Meiosis and gametes

Copying doubles the chromatids, not the chromosomes: each of the 46 chromosomes is now two sister chromatids joined at a centromere. The count reaches 92 chromosomes only in anaphase, when the sisters separate.

• Correct: 46 chromosomes, 92 chromatids:
• 92 chromosomes, 92 chromatids:
• 23 chromosomes, 46 chromatids:

DNA replication in S phase copies each chromosome. The two identical copies stay joined at the centromere and are called sister chromatids.

• The two chromosomes of a pair, one from each parent:
• Correct: Two identical copies of one chromosome made by DNA replication:
• The two strands of one DNA double helix:

The gonads are the testes and ovaries. They make the gametes and the sex hormones: testosterone from the testes, estrogens and progesterone from the ovaries.

• The adrenal glands:
• Correct: The testes and ovaries:
• The pituitary and hypothalamus:

In anaphase I, the homologs move apart but each is still two sister chromatids joined at a centromere. No chromatids split, so the count stays at 46 chromosomes (92 chromatids), 23 moving to each end.

• 23: 23 is the number that ends up at each end, or in each cell after telophase I, not the total in the cell during anaphase I.
• Correct: 46: Correct. 46 double chromosomes: the homologs separate but the sisters do not.
• 92: 92 is the count in anaphase of mitosis, when sister chromatids split and each becomes a chromosome. In anaphase I they stay joined.
• 69: 69 would be one and a half sets, which never occurs in a normal division.

Anaphase I separates the homologs, not the sister chromatids. The sisters stay joined at the centromere until anaphase II. That is why each cell after meiosis I has 23 chromosomes that are still double.

• Each chromosome pairs with its homolog in prophase I: This step is right. Pairing of homologs is the hallmark of prophase I.
• Crossing over swaps pieces between homologs: This step is right. Crossing over happens while the homologs are paired.
• The pairs line up across the middle of the cell in metaphase I: This step is right. In metaphase I, chromosomes line up as pairs.
• Correct: Sister chromatids are pulled apart in anaphase I: This is the error. Anaphase I separates homologs; sister chromatids separate in anaphase II.
• Two cells form, each with 23 chromosomes: This step is right. Each cell gets one member of each of the 23 pairs.

Meiosis I is the reduction division. It halves the number of chromosomes and chromatids and the DNA in each cell, but each chromosome stays double until meiosis II.

• Number of chromosomes in the cell: down. Meiosis I sends one homolog of each pair to each cell, so the count falls from 46 to 23.
• Number of chromatids in the cell: down. Each of the 23 chromosomes still has two chromatids, so the cell has 46 chromatids instead of 92.
• Chromatids per chromosome: no change. Sister chromatids stay joined through meiosis I, so each chromosome is still made of two chromatids.
• Amount of DNA in the cell: down. Half the chromatids went to the other cell, so each cell has half the DNA: back to the amount of a G1 body cell.

In a man's meiosis, the X and the Y behave as a homologous pair and separate in anaphase I. A sperm carrying both an X and a Y means that pair failed to separate: nondisjunction in meiosis I.

• Correct: In the father's meiosis I: Correct. The X and Y are homologs, and homologs separate in meiosis I.
• In the father's meiosis II: In meiosis II, sister chromatids separate. A failure there would give a sperm with two X chromosomes or two Y chromosomes, not one of each.
• In the mother's meiosis I: The mother gave only her normal single X. The extra chromosome came from the father.
• In the boy's own early cell divisions: An error after the egg and sperm joined could not supply a Y from the father alongside his X, and it would usually give a mix of cell lines rather than XXY in every cell.

2n = 12 means 6 pairs. Each pair can send either homolog to a gamete, and the pairs assort independently: 2⁶ = 64.

• Correct: 64: Correct. 6 pairs, 2 choices each: 2 × 2 × 2 × 2 × 2 × 2 = 64.
• 12: 12 is the diploid number, not the number of combinations.
• 24: 24 is 2 × 12. The choices for each pair multiply; they are not added or doubled once.
• 4,096: 4,096 is 2¹², which treats all 12 chromosomes as independent choices. Only the 6 pairs are; one member of each pair goes to each gamete.

Every egg carries an X, because a woman's cells have two X chromosomes and no Y. A man's X and Y separate in meiosis I, so half his sperm carry an X and half a Y. The sperm that joins the egg decides: X gives XX, Y gives XY.

• The mother, because the egg carries the X or the Y: An egg never carries a Y chromosome; a woman has none to give.
• Both equally, because each gives one sex chromosome: Each parent does give one sex chromosome, but the mother's is always an X, so only the father's contribution varies.
• Correct: The father, because his sperm carry an X or a Y: Correct. The sperm carries either an X or a Y; the egg always carries an X.
• Neither: it depends on hormone levels in pregnancy: The chromosomes present at the start, and SRY on the Y, set whether the gonad becomes a testis. Hormones follow from that.

Homologous chromosomes are the two members of a pair, one inherited from each parent: same genes in the same order, but possibly different alleles. Sister chromatids are the two identical copies of one chromosome made in S phase and joined at the centromere.

• Correct: Homologs come one from each parent; sisters are copies from DNA replication: Correct. Homologs are a matched pair from two parents; sisters are copies of one chromosome.
• Homologs are identical copies; sisters come one from each parent: This reverses the two. Sister chromatids are the identical copies.
• Homologs carry different genes; sister chromatids carry the same genes: Homologs carry the same genes, often in different versions (alleles).
• Homologs are found only in gametes; sisters are found only in body cells: Body cells hold homologous pairs, and every cell that copies its DNA has sister chromatids. Gametes have no homologous pairs.

The error in meiosis I puts both homologs into one cell and neither into the other. Meiosis II then splits each cell's chromosomes normally: the first cell gives two gametes with 24 chromosomes, and the second gives two with 22. All four are abnormal.

• None of the four: An error that puts both homologs into one cell must change the count in that cell's products and in its partner's.
• One of the four: One abnormal gamete cannot arise, because an extra chromosome in one cell always means a missing one in another.
• Two of the four: Two of four abnormal is the result of an error in meiosis II in one of the two cells: one gamete with 24, one with 22 and two normal.
• Correct: All four: Correct. Meiosis I nondisjunction makes every gamete from that cell abnormal: two with 24 and two with 22.