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.

In a species with $2n = 16$ chromosomes, two individuals heterozygous for two different traits ($AaBb$) are analysed:

• In Individual 1, gene $A$ and gene $B$ are located on different pairs of homologous chromosomes.

• In Individual 2, both genes are located within the same pair of homologues (linked genes), with alleles $AB$ on one chromosome and $ab$ on the homologue.

1. a) Deduce how many genetically distinct types of gametes for these two traits Individual 1 can produce and in what relative proportions.
2. b) If no crossing over occurs during prophase I in Individual 2, predict what types of gametes it will form and compare this with Individual 1.
3. c) Justify which meiotic event allows Individual 2 to generate recombinant gametes ($Ab$ and $aB$) and explain why the probability of their occurrence depends on the physical distance between both genes.

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 fertilised by a normal sperm of the species, calculate the chromosome endowment 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 endowment prevents proper meiosis.
3. c) Based on this case, argue why the chromosome reduction of meiosis and subsequent fertilisation are not just a reproductive mechanism, but an essential barrier for the stability of sexually reproducing species.

The karyotype of a newborn is analysed 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 presents alleles 1 and 2.

• The father is homozygous and presents only allele 3.

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

1. a) Determine with justification in which of the two parents the meiotic non-disjunction occurred.
2. b) Reasonedly 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 non-disjunction had occurred in anaphase II of the same parent, predict which maternal allelic combinations could have appeared in the baby instead of presenting alleles 1 and 2 at the same time.