The Question is from neet-botany cell-cycle-and-cell-division , and it was created by the author K SURESH KUMAR

Question

In a certain species, a reciprocal translocation has occurred between chromosome 1 and chromosome 2. During meiosis in a heterozygote for this translocation, what structure is MOST likely to be observed during Prophase I?

K SURESH KUMAR · Accepted Answer

## CORE INFORMATION
✓ **Answer**: D
## CONCEPT FRAMEWORK
 **Primary Concept**: Meiosis in individuals heterozygous for reciprocal translocations
 **Related Topics**: Chromosome structure, meiosis I, chiasmata, genetic recombination
 **Prerequisites**: Understanding of meiosis, chromosome structure, and reciprocal translocations.
## SOLUTION PATHWAY
**Given Information**:
  A reciprocal translocation has occurred between chromosome 1 and chromosome 2.
  The organism is a heterozygote for this translocation (meaning it has one normal set of chromosomes and one set with the translocation).
  We are observing Prophase I of meiosis.
**Method & Steps**:
 1. Understanding Reciprocal Translocations:
A reciprocal translocation involves the exchange of segments between two non-homologous chromosomes.  In a heterozygote, one pair of chromosomes will have the normal arrangement, and the other pair will have the translocated segments.
 2. Meiosis I in a Heterozygote:
During Prophase I of meiosis, homologous chromosomes pair up to form bivalents. However, in a heterozygote for a reciprocal translocation, the homologous chromosomes cannot pair normally because of the exchanged segments.  Instead, they form a complex structure involving all four chromosomes (two homologous pairs).
 3. Formation of a Quadrivalent:
To allow for pairing, the chromosomes form a quadrivalent structure.  This involves the formation of chiasmata (crossovers) between the translocated segments and the normal segments to ensure proper segregation (or at least attempt to) during anaphase I.
  Working: The four chromosomes involved (two normal and two translocated) interact to create a complex structure.
  Key Point:  The quadrivalent is crucial for allowing some degree of proper segregation of the chromosomes, although it often leads to unbalanced gametes.
 4. Observation during Prophase I:
The most likely structure to be observed in Prophase I is the quadrivalent, a four-chromosome structure formed due to the reciprocal translocation.
  Result: A quadrivalent
  Verification: The formation of a quadrivalent is a direct consequence of a reciprocal translocation in a heterozygote during meiosis I.
 ## OPTION ANALYSIS
A :x: A bivalent is formed by the pairing of two homologous chromosomes. This would only occur if the translocation wasn't present.
B :x: A univalent is a single unpaired chromosome. This is less likely in this case as the chromosomes will attempt to pair.
C :x: A trivalent is a group of three chromosomes paired together. This is not typical for reciprocal translocations.
D ✓: A quadrivalent (tetravalent) is a structure formed by the pairing of four chromosomes, which is the expected outcome of a reciprocal translocation heterozygote during meiosis I.
 ## LEARNING AIDS
 **Common Mistakes**:
  1.Confusing reciprocal translocations with other chromosomal rearrangements:  Understanding the specific nature of a reciprocal translocation is crucial.
  2.Assuming normal bivalent formation:  Recognizing that the translocation disrupts normal pairing is key.
 **Quick Tips**:
  1.Remember that in a reciprocal translocation heterozygote, four chromosomes are involved in pairing during meiosis I.
  2.Visualize the structure: Drawing a diagram of the four chromosomes and their pairing can help understand the formation of the quadrivalent.

K SURESH KUMAR · Answer

A bivalent

K SURESH KUMAR · Answer

A univalent

K SURESH KUMAR · Answer

A trivalent

K SURESH KUMAR · Answer

The correct Option is "A quadrivalent"

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