Mendelian Genetics Overview

Mendelian Genetics

• Gregor Mendel, an Austrian monk, experimented with pea plants in the 1800s
• Pea plants reproduce through self-fertilization, meaning a flower's male and female gametes fertilize each other
• Mendel manually cross-pollinated pea plants to study inheritance
• Mendel's experiments in 1866 marked the beginning of genetics

Inheritance of Traits

• Mendel used true-breeding pea plants to study traits that remain constant over generations
• He studied seven traits: flower color, seed color, seed pod color, seed shape, seed pod shape, stem length, and flower position

How Mendel's Experiments Worked

• Mendel considered the parent (P) generation and the first filial (F1) generation
• F1 offspring had one trait from each parent (yellow seeds from a cross between yellow-seeded plants and green-seeded plants)
• Mendel allowed F1 plants to self-fertilize, resulting in an F2 generation with a 3:1 ratio of the expressed trait
• Mendel repeated this process for several characteristics.

How Mendel explained results

• Mendel proposed each trait had two forms (alleles)
• Alleles are different forms of a gene passed from generation to generation
• Yellow-seed and green-seed plants have different alleles
• Mendel proposed each trait is controlled by two alleles and there is a dominant trait which appears in the F1 generation and a recessive trait that hides in the F1 generation

Punnett Squares

• Punnett Squares were created to predict genotypes and phenotypes of offspring
• The genotype of one parent is displayed vertically, and the genotype of the other parent is horizontally across the top
• Each small square in the Punnett Square represents a possible combination of alleles for offspring

Probability in Genetics

• Genetic traits follow the rules of probability (chance)
• The larger the number of offspring, the closer the results will be to the prediction made by Punnett Squares

Genetic Recombination

• Genetic recombination occurs through crossing over and independent assortment during meiosis of genes
• Genetic recombination creates new gene combinations
• Pea plants have seven pairs of chromosomes. Possible combinations for chromosome combinations are 2⁷ or 128

Genes on Chromosomes

• Genes located close together on the same chromosome (linked genes) tend to travel together during gamete formation
• Linked genes are an exception to Mendel's law of independent assortment
• Crossing over occurs more often between genes that are far apart on a chromosome

Polyploidy

• Polyploidy means having multiple sets of chromosomes (3n or more)
• It is common in flowering plants such as wheat, oats, and sugarcane

Applied Genetics

• Selective Breeding: Modifying animals and plants to have desired traits. Examples include huskies bred for speed and Saint Bernards for smell
• Hybridization: Crossing organisms with different traits to create hybrid offspring with combined traits. Example: a disease-resistant tomato plant crossed with a fast-growing tomato plant.

Inbreeding

• Inbreeding is a selective breeding technique that involves closely related organisms that show desired traits
• Inbreeding is used to ensure desired traits are passed on, but it can also increase the chances of harmful recessive traits being passed on

Test Cross

• Used to determine the genotype of an organism.
• A test cross is conducted by crossing an organism with an unknown genotype with an organism whose genotype for the trait is known (two recessive alleles)

Complex Inheritance Patterns

• Incomplete Dominance: The heterozygous phenotype is an intermediate between the two homozygous phenotypes. Example: red snapdragons crossed with white snapdragons produce pink snapdragons
• Codominance: Both alleles are expressed in the heterozygous condition. Example: sickle-cell anemia
• Multiple alleles: A gene has more than two alleles. Example: blood type

Environmental Influence

• Environmental factors like sunlight, temperature, and water can affect an organism's phenotype.
• Example: Siamese cats have darker coloring in cooler parts of their body

Twin Studies

• Twin studies can identify if traits are inherited or influenced by the environment.
• Identical twins share the same genes, so differences are often due to environment.

Epistasis

• When the allele of a second gene hides the effect of the allele of another gene. This effect is from different genes influencing a trait. Example: Labrador coat colors.

Sex-linked Traits

• Traits carried on the X chromosome (sex-linked traits or X-linked).
• Males are more likely to express recessive sex-linked traits than females, because they only have one X chromosome. Example: red-green color blindness and hemophilia.

Polygenic Traits

• Traits from more than one gene which influence a trait. Example: skin color and height