Question

If you knew that a locus that affected earlobe shape was tightly linked to a locus that affected susceptibility to cardiovascular disease in humans, would a test cross of a dihybrid for these genes result in a 1:1:1:1 phenotypic ratio? Under what circumstances would this information be clinically useful? Why would the dihybrid test cross be problematic from an ethical stance?

Answer 1

Ans: To answer in a single sentence would be no, you should not get a 1:1:1:1 phenotypic ratio if the gene pair are linked, but that would not help you. So, let us briefly discuss how you get a 1:1:1:1 ratio from a test cross. If you understand Mendel's Independent assortment it states that gene pairs, if not linked, (different chromosomes) assort independently at meiosis. Hence in this case, if the ear-lobe gene and the C gene (C for cardiovascular susceptible disease gene) are on separate chromosomes then upon test cross you will get the ratio but since the question states clearly that they are tightly linked hence we arrive at the above conclusion.
Actually what happens is if you take the F₁ hybrid (E C; e c) and cross it with a recessive parent (e c) then what would happen is during meiosis the F₁ hybrid would produce gametes and if the gene pairs are not linked then they can combine with each other in a number of ways and can produce 4 types of gametes which are the basis of the 1:1:1:1 ratio, but if they are linked they can only produce two types hence no 1:1:1:1 ratio.

Think about it you know that the genes are linked then it makes it so much easier for detecting whether a person has inherited the disease. For your information, I can say that geneticists always rely on this type of technique where if they can somehow identify a disease-causing gene linked to another gene that can be phenotypically identified. This removes all types of time-consuming tests and helps in quick treatment if required but mostly the family in which the disease runs can know which of their family members has inherited the disease.

The dihybrid cross is very helpful but not applicable in the case of human beings since conducting such a test would break all ethical values. Unlike plants and rodents, human beings are not subjected to experimentation and you can probably understand that to identify a person with a disease that person can't mate with a double recessive partner.

I hope this clears your doubt without me violating any rules, but feel free to refer to the comment section if you don't understand any part & I will get back to you.