Unit 5 · Practice for the Topic 5.2 end-of-topic test
You’ve gone through everything in this topic. The summary video below recaps it all, so you’re ready for the questions.
Watch first: Meiosis and genetic diversity, summed up
Pairs that line up on their own and the two to the power of the pairs; non-sister chromatids trading pieces; any egg with any sperm; when a pair fails to separate, and how the counts say which division failed.
A gamete from a dragonfly carries one chromosome of every pair.
Which statement describes where the chromosomes in that gamete came from?
A slug has 3 pairs of chromosomes.
How many kinds of gamete can one slug make by independent orientation alone?
In prophase I in a bee, two chromatids break at the same point and exchange pieces.
Which two chromatids are they?
A rat’s homologous pair carries R and S on one homolog and r and s on the other. After one crossover between the two positions, the four chromatids are RS, Rs, rS and rs.
Which two are the parental combinations?
In a species of crab, each parent makes 16 kinds of gamete by independent orientation.
How many kinds of zygote can two crabs make, counting independent orientation and random fertilization only?
One chromatid in a guinea pig’s egg carries the mother’s allele of one gene and the father’s allele of the next gene along the same chromosome.
Which source of variation made that chromatid?
A plant’s body cells hold 16 chromosomes. In one cell, one homologous pair undergoes nondisjunction in meiosis I; meiosis II runs normally.
What are the chromosome counts of the four cells produced?
A fly’s body cells hold 8 chromosomes. The four cells from one meiosis hold 5, 3, 4 and 4.
Which division did the failure happen in?
A sheep’s body cells hold 54 chromosomes. A sheep egg with 28 chromosomes is fertilized by a normal sperm with 27.
What does the zygote hold?
(a) Calculate the number of chromosomes in each of the four cells when meiosis runs normally. (1 point)
Write down the values in the question:
Write down the equation:
Substitute the values into the equation:
A full-credit answer: 5 chromosomes in each cell.
(b) Predict the four counts if one homologous pair undergoes nondisjunction in meiosis I and meiosis II runs normally. (1 point)
A full-credit answer: 6, 6, 4 and 4: one cell from meiosis I holds 6 and the other 4, and meiosis II gives two cells of each.
Check the box for each point your answer earns
Common slip: Writing 6, 4, 5, 5. That is the pattern of a failure in meiosis II.
(c) Predict the four counts if instead meiosis I is normal and, in one of the two cells, the sister chromatids of one chromosome undergo nondisjunction in meiosis II. (1 point)
A full-credit answer: 6, 4, 5 and 5: the cell that failed gives one cell of 6 and one of 4, and the other cell gives two normal cells of 5.
Check the box for each point your answer earns
Common slip: Writing 6, 6, 4, 4. Only one cell failed, so two of the four are normal.
(d) A set of four cells from a second cell of this plant holds 6, 6, 4 and 4. Identify the division in which the failure happened, and justify it from the counts. (1 point)
A full-credit answer: Meiosis I: no cell holds the normal count of 5, so the pair failed to part in meiosis I and both cells from meiosis I carried the error into meiosis II, which copied each count into two cells.
Check the box for each point your answer earns
Common slip: Choosing meiosis II because a cell is over and a cell is under. Both patterns have that; the cells with the normal count place the failure.
(e) Predict the chromosome number of the zygote made when the 6-chromosome gamete fuses with a normal gamete, and identify it as a trisomy or a monosomy. (1 point)
A full-credit answer: 11 chromosomes: 6 from the abnormal gamete and 5 from the normal one, so three copies of one chromosome, a trisomy.
Check the box for each point your answer earns
Common slip: Calling it a monosomy. An extra chromosome in a gamete gives three copies in the zygote.
(a) Write a testable question about this observation that names what would be changed and what would be measured. (1 point)
A full-credit answer: Does the amount of water given to the plants change the mean number of chiasmata per cell in prophase I? The amount of water is changed and the number of chiasmata per cell is measured.
Check the box for each point your answer earns
Common slip: Asking why dry soil matters. A “why” question names nothing to change or measure.
(b) Identify the independent variable and the dependent variable in your question. (1 point)
A full-credit answer: The independent variable is the amount of water given to the plants; the dependent variable is the mean number of chiasmata per cell.
Check the box for each point your answer earns
Common slip: Swapping the two. What the experimenter changes is independent; what is measured is dependent.
(c) State the null hypothesis for the experiment. (1 point)
A full-credit answer: The amount of water given to the plants makes no difference to the mean number of chiasmata per cell; any difference between the groups is due to chance.
Check the box for each point your answer earns
Common slip: Stating that dry soil lowers the number of chiasmata. That is the alternative hypothesis, the prediction, not the null.
(d) Predict what fewer chiasmata per cell would mean for the fraction of recombinant gametes the dry-soil plants make, and explain why. (1 point)
A full-credit answer: Fewer chiasmata mean fewer crossovers, so fewer chromatids carry a combination of alleles neither homolog had, and a smaller fraction of the gametes are recombinant.
Check the box for each point your answer earns
Common slip: Saying the alleles themselves change. Crossing over recombines existing alleles; the number of alleles is unchanged.