Meiosis and Genetics: Structured Study Notes

Meiosis

Purpose and Overview of Meiosis

Meiosis is a specialized type of cell division that reduces the chromosome number by half, producing four genetically distinct gametes. It is essential for sexual reproduction and genetic diversity.

• Purpose of Meiosis: To produce gametes (sperm and egg cells) with half the chromosome number of the parent cell.

• Gametes: Haploid reproductive cells (n) formed from diploid (2n) cells.

• Fertilization: The fusion of two gametes restores the diploid chromosome number in offspring.

Chromosomes and Stages of Meiosis

Meiosis consists of two sequential divisions: Meiosis I and Meiosis II, each with distinct phases.

• Homologous Chromosomes: Chromosome pairs, one from each parent, with the same genes but possibly different alleles.

• Stages of Meiosis: Prophase, Metaphase, Anaphase, Telophase (I and II).

• Meiosis I: Homologous chromosomes separate, reducing chromosome number by half.

• Meiosis II: Sister chromatids separate, similar to mitosis.

Genetic Variation in Meiosis

Meiosis introduces genetic variation through independent assortment and crossing over.

• Mendel's Law of Segregation: Each gamete receives only one allele of each gene.

• Independent Assortment: Homologous chromosomes are randomly distributed to gametes.

• Crossing Over: Exchange of genetic material between homologous chromosomes during Prophase I.

DNA and Chromosome Amounts During Meiosis

The amount of DNA and chromosome number changes throughout meiosis.

• DNA Amount: DNA is replicated before meiosis begins, then halved after Meiosis I and II.

• Chromosome Number: Diploid (2n) cells become haploid (n) after meiosis.

• Example: In humans, cells start with 46 chromosomes (2n), and gametes end with 23 (n).

Genetics

Genotype and Phenotype

Genotype refers to the genetic makeup of an organism, while phenotype is the observable trait.

• Genotype Examples: AA, Aa, aa (for a single gene); AABB, AaBb (for two genes).

• Phenotype: The physical expression of the genotype (e.g., green peas, wrinkled seeds).

Probability and Punnett Squares

Punnett squares are used to predict the probability of offspring genotypes and phenotypes.

• Probability Calculation: Use the formula $P = \frac{\text{number of desired outcomes}}{\text{total number of outcomes}}$.

• Example: For a monohybrid cross Aa x Aa, the probability of aa offspring is $\frac{1}{4}$.

Autosomal vs. Sex-Linked Traits

Genes can be located on autosomes (non-sex chromosomes) or sex chromosomes (X and Y).

• Autosomal Traits: Traits determined by genes on chromosomes 1-22.

• Sex-Linked Traits: Traits determined by genes on the X or Y chromosome.

• Example: Color blindness is an X-linked recessive trait.

Dominance and Codominance

Dominance describes how alleles interact to produce phenotypes.

• Complete Dominance: One allele completely masks the other.

• Codominance: Both alleles are expressed equally (e.g., AB blood type).

• Incomplete Dominance: Heterozygotes show an intermediate phenotype (e.g., pink flowers from red and white parents).

Test Crosses and Multiple Alleles

Test crosses help determine the genotype of an organism with a dominant phenotype.

• Test Cross: Cross the individual with a homozygous recessive to reveal genotype.

• Multiple Alleles: Some genes have more than two alleles (e.g., ABO blood group).

Pedigree Analysis

Pedigrees are diagrams that show inheritance patterns in families.

• Symbols: Squares for males, circles for females; shaded for affected individuals.

• Autosomal vs. Sex-Linked: Autosomal traits appear equally in both sexes; sex-linked traits often affect males more.

• Example: In a pedigree, a filled square indicates an affected male.

Complex Crosses and Probability

Genetic crosses can involve multiple genes; probabilities are calculated for each gene and multiplied together.

• Two-Factor, Three-Factor, Four-Factor Crosses: Analyze each gene separately, then multiply probabilities for combined outcomes.

• Example: For AaBb x AaBb, probability of aabb offspring is $\frac{1}{16}$.

Pedigree Interpretation and Genetic Disorders

Pedigrees can be used to infer inheritance patterns and identify carriers of genetic disorders.

• Autosomal Dominant: Trait appears in every generation.

• Autosomal Recessive: Trait may skip generations; carriers are heterozygous.

• Sex-Linked: More males affected; females can be carriers.

Additional info:

• For linked genes, recombination frequency can be used to estimate genetic distance.

• Common pedigree symbols include: X-linked recessive, X-linked dominant, Y-linked, and mitochondrial inheritance.