Mendelian Genetics and Patterns of Inheritance

Introduction to Mendelian Genetics

Mendelian genetics forms the foundation of classical genetics, describing how traits are inherited from one generation to the next. Gregor Mendel's experiments with pea plants revealed the basic principles of heredity, including the concepts of genes, alleles, dominance, and segregation.

PART 1 · MENDEL’S APPROACH

Blending vs. Particulate Inheritance

Early theories of inheritance included the blending hypothesis, which suggested that parental traits mix in offspring, and the particulate hypothesis, which proposed that parents pass on discrete units (genes) that retain their identity across generations. Mendel's experiments supported the particulate model.

• Blending hypothesis: Traits mix like paint; offspring are intermediate.

• Particulate hypothesis: Traits are inherited as distinct units (genes).

• Mendel’s experiments: Used garden peas for controlled crosses and observed inheritance patterns.

Mendel’s Experimental Approach

Mendel chose pea plants for their short generation time, many offspring, and ability to self- or cross-pollinate. He distinguished between characters (heritable features) and traits (variants of characters), and used true-breeding plants for his crosses.

• Character: Heritable feature (e.g., flower color).

• Trait: Variant of a character (e.g., purple or white flowers).

• True-breeding: Plants that produce only their own variety when self-pollinated.

• Hybridization: Crossing two contrasting true-breeding varieties.

• P generation: True-breeding parents.

• F1 generation: Hybrid offspring.

• F2 generation: Offspring of F1 × F1.

The F2 Generation: 3:1 Ratio

Mendel observed that crossing purple and white true-breeding pea plants produced all purple F1 offspring. Self-pollinating F1 plants yielded F2 offspring in a 3:1 ratio of purple to white, demonstrating dominance and recessiveness.

• Dominant trait: Purple flowers.

• Recessive trait: White flowers.

• Ratio: 705 purple : 224 white (approx. 3:1).

• Gene: Mendel’s “factor” is now called a gene.

Mendel’s Seven Characters

Mendel studied seven characters in pea plants, each showing a similar dominant:recessive ratio in the F2 generation.

PART 2 · SEGREGATION AND VOCABULARY

Mendel’s Model: Four Concepts

Mendel’s model of inheritance is based on four key concepts:

• Alleles: Alternative versions of a gene account for variation in inherited characters.

• Inheritance: Each organism inherits two alleles, one from each parent.

• Dominance: If alleles differ, the dominant allele determines appearance.

• Law of Segregation: The two alleles separate during gamete formation; each gamete gets one allele.

Alleles and Locus

Each gene is located at a specific locus on a chromosome. Homologous chromosomes carry the same gene at the same locus, but may have different alleles.

• Allele for purple flowers: Produces a functional pigment enzyme.

• Allele for white flowers: DNA differs, no enzyme produced.

Genetic Vocabulary

Phenotype vs. Genotype

Phenotype is the observable trait, while genotype is the genetic makeup. Different genotypes can produce the same phenotype.

• Phenotype ratio: 3 purple : 1 white

• Genotype ratio: 1 PP : 2 Pp : 1 pp

PART 3 · THE LAW OF SEGREGATION IN A PUNNETT SQUARE

Punnett Squares and Segregation

Punnett squares are used to predict the outcome of genetic crosses. The law of segregation explains the 3:1 ratio observed in the F2 generation.

• P generation: PP × pp

• F1 generation: All Pp

• F2 generation: ¼ PP, ½ Pp, ¼ pp

Reading a Punnett Square

Each parent’s gametes are listed along the top and side. Offspring genotypes are calculated by combining alleles from each parent.

• Gamete frequency: Heterozygote (Pp) makes P and p equally (½ each).

• Genotype ratio: ¼ PP : ½ Pp : ¼ pp

• Phenotype ratio: ¾ purple : ¼ white

Probability Rules

• Multiplication rule: Probability of two independent events occurring together: multiply probabilities.

• Addition rule: Probability of one outcome reached in different ways: add probabilities.

Testcross

A testcross determines whether an individual with a dominant phenotype is homozygous or heterozygous by crossing it with a homozygous recessive individual.

• All offspring purple: Parent is PP.

• Half purple, half white: Parent is Pp.

PART 4 · DIHYBRID AND MULTIHYBRID CROSSES

Dihybrid Crosses and Independent Assortment

Dihybrid crosses involve two genes and test whether they are inherited together or independently. Mendel’s law of independent assortment states that alleles of different genes segregate independently during gamete formation.

• Example: YYRR × yyrr → F1: YyRr

• F2 ratio: 9:3:3:1 (independent assortment)

FOIL Method for Gametes

The FOIL (First, Outer, Inner, Last) method is used to determine all possible gamete combinations for individuals heterozygous for multiple genes.

• Number of gamete types: where n = number of heterozygous genes.

Multihybrid Crosses

For crosses involving three or more genes, calculate each gene separately and multiply probabilities.

• Example: AaBbCc × AaBbCc

• Probability of aabbcc:

• Probability of AaBbCc:

PART 5 · EXTENSIONS OF MENDELIAN GENETICS

Patterns of Inheritance

Incomplete Dominance

In incomplete dominance, the heterozygote phenotype is intermediate between the two homozygotes. For example, crossing red and white snapdragons produces pink flowers.

Codominance and Multiple Alleles: ABO Blood Types

Codominance occurs when both alleles are fully expressed in the heterozygote. The ABO blood group system is an example, with three alleles (IA, IB, i) and codominant expression of IA and IB.

Pleiotropy

Pleiotropy occurs when one gene affects multiple traits. Sickle-cell disease is an example, where a single amino acid change in hemoglobin leads to multiple symptoms.

Epistasis

Epistasis is when the expression of one gene alters the expression of another gene. Labrador retriever coat color is determined by two genes: one for pigment color and one for pigment deposition.

Polygenic Inheritance

Polygenic inheritance involves multiple genes contributing additively to a single trait, such as human height or skin color. The result is a continuous variation in phenotype.

PART 6 · PEDIGREE ANALYSIS AND HUMAN GENETIC DISORDERS

Pedigree Analysis

Pedigrees are family trees used to study inheritance patterns in humans. They help determine whether traits are dominant or recessive and predict the probability of future offspring inheriting a trait.

• Square: Male

• Circle: Female

• Filled: Affected

• Open: Unaffected

• Horizontal line: Mating

• Vertical line: Offspring

• Roman numerals: Generations

Recessively Inherited Disorders

Recessive disorders appear only in individuals homozygous for the allele. Carriers are heterozygous and phenotypically normal. Examples include albinism, cystic fibrosis, and sickle-cell disease.

• Cystic fibrosis: Defective chloride channels, mucus buildup.

• Sickle-cell disease: Hemoglobin mutation, heterozygote advantage against malaria.

Dominantly Inherited Disorders

Dominant disorders require only one copy of the allele to be expressed. Examples include achondroplasia (dwarfism).

• Achondroplasia: Dd × dd → ½ dwarf : ½ normal

Summary

• Mendelian genetics explains inheritance patterns using genes, alleles, dominance, and segregation.

• Punnett squares and probability rules predict genotype and phenotype ratios.

• Extensions include incomplete dominance, codominance, multiple alleles, pleiotropy, epistasis, and polygenic inheritance.

• Pedigree analysis is used to study human inheritance and genetic disorders.