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Question Video: Completing a Punnett Square to Calculating the Probability of a Homozygous Dominant Genotype Being Inherited Biology

The allele that gives mice yellow fur (Y) is dominant to the allele that gives brown fur (y). However, if a mouse embryo inherits two dominant alleles, it will die in utero. Complete the Punnett square provided and state the probability that an offspring born to these parents will die in utero.

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Video Transcript

The allele that gives mice yellow fur, uppercase Y, is dominant to the allele that gives brown fur, lowercase y. However, if a mouse embryo inherits two dominant alleles, it will die in utero. Complete the Punnett square provided and state the probability that an offspring born to these parents will die in utero.

Let’s begin by reviewing some basic genetics. Genes are regions of DNA that code for functional units like proteins. Genes can exist in different forms called alleles, which can be dominant or recessive. So what determines the combination of alleles that we have for each of our genes? Here, we can see matching pairs of chromosomes, one from each of our parents. All humans have 23 pairs of chromosomes. Each of our parents provide us with one chromosome from each pair.

Let’s take an example of a gene that we’ll call P. We’ll represent the dominant allele with an uppercase P and the recessive allele with a lowercase p. In this example, the offspring received a recessive allele from their biological father and a dominant allele from their biological mother for the highlighted gene. The offspring’s combination of alleles will be uppercase P lowercase p. This is called the genotype, while the observable trait that this genotype causes is called the phenotype.

In this question, we’re being asked to complete a Punnett square, which can help us figure out the possible genotypes of an offspring with respect to the gene that controls the color of fur in mice. This Punnett square has alleles along the top row and along the left-hand column that represents the alleles that each parent has. The four boxes will give us the possible combinations of the alleles or genotypes that an offspring of these parents could have. We can fill in each of these boxes using the corresponding alleles from the top and left side like this. Remember, the yellow fur allele, uppercase Y, is dominant to the brown fur allele, lowercase y.

The question tells us that if a mouse has two dominant alleles, it will die in utero. This means that the fetus will die before it can be born. We can see that the pink square indicates that the mouse will die in utero. The question has asked us to find out the probability that an offspring born to these parents will die in utero. This will happen one out of four times. To work out the probability that this offspring will die in utero, we multiply one divided four by 100 percent. This means that the probability that the offspring born to these parents will die in utero is 25 percent.

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