definitely has a normal gene B, which is dominant over the disease gene b.