Mendelian Genetics: Principles of Inheritance chapter 11

Mendelian Genetics

Introduction to Mendelian Genetics

Mendelian genetics is the study of how traits are inherited from one generation to the next, based on the pioneering work of Gregor Mendel in the 19th century. Mendel's experiments with pea plants established the fundamental laws of inheritance, which form the basis of classical genetics.

• Gregor Mendel (1822–1884) is known as the father of genetics.

• He used pea plants (Pisum sativum) to study inheritance patterns.

• His work led to the discovery of predictable ratios in the inheritance of traits.

Key Terms and Definitions

• Gene: A unit of hereditary information consisting of a specific nucleotide sequence in DNA.

• Locus: The specific physical location of a gene on a chromosome.

• Allele: Alternative versions of a gene that produce distinguishable phenotypic effects.

• Genotype: The genetic makeup of an organism (e.g., PP, Pp, or pp).

• Phenotype: The observable traits of an organism (e.g., purple or white flowers).

• True-breeding: Organisms that produce offspring of the same variety when they self-pollinate.

• Hybridization: The crossing of two true-breeding varieties.

• P Generation: Parental generation (true-breeding parents).

• F1 Generation: First filial generation, offspring of the P generation.

• F2 Generation: Second filial generation, offspring of the F1 generation.

Mendel's Experiments and Observations

• Mendel studied characters (heritable features, e.g., flower color) and their traits (variants, e.g., purple or white).

• He observed that crossing true-breeding plants with different traits produced F1 hybrids that all showed only one of the parental traits (e.g., all purple flowers).

• Self-pollination of F1 hybrids produced F2 offspring with a 3:1 ratio of dominant to recessive phenotypes (e.g., 705 purple:224 white).

Table: Mendel's Seven Pea Plant Characters and Dominant Traits

Character	Dominant Trait
Flower color	Purple
Seed color	Yellow
Seed shape	Round
Pod shape	Inflated
Pod color	Green
Flower position	Axial
Stem length	Tall

Mendel's Model of Inheritance

Mendel proposed a model to explain the patterns of inheritance he observed:

• Alternative versions of genes (alleles) account for variations in inherited characters.

• For each character, an organism inherits two alleles, one from each parent.

• If the two alleles differ, the dominant allele determines the organism's appearance; the recessive allele has no noticeable effect.

• The two alleles for a heritable character segregate (separate) during gamete formation and end up in different gametes.

Law of Segregation

The law of segregation states that the two alleles for a gene separate during gamete formation, so that each gamete receives only one allele.

• Explains the 3:1 ratio observed in the F2 generation.

• Each parent contributes one allele to their offspring.

Law of Independent Assortment

The law of independent assortment states that alleles of different genes assort independently of one another during gamete formation.

• Applies to genes on different chromosomes or far apart on the same chromosome.

• Explains the inheritance of two or more traits simultaneously.

Punnett Squares and Genetic Crosses

Punnett squares are diagrams used to predict the genotypic and phenotypic outcomes of genetic crosses.

• Monohybrid cross: A cross involving one character (e.g., flower color).

• Dihybrid cross: A cross involving two characters (e.g., seed color and seed shape).

Example: Crossing two heterozygous purple-flowered plants (Pp x Pp):

• Genotypic ratio: 1 PP : 2 Pp : 1 pp

• Phenotypic ratio: 3 purple : 1 white

Summary of Key Concepts

• Traits are inherited as discrete units (genes), not blended.

• Dominant and recessive alleles determine the expression of traits.

• Genotype determines phenotype, but environmental factors can also influence traits.

• Punnett squares are useful tools for predicting inheritance patterns.

Additional info:

• The "wild type" refers to the most common phenotype in a population.

• Mendel's laws apply to genes that are not linked (i.e., not located close together on the same chromosome).