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?
An animal’s body cells hold 10 pairs of chromosomes.
How many kinds of gamete can one of these animals 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?
In a squirrel, the maternal homolog of a pair carries R at one position and T at the next position along the same arm; the paternal homolog carries r and t. After one crossover between the two positions, the four chromatids read R with T, R with t, r with T and r with t.
Which two are the parental combinations?
In a species of animal, each parent makes 16 kinds of gamete by independent orientation.
How many kinds of zygote can two of these animals 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.
In which stage of meiosis did that chromatid form?
An animal’s body cells hold 30 chromosomes. In one cell, one homologous pair undergoes nondisjunction in meiosis I; meiosis II is normal.
What are the chromosome counts of the four cells produced?
An insect’s body cells hold 14 chromosomes. The four cells from one meiosis hold 8, 6, 7 and 7.
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?
One parent of a species of animal makes 2,048 kinds of gamete by independent orientation alone.
How many pairs of chromosomes does this animal have?
(a) Calculate the number of chromosomes in each of the four cells when meiosis is normal. (1 point)
Write down the values in the question:
Write down the equation:
Substitute the values into the equation:
A full-credit answer: Each of the four cells holds 4 chromosomes, one of every pair.
(b) Predict the four counts if one homologous pair undergoes nondisjunction in meiosis I and meiosis II is normal. (1 point)
A full-credit answer: The cell that receives both members holds 5, the other holds 3, and meiosis II copies each into two cells: 5, 5, 3, 3.
Check the box for each point your answer earns
Common slip: Writing 5, 3, 4, 4. 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: The cell that failed gives 5 and 3; the other cell gives 4 and 4, the normal count.
Check the box for each point your answer earns
Common slip: Writing 5, 5, 3, 3. Only one cell failed, so two of the four are normal.
(d) A set of four cells from a second plant, whose body cells hold 34 chromosomes, holds 18, 18, 16 and 16. Determine the division in which the failure happened, using the counts. (1 point)
A full-credit answer: The normal count for this plant is 17.
None of the four cells holds 17, so all four are abnormal.
Only a failure in meiosis I makes both cells it produces abnormal.
So the failure was in meiosis I.
Meiosis II then copied each count into two cells: 18, 18, 16 and 16.
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) Return to the first plant, with 8 chromosomes. Predict the chromosome number of the zygote made when one of the gametes with the extra chromosome from part (b) fuses with a normal gamete, and identify it as a trisomy or a monosomy. (1 point)
A full-credit answer: The zygote holds 5 from the abnormal gamete and 4 from the normal one: 9 chromosomes, a trisomy, with three copies of one chromosome.
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) Construct 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 salt in the soil change the mean number of chiasmata per cell in prophase I?
The amount of salt in the soil is changed and the number of chiasmata per cell is measured.
Check the box for each point your answer earns
Common slip: Asking why salty 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 salt in the soil; 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 salt in the soil 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 salty 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 salty-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.