Meiosis and Sources of Genetic Variation (1C)
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MCAT Biological and Biochemical Foundations of Living Systems › Meiosis and Sources of Genetic Variation (1C)
In a population study of wild Drosophila melanogaster, investigators crossed females heterozygous for two linked markers on chromosome 2 (A and B) with males homozygous recessive (ab/ab). Female germ cells were exposed to a brief heat pulse during early prophase I, a condition previously associated with increased double-strand break formation. Offspring were scored for parental (AB, ab) versus recombinant (Ab, aB) phenotypes. Across replicates, the recombinant fraction increased from 8% (control) to 18% (heat pulse), while total offspring number was unchanged.
Which outcome is most likely due to crossing over during meiosis in the heat-pulse group?
A higher proportion of Ab and aB offspring without a change in the parental AB and ab allele combinations present in the parents
No change in offspring haplotypes because recombination occurs only during mitotic S phase
A higher proportion of AB offspring because sister chromatids exchange segments during anaphase II
A decrease in genetic variation because crossing over restores original linkage between A and B
Explanation
This question tests understanding of meiosis and genetic variation, fundamental to biological systems. Meiosis introduces genetic variation through mechanisms like crossing over and independent assortment. In the passage, meiosis was observed in Drosophila melanogaster, highlighting how heat pulse increased recombination from 8% to 18%. The correct answer, A, aligns with how crossing over between homologous chromosomes creates recombinant gametes (Ab and aB) without changing the parental allele combinations in the parents themselves. Choice B fails as it incorrectly describes sister chromatid exchange during anaphase II, when crossing over actually occurs between homologs in prophase I. To verify crossing over effects, look for increased recombinant frequencies while parental genotypes remain unchanged.
In an experimental observation of Arabidopsis male meiosis, researchers introduced a fluorescent reporter that marks a specific chromosomal interval flanked by two polymorphic sites. Meiocytes were isolated and the interval was sequenced from individual haploid microspores. Compared with a control line, a mutant line showed a marked reduction in microspores carrying mixed combinations of the flanking polymorphisms, while overall viability and chromosome number appeared normal. Which interpretation is most consistent with meiosis processes in the mutant line?
Enhanced DNA replication fidelity during mitosis leading to fewer recombinant haplotypes
Failure of fertilization leading to fewer recombinant haplotypes among surviving diploid embryos
Reduced homologous recombination during prophase I leading to fewer recombinant haplotypes
Increased independent assortment during meiosis II leading to fewer recombinant haplotypes
Explanation
This question tests understanding of meiosis and genetic variation, fundamental to biological systems. Meiosis introduces genetic variation through mechanisms like crossing over and independent assortment. In the passage, meiosis was observed in Arabidopsis male meiosis, highlighting reduced microspores with mixed polymorphisms in the mutant line. The correct answer, A, aligns with how reduced homologous recombination during prophase I would lead to fewer recombinant haplotypes between flanking markers. Choice B fails as it incorrectly places independent assortment in meiosis II (it occurs in meiosis I) and this wouldn't affect recombination between linked markers. Consider that crossing over between flanking polymorphisms creates recombinant intervals; reduced recombination frequency maintains parental combinations.
A field study of a diploid amphibian quantified genotype frequencies at two loci in offspring from multiple mating pairs. The loci were on different chromosomes. Across families, offspring frequently carried allele combinations not present together on either parental homolog, yet within each family the four possible allele combinations appeared in roughly equal proportions when averaged over many offspring. No evidence supported postzygotic selection against any genotype. Based on the study, what is the most significant source of genetic variation in meiosis explaining the near-equal representation of all allele combinations?
Crossing over at centromeres during meiosis II producing equal allele combinations across families
Deterministic segregation of homologs that preserves parental allele combinations in all gametes
Random orientation of homologous chromosome pairs at metaphase I generating diverse gamete combinations
Mitotic nondisjunction in germline stem cells producing balanced haplotype frequencies in gametes
Explanation
This question tests understanding of meiosis and genetic variation, fundamental to biological systems. Meiosis introduces genetic variation through mechanisms like crossing over and independent assortment. In the passage, meiosis was observed in diploid amphibians, highlighting equal proportions of all four allele combinations for unlinked loci across families. The correct answer, A, aligns with how random orientation of homologous chromosome pairs at metaphase I generates diverse gamete combinations through independent assortment. Choice D fails as it suggests deterministic segregation that would preserve parental combinations, contradicting the observed novel allele combinations. Consider that chromosomes on different homologs align independently at metaphase I; this random orientation produces $2^n$ possible gamete types, explaining equal representation of all combinations.
A comparative study in zebrafish examined genetic variation among cells derived from a single fertilized egg. One group of cells was collected from early embryos after several rounds of mitotic division. A second group consisted of gametes collected from adult fish. Whole-genome SNP phasing showed that gametes carried novel combinations of maternal and paternal haplotype blocks along the same chromosome, whereas embryonic somatic cells largely preserved the original parental haplotype blocks aside from rare point mutations.
