Inheritance and Sources of Genetic Variation

Meiosis: Reduction of Genetic Information

Meiosis is a specialized type of cell division that reduces the chromosome number by half, resulting in the formation of haploid gametes (sperm and egg). This process is essential for sexual reproduction and ensures genetic diversity in offspring.

• Diploid (2n): Cells contain two copies of every gene.

• Haploid (n): Gametes contain one copy of every gene.

• Meiosis occurs in gonads: Ovaries and testes.

• Meiosis generates: Haploid gametes for sexual reproduction.

Additional info: The graph illustrates the reduction in DNA content and ploidy during meiosis, highlighting the transition from diploid to haploid cells.

Sexual vs. Asexual Reproduction

Reproduction can occur via asexual or sexual means. Asexual reproduction is faster and produces genetically identical offspring, while sexual reproduction increases genetic variation.

• Asexual reproduction: Involves mitosis, producing identical diploid cells.

• Sexual reproduction: Involves meiosis and fertilization, producing genetically diverse offspring.

Mechanisms Increasing Genetic Diversity

Sexual reproduction increases genetic diversity through three key mechanisms:

1. Homologous recombination (Crossing over): Exchange of genetic material between homologous chromosomes during Prophase I of meiosis.

2. Independent assortment: Random orientation of chromosome pairs during Metaphase I.

3. Random fertilization: Any sperm can fertilize any egg, further increasing genetic variation.

4. Mutation: The ultimate source of genetic variation.

Homologous Recombination (Crossing Over)

During Prophase I of meiosis, homologous chromosomes pair up and exchange segments, creating new combinations of alleles.

• Creates recombinant chromatids: Chromatids with new allele combinations not found in either parent.

• Occurs only in meiosis: Not in mitosis.

Independent Assortment

Independent assortment refers to the random orientation of non-homologous chromosome pairs during Metaphase I of meiosis, resulting in different combinations of chromosomes in gametes.

• Law of Independent Assortment: Each pair of chromosomes segregates independently of the others.

• Results in genetic variation: Each gamete receives a random mix of maternal and paternal chromosomes.

Random Fertilization

Random fertilization further increases genetic diversity, as any sperm can fertilize any egg, resulting in a vast number of possible genetic combinations.

• Number of possible combinations: For humans, more than 70 trillion possible combinations due to independent assortment and crossing over.

Mendel’s Laws and Meiosis

Mendel’s laws of inheritance are explained by the behavior of chromosomes during meiosis:

• Law of Segregation: Homologous chromosomes separate during Anaphase I, so each gamete receives only one allele of each gene.

• Law of Independent Assortment: Chromosome pairs align independently during Metaphase I, leading to genetic variation.

Punnett Squares and Genetic Outcomes

Punnett squares are used to predict the genetic outcomes of crosses, based on Mendel’s laws and random fertilization.

• Marginal letters: Represent the alleles contributed by each parent.

• Combinations: Represent possible genotypes of offspring.

Phases of Meiosis and Mitosis

Meiosis consists of two divisions: Meiosis I (homologs separate) and Meiosis II (sister chromatids separate). Mitosis involves a single division where sister chromatids separate.

• Meiosis I: Reductive division (2n → 1n).

• Meiosis II: Equational division (1n → 1n).

• Mitosis: Maintains ploidy (2n → 2n).

Comparison of Mitosis and Meiosis

Mitosis and meiosis differ in their purpose, location, and outcomes:

Additional info: Some organisms, like yeast, can undergo mitosis as haploids and only become diploid under stress.

Order of Events in Meiosis

The key events in meiosis occur in the following order:

1. Homologs synapse and cross over (Prophase I).

2. Homologous chromosomes pair and align in the middle (Metaphase I).

3. Homologous chromosomes separate to opposite poles and cells divide (Anaphase I and Telophase I).

4. Sister chromatids separate (Meiosis II).

Key Definitions

• Homologous chromosomes: Chromosomes with the same genes but possibly different alleles.

• Sister chromatids: Identical copies of a chromosome, joined at the centromere.

• Ploidy: Number of sets of chromosomes in a cell (haploid = 1n, diploid = 2n).

Summary Table: Mitosis vs. Meiosis

Equations and Formulas

• Number of possible gamete combinations: (where n = number of chromosome pairs)

• Number of possible zygote combinations:

Study Checklist

• Differentiating sister chromatids and homologous chromosomes

• Understanding haploid (n) and diploid (2n)

• Advantages of meiosis

• Overview of meiosis: 1 replication, 2 divisions

• Phases of Meiosis I and II

• Comparison of mitosis and meiosis

• Locations and products of meiosis and mitosis