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.