Genetics and Inheritance

Genotype and Phenotype

  • Genotype: Genetic makeup of an organism.
  • Phenotype: Observable characteristics of an organism.
  • Example: Flower color (purple or white).
    • Purple: Dominant P allele.
    • White: Two recessive p alleles.
  • Genotype is responsible for phenotype, but they can be different.

Hypotheses About Genetic Material

  • Blending Hypothesis: Genetic material from parents blends together.
  • Particular Hypothesis: Parents pass on discrete heritable units (genes).
  • Gregor Mendel: Documented particulate mechanism through experiments with garden peas.

Mendel's Experimental Organisms

  • Pea Plants: Good experimental organisms.
    • Lots of varieties.
    • Many varieties were pure breeding.
  • Heritable Features (Characters): Varies among individuals (e.g., flower color).
  • Trait: Each variant of a character (e.g., purple or white flowers).

Advantages of Pea Plants

  • Available in many varieties.
  • Short generation time.
  • Large numbers of offspring.
  • Controlled mating (self-pollination or cross-pollination).
  • Tracked characters with two distinct alternative forms.
  • Used true-breeding varieties (plants that produce offspring of the same variety when bred together).

Mendel's Experiments

  • Hybridization: Mating two contrasting, pure-breeding varieties.
  • P Generation: True-breeding parents.
  • F1 Generation: Hybrid offspring of the P generation.
  • F2 Generation: Offspring when F1 individuals self-pollinate or cross-pollinate.

Example Experiment

  • Crossed pure-breeding purple flower plant with pure-breeding white flower plant.
  • F1 Generation: All plants had purple flowers.
  • F2 Generation: Ratio of approximately 3:1 (purple to white).
  • Purple flower color: Dominant trait.
  • White flower color: Recessive trait.

Mendel's Conclusions

  • Repeated experiment with six other pea plant characters, always getting roughly the same 3:1 ratio in the F2 generation.

Mendel's Model

  • Explains the 3:1 inheritance pattern.
  • Four concepts:
    • Alternative versions of genes (alleles) account for variations in inherited characters.
      • Gene for flower color exists in two versions: purple and white.
      • Each gene resides on a specific locus on a specific chromosome.
    • For every character, an organism inherits two alleles, one from each parent.
      • Two alleles of a particular locus may be identical (true-breeding plants).
      • Alleles can differ (F1 hybrid).
    • If two alleles at a locus differ, one is dominant (determines appearance) and the other is recessive (no noticeable effect).
      • F1 generation: Purple is dominant over white, so all offspring appear purple.
    • Law of Segregation: Two alleles for a character separate during gamete formation.
      • Egg or sperm only gets one of the two alleles present in the parent.
      • Segregation corresponds to distribution of homologous chromosomes to different gametes in meiosis.

Punnett Square

  • Possible combinations of sperm and egg.
  • Dominant allele: Capital letter.
  • Recessive allele: Lowercase letter.
  • Example
    • Purple Flowers: Two dominant alleles (PP).
    • White Flowers: Two recessive alleles (pp).

F1 Generation

  • One parent gives a dominant allele, and the other gives a recessive allele.
  • Offspring have one dominant and one recessive allele (Pp). They are all purple.

F2 Generation

  • More complex due to two different alleles in both parents.
  • Offspring can have:
    • Both dominant alleles (PP).
    • Both recessive alleles (pp).
    • One dominant and one recessive allele (Pp).
  • Results in a 3:1 phenotypic ratio (three purple to one white).

Genotype vs. Phenotype Ratio

  • Phenotype: 3 purple to 1 white.
  • Genotype: 1 homozygous dominant (PP), 1 homozygous recessive (pp), 2 heterozygous (Pp).

Test Cross

  • Used to determine if a purple-flowered plant is pure (homozygous).
  • Cross the unknown plant with a homozygous recessive plant (white-flowered).
  • If any offspring are white, the purple-flowered plant was heterozygous.

Monohybrid Cross

  • Focuses on one trait.
  • Punnett square with four squares.

Monohybrid Cross Example

  • Crossing two heterozygous guinea pigs (Hh).
  • Genotypes of parents: Hh x Hh
  • Possible genotypes of offspring: HH, Hh, hh
  • Genotype ratio: 1 homozygous dominant : 2 heterozygous : 1 homozygous recessive
  • Phenotype ratio: 3 have hair : 1 hairless

Dihybrid Cross

  • Involves two characters.
  • Cross two pure-breeding parents that differ in two characters, producing dihybrids in the F1 generation.
  • Cross F1 dihybrids to determine if the characters are transmitted together or independently.
  • Example: Seed color (yellow or green) and seed shape (smooth or wrinkled).

Hypothesis

  • Inherited Together: Get a different ratio in the offspring than if they were inherited independently.
  • Inherited Independently: See four different versions in both the sperm and the egg.

Mendel's Second Law

  • Law of Independent Assortment: Each pair of alleles segregates independently of each other pair of alleles during gamete formation.
  • Applies to genes on different, non-homologous chromosomes or far apart on the same chromosome.
  • Genes located near each other on the same chromosome tend to be inherited together.

Variations from Mendelian Patterns

  • Alleles not completely dominant or recessive.
  • Gene has more than two alleles.
  • Genes produce multiple phenotypes.

Degrees of Dominance

  • Complete Dominance: Phenotype of heterozygote and dominant homozygote are the same.
  • Incomplete Dominance: Phenotype of F1 hybrid is somewhere between the phenotypes of the two parents.
  • Codominance: Two dominant alleles affect the phenotype in separate, distinguishable ways.

Incomplete Dominance Example

  • Snapdragons: Red and white pure-breeding varieties produce pink F1 generation.

Multiple Alleles Example

  • ABO blood groups: Determined by three alleles (IA, IB, i).
  • Four phenotypes: A, B, AB, O.

Human Genetics

  • Basic Mendelian genetics are the foundation for human genetics.
  • Recessively inherited disorders show up only in individuals homozygous for the allele.
  • Carriers: Heterozygous individuals carry the recessive allele but are phenotypically normal.
  • Consanguineous mating: Mating between close relatives increases the chance of two carriers mating.

Sickle Cell Disease

  • Caused by a substitution in a single amino acid in hemoglobin.
  • Homozygous individuals have abnormal hemoglobin; red blood cells are sickle-shaped.
  • Heterozygotes (sickle cell trait) are usually healthy but can suffer some symptoms.
  • Heterozygotes are less susceptible to malaria parasites.

Genetic and Environmental Components of Diseases

  • Many diseases have both genetic and environmental components.
  • Lifestyle can have a tremendous effect on the phenotype displayed.