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Biology Question 122 – NEET-UG 2024

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In a plant, black seed color (BB/Bb) is dominant over white seed color (bb). In order to find out the genotype of the black seed plant, with which of the following genotype will you cross it?

Black seed color is dominant, meaning a plant with at least one 'B' allele will have black seeds. White seed color is recessive, requiring two 'b' alleles.

Step 1: Identify the Unknown Genotype and Goal✦ Active

The problem states that black seed color is dominant (BB or Bb) and white seed color is recessive (bb). We have a plant with black seeds, but its genotype is unknown (it could be BB or Bb). The goal is to determine this unknown genotype.

Step 2: Recall the Purpose of a Test Cross○ Expand

To determine the unknown genotype of an individual expressing a dominant phenotype, a specific type of cross called a 'test cross' is performed. In a test cross, the individual with the unknown dominant genotype is crossed with a homozygous recessive individual.

💡 Teacher's Secret Hint

A test cross helps differentiate between homozygous dominant and heterozygous individuals with the same phenotype.

Step 3: Apply Test Cross to the Given Scenario○ Expand

In this case, the homozygous recessive genotype for seed color is bb (white seeds). Therefore, to find out the genotype of the black seed plant, it must be crossed with a plant having the genotype bb. If the offspring are all black, the unknown parent was BB. If there is a 1:1 ratio of black to white offspring, the unknown parent was Bb.

💡 Teacher's Secret Hint

Crossing with any other genotype (like BB or Bb) would not definitively reveal the unknown genotype if the unknown plant is BB.

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