Consider a diploid organism with the genotype AaBb, where genes A and B are located on sep… — JAMB UTME

Consider a diploid organism with the genotype $AaBb$ , where genes $A$ and $B$ are located on separate homologous chromosome pairs. During meiotic prophase I, crossing over does not occur between these loci. Which of the following statements correctly accounts for the relationship between gene loci, alleles, and sister chromatids during gamete formation in this organism?

The alleles $A$ and $a$ occupy identical loci on homologous chromosomes, whereas identical copies of allele $A$ exist on sister chromatids prior to anaphase I segregation.Answer
B
The alleles $A$ and $a$ represent alternative structural forms of the same gene located on sister chromatids of a single replicated chromosome.
C
The phenotype expressed by the heterozygous state $Aa$ is always an exact intermediate blending of the homozygous dominant $AA$ and homozygous recessive $aa$ traits.
D
The alleles $A$ and $B$ are codominant alternatives located at the same physical gene locus on a single chromosome pair.

Answer

The alleles

A

and

a

occupy identical loci on homologous chromosomes, whereas identical copies of allele

A

exist on sister chromatids prior to anaphase I segregation.

Homologous chromosomes contain corresponding gene loci where alternative alleles (

A

and

a

) reside. Following S-phase replication prior to meiosis I, each chromosome consists of two sister chromatids carrying identical copies of its allele (e.g., two copies of

A

on one chromosome and two copies of

a

on its homologue).

Step-by-Step Solution

Analyze the relationship between homologous chromosomes and gene loci.

In diploid organisms, homologous chromosomes occur in pairs, carrying genes for the same traits at identical physical locations termed loci. Therefore, alternative forms of a gene (alleles

A

and

a

) are located at corresponding loci on paternal and maternal homologous chromosomes.

Establishing the physical location of alleles on homologous chromosomes is essential to distinguish homologous pairs from replicated chromatids.

Analyze the composition of sister chromatids following S-phase replication.

During the S phase of interphase, each chromosome replicates to form two sister chromatids joined at the centromere. Sister chromatids carry genetically identical nucleotide sequences and thus identical alleles (e.g.,

A

and

A

Differentiating sister chromatids (identical duplicated strands) from homologous chromosomes (maternal and paternal counterparts) prevents confusing allele variation with chromatid duplication.

Evaluate independent assortment and locus position for unlinked genes

A

and

B

Genes

A

and

B

reside on distinct chromosome pairs. They represent separate loci rather than alleles of the same gene locus.

Alleles compete for the same locus, whereas separate genes reside at different loci across the genome.

Key Concept

Basic Genetics Terminology and Concepts

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