explain that crossing over and random orientation (independent assortment) of pairs of homologous chromosomes and sister chromatids during meiosis produces genetically different gametes

Passage of Information from Parents to Offspring

Learning Objective

Explain how crossing‑over and the random orientation (independent assortment) of homologous chromosome pairs and sister chromatids during meiosis produce genetically different gametes.

Key Concepts

• Meiosis I – reductional division: homologous chromosomes separate.
• Meiosis II – equational division: sister chromatids separate.
• Crossing‑over (recombination): exchange of DNA between non‑sister chromatids of homologous chromosomes.
• Independent assortment: random alignment of chromosome pairs on the metaphase plate.

Mechanism of Genetic \cdot ariation

1. Crossing‑over

During prophase I, homologous chromosomes pair to form a tetrad. At points called chiasmata, non‑sister chromatids exchange segments of DNA.

Resulting chromatids are a mixture of maternal and paternal alleles, creating new allele combinations on a single chromosome.

2. Independent Assortment

In metaphase I, each pair of homologous chromosomes aligns randomly with respect to the cell’s poles. The orientation of one pair does not affect the orientation of another.

For a diploid organism with \$n\$ chromosome pairs, the number of possible gamete genotypes arising from independent assortment alone is:

\$\$

2^{n}

\$\$

For humans (\$n = 23\$) this gives \$2^{23} \approx 8.4 \times 10^{6}\$ possible combinations.

Combined Effect

The total number of genetically distinct gametes that can be produced by a single meiosis is the product of the possibilities from crossing‑over and independent assortment. While the exact number of crossover combinations is difficult to calculate, the theoretical maximum can be expressed as:

\$\$

\text{Total gametes} = 2^{n} \times \text{(number of crossover patterns)}

\$\$

Summary Table

Implications for Inheritance

1. Each gamete carries a unique set of genetic information.
2. Fertilisation restores the diploid chromosome number but combines two genetically distinct gametes, increasing variability in the offspring.
3. Genetic variation is the raw material for evolution and natural selection.

Key Points to Remember

• Crossing‑over occurs only between non‑sister chromatids of homologous chromosomes.
• The orientation of each homologous pair on the metaphase plate is independent of the others.
• Both mechanisms together ensure that no two gametes (and therefore no two offspring) are genetically identical, except in the case of identical twins.