Mendel and the Gene: Patterns of Inheritance

Chapter 14: Mendel and the Gene

Introduction to Mendel and the Gene

This chapter explores how Mendel’s principles can predict patterns of inheritance. Gregor Mendel established the rules of inheritance through experiments on pea plants, laying the foundation for modern genetics. The Chromosomal Theory of Inheritance, later proposed by Sutton and Boveri, linked the transmission of genetic information to the behavior of chromosomes during meiosis.

• Genes are located on chromosomes.

• Inheritance patterns can be explained by the movement of chromosomes during meiosis.

Historical Context: Competing Hypotheses

Before Mendel, two main hypotheses attempted to explain inheritance:

1. Blending Inheritance: Parental traits blend in offspring, resulting in intermediate traits.

2. Inheritance of Acquired Characteristics: Traits modified through use are passed to offspring (Lamarck).

Model Organism: The Garden Pea (Pisum sativum)

Mendel used the garden pea as a model organism due to its practical features and polymorphic traits (traits with two or more common forms, e.g., purple vs. white flowers). The observable features of an individual are called its phenotype.

Trait	Form 1	Form 2
Seed Shape	Round	Wrinkled
Seed Cotyledon Color	Yellow	Green
Flower Color	White	Violet
Pod Form	Full	Constricted
Pod Color	Green	Yellow
Stem Place	Axial	Terminal
Stem Size	Tall	Short

Mendel’s Experimental System

• Self-fertilization: Peas normally self-pollinate.

• Cross-fertilization: Mendel controlled mating by transferring pollen from one plant to another, allowing for controlled crosses.

Key Genetic Terms (Table 14.1)

Term	Definition	Example or Comment
Autosomal inheritance	Inheritance of genes not on sex chromosomes	Standard Mendelian patterns
Gene	Hereditary factor influencing a trait	Flower color gene
Allele	Different versions of a gene	Alleles for purple or white flowers
Genotype	Allele combination in an individual	RR, Rr, or rr
Phenotype	Observable traits	Round or wrinkled seeds
Homozygous	Two identical alleles	RR or rr
Heterozygous	Two different alleles	Rr
Dominant allele	Expressed in heterozygotes	R (round)
Recessive allele	Masked in heterozygotes	r (wrinkled)
Pure line	Individuals produce offspring identical to themselves	RR or rr self-crossed
Hybrid	Offspring from parents with different genotypes	Rr
Reciprocal cross	Cross where parental sexes are switched	Male RR x Female rr and vice versa
X-linked	Gene on X chromosome	Color blindness gene
Y-linked	Gene on Y chromosome	SRY gene

Mendel’s Experiments with One Trait: The Monohybrid Cross

Mendel crossed parents with different phenotypes for a single trait and followed the inheritance through generations (P, F1, F2, etc.).

• Dominant and Recessive Traits: The trait that appears in the F1 generation is dominant; the one that is masked is recessive.

• F2 generation shows a 3:1 ratio of dominant to recessive phenotypes.

• Reciprocal crosses showed that gender did not influence inheritance.

Particulate Inheritance

Mendel proposed that hereditary determinants (genes) do not blend or change through use. They act as discrete, unchanging particles (now called genes).

Mendel’s Principle of Segregation

The two members of each gene pair segregate (separate) during gamete formation (anaphase I of meiosis), so each gamete carries only one allele for each gene.

Genotype and Phenotype Ratios

• Homozygote Cross (RR x rr): All offspring are Rr (heterozygous), all round seeds (phenotype 100% round).

• Heterozygote Cross (Rr x Rr): Offspring genotypes: 1/4 RR, 1/2 Rr, 1/4 rr (1:2:1 ratio). Phenotypes: 3/4 round, 1/4 wrinkled (3:1 ratio).

Genes, Alleles, and Genotypes

• Gene: Hereditary determinant for a trait.

• Allele: Different versions of a gene.

• Genotype: Combination of alleles in an individual.

• Phenotype: Observable features determined by genotype.

Mendel’s Experiments with Two Traits: The Dihybrid Cross

Mendel crossed individuals differing in two traits to test whether alleles of different genes are transmitted independently.

• Independent Assortment: Alleles of different genes are transmitted independently.

• Dependent Assortment: Transmission of one allele depends on another.

Mendel’s results supported independent assortment, with a 9:3:3:1 phenotypic ratio in the F2 generation.

Punnett Squares and Testcrosses

• Punnett squares predict offspring genotypes and phenotypes.

• Testcrosses (crossing to a homozygous recessive) can