A. Correct! Genetically a female is XX, and has 22 pairs of autosomes.

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1 MCAT Biology - Problem Drill 08: Meiosis and Genetic Variability Question No. 1 of A human female has pairs of autosomes and her sex chromosomes are. Question #01 (A) 22, XX. (B) 23, X. (C) 23, XX. (D) 22, XY. A. Correct! Genetically a female is XX, and has 22 pairs of autosomes. X is a chromosome required for female traits. However, the female genome consists of two XX. There are a total of 23 pairs, including the sex chromosomes. XY is the chromosomal make up of a male. In humans, each cell normally contains 23 pairs of chromosomes or 46 individual chromosomes. The autosomes (non-sex determining chromosomes) make up 22 of these pairs. The remaining two chromosomes are for sex determination and in human females sex is determined by XX and in males by XY. The correct answer is (A).

2 Question No. 2 of Meiosis is best described by which of the following answers? Question #02 (A) Results in chromosome changes and recombination. (B) There are two nuclear divisions required to complete the process. (C) Occurs only in gamete production. (D) All of the above. This answer is correct, but the choice is incomplete. This answer is correct, but the choice is incomplete. This answer is correct, but the choice is incomplete. D. Correct! This answer is complete. All of the answers describe the process of meiosis. Meiosis occurs in germ cells, such as sperm and egg, or in plants spores. Before division begins the chromosomes have already replicated. Chromosome replication is followed by two successive nuclear divisions. The chromosome number in the resulting germ cells is reduced by one-half. That is, the original cell goes from diploid (2N) to haploid (1N). The correct answer is (D).

3 Question No. 3 of Which of the statements is correct with regards to mitosis and meiosis? Question #03 (A) Mitosis and meiosis are identical processes. (B) Cells divide once in mitosis and twice in meiosis. (C) Mitosis is reduction division. (D) Meiosis results in 4 diploid cells. Mitosis is simply cell replication and meiosis is gamete formation. B. Correct! Following chromosome replication, cells divide once in mitosis and twice in meiosis. Meiosis is reduction division. Meiosis results in 4 haploid cells, not diploid cells. Following cell replication, cells divide once in mitosis to form 2 diploid daughter cells that are identical to the original cell, while in meiosis cells divide twice to form 4 haploid gamete cells. The correct answer is (B).

4 Question No. 4 of Genetic diversity occurs when chromosomes are rearranged in and fertilization. Question #04 (A) Natural selection. (B) Meiosis. (C) Mutations. (D) Mitosis. Natural selection is the process where more beneficial traits have a greater probability of surviving into the next generation. B. Correct! Recombination and independent assortment during meiosis result in increased diversity of genotypes in the population. Mutations can be a source of genetic diversity, but the question is referring to recombination, which is not a mutation. Mitosis produces two identical daughter cells. There are several ways that genetic diversity is created. Mutation, recombination, and selective breeding are just a few of the mechanisms. However only meiosis is represented during fertilization and rearrangement, which is another way of referring to recombination. The correct answer is (B).

5 Question No. 5 of Which word sequence below is arranged correctly regarding genetic structure, from smallest to largest? Question #05 (A) Nucleotide chromosome---gene---codon. (B) Nucleotide codon gene chromosome. (C) Gene--chromosome codon nucleotide. (D) Codon chromosome genenucleotide. Chromosomes are larger than genes. B. Correct! The smallest genetic structure is the nucleotide. Three nucleotides together form a codon. Several codons combine to make a gene. Ultimately the chromosome is made up of several genes. Genes are larger than nucleotides. Chromosomes are larger than genes. Nucleotides are the smallest unit within genetic molecules. Three nucleotides make up a codon. Codons taken together form genes. Many genes together form chromosomes. The correct answer is (B).

6 Question No. 6 of Seedless watermelons are triploid (3n) and are produced by crossing a tetraploid (4n) watermelon with a diploid (2n) watermelon. Mitosis can proceed normally in triploid cells but meiosis cannot. Which answer best explains why meiosis cannot proceed? Question #06 (A) Homologues would be unequal in 3n, or have an odd number, so meiosis is not possible. (B) Since there are no seeds meiosis is not possible. (C) Only one of the reduction division cell lines would have the correct number of chromosomes. (D) Seedless watermelons are sterile because they do not undergo either meiosis or mitosis. A. Correct! Because a requirement of meiosis is that the cells receive half the genetic complement it cannot proceed. This is a chicken or egg argument. The question is asking why there are no seeds. There would be no reduction division of these cell lines. Seedless watermelons do exist. Therefore they must undergo mitosis. Seedless watermelons cannot reproduce by themselves. Normal watermelons are diploid. They have two sets of 11 chromosomes. During meiosis the cells require precise alignment of chromosome pairs in the cells equatorial plane. This cannot occur with an odd number of chromosomes. The triploids have three sets of chromosomes, so meiosis cannot proceed and seeds are not formed. The correct answer is (A).

