Question Video: Determining the Missing Genotype in a Punnett Square | Nagwa Question Video: Determining the Missing Genotype in a Punnett Square | Nagwa

Question Video: Determining the Missing Genotype in a Punnett Square Biology • First Year of Secondary School

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Huntington’s disease is an inherited disease caused by a dominant allele (H). The Punnett square shows the genotypes of a male and a female and the predicted genotypes for their offspring. What is the missing genotype?

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Video Transcript

Huntington’s disease is an inherited disease caused by a dominant allele uppercase H. The Punnett square shows the genotypes of a male and a female and the predicted genotypes for their offspring. What is the missing genotype?

This question discusses Huntington’s disease, a condition that begins in midlife and causes progressive nerve damage. This disease is caused by the inheritance of a dominant allele, designated here by the uppercase letter H, since dominant alleles are typically denoted by uppercase letters. An allele refers to different variations of a particular gene. Dominant alleles are always expressed if they are inherited. This contrasts with recessive alleles, which require two copies, one inherited from each parent, in order to be expressed.

We can see that this Punnett square contains the genotypes of the mother and father of these offspring. The mother has a dominant allele, so apparently she has or will likely develop Huntington’s disease, while the father, with two recessive alleles, does not have Huntington’s disease. We may remember that a parent’s two alleles for a gene will split up when gametes are formed so that only one allele for each gene will be passed on to offspring. And when the single alleles from each parent combine, we get the genotype of the offspring.

The Punnett square is a tool that helps us quickly determine how the alleles from the parents may be combined. When filling out a Punnett square, we combine the letter from the row header of each square with the letter of the column header. Three possibilities in this Punnett square have already been filled out. If we fill out this last square in the grid in the same way by combining the row and column header letters, we should get our answer, which is uppercase H lowercase h. The missing genotype is uppercase H lowercase h.

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