Meiosis

Explain what meiosis is and in which organs of the human body it takes place, stating which cells it produces in males and females.

Look at the diagram of meiosis, in which an initial cell with four chromosomes ($2n = 4$) divides twice in succession. Using the diagram, state how many cell divisions meiosis includes, how many daughter cells are produced from the initial cell, and how many chromosomes each one has compared to it.

During prophase I of meiosis, homologous chromosomes exchange fragments in a process called crossing over. Explain what this consists of and what consequence it has for the genetic variability of gametes.

State two fundamental differences between mitosis and meiosis: one referring to the number of divisions and resulting cells, and another to the chromosome complement of the daughter cells.

Explain the difference between a haploid cell and a diploid cell, and state how many chromosomes a human diploid cell has and how many a human gamete has.

Why is it essential for gametes to be haploid? Reason what would happen to the number of chromosomes in a species, generation after generation, if gametes were diploid.

In an amphibian population with $2n = 28$ chromosomes, a female is detected whose eggs do not reduce their chromosome number due to a cytokinesis failure in meiosis I, giving rise to viable diploid eggs with 28 chromosomes.

1. a) If one of these abnormal eggs is fertilized by a normal sperm of the species, calculate the chromosome number of the resulting zygote and indicate what genetic term this numerical chromosomal alteration receives.
2. b) Although the resulting individual may be viable, it is usually completely sterile when attempting to undergo meiosis to form gametes. Justify why an odd (triploid) chromosome number prevents proper meiosis.
3. c) Based on this case, argue why the chromosome reduction of meiosis and subsequent fertilization are not just a reproductive mechanism, but an essential barrier for the stability of sexually reproducing species.

The karyotype of a newborn is analyzed and a trisomy in pair 21 ($47, XX, +21$) is detected. To find out the origin of the error, a genetic marker on chromosome 21 with three possible alleles (1, 2, and 3) is studied:

• The mother is heterozygous and has alleles 1 and 2.

• The father is homozygous and has only allele 3.

• The baby has all three alleles simultaneously: 1, 2, and 3.

1. a) Determine, with justification, in which of the two parents the meiotic nondisjunction occurred.
2. b) Use reasoning to deduce whether the failure occurred in anaphase I or in anaphase II of meiosis, taking into account the difference between the separation of homologous chromosomes and that of sister chromatids.
3. c) If nondisjunction had occurred in anaphase II of the same parent, predict which maternal allele combinations could have appeared in the baby instead of having alleles 1 and 2 at the same time.

Unit challenge. How do cell division failures impact public health? Become a biomedical consultant and analyze demographic data from Spain to highlight the societal importance of the cell cycle.

1. Data collection: Go to the official INE website (ine.es). In the demography section, record the average age of childbearing over the last decade (around $32.6$ years) and check the death rate from tumors according to causes-of-death statistics.

2. Cellular analysis: Relate cancer to the loss of control during interphase stages ($G_1$, $S$, $G_2$) and uncontrolled mitosis. Then, justify how the advancing age of female gametes increases errors during meiosis (nondisjunction in anaphase) and the occurrence of aneuploidies.

3. Final product: Produce a one-page informational report containing: a table and graph with the INE data, an infographic comparing mitosis and meiosis, and two reasoned guidelines to prevent cellular alterations (exposure to mutagens, lifestyle).

Assessment criteria:

• Scientific rigor when describing the stages of the cell cycle, mitosis, and meiosis.

• Accuracy in searching, analyzing, and plotting INE data.

• Clarity and communicative quality of the report and preventive proposals.