Bio 160- Meiosis and Genetic Diversity

What types of cells divide by meiosis?

Gamete-producing cells—cells in the ovaries and testes that give rise to sperm and egg. These are germ cells, not somatic (body) cells. Meiosis produces haploid (n) gametes from diploid (2n) parent cells.

What are the phases of Meiosis I and meiosis II? What major events are happening at each phase? One parent cell can give rise to how many daughter cells in meiosis?

Meiosis I (reduction division):

• Prophase I: Homologs pair (synapsis) and crossing over occurs.

• Metaphase I: Homologous pairs align randomly (independent assortment).

• Anaphase I: Homologous chromosomes separate (sister chromatids stay together).

• Telophase I & Cytokinesis: Two haploid cells form.

Meiosis II (equational division):

• Prophase II: New spindle forms.

• Metaphase II: Chromosomes align individually.

• Anaphase II: Sister chromatids separate.

• Telophase II & Cytokinesis: Four genetically unique haploid daughter cells are produced from one parent cell.

What is a crossover event and what does it result in?

A crossover (recombination) occurs during Prophase I when non-sister chromatids of homologous chromosomes exchange segments. This swaps alleles between maternal and paternal chromosomes, creating recombinant chromosomes. It increases genetic variation by producing new combinations of traits not found in either parent.

What is independent assortment and how does it create variation?

Independent assortment occurs during Metaphase I, when homologous chromosome pairs align randomly. Each gamete receives a random mix of maternal and paternal chromosomes.

• Humans can form 2²³ (~8.4 million) different chromosome combinations in gametes.

• Random fusion of sperm and egg further amplifies variation, producing ~70 trillion possible genetic combinations in offspring.

What is nondisjunction, and what happens if it occurs in Meiosis I vs. Meiosis II?

Nondisjunction is when chromosomes fail to separate properly.

• Meiosis I: Homologous pairs fail to separate → all gametes abnormal (two with an extra chromosome, two missing one).

• Meiosis II: Sister chromatids fail to separate → half normal gametes, half abnormal (one extra, one missing).
  Results in aneuploidy (abnormal chromosome number).

What is Trisomy 21, and why does its risk increase with maternal age?

Trisomy 21 (Down syndrome) results from nondisjunction leading to an extra chromosome 21. Risk increases with maternal age because oocytes remain arrested in Prophase I for decades, increasing chances of spindle errors during chromosome segregation.

What types of structural chromosome changes can occur?

• Deletion: Segment missing.

• Duplication: Segment repeated.

• Inversion: Segment flipped in orientation.

• Translocation: Segment moved to a nonhomologous chromosome.
  These changes can disrupt gene function and lead to developmental or genetic disorders.

What is Cri du chat syndrome?

A genetic disorder caused by a deletion on chromosome 5. Infants have a high-pitched cry (“cry of the cat”), intellectual disability, and developmental delays. It’s an example of a structural chromosome abnormality.

Why are abnormal numbers of sex chromosomes more common than autosomal abnormalities?

Extra or missing sex chromosomes (X or Y) are often less lethal because of X-chromosome inactivation and the relatively small Y chromosome. Autosomal abnormalities usually disrupt essential gene balance and are typically fatal early in development.