Chpt. 14 Mendel's Experiments in Genetics

Mendel's Experiments

• Mendel studied pea plants because they have a short generation time, allowing him to observe several generations in a single growing season.
• He could control mating in pea plants by removing the male organs to prevent self-fertilization.
• He meticulously recorded data and kept track of thousands of plants throughout his experiments.
• He only studied traits with distinct forms, such as purple or white flowers, green or yellow seeds, eliminating ambiguity in observation.

Terminology Used

• Character: A heritable feature, such as flower color.
• Trait: A variant of a character, such as purple or white flowers.
• Parental generation (P): The original plants used for crossing.
• First filial generation (F1): The offspring of the parental generation.
• Second filial generation (F2): The offspring of the F1 generation.

Parental Generation (P)

• The parental plants were true-breeding, meaning they consistently produced offspring with the same traits.
• For example, a purple-flowered plant only produced purple-flowered offspring, and a white-flowered plant only produced white-flowered offspring.
• Mendel crossed a purple-flowered plant with a white-flowered plant.

Results of Crossing

• All F1 generation plants had purple flowers, demonstrating that the white trait was not lost but masked.
• No light purple flowers were observed, contradicting the blending hypothesis.
• In the F2 generation, some plants exhibited white flowers again, indicating that the white "particle" was present but masked in the F1.
• The F2 generation had a 3:1 ratio of purple to white flowers, suggesting that the white trait was not lost but recessive.
• This pattern was consistent across six other characters.

Key Concepts from Mendel's Experiments and Observations

• Particulate inheritance: Traits are determined by discrete units of inheritance, now known as genes.
• Alleles: Different versions of a gene, accounting for variations in a character.
• Law of Segregation: Each organism inherits two alleles for each character, one from each parent, which are separated during gamete formation.
• Law of Independent Assortment: Alleles for different genes assort independently of each other during gamete formation.

Polygenic Inheritance

• When the phenotype is influenced by multiple genes.
• Examples include height, skin color, and weight.

Probability and Genetics

• The multiplication rule calculates the probability of two independent events occurring together.
• The addition rule calculates the probability of one event occurring given two or more mutually exclusive events.

Epistasis

• A gene at one locus can affect the expression of a gene at another locus.
• For example, in Labrador Retrievers, the "B" gene controls black (B) or brown (bb) coat color, however, a separate gene (C) determines if pigment is deposited in the hair. If the "C" gene is homozygous recessive (cc), the dog will be albino regardless of its genotype for the "B" gene.

Mendelian Inheritance in Humans

• Many human traits follow Mendelian inheritance patterns.
• Studying pedigrees can help determine the mode of inheritance for human traits.

Genes and the Environment

• Genes play a major role in our lives, but we are not simply products of our genes.
• The environment can influence how a gene is expressed.
• Example: Hydrangeas of the same genetic variety grown in acidic soil with free aluminum will have blue flowers, while hydrangeas grown in basic soil will have pink flowers.

Alleles and Genetic Expression

• Complete dominance: One allele completely masks the effect of another allele.
  • Example: In pea plants, the allele for purple flowers (P) is dominant over the allele for white flowers (p). A plant with the genotype PP or Pp will have purple flowers.
• Incomplete dominance: The heterozygous phenotype is intermediate between the two homozygous phenotypes.
  • Example: Snapdragon flowers with the genotype CRCR are red, CWCW are white, and CRCW are pink.
• Codominance: Both alleles are expressed in the heterozygote.
  • Example: In humans, the ABO blood group is determined by three alleles: IA, IB, and i. Individuals with the genotype IAIB have both A and B antigens on their red blood cells.
• Multiple alleles: Some genes have more than two alleles in the population.
  • Example: The ABO blood group system is an example of multiple alleles.
• Pleiotropy: One gene affects multiple phenotypic characters.
  • Example: Sickle-cell disease, which affects red blood cell shape, can also cause anemia, joint pain, and organ damage.

Interactions Between Genes

• Epistasis: The phenotypic expression of one gene affects the expression of another gene.
  • Example: In Labrador retrievers, the gene for coat color (B) is epistatic to the gene for pigment deposition (E). A dog with the genotype BbEe will have black fur if it inherits at least one dominant E allele, but will have brown fur if it inherits two recessive e alleles.
• Polygenic inheritance: A single phenotypic character is affected by two or more genes.
  • Example: Skin color, height, and eye color are examples of polygenic inheritance.

Explore the groundbreaking experiments conducted by Gregor Mendel on pea plants, which laid the foundation for modern genetics. This quiz covers key terminology, important generations, and the meticulous methods Mendel employed to study inheritance patterns. Test your understanding of Mendel's work and its significance in the field of genetics.