Mendelian Genetics & Inheritance Quiz

Mendelian Genetics & Inheritance

• Gregor Mendel (1822-1884) documented inheritance mechanisms before chromosomes and genes were observed.
• Mendel used garden peas (Pisum sativum) for his experiments.
• Traits of organisms used in experiments should show variation in characteristics. They should produce a large number of offspring. They need to allow controlled matings to be conducted and have a short life cycle. They should be convenient to handle as well.
• True-breeding varieties exist for particular traits that could be controlled or mated easily.
• Mendel studied the inheritance of 7 characters that exist in two forms (either/or traits).

Genetic Principles

• Mendel's work examined genetic principles that account for the passing of traits from parents to offspring.
• Two hypotheses were considered: blending and particulate.

Mendel's Experimental Approach

• Mendel performed planned breeding experiments.
• He chose garden peas because True-breeding varieties exist, variations exist for particular traits, controlled mating was possible, short life cycles, and they are easy to handle and grow a lot of offspring.
• He studied 7 characters with two contrasting forms (e.g., purple vs. white flowers).

Results of Mendel's Pea Plant Crosses

• 7 characteristics were studied by Mendel.
• Dominant trait to recessive ratios are shown.
• Data from dominant vs. recessive crosses are shown using example ratios like 3:1.

Mendel's Experimental Design

• True-breeding parental generation (P)
  • Used plants with one particular trait.
  • Example: Purple-flowered plants crossed with white-flowered plants.
• First filial generation (F1)
  • All offspring from P generation exhibit one trait.
  • Example: All F1 plants had purple flowers.
• Second filial generation (F2)
  • The trait that was hidden in F1 started reappearing.
  • Example: A ratio of 3:1 (purple flower to white flower) of offspring is observed.

Laws of Inheritance

• Law of Segregation

  • Parents don't transmit traits directly but as "heritable factors" (genes).
  • Alleles are different forms of a gene.
  • Each individual inherits two alleles per gene (one from each parent).
  • During gamete formation, alleles separate, so each gamete gets only one allele for each gene.

• Law of Independent Assortment

  • Inheritance of one character has no effect on the inheritance of another.
  • Alleles of different genes are passed independently to the offspring.
  • Used in dihybrid cross, examining two traits.
  • Example: seed color (yellow or green) and seed shape (round or wrinkled) were both transmitted independently.

Concepts Related To Inheritance

• First Concept: Parents pass heritable factors (genes) to offspring, not traits directly.
• Second Concept: Different forms of genes (alleles) account for variations in an inherited character. Alleles are located at a particular gene locus on homologous chromosomes.
• Third Concept: For each character, an organism inherits two alleles (from each parent). The alleles at the same locus might be identical or different. (Homozygous or Heterozygous)
• Fourth Concept: If alleles at a locus are different, the dominant allele determines the trait's appearance, while the recessive allele has no noticeable effect. This is determined by genotypes (the genes present in an individual) resulting in phenotypes (the observable characteristics present).
• Fifth Concept: The two alleles for a heritable character segregate during gamete formation. Each gamete receives only one allele from each parent.

Terminology

• Allele: Different versions of a gene.
• Locus: Specific location of a gene on a chromosome.
• Homozygous: Two identical alleles for a gene.
• Heterozygous: Two different alleles for a gene.
• Genotype: Genetic makeup of an organism (e.g., Pp).
• Phenotype: Observable characteristics of an organism (e.g., purple flower).
• Dominant Allele: An allele that expresses its phenotype even when paired with a recessive allele.
• Recessive Allele: An allele that only expresses its phenotype when paired with another recessive allele
• Monohybrid cross: A cross between individuals heterozygous for one trait.
• Dihybrid cross: A cross between individuals heterozygous for two traits.
• Test cross: A cross between an individual with a dominant phenotype (but unknown genotype) and a homozygous recessive individual.

Punnett Squares

• Diagrams to predict the genotypes and phenotypes of offspring in genetic crosses.
  • Shows the distribution of possible genotypes from a cross.
  • Shows the phenotypic ratio result of a cross.
  • A way to predict genetic inheritance.