Which law explains how alleles separate during gamete formation?
When studying the inheritance of a single gene, Mendel's Law of Segregation explains how the alleles for that gene separate during gamete formation.. This law states that alleles for different genes assort independently during gamete formation, provided the genes are located on different chromosomes. Each individual has two alleles for a gene, one inherited from the biological mother and one from the biological father. These alleles are separated during meiosis, which is the process of nuclear division that produces haploid gametes.
$\underline{\textrm{Meiosis I: Separation of alleles}}$:
Before meiosis begins, each chromosome makes an identical copy of itself, known as **sister chromatids**, which are joined together at the **centromere**. During **prophase I**, the chromosomes condense and become visible, forming **tetrads** (pairs of homologous chromosomes), each consisting of two sister chromatids. These homologous chromosomes line up, in their tetrads, across the equator of the cell during **metaphase I**.
In **anaphase I**, the homologous chromosomes are pulled to opposite poles of the cell. Importantly, the sister chromatids remain attached during this phase. This ensures that each resulting cell will contain only one allele from each gene pair.
The separation of homologous chromosomes during **meiosis I** explains **Mendel's Law of Segregation**, which states that alleles for a gene separate during gamete formation. For example, if an individual has the genotype **Aa** for a gene, the **A** and **a** alleles will be carried on different homologous chromosomes. During meiosis I, these chromosomes are separated, so the resulting two cells will contain either **A** or **a**, but not both.
The **Law of Independent Assortment** also occurs during meiosis I, specifically during metaphase I, when homologous chromosomes align randomly along the equator of the cell. During anaphase I, these chromosomes are separated to opposite poles of the cell. However, this law does not explain the separation of alleles for a single gene; rather, it explains the **genetic variation** that results from the independent assortment of alleles for different genes located on different chromosomes.
$\underline{\textrm{Meiosis II: Formation of Gametes and Separating Sister Chromatids}}$:
After **meiosis I**, the two haploid cells enter **meiosis II**. During **metaphase II** the pairs of sister chromatids form a single line across the equator of the cell.
In **anaphase II**, the centromeres split, and the sister chromatids (now regarded as individual chromosomes) are pulled to opposite poles of the cell. This ensures that each **gamete** will contain a single **chromosome** from each pair, contributing to the haploid number of chromosomes.
The result of meiosis II is the formation of **four non-identical haploid gametes**, each containing one allele for each gene.
In summary, **Mendel's Law of Segregation** explains the separation of homologous chromosomes during meiosis I, which ensures that each gamete carries only one allele for each gene. **Mendel's Law of Independent Assortment** explains how genetic variation is increased by the independent inheritance of alleles for different genes, depending on their location on different chromosomes.