Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
Meiosis is a reduction division from a diploid () nucleus to four haploid () nuclei, occurring in two stages: Meiosis I and Meiosis II.
During Prophase I, homologous chromosomes undergo synapsis to form bivalents (tetrads). Crossing over occurs between non-sister chromatids at points called chiasmata, resulting in the exchange of genetic material.
In Metaphase I, homologous pairs align at the equatorial plate. Their orientation is random, leading to independent assortment of maternal and paternal chromosomes. The number of possible combinations is .
In Anaphase I, homologous chromosomes are separated and pulled to opposite poles, reducing the chromosome number from to .
Meiosis II resembles mitosis, where sister chromatids are separated during Anaphase II, resulting in four genetically distinct haploid () cells.
Non-disjunction is the failure of chromosomes to separate properly during Anaphase I or II. This leads to aneuploidy, such as Trisomy 21 (Down Syndrome), where an individual has chromosomes.
Genetic variation is promoted by three main mechanisms: crossing over in Prophase I, random orientation in Metaphase I, and the random fusion of gametes during fertilization.
📐Formulae
💡Examples
Problem 1:
A cell from an organism with a diploid number of undergoes meiosis. Calculate the number of possible chromosome combinations in the gametes due to independent assortment alone, assuming no crossing over occurs.
Solution:
The haploid number is . The number of combinations is .
Explanation:
Independent assortment occurs during Metaphase I. The number of ways pairs of homologous chromosomes can align is given by the formula .
Problem 2:
In a human cell (), non-disjunction occurs during Meiosis I for chromosome 21. Describe the resulting gametes.
Solution:
Two gametes will contain chromosomes (two copies of chromosome 21), and two gametes will contain chromosomes (zero copies of chromosome 21).
Explanation:
When non-disjunction occurs in Meiosis I, the homologous pair fails to separate. Consequently, all four daughter cells are affected: half have an extra chromosome and half are missing one.