Mendel and the Gene Idea: Principles of Inheritance

Chapter 14: Mendel and the Gene Idea

Introduction

This chapter explores the foundational principles of heredity as discovered by Gregor Mendel through his experiments with garden peas. Mendel's work established the basic laws of inheritance, which remain central to genetics today.

CONCEPT 14.1: Mendel's Scientific Approach and Laws of Inheritance

Mendel’s Experimental, Quantitative Approach

• Character: A heritable feature that varies among individuals (e.g., flower color).

• Trait: Each variant for a character (e.g., purple or white flowers).

• Mendel used garden peas due to their many varieties, short generation time, large number of offspring, and the ability to control mating (self-pollination or cross-pollination).

Advantages of Using Peas

• Short generation time

• Large numbers of offspring

• Controlled mating (self- or cross-pollination)

Experimental Design

• Mendel tracked characters with two distinct forms and started with true-breeding varieties (offspring identical to parents when self-pollinated).

• Hybridization: Mating two contrasting, true-breeding varieties.

• P generation: True-breeding parents.

• F1 generation: Hybrid offspring of the P generation.

• F2 generation: Offspring from self- or cross-pollination of F1 hybrids.

The Law of Segregation

• Contradicted the "blending" hypothesis of inheritance.

• Crossing true-breeding purple and white flowered plants produced all purple F1 hybrids.

• F2 generation showed a 3:1 ratio of purple to white flowers.

• Dominant trait: Purple flower color.

• Recessive trait: White flower color.

• The "heritable factor" is now known as a gene.

Table: Mendel's Crosses for Seven Characters in Pea Plants

Character	Dominant Trait	Recessive Trait	F2 Ratio
Flower color	Purple	White	3:1
Seed color	Yellow	Green	3:1
Seed shape	Round	Wrinkled	3:1
Pod color	Green	Yellow	3:1
Pod shape	Inflated	Constricted	3:1
Flower position	Axial	Terminal	3:1
Stem length	Tall	Dwarf	3:1

Mendel’s Model of Inheritance

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

• Second: Each organism inherits two alleles for each character, one from each parent.

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

• Fourth (Law of Segregation): The two alleles for a heritable character segregate during gamete formation and end up in different gametes.

Punnett Square: A diagram used to predict the results of a genetic cross between individuals of known genotype.

Genetic Vocabulary

• Homozygote: An organism with two identical alleles for a gene (homozygous).

• Heterozygote: An organism with two different alleles for a gene (heterozygous).

• Phenotype: Physical appearance or observable traits.

• Genotype: Genetic makeup.

Example: Both PP and Pp genotypes produce purple flowers (same phenotype, different genotype).

The Testcross

• Used to determine the genotype of an individual with a dominant phenotype.

• Cross the individual with a homozygous recessive; if any offspring display the recessive phenotype, the parent is heterozygous.

The Law of Independent Assortment

• Monohybrid cross: Cross between heterozygotes for one character.

• Dihybrid cross: Cross between F1 dihybrids (heterozygous for two characters).

• Law of Independent Assortment: Each pair of alleles segregates independently during gamete formation.

• This law applies to genes on different chromosomes or those far apart on the same chromosome.

CONCEPT 14.2: Probability Laws in Mendelian Inheritance

Rules of Probability

• Multiplication Rule: The probability of two independent events occurring together is the product of their individual probabilities.

• Addition Rule: The probability of any one of two or more mutually exclusive events is the sum of their individual probabilities.

Example: In a monohybrid cross, the probability of producing a heterozygote (Rr) is calculated using the addition rule.

Solving Complex Genetics Problems

• For multicharacter crosses, treat each character separately and multiply the probabilities.

Example Calculations:

• Probability of YYRR: $\frac{1}{4} \times \frac{1}{4} = \frac{1}{16}$

• Probability of YyRR: $\frac{1}{2} \times \frac{1}{4} = \frac{1}{8}$

• Chance of at least two recessive traits in a trihybrid cross: $\frac{6}{16} = \frac{3}{8}$

Summary Table: Key Terms and Concepts

Term	Definition	Example
Character	Heritable feature that varies among individuals	Flower color
Trait	Variant of a character	Purple or white flowers
Allele	Alternative version of a gene	P (purple), p (white)
Genotype	Genetic makeup	PP, Pp, or pp
Phenotype	Physical appearance	Purple or white flowers

Additional info: These notes cover the first half of Mendelian genetics, focusing on Mendel's experiments, the laws of segregation and independent assortment, and the application of probability to genetic crosses. Further sections would include extensions to Mendelian genetics, human inheritance, and genetic disorders.