Cell Division and Genetics Review

Ploidy of Daughter Cells After Mitosis and Meiosis

• Mitosis:
  • Starting with 2n = 46, the daughter cells will also have 2n = 46.
  • If n = 20, daughter cells after mitosis will still have n = 20.
  • Mitosis always produces two daughter cells.
  • Starting with 4n, the concept remains the same.
• Meiosis:
  • Involves interphase followed by prophase I.
  • Prophase I:
    • Nuclear membrane disappears.
    • Spindle fibers appear.
    • Synapsis or crossing over occurs between chromosomes.
  • Metaphase I:
    • Homologous pairs line up in the middle.
  • Anaphase I:
    • Homologous pairs are pulled apart, resulting in one of each pair on opposite ends.
  • Telophase I:
    • Spindle fibers disappear, nuclear membrane reappears.
    • Cleavage furrow forms, followed by cytokinesis.
• Meiosis I Outcomes:
  • Starting with a parent cell having 2n = 46, meiosis I produces two daughter cells.
  • Each daughter cell has a ploidy of n = 23. To find the 'n' value, divide the chromosome number by two.

Ploidy Considerations in Gamete Formation and Hybridization

• Meiosis always halves the chromosome number during gamete formation.
• Hybrid ploidy involves combining two different ploidies (e.g., 1n + 3n).

Genetic Traits and Inheritance

• For somatic or autosomal traits, sex-linked inheritance principles do not apply unless specified in the question.
• F2 Generation:
  • If a question asks about the F2 generation, the parental (P) generation will likely be provided.
  • Example: If the parent generation is Bb \times bb, the F1 generation is derived from this cross.
  • F2 phenotypic ratio for a dihybrid cross is typically 9:3:3:1.
    • 9 offspring will have at least one dominant allele for both genes (e.g., one Big A and one Big B allele).
    • 3 offspring will have one dominant allele for one gene and homozygous recessive for the other gene (e.g., one Big A and two little b alleles).
    • Another 3 will have a reciprocal combination (e.g., two little a alleles and one Big B allele).
    • 1 will be homozygous recessive for both traits.

Complex Inheritance Example: Two-Gene Deafness

• In the mentioned deafness example, deafness occurs if at least one dominant allele is present for either of the two genes.
• The question likely involves determining whether deafness is controlled by one or two genes based on the provided data.

Sex-Linked Traits and Inheritance

• For sex-linked traits, the father's X chromosome always goes to the daughters.
• If the father is affected, it directly impacts the daughters, as they receive one X chromosome from each parent.
• Sons receive their X chromosome from the mother; therefore, the mother's genotype is crucial for determining the son's phenotype.
• The daughter's phenotype depends on which X chromosome she receives from the mother (depending on whether the mother is heterozygous or homozygous).
• If both parents have the trait, the mother's zygosity (heterozygous or homozygous) greatly affects the possible outcomes for their offspring.