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You are consulting with a family where sometimes a mutation that leads to deafness (D) and a mutation that predisposes individuals to early heart failure (H) are linked.  Normally these two traits sort independently but, in some individuals, a reciprocal translocation has moved them to the same chromosome. The chromosome maps for an individual with two normal chromosome pairs (Normal Arrangement), and a translocation heterozygote (Translocation Arrangement) are shown below.

Deafness and early heart failure are both inherited in a dominant fashion (with respect to hearing and normal heart function). However, deafness is normally present at birth and early heart failure occurs on average at 40 years of age. A pedigree of the family is shown below.

Below are chromosome maps (centromeres shown as blue dot) and pedigrees. Use what you know about translocation heterozygotes, and the chromosome maps and pedigrees, to answer the questions that follow. Note: The images are also in a separate PDF file (with text descriptions) posted under the Online Tutorial #2 link on Moodle.

Pedigree description:

Three generations are shown.

In generation I: I-1 and I-2 have mated. I-1 has died. She is deaf and had early heart failure. I-2 is male, still alive, not deaf and no signs of early heart failure.

In generation II: Children of I-1 and I-2 are shown. These children are: II-2 (male), II-3 (female), II-4 (male) and II-5 (male). II-2 is deaf, no signs of early heart failure; II-3 is deaf with early heart failure, II-4 has died and was deaf with early heart failure; II-5 has died and was deaf with early heart failure.

II-1 is a female without deafness or early heart failure. She has mated with II-2. II-6 is a male without deafness or early heart failure. She has mated with II-5.

In generation III: The children of II-1 and II-2 are shown. They are III-1 and III-2; two females, one is deaf and one is not; both have no signs of early heart failure.

The child of II-5 and II-6 is shown (III-3). III-3 is a deaf male with no signs of early heart failure. III-3 has partnered with III-5 (a female without deafness or early heart failure).

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III-3 and III-5 (from the pedigree) have been having fertility difficulties. What is the most likely explanation?

Select one:

a. III-3 and III-5 are both translocation heterozygotes and their gametes are incompatible.

b. III-3 is a translocation heterozygote and many of the gametes they form during meiosis are non-viable.

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Deafness is normally present at birth and early heart failure occurs on average at 40 years of age. III-3 (from the pedigree) is only 24. He could have the H allele that leads to heart failure but be too young for the phenotype to be seen. Based on the pedigree, chromosome maps, and the fact that III-3 and III-5 are having fertility difficulties, is III-3 (from the pedigree) likely to suffer early heart failure later in life?

Select one:

a. III-3's parents are likely both heterozygotes so there is a 1/4 chance he'll suffer from early heart failure.

b. One of III-3's parents is a heterozygote for deafness and early heart failure (both dominant traits), and the other is homozygous recessive for both. Therefore, at most, III-3 has a 1/2 chance of early heart failure.

c. If III-3's fertility difficulty is due to being a translocation heterozygote then he will most likely suffer from early heart failure.

d. If III-3's fertility difficulty is due to being a translocation heterozygote then he is unlikely to suffer from early heart failure.

Answer 1

ANSWER 1 :-

1. Lets consider there are two chromosomes which are non-homologous or may be having different gene sequences which are not similar for both the chromosomes.
2. When there is an interchange of segment between both the chromosomes, it is called as Translocation. As the event occurs with respect to the two non-homologous chromosomes, it is being termed as Reciprocal Translocation.
3. In the above mentioned case, the phenomena which is seen is translocation heterozygotes which is characterized by formation of a fraction of gametes to be non-viable or which are not able to survive. This is usually characterized by formation of two chromosomes which will have a normal appearance and the remaining two will show the reciprocal translocation thereby rendering them non-viable.
4. Thus, the possible chances for this fertility problem is due to the formation of non-viable gametes during the process of meiosis and hence option 'B' is the most appropriate answer.
5. Incompatibility usually arises in cases such as polyploidy wherein the number of chromosomes in the mating partners is different and usually marks the process of non-disjunction. So, this is the reason why option 'A' is not appropriate.

So, the most appropriate answer is option 'B'.

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