Which statement best explains the difference in genetic variation between the two groups?
Mitosis produces haplotype blocks only when chromosomes fail to replicate, whereas meiosis always introduces new point mutations
Meiosis generates new combinations of linked alleles via homologous recombination, whereas mitosis generally preserves linkage across divisions
Mitosis increases variation by independent assortment of homologs, whereas meiosis produces identical daughter cells
Meiosis prevents any reshuffling of haplotypes by keeping homologs paired through all divisions
Explanation
This question tests understanding of meiosis and genetic variation, fundamental to biological systems. Meiosis introduces genetic variation through mechanisms like crossing over and independent assortment. In the passage, meiosis was observed in zebrafish gametes showing novel haplotype block combinations, while mitotic cells preserved parental blocks. The correct answer, A, aligns with how meiosis generates new combinations through homologous recombination, while mitosis maintains genetic fidelity across divisions. Choice B fails as it incorrectly attributes independent assortment to mitosis and claims meiosis produces identical cells, reversing their actual roles. Remember that meiosis uniquely features homolog pairing and recombination, while mitosis replicates existing genetic information faithfully.
A laboratory colony of mice was established from a single pair heterozygous at many loci. Over multiple generations, investigators compared genetic diversity in offspring produced by (i) normal germline meiosis and (ii) clonal expansion of a single embryonic stem cell line followed by nuclear transfer to generate genetically matched animals. Whole-genome SNP analysis showed substantially higher within-litter genotype diversity in the meiosis-derived offspring than among animals derived from the stem-cell nuclear transfer protocol. Which feature of meiosis is most consistent with the increased within-litter diversity observed in the normal breeding group?
Random alignment of homologous chromosome pairs at metaphase I producing varied combinations of maternal and paternal chromosomes in gametes
Equal segregation of sister chromatids during mitotic anaphase generating genetically distinct daughter cells
A reduction in genetic diversity because homologous chromosomes remain paired through all divisions
Cytokinesis in somatic tissues creating new alleles through unequal partitioning of cytoplasm
Explanation
This question tests understanding of meiosis and genetic variation, fundamental to biological systems. Meiosis generates genetic diversity through two key mechanisms: crossing over during prophase I and independent assortment at metaphase I, where homologous pairs align randomly. In the passage, mice produced through normal meiosis showed substantially higher within-litter diversity compared to clonally-derived animals from nuclear transfer. The correct answer, A, identifies the key feature: random alignment of homologous chromosome pairs at metaphase I produces varied combinations of maternal and paternal chromosomes in gametes. Choice B incorrectly describes mitotic division, which maintains genetic identity rather than creating diversity. When comparing meiotic and non-meiotic reproduction, focus on how random chromosome segregation and crossing over create unique gamete genotypes.
In a fish species, researchers tracked two unlinked loci using barcoded sequencing of single sperm. They observed that each sperm carried exactly one allele at each locus and that the combination of alleles across loci varied widely among sperm from the same male. No evidence suggested recombination within either locus region. How does independent assortment best explain the observed sperm-to-sperm variation?
A) It produces different allele combinations by random distribution of maternal and paternal homologs into haploid gametes
B) It produces different allele combinations by exchanging DNA segments between sister chromatids
C) It produces different allele combinations by introducing random point mutations during cytokinesis
D) It produces different allele combinations by copying one homolog twice and discarding the other
It produces different allele combinations by random distribution of maternal and paternal homologs into haploid gametes
It produces different allele combinations by exchanging DNA segments between sister chromatids
It produces different allele combinations by introducing random point mutations during cytokinesis
It produces different allele combinations by copying one homolog twice and discarding the other
Explanation
This question tests understanding of meiosis and genetic variation, fundamental to biological systems. Meiosis introduces genetic variation through mechanisms like crossing over and independent assortment. In this scenario, meiosis was observed in a fish species, highlighting varied allele combinations in sperm from unlinked loci. The correct answer, A, aligns with how independent assortment contributes to variation by random homolog distribution. Choice B fails as it incorrectly involves sister chromatid exchange for unlinked variation. Consider stages of meiosis and their unique contributions to diversity; avoid confusing mitotic and meiotic events.
A mouse spermatocyte line was engineered so that a pair of homologous chromosomes carries distinguishable centromere tags (red vs blue) and a distal arm marker (green vs yellow). Live-cell imaging showed random orientation of the red and blue homologs at the meiosis I spindle, while no crossing over was detectable on that chromosome pair. Which statement best describes how independent assortment contributes to genetic diversity in the resulting sperm?
