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The following pedigree is for myopia (nearsightedness) in humans.
Predict whether the disorder is inherited as the result of a dominant or recessive trait. Determine the most probable genotype for each individual based on your prediction.
HINT: This problem involves the analysis of a pedigree to deter-mine the mode of inheritance of myopia. The key to its solution is to identify whether or not there are individuals who express the trait but neither of whose parents also express the trait. Such an observation is a powerful clue and allows you to rule out one mode of inheritance.
Let's consider an inheritance pattern of autosomal dominant inheritance. This occurs when the altered allele is dominant over average, and a single copy is enough to express the inherited trait, the father (blue box) in this case. Being autosomal, it should affect children equally, male or female. So, in this case, the hereditary pattern is not autosomal dominant.
Autosomal recessive inheritance occurs when the altered allele is recessive over normal so that a single copy of the modified allele of myopia is not expressed. Being autosomal, the gene is found in one of the autosomes, affecting sons and daughters equally. To express the trait, the altered allele has to be inherited from both the father and the mother. In this case, the inheritance could be autosomal recessive because the father (blue box) is homozygous recessive and expresses the disease. The mother (open circle) is unaffected but carriers of the trait. There is a 50% probability that a descendant could be affected, male or female, and the rest of the offspring will be carriers without expressing the condition (open boxes and circles).
Likewise, the inheritance diagram could not respond to an X-linked dominant trait because not all pedigree men are affected, which excludes the possibility that the gene causing the condition is located on the X chromosome, which in males comes from the mother. It cannot be linked to Y because there are daughters affected.
And finally, in recessive inheritance linked to the X, the altered allele is recessive. The condition is not expressed with a single copy of the modified allele, and the gene is on the X chromosome. It usually occurs more frequently in men since they have only one X chromosome, so if they inherit the mutated allele, they will express the trait, like the blue box in the diagram. However, x-linked recessive inheritance does not respond to the inheritance pattern in this case. Women having two X chromosomes will only inherit a mutated allele from the father and will be carriers but not show the condition.