Study Guide: Heredity and Genetics

Study Guide: Heredity and Genetics

Introduction to Genetics

  • Genetics: The science of heredity.
  • Heredity/Inheritance: The process by which traits are passed from one generation to the next.

Key Definitions

  • Genotype: The genetic makeup of an individual, expressed as alleles (e.g., Hh). These are coded by genes located on chromosomes.
  • Phenotype: The observable traits of an individual, such as brown eyes or sickled red blood cells.

Alleles

  • Every individual possesses two alleles for a given trait, one inherited from the mother and one from the father.
  • Homozygous: An organism with two identical alleles for a trait (e.g., HH or hh).
  • Heterozygous (or Hybrid): An organism with two different alleles for a trait (e.g., Hh).

Mendel's Laws

  • Law of Segregation/Independent Assortment: The principle stating that alleles for a trait separate during meiosis, resulting in gametes that carry only one allele for each trait.
    • During fertilization, the alleles from both parents unite, restoring the pair of alleles in the offspring.
  • Crossing Over: The exchange of genetic material between homologous chromosomes during meiosis, leading to genetic diversity.
    • More frequent between genes that are far apart; genes that are close together are inherited as a group.

Punnett Squares

  • A tool used to predict the probability of inheritance of genes.
  • Example Case: Predicting offspring hair color based on parental genotypes (e.g., green hair vs. yellow hair).
  • You should be able to:
    • Draw and analyze a monohybrid Punnett square (involves one trait).
    • Draw and analyze a dihybrid Punnett square (involves two traits).

Complex Forms of Inheritance

  • Multiple Alleles: When more than two possible alleles exist for a trait.
    • Example - ABO Blood Type:
    • Alleles: IA, IB, and i.
    • Phenotypes:
      • Blood type A: Genotypes IAIA or IAi.
      • Blood type B: Genotypes IBIB or IBi.
      • Blood type O: Genotype ii.
    • Dominance: IA and IB are dominant over i; therefore, only one dominant allele results in the respective phenotype.
    • Codominance: Both alleles express their traits fully when present.
    • Example: Genotype IAIB expresses blood type AB (both A and B present).
    • Example: Sickle Cell Disease: An individual with both normal hemoglobin and sickle cell alleles (HbHbS) expresses both traits (normal and sickled red blood cells).
    • Incomplete Dominance: Neither allele is completely dominant, resulting in a blended phenotype.
    • Example: If red flowers and white flowers are crossed, the offspring will be pink.

X-linked Traits

  • Traits inherited on the X chromosome.
    • Females (XX) are less likely to express X-linked recessive disorders because they have two X chromosomes.
    • Males (XY) will express the disorder if they inherit a recessive trait since they only possess one X chromosome.

Polygenic Traits

  • Traits that are determined by the combined effects of multiple genes.
    • Example: Skin color shows a spectrum due to variations in several genes.

Environmental Influence

  • Inheritance and expression of traits can be affected by environmental factors.

Pedigrees

  • Pedigree: A diagram that illustrates the inheritance of a trait within a family.
  • Recessive Disorders: Require two alleles to be expressed in the phenotype (e.g., Bb where the lowercase b represents the disorder).
  • Dominant Disorders: Only one allele is required for the disorder to appear (e.g., Bb where the capital B indicates the disorder).
  • Carrier: An individual who possesses one allele for a recessive disorder but does not exhibit symptoms.
  • Skills: You should be able to:
    • Draw a pedigree diagram.
    • Fill in genotypes for family members.
    • Predict who will be affected by the disorder based on the pedigree.
    • Determine if the trait represented in a pedigree is recessive or dominant.