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MCAT Biology Question — Chromosomes

  • by John
  • Jul 31, 2014
  • MCAT Question of the Day

An organism is heterozygotic at each of its chromosomes, and can create 16 genetically unique haploid cells. How many homologous pairs of chromosomes are in each diploid cell? Assume that no recombination occurs.

 

a) 2

 

b) 4

 

c) 8

 

d) 16

 

Explanation

 

This organism is heterozygotic at each of its chromosomes, and it can create 16 genetically unique haploid cells. Let’s try out an example genotype. If the organism had 2 pairs of chromosomes, its genotype could be represented as AaBb. This would only yield 4 genetically unique gametes: AB, Ab, aB, and ab. In fact, the number of potential unique gametes is equal to 2n, where “n” is the number of homologous pairs in the original cell. Since the organism described in the question can form 16 possible haploid genotypes, there must be 4 homologous pairs in the diploid cell.

 

a) 2, incorrect, 2 homologous pairs could only make 4 genetically unique haploid cells.

 

b) 4, correct.

 

c) 8, incorrect, 8 homologous pairs would require some homozygotic chromosomes in order to allow 16 haploid cells.

 

d) 16, incorrect, This would only be possible if the organism were homozygotic at every chromosome, which contradicts the question stem.

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