Meditaliano IMAT Prep

Part 1: The Language of Genetics

1.1 Mendel's Model and Basic Terminology

Mendel's genius was in his methodical approach and his formulation of a particulate theory of inheritance. He proposed that parents pass on discrete heritable units, which we now call genes. It's crucial to understand the vocabulary used to describe these concepts.

• Gene: A specific sequence of nucleotides in DNA, located at a particular position (locus) on a chromosome, that codes for a heritable trait.
• Allele: Alternative versions of a gene that account for variations in inherited characters (e.g., the allele for purple flowers vs. the allele for white flowers). An organism inherits two alleles for each character, one from each parent.
• Genotype: The genetic makeup of an organism; the combination of alleles it possesses (e.g., PP, Pp, or pp).
• Phenotype: The observable physical and physiological traits of an organism, which are determined by its genotype and environmental influences (e.g., purple flowers).
• Homozygous: Having two identical alleles for a given gene (e.g., PP or pp). Also called a true-breeder.
• Heterozygous: Having two different alleles for a given gene (e.g., Pp). Also called a hybrid.

Part 2: Mendel's Laws of Inheritance

2.1 The Law of Dominance

When an organism is heterozygous for a trait, one allele, the dominant allele, determines the organism's appearance. The other allele, the recessive allele, has no noticeable effect on the appearance. For example, in pea plants, the allele for purple flowers (P) is dominant to the allele for white flowers (p). Therefore, both PP and Pp genotypes result in a purple phenotype.

2.2 The Law of Segregation and the Monohybrid Cross

The Law of Segregation states that the two alleles for a heritable character separate (segregate) from each other during gamete formation (meiosis) and end up in different gametes. This is a direct result of the separation of homologous chromosomes during Anaphase I of meiosis. This means an egg or a sperm gets only one of the two alleles present in the parent's somatic cells.

The Test Cross

How can we determine the genotype of an individual with a dominant phenotype (e.g., a purple flower that could be PP or Pp)? We can perform a test cross, breeding the mystery individual with a homozygous recessive individual (pp). The phenotypes of the offspring will reveal the unknown genotype.

2.3 The Law of Independent Assortment and the Dihybrid Cross

The Law of Independent Assortment states that each pair of alleles segregates independently of each other pair of alleles during gamete formation. This law is a result of the random orientation of homologous pairs at the metaphase plate during Metaphase I of meiosis and applies to genes located on different, nonhomologous chromosomes.

A dihybrid cross between two individuals heterozygous for two characters (e.g., YyRr x YyRr) results in a characteristic phenotypic ratio of 9:3:3:1.

Part 3: Beyond Simple Mendelian Genetics

Mendel's laws are the foundation, but inheritance patterns are often more complex.

• Incomplete Dominance: The phenotype of the heterozygote is intermediate between the phenotypes of the two homozygotes (e.g., red and white snapdragons producing pink offspring). The F₂ generation shows a 1:2:1 phenotypic ratio (Red:Pink:White).
• Codominance: Both alleles affect the phenotype in separate, distinguishable ways. For example, in the human ABO blood group system, the Iᴬ and Iᴮ alleles are codominant, and both are dominant to the i allele. This is also an example of Multiple Alleles, as there are three alleles in the population.
• Pleiotropy: A single gene having multiple effects on an individual's phenotype. For example, sickle-cell disease is caused by a single gene mutation but results in a wide range of symptoms, including anemia, pain, and organ damage.
• Epistasis: A gene at one locus alters the phenotypic expression of a gene at a second locus. In Labrador retrievers, one gene (B/b) determines black/brown pigment, but a second gene (E/e) determines if the pigment will be deposited. An individual with 'ee' genotype will be golden regardless of the B/b alleles.
• Polygenic Inheritance: An additive effect of two or more genes on a single phenotypic character. These are called quantitative characters and usually show a continuous variation in a population (e.g., skin color, height).
• Sex-Linked Traits: A gene located on either sex chromosome (X or Y) is called a sex-linked gene. Recessive X-linked traits are far more common in males than in females because males have only one X chromosome. Fathers pass X-linked alleles to all of their daughters but none of their sons.

Part 4: Interactive Quiz