Video Transcript
Duchenne muscular dystrophy, DMD, is an X-linked recessive condition in humans that causes muscle weakness and wasting. The allele that correctly produces the dystrophin protein, uppercase D, is dominant to the allele that causes DMD, lowercase d. A female with the genotype X uppercase D X lowercase d reproduces with a male with the genotype X uppercase D Y. What is the probability, in percent, that the offspring will be a male with the disease?
The question describes a condition called Duchenne muscular dystrophy, or DMD as we’ll be referring to it throughout this video. We’re told that DMD is an X-linked recessive condition or trait. We know that there are two types of sex chromosomes, X and Y. X-linked traits are caused by genes found only on the X chromosome. So, the gene that causes DMD will be found only on the X chromosome, since the Y chromosome contains a different set of genes.
We know that recessive traits are only expressed when the dominant allele is absent. So, individuals will only be affected by DMD if they have no copies of the uppercase D allele. This dominant allele codes for the correct structure of dystrophin, a protein that strengthens muscles and protects them from injury. In most cases, as long as one copy of the dominant allele is present, dystrophin will be made normally.
The recessive allele contains a mutation for dystrophin production. This mutation leads to a lack of dystrophin, causing abnormal muscle growth and function. Biological females have two X chromosomes. So, if an individual female carries a recessive allele for DMD on one X chromosome and a dominant allele on the other, the dominant allele will mask the recessive allele, allowing for normal dystrophin production and muscle function. We call individuals of this genotype carriers of DMD because they carry the recessive allele but typically are unaffected by the condition due to the presence of the dominant allele.
Biological males have one X chromosome and one Y chromosome. So, if an individual male carries the recessive allele on its X chromosome, they will be unable to produce enough dystrophin for normal muscle function. Now that we’ve seen how DMD is inherited as an X-linked recessive condition, let’s return to our question.
We’re given two genotypes and asked to find the probability or mathematical chance that the offspring will be a male with DMD. Let’s use a Punnett square to help us visually represent the cross of the two genotypes. We’ll add the female genotype to the top of our Punnett square and the male genotype to the side of our Punnett square. Now, let’s fill in our Punnett square by crossing the genotypes for each row and column.
Each square now contains the genotype that would come from combining a potential female gamete or egg with a potential male gamete or sperm. Next, we need to find the probability that the offspring will be a male with the disease. The four different genotypes from our Punnett square are an unaffected female, a carrier female, an unaffected male, and an affected male, who will have DMD. So, one out of four offspring or one-quarter will be a male with DMD.
The question asked us to give our answer as a percent, so let’s multiply by 100 and express the result as a percentage. The probability that the offspring of these two genotypes will be a male with DMD is 25 percent.
