Mendelian Genetics and Patterns of Inheritance: Study Guide (Ch. 11-12)

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Chapter 11 – Mendel and the Gene

Introduction to Mendelian Genetics

Mendelian genetics explains how traits are inherited through discrete units called genes. Gregor Mendel's experiments with pea plants established foundational principles of inheritance, including the concepts of dominant and recessive alleles, segregation, and independent assortment.

Key Point 1: Mendel’s Law of Segregation states that each individual has two alleles for each gene, which segregate during gamete formation so that each gamete receives only one allele.
Key Point 2: Dominant alleles mask the expression of recessive alleles in heterozygotes.
Example: Crossing true-breeding purple-flowered (PP) and white-flowered (pp) pea plants produces F1 offspring (Pp) with purple flowers. When F1 plants are crossed, the F2 generation shows a 3:1 ratio of purple to white flowers.

Genotype and Phenotype

Genotype refers to the genetic makeup of an organism, while phenotype is the observable trait.

Genotype: The combination of alleles (e.g., PP, Pp, pp).
Phenotype: The physical expression (e.g., purple or white flowers).
Example: Heterozygous plants (Pp) have purple flowers, but carry the recessive allele for white.

Monohybrid and Dihybrid Crosses

Monohybrid crosses involve one trait, while dihybrid crosses involve two traits.

Monohybrid Cross: PP x pp produces all Pp (purple) in F1; F2 ratio is 3:1.
Dihybrid Cross: YyRr x YyRr produces a 9:3:3:1 phenotypic ratio in F2.

Law of Independent Assortment

Alleles of different genes assort independently during gamete formation, leading to genetic variation.

Key Point: Applies to genes on different chromosomes.
Example: Dihybrid cross (YyRr x YyRr) yields four possible phenotypes.

Definitions of Key Terms

Allele: Variant of a gene.
Gene: Segment of DNA coding for a trait.
Homozygous: Two identical alleles (e.g., PP or pp).
Heterozygous: Two different alleles (e.g., Pp).
Genotype: Genetic makeup (e.g., PP, Pp, pp).
Phenotype: Observable traits.
Parental Cross: Cross between true-breeding parents.
Monohybrid Cross: Cross involving one trait.
Dihybrid Cross: Cross involving two traits.

Patterns of Inheritance

Incomplete Dominance

Incomplete dominance occurs when the heterozygote phenotype is intermediate between the two homozygotes.

Key Point: Neither allele is completely dominant.
Example: Crossing red and white snapdragons produces pink flowers.

Multiple Alleles

Some genes exist in more than two allelic forms.

Example: ABO blood group in humans (alleles: IA, IB, i).

Pedigree Analysis

Pedigrees are used to track inheritance patterns in families and determine whether traits are dominant or recessive.

Key Point: Recessive traits often skip generations; dominant traits appear in every generation.
Example: Albinism is recessive; affected individuals have two copies of the recessive allele.

Punnett Squares and Probability

Punnett squares are used to predict the probability of offspring genotypes and phenotypes.

Key Point: Probability of combined independent events is the product of their individual probabilities.
Example: Probability of being white AND short in snapdragons:

True-Breeding Individuals

True-breeding individuals are homozygous for the trait and produce offspring with the same phenotype when self-pollinated.

Key Point: No genetic variation for the trait in true-breeding lines.

Blood Group Inheritance

Blood type inheritance demonstrates multiple alleles and codominance.

Key Point: Offspring can inherit any combination of parental alleles.
Example: Cross between type A (IAIA or IAi) and type B (IBIB or IBi) can produce children with type A, B, AB, or O blood.

Genetic Crosses in Animals and Plants

Genetic crosses can be used to predict the inheritance of traits in animals and plants using Punnett squares and probability calculations.

Example: In horses, crossing BbTt x BbTt (black trotting) yields a 9:3:3:1 ratio for color and gait.
Probability Calculation: Probability of black trotting horse:

Interpreting Phenotypic Ratios

Phenotypic ratios help determine the mode of inheritance and dominance relationships.

Example: For flower color and seed shape, a 9:3:3:1 ratio indicates independent assortment and dominance.

Table: Mendelian Crosses and Ratios

Cross	Phenotypic Ratio
Monohybrid (Pp x Pp)	3 purple : 1 white
Dihybrid (YyRr x YyRr)	9 yellow round : 3 yellow wrinkled : 3 green round : 1 green wrinkled
Incomplete Dominance (Rr x Rr)	1 red : 2 pink : 1 white
Blood Type (IAi x IBi)	1 AB : 1 A : 1 B : 1 O

Formulas and Equations

Probability of independent events:
Genotypic ratio for monohybrid cross:

Additional info:

Pedigree analysis is a key tool in human genetics for determining inheritance patterns.
Independent assortment increases genetic diversity in sexually reproducing organisms.