An animal heterozygous for a reciprocal balanced translocation has the following chromosomes:
MN • OPQRST           
MN • OPQRjkl           
cdef • ghijkl           
cdef • ghiST
Among the three segregation patterns, which is least likely to occur? Why?

Verified step by step guidance

1. Understand the problem: A reciprocal balanced translocation involves the exchange of segments between two non-homologous chromosomes. The individual is heterozygous, meaning one set of chromosomes is normal, and the other set carries the translocation. The question asks about the least likely segregation pattern among the three possible patterns.
2. Recall the three possible segregation patterns for a heterozygous reciprocal translocation during meiosis: (1) Alternate segregation, (2) Adjacent-1 segregation, and (3) Adjacent-2 segregation. These patterns determine how the chromosomes separate into gametes.
3. Analyze the segregation patterns: (1) Alternate segregation produces gametes with either normal chromosomes or balanced translocation chromosomes, which are viable. (2) Adjacent-1 segregation involves the separation of homologous centromeres, leading to unbalanced gametes. (3) Adjacent-2 segregation involves the separation of non-homologous centromeres, also leading to unbalanced gametes.
4. Consider the likelihood of each pattern: Alternate segregation is the most common because it ensures balanced gametes. Adjacent-1 segregation is less common but still occurs. Adjacent-2 segregation is the least likely because it requires non-homologous centromeres to separate, which is less favorable during meiosis.
5. Conclude that Adjacent-2 segregation is the least likely to occur because it results in the highest degree of unbalanced gametes and is mechanically less favorable during meiotic division.

Key Concepts

Heterozygous Reciprocal Balanced Translocation

A heterozygous reciprocal balanced translocation occurs when two non-homologous chromosomes exchange segments, resulting in a rearrangement of genetic material without loss. In this scenario, one chromosome carries the original segments, while the other carries the exchanged segments. This can lead to unique segregation patterns during meiosis, affecting gamete viability and genetic diversity.

Segregation Patterns

Segregation patterns refer to the ways in which chromosomes are distributed into gametes during meiosis. In the case of a heterozygous translocation, different segregation patterns can arise, including alternate, adjacent-1, and adjacent-2 segregation. Each pattern has different implications for the genetic composition of the resulting gametes, influencing the likelihood of producing viable offspring.

Viability of Gametes

The viability of gametes is crucial in understanding the outcomes of different segregation patterns. In the context of a reciprocal balanced translocation, certain segregation patterns may produce gametes with unbalanced genetic material, leading to non-viable embryos. Typically, alternate segregation produces viable gametes, while adjacent segregation patterns are more likely to result in aneuploidy, making them less favorable.