AP Biology · Lesson 1

Meiosis, variation and chromosome errors

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Meiosis makes gametes. The DNA is copied once, then the cell divides twice, giving four haploid cells (n) from one diploid cell (2n). A human cell with 46 chromosomes makes gametes with 23, one from each pair. Fertilization joins two gametes and restores the diploid number.

Meiosis I separates homologous chromosomes: the two members of each pair go to opposite poles, so the number of chromosomes is halved. Each chromosome still has two sister chromatids. Meiosis II separates the sister chromatids, much as mitosis does. Mitosis, by contrast, gives two diploid cells that are genetically identical to the parent cell.

Meiosis makes gametes that differ from one another in three ways. In prophase I, crossing over swaps segments between non-sister chromatids of a homologous pair. In metaphase I, each pair lines up on its own, so maternal and paternal chromosomes of different pairs sort into gametes at random: independent assortment, which gives 2 to the power n combinations, where n is the number of pairs (16 when n = 4). Then random fertilization joins any one sperm with any one egg.

Sometimes chromosomes fail to separate: nondisjunction. If a homologous pair fails to separate in meiosis I, all four gametes are abnormal, two with an extra chromosome (n + 1) and two missing one (n - 1). If sister chromatids fail to separate in one cell in meiosis II, two gametes are normal, one is n + 1 and one is n - 1. A gamete that is n + 1 joining a normal gamete gives a trisomy (2n + 1), such as trisomy 21; an n - 1 gamete gives a monosomy (2n - 1). A karyotype, a picture of the chromosomes sorted by size and shape, shows these changes in number.

Trap Meiosis I already halves the number

Students often say the cells become haploid only after meiosis II. In fact each cell after meiosis I has one member of each homologous pair, so it is already haploid, even though every chromosome still has two sister chromatids. Meiosis II separates those chromatids but does not change the number of chromosomes per cell.

Worked example Easy

A cell from a fictional fern has 24 chromosomes (2n = 24). How many chromosomes are in each cell made when it completes meiosis?

  1. 6
  2. 12Answer
  3. 24
  4. 48

How to solve it

  1. Meiosis makes haploid cells, with one chromosome from each pair.
  2. The fern's cell has 24 chromosomes, or 12 pairs.
  3. Each finished cell has 24 / 2 = 12 chromosomes.

Why each choice is right or wrong

  • A. Incorrect. 6 would mean the number was halved twice, but meiosis halves it only once, in meiosis I.
  • B. Correct. Meiosis gives haploid cells, so 24 is halved to 12, one chromosome from each pair.
  • C. Incorrect. 24 is the diploid number of the parent cell; keeping it unchanged describes mitosis, not meiosis.
  • D. Incorrect. 48 is the number of chromatids after DNA copying, not the number of chromosomes in a finished gamete.

Explain Explain it

Task verbs: explain, predict

To explain variation, name the process, say when it happens and say what it shuffles. Crossing over happens in prophase I and mixes alleles within a chromosome. Independent assortment happens in metaphase I and mixes whole chromosomes. Random fertilization mixes the gametes of two parents.

To predict the result of nondisjunction, count chromosomes cell by cell. Ask which division failed, follow the extra or missing chromosome into each gamete, and then add the normal gamete it joins at fertilization.

Worked example Medium

A fictional insect has 2n = 6, so it has three pairs of chromosomes. Ignoring crossing over, how many genetically different kinds of gametes can one insect make through independent assortment alone?

  1. 3
  2. 6
  3. 8Answer
  4. 9

How to solve it

  1. Each pair lines up on its own at metaphase I, so each pair has 2 ways to go.
  2. With n = 3 pairs, the number of kinds is 2 x 2 x 2.
  3. 2 x 2 x 2 = 8 kinds of gametes.

Why each choice is right or wrong

  • A. Incorrect. 3 counts the chromosome pairs, not the ways the pairs can be combined in a gamete.
  • B. Incorrect. 6 is the diploid number of chromosomes, not the number of possible gamete combinations.
  • C. Correct. Each of the 3 pairs can send either member to a gamete, so there are 2 x 2 x 2 = 8 kinds.
  • D. Incorrect. 9 is 3 squared, but the rule is 2 raised to the number of pairs, which is 2 x 2 x 2.

Explain Your turn

One parent makes many gametes. Explain how crossing over and independent assortment each make those gametes differ genetically from one another. For each process, say when in meiosis it happens.

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A full answer does this

  • Explains that crossing over in prophase I swaps segments between non-sister chromatids of a homologous pair, giving chromatids with new combinations of alleles
  • Explains that in independent assortment at metaphase I each homologous pair lines up independently, so the maternal and paternal chromosomes of different pairs are sorted into gametes in random combinations

Model answer

Crossing over happens in prophase I, when homologous chromosomes are paired. Non-sister chromatids swap matching segments, so a chromatid can end up with a new combination of alleles, such as A with b instead of A with B. Independent assortment happens in metaphase I. Each homologous pair lines up on its own, so whether the maternal or paternal chromosome of one pair goes to a given pole does not depend on the other pairs. The gametes therefore get different mixes of maternal and paternal chromosomes.

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