Untitled

Genetics Terminology

  • Recessive: A trait that is expressed only when two copies of the recessive allele are present. It may not be observed in individuals who possess only one copy of the recessive allele alongside a dominant allele.

  • Dominant: A trait that is expressed when at least one copy of the dominant allele is present. In genetic notation, a dominant allele may be represented by an uppercase letter (e.g., A).

Mendelian Genetics

  • Recessive and Dominant Relationships:

    • If both parents are unaffected (ii) but have affected offspring (i.e., recessive), it suggests both parents are carriers (Ii).
    • A genotype of two unaffected parents could have affected children, indicating a recessive inheritance pattern.
    • Notation: JJ = Dominant, O = Recessive or rr.
    • Affected offspring can arise from two unaffected parents due to the skipping of generations in recessive traits.
    • Parental genotypes may be represented as RR or Pr, with offspring appearing less frequently.

  • X-Linked Traits:

    • Traits where the gene causing the trait is located on the X chromosome.
    • For affected males inheriting this trait, all sons will be unaffected because they inherit the Y chromosome from their father.
    • The affected individual must pass the X-linked recessive trait to all daughters but may not pass it to sons.
Autosomal Traits
  • Autosomal Inheritance:
    • Traits associated with autosomes (non-sex chromosomes).
    • In autosomal recessive conditions, both sexes (males and females) are affected equally.
    • If one individual is affected, every offspring has a probability based on parental genotypes.
    • An affected individual is often represented as ‘a’ or ‘aa’.
    • A carrier is represented as ‘Aa’ and unaffected individuals as ‘AA’.
    • Affected offspring can be produced from unaffected parents if both are carriers.

Punnett Squares

  • A tool used to predict the genetic makeup of offspring from parental genotypes involving cross-pollination or mating.
    • Monohybrid Cross: A cross focusing on a single trait that has two alleles.
    • Alleles can be homozygous (same alleles, e.g., RR or rr) or heterozygous (different alleles, e.g., Rr).
Constructing a Punnett Square
  1. Identify Parents' Genotypes: Write down the genotypes of the mother and father (e.g., Rr x Rr).
  2. Create the Square: Fill in a 2x2 grid and place alleles on the top and side.
  3. Complete the Boxes: Indicate possible genotypes for the offspring in each box considering combinations.
  4. Summarize Results: Calculate the phenotypic (observable traits) and genotypic (genetic makeup) ratios from the completed square.
    • Example Ratios:
      • 25% Homozygous Dominant (RR)
      • 50% Heterozygous (Rr)
      • 25% Homozygous Recessive (rr)

Pedigrees

  • A diagram that traces the inheritance of a trait through several generations.
    • Autosomal Traits:
    • Located on autosomes.
    • Males and females are affected equally.
    • Autosomal Recessive Conditions:
    • Individuals with genotype rr are affected, Aa are carriers, and AA are unaffected individuals.
    • Two affected parents will always have affected children, often skipping generations.
    • Sex-Linked Traits:
    • Traits that may be linked to the X or Y chromosomes.
    • X-Linked Recessive:
      • Affected females are XX, while affected males are XY.
      • X-linked traits may appear more frequently in males.
    • X-Linked Dominant:
      • Affected individuals (XX) can transmit the trait, and males will pass on the Y chromosome.
    • Y-Linked Traits:
      • Traits found only on the Y chromosome affecting only males.

Summary of Terminology

  • Autosomal Dominant:

    • RR (affected), Rr (carrier), rr (unaffected).
    • Parents can have unaffected children despite being affected themselves.
    • Traits generally do not skip generations.

  • Sex-Limited Traits:

    • Specifically located on sex chromosomes and can also be dominant or recessive.