Mendelian Genetics: Patterns of Inheritance

Historical Foundations of Genetics

The study of classical inheritance began in the 19th century, before the discovery of cells and chromosomes. Gregor Mendel, an Austrian monk, is considered the father of genetics due to his pioneering experiments with garden peas (Pisum sativum). Mendel's work established the basic principles of heredity, using mathematical analysis to deduce how traits are passed from parents to offspring.

• Gregor Mendel: Conducted breeding experiments with peas to study inheritance.

• Particulate Theory of Inheritance: Traits are inherited as discrete units (now known as genes), not blended.

• True Breeding (Homozygous): Plants that consistently produce offspring with the same trait.

• Monohybrid Cross: Cross between parents differing in a single trait.

• Reciprocal Crosses: Controlled crosses to track trait inheritance.

Example: Mendel crossed pea plants with different flower colors (purple vs. white) and observed the resulting offspring.

Key Traits Studied by Mendel

Mendel selected pea plants for their simple, easily distinguishable traits and ability to self- or cross-pollinate. He focused on traits controlled by single genes, such as:

• Stem Length: Tall vs. short

• Pod Shape: Inflated vs. constricted

• Seed Shape: Round vs. wrinkled

• Seed Color: Yellow vs. green

• Flower Position: Axial vs. terminal

• Flower Color: Purple vs. white

• Pod Color: Green vs. yellow

Additional info: These traits are examples of single-gene (monogenic) inheritance, making them ideal for Mendel's analysis.

Fundamental Laws of Heredity

Law of Segregation

The Law of Segregation states that each individual has two alleles for each trait, which segregate during gamete formation so that each gamete receives only one allele. Fertilization restores the pair in the offspring.

• Each individual has a pair of factors (alleles) for each trait.

• Factors segregate during gamete formation (meiosis).

• Each gamete contains only one factor from each pair.

• Fertilization gives offspring two factors for each trait.

Example: Crossing two heterozygous pea plants (Tt x Tt) yields a 3:1 ratio of tall to short plants in the F2 generation.

Equation:

Law of Independent Assortment

The Law of Independent Assortment states that alleles for different traits segregate independently during gamete formation. This allows for new combinations of traits in offspring.

• Traits are inherited independently of one another.

• Multiple traits can be mixed in offspring (e.g., tall with yellow pods, short with green pods).

Example: Dihybrid cross of pea plants differing in height and pod color produces offspring with all possible combinations of these traits.

Equation:

Additional info: This ratio is observed when both parents are heterozygous for two traits.

Genotype, Phenotype, and Alleles

Definitions and Relationships

Genotype refers to the genetic makeup (alleles) of an individual for a trait, while phenotype is the observable physical expression of that trait.

• Allele: Alternate form of a gene.

• Dominant allele: Masks the expression of the recessive allele (represented by uppercase letter).

• Recessive allele: Expressed only when two copies are present (lowercase letter).

• Homozygous: Two identical alleles at a locus (e.g., TT or tt).

• Heterozygous: Two different alleles at a locus (e.g., Tt).

• Locus: Physical location of a gene on a chromosome.

Example: A pea plant with genotype PP or Pp will have purple flowers (dominant), while pp will have white flowers (recessive).

Dominant vs. Recessive Alleles

Dominant alleles code for functional proteins, while recessive alleles often represent a loss of function or altered protein.

• Dominant allele: Produces normal protein, masks recessive.

• Recessive allele: Loss of function or altered protein, only expressed when homozygous.

Additional info: Many human traits, such as albinism, follow simple Mendelian inheritance.

Punnett Squares: Predicting Inheritance

Monohybrid Crosses

Punnett squares are tables used to predict the probability of genotypes and phenotypes among offspring from a genetic cross.

• List possible gametes from each parent.

• Combine gametes to predict offspring genotypes.

• Calculate ratios of phenotypes and genotypes.

Example: Cross between two heterozygous parents (Tt x Tt):

Genotype ratio: 1 TT : 2 Tt : 1 tt Phenotype ratio: 3 tall : 1 short

Dihybrid Crosses

Dihybrid crosses involve two traits and produce more complex Punnett squares.

• Each parent produces four types of gametes.

• Offspring show a 9:3:3:1 phenotype ratio.

Example: Guinea pig coat color and length:

Additional info: The 9:3:3:1 ratio is characteristic of dihybrid crosses with independent assortment.

Test Crosses

A test cross is used to determine the genotype of an individual with a dominant phenotype by crossing it with a homozygous recessive individual.

• If all offspring show the dominant phenotype, the unknown parent is homozygous dominant.

• If offspring show a mix of dominant and recessive phenotypes, the unknown parent is heterozygous.

Example: Crossing a yellow pea plant (unknown genotype) with a green pea plant (yy):

Summary Table: Mendelian Principles

Conclusion

Mendel's experiments with pea plants established the foundation of classical genetics, including the laws of segregation and independent assortment. These principles explain how traits are inherited and can be predicted using tools like Punnett squares. Understanding genotype, phenotype, and allele relationships is essential for analyzing inheritance patterns in both plants and animals.