biology : class12 Chromosomal Basis of Assortment : Dihybrid Cross, Independent Assortment : Law of Independent Assortment MCQ Questions and Answers
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Chromosomal Basis of Assortment
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MCQ Questions
1. In a diploid organism with genotype AaBb, the chromosomal basis of independent assortment is best represented by which meiotic event?
A.
The two homologues of chromosome pair carrying A/a assort independently of the two homologues of chromosome pair carrying B/b at metaphase I
B.
The sister chromatids carrying A and a separate independently at metaphase I
C.
The alleles A and B separate from each other because they occupy different loci on the same chromosome
D.
The homologous chromosomes carrying A and B always migrate to the same pole during meiosis I
Correct Answer:
The two homologues of chromosome pair carrying A/a assort independently of the two homologues of chromosome pair carrying B/b at metaphase I
Explanation:
Independent assortment results from the random orientation of each homologous chromosome pair at metaphase I. The orientation of the chromosome pair carrying A/a is independent of the orientation of the nonhomologous pair carrying B/b.
2. An individual has genotype AaBb, with the A/a genes located on one chromosome pair and B/b genes on a different chromosome pair. Ignoring crossing over, how many genetically distinct gamete types can be produced solely because of independent assortment?
A.
2
B.
3
C.
4
D.
8
Correct Answer:
4
Explanation:
Each of the two chromosome pairs can orient in two alternative ways at metaphase I. The resulting combinations are AB, Ab, aB, and ab, giving 2^2 = 4 gamete types.
3. For an organism heterozygous at three loci, AaBbCc, where each gene is on a different chromosome pair, how many equally probable chromosome-derived gamete combinations are expected in the absence of selection?
A.
6
B.
8
C.
9
D.
12
Correct Answer:
8
Explanation:
Each chromosome pair contributes one of two alternative homologues to a gamete. Independent assortment therefore produces 2^3 = 8 combinations, each expected at equal frequency when no other factor alters viability or segregation.
4. At metaphase I, homologous pairs carrying alleles A/a and B/b can orient in two fundamentally different ways. Which pair of pole-directed combinations correctly describes these orientations?
A.
AB toward one pole and ab toward the other; or Ab toward one pole and aB toward the other
B.
A and a toward one pole; B and b toward the other
C.
AB and Ab toward one pole; aB and ab toward the other
D.
A and B must always move toward opposite poles, regardless of orientation
Correct Answer:
AB toward one pole and ab toward the other; or Ab toward one pole and aB toward the other
Explanation:
The two nonhomologous chromosome pairs orient independently at metaphase I. Consequently, either the A-bearing and B-bearing homologues face the same pole, or the A-bearing homologue faces the same pole as the b-bearing homologue.
5. Why does independent assortment generate different allele combinations in the gametes of an AaBb individual even if no crossing over occurs?
A.
Because homologous chromosomes replicate unevenly before meiosis
B.
Because nonhomologous chromosome pairs independently orient at metaphase I and segregate toward opposite poles
C.
Because sister chromatids exchange alleles during metaphase I
D.
Because alleles mutate during chromosome condensation
Correct Answer:
Because nonhomologous chromosome pairs independently orient at metaphase I and segregate toward opposite poles
Explanation:
Independent assortment does not require crossing over. Random orientation and segregation of different homologous chromosome pairs at meiosis I can place different maternal and paternal chromosomes into gametes.
6. A diploid cell is heterozygous for genes A and B, which are located on nonhomologous chromosome pairs. If the maternal chromosome carrying A enters a particular pole during meiosis I, what determines whether the B or b homologue enters that same pole?
A.
The allele A directly controls the orientation of the B/b chromosome pair
B.
The orientation of the B/b chromosome pair is independent and therefore either homologue may enter that pole
C.
The B homologue must enter the same pole because both genes are heterozygous
D.
The b homologue must enter the same pole because homologues always segregate in pairs
Correct Answer:
The orientation of the B/b chromosome pair is independent and therefore either homologue may enter that pole
Explanation:
Because A/a and B/b reside on different chromosome pairs, the orientation of one pair does not determine the orientation of the other. Either B or b may accompany A into the specified pole.
7. Which observation most directly supports the chromosomal explanation of Mendel's law of independent assortment?
A.
Homologous chromosomes pair and segregate during meiosis, while different chromosome pairs orient independently
B.
DNA replication produces identical sister chromatids before every cell division
C.
All genes on a chromosome are transmitted with identical frequency in every cross
D.
Sister chromatids always separate before homologous chromosomes pair
Correct Answer:
Homologous chromosomes pair and segregate during meiosis, while different chromosome pairs orient independently
Explanation:
Mendelian independent assortment is explained cytologically by the independent orientation and segregation of nonhomologous chromosome pairs during meiosis I.
8. In a species with 2n = 6, how many different maternal-paternal chromosome combinations can be produced in gametes by independent assortment alone, assuming all three chromosome pairs are distinguishable?
A.
3
B.
6
C.
8
D.
12
Correct Answer:
8
Explanation:
The haploid number is n = 3, so independent assortment can generate 2^n = 2^3 = 8 combinations of maternal and paternal chromosomes in gametes.
9. Two genes are located on the same chromosome and are very close together. Which conclusion most accurately distinguishes this situation from the chromosomal basis of independent assortment?
A.
Their alleles are expected to assort independently because all loci separate at metaphase I
B.
Their transmission may deviate from independent assortment because the genes tend to travel on the same chromosome
C.
They cannot be inherited together because homologous chromosomes always separate before replication
D.
Their alleles assort independently only when they are homozygous
Correct Answer:
Their transmission may deviate from independent assortment because the genes tend to travel on the same chromosome
Explanation:
Independent assortment applies most directly to genes on different chromosome pairs. Genes close together on the same chromosome are physically associated and may not show the expected independent combinations.
10. A cell with genotype AaBb undergoes meiosis. If the A-bearing chromosome and the b-bearing chromosome move toward the same pole at meiosis I, which chromosome combination must move toward the opposite pole?
A.
A and b
B.
A and B
C.
a and B
D.
a and b
Correct Answer:
a and B
Explanation:
Each homologous chromosome pair segregates so that its two homologues move to opposite poles. If A and b move together, the alternative homologues a and B must move together toward the opposite pole.
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