A) It generates new allele combinations only when a crossover occurs between the distal markers
B) It randomizes which homolog (red-green vs blue-yellow) enters each haploid cell at meiosis I
C) It randomizes separation of sister chromatids at meiosis I, producing recombinant chromatids
D) It ensures that all sperm receive both homologs to maintain diploidy
It ensures that all sperm receive both homologs to maintain diploidy
It generates new allele combinations only when a crossover occurs between the distal markers
It randomizes which homolog (red-green vs blue-yellow) enters each haploid cell at meiosis I
It randomizes separation of sister chromatids at meiosis I, producing recombinant chromatids
Explanation
This question tests understanding of meiosis and genetic variation, fundamental to biological systems. Meiosis introduces genetic variation through mechanisms like crossing over and independent assortment. In this scenario, meiosis was observed in mouse spermatocytes, highlighting random homolog orientation without crossing over. The correct answer, B, aligns with how independent assortment contributes to variation by randomizing homolog distribution at meiosis I. Choice A fails as it incorrectly requires crossover for new combinations, but assortment alone suffices. Consider stages of meiosis and their unique contributions to diversity; avoid confusing mitotic and meiotic events.
In a yeast strain heterozygous for two linked markers, investigators observed tetrads with a 3:1 segregation at one marker but a 2:2 segregation at the other, while overall recombinant frequency between markers remained unchanged. They ruled out selection among spores. Which conclusion is most consistent with meiosis as the source of recombinants between the two markers?
A) Recombinant frequency reflects crossing over between homologs, which can occur independently of unusual segregation at a single marker
B) Recombinant frequency must drop to zero whenever any marker shows non-2:2 segregation
C) Recombinant frequency is determined by independent assortment, so linkage between markers is irrelevant
D) Recombinants arise primarily from mitotic crossing over after spore germination
Recombinant frequency reflects crossing over between homologs, which can occur independently of unusual segregation at a single marker
Recombinants arise primarily from mitotic crossing over after spore germination
Recombinant frequency is determined by independent assortment, so linkage between markers is irrelevant
Recombinant frequency must drop to zero whenever any marker shows non-2:2 segregation
Explanation
This question tests understanding of meiosis and genetic variation, fundamental to biological systems. Meiosis introduces genetic variation through mechanisms like crossing over and independent assortment. In this scenario, meiosis was observed in a yeast strain, highlighting recombinants despite unusual segregation at one marker. The correct answer, C, aligns with how crossing over contributes to variation independently of single-marker segregation. Choice D fails as it incorrectly attributes to post-meiotic mitosis. Consider stages of meiosis and their unique contributions to diversity; avoid confusing mitotic and meiotic events.
In a frog species, investigators genotyped eggs from a single female heterozygous at two loci on different chromosomes. They observed that eggs carrying allele M at locus 1 were just as likely to carry allele N as allele n at locus 2. The study design excluded recombination by selecting loci on separate chromosomes. How does independent assortment contribute to the observed pattern?
A) It ensures alleles at different loci are always inherited together as a unit
B) It generates random combinations of homologs across chromosome pairs in the haploid eggs
C) It requires crossing over between the loci to break linkage
D) It reduces variation by forcing equal numbers of each genotype among eggs
It generates random combinations of homologs across chromosome pairs in the haploid eggs
It reduces variation by forcing equal numbers of each genotype among eggs
It requires crossing over between the loci to break linkage
It ensures alleles at different loci are always inherited together as a unit
Explanation
This question tests understanding of meiosis and genetic variation, fundamental to biological systems. Meiosis introduces genetic variation through mechanisms like crossing over and independent assortment. In this scenario, meiosis was observed in a frog species, highlighting random allele combinations in eggs from unlinked loci. The correct answer, B, aligns with how independent assortment contributes to variation by generating random homolog combinations. Choice C fails as it incorrectly requires crossing over for unlinked loci. Consider stages of meiosis and their unique contributions to diversity; avoid confusing mitotic and meiotic events.
In a barley experiment, two linked loci were genotyped in gametes from a heterozygous plant. Recombinant gametes appeared only when the plant expressed a functional Spo11 homolog; in a Spo11-deficient background, recombinant gametes were nearly absent though gamete production still occurred. Which outcome is most likely due to crossing over during meiosis in the functional background?
A) Gametes containing chromosome sets identical to somatic cells due to mitotic division
B) Gametes containing non-parental combinations of alleles at the linked loci
C) Gametes that all carry the same allele at each locus due to deterministic segregation
D) Gametes showing altered cytoplasmic content that changes phenotype without changing genotype
Gametes containing non-parental combinations of alleles at the linked loci
Gametes containing chromosome sets identical to somatic cells due to mitotic division
Gametes that all carry the same allele at each locus due to deterministic segregation
Gametes showing altered cytoplasmic content that changes phenotype without changing genotype
Explanation
This question tests understanding of meiosis and genetic variation, fundamental to biological systems. Meiosis introduces genetic variation through mechanisms like crossing over and independent assortment. In this scenario, meiosis was observed in barley, highlighting non-parental combinations dependent on Spo11. The correct answer, B, aligns with how crossing over contributes to variation by producing recombinant gametes. Choice A fails as it incorrectly associates mitotic-like gametes with crossing over. Consider stages of meiosis and their unique contributions to diversity; avoid confusing mitotic and meiotic events.