7 Question No. 7 of Which of the following assumptions is not correct for a population to be in Hardy-Weinberg equilibrium? Question #07 (A) There must be migration between populations. (B) There can be no mutation. (C) There can be no natural selection. (D) The population must be infinitely large. A. Correct! If migration occurs between populations, they are not in Hardy-Weinberg equilibrium. This assumption is correct. If there is natural selection the population is not in Hardy-Weinberg equilibrium. If the population is not infinitely large then genetic drift will cause the population to be out of Hardy-Weinberg equilibrium. The following assumptions must be made in order for a population to be in Hardy- Weinberg equilibrium: 1. Population is infinitely large, 2. Mating is random, 3. No mutation, 4. No natural selection and 5. No migration. If any of these assumptions are violated, then the population is evolving. Remember that populations evolve, not individuals. The correct answer is (A).

8 Question No. 8 of Calculate the frequency for gene A and a using the Hardy-Weinberg equation for a species at a single locus that has two alleles. For each genotype, AA=1500 individuals, Aa=130 and aa=2. Question #08 (A) A = 1500/1632, a= 132/1632. (B) A = (3/4)*1632, a= (1/4)*1632. (C) A = 0.962, a = (D) A = 0.75, a =.25. Remember that each individual having genotype AA carries 3000 AA total alleles. These numbers do not make sense when determining genotype and allele frequencies in a diploid organism. C. Correct! You can check to see that your answer is correct by plugging these allele frequencies into the Hardy-Weinberg equation. Given the high proportion of individuals with the AA geneotype, this answer could not be true. First estimate the frequencies of the A and a alleles from the distribution of genotypes. p = frequency(a) = ((2*1500)+140))/(2*1632)=0.962 q = frequency(a) = 1-p=0.038 p² + 2pq + q² = 1 p + q = 1 The correct answer is (C).

9 Question No. 9 of An affected male parent has affected boy and girl offspring in the same proportion that he has no unaffected children. This information indicates that this trait is most likely to be. Question #09 (A) X-linked recessive. (B) X-linked dominant. (C) Autosomal recessive. (D) Autosomal dominant. Since the males and females are affected equally the trait is unlikely to be sexlinked. Since the males and females are affected equally the trait is unlikely to be sexlinked. Since both genders inherit the trait it is not likely that it is sex linked. However, since it appears in both genders with equal frequency it probably also a dominant gene. D. Correct! Since the question says that unaffected and affected children occur in the same proportions, the gene is dominant. Since both the boys and the girls are expressing the trait it is clear that this is not a sex-linked trait, which means that it is autosomal. The next thing to consider is weather the trait is recessive or dominant. Since half of the children in this case express the trait, it is likely to be dominant. The correct answer is (D).

10 Question No. 10 of The four phases of mitosis in the correct order are. Question #10 (A) Metaphase, anaphase, telophase, prophase. (B) Anaphase, telophase, metaphase, prophase. (C) Prophase, metaphase, anaphase, telophase. (D) Telophase, anaphase, metaphase, prophase. The first phase is prophase. The first phase is prophase. C. Correct! This is the correct order. Good job. Telophase is the final stage not the first. Mitosis described in order. Prophase: The chromatin condenses to form chromosomes. The chromosome has duplicated and consists of two sister chromatids. By the end of prophase, the nuclear envelope breaks down. Metaphase: The chromosomes align at the equatorial plate and are held in place by microtubules. The microtubules are attached to the mitotic spindle at the centromere. Anaphase: The centromeres divide. Sister chromatids separate and go to their respective poles. Telophase: Daughter chromosomes reach the cell poles and the microtubules disappear. The nuclear envelope reappears. The cytoplasm divides and the cell membrane is formed and pinches into two cells. These are two identical daughter cells. The correct answer is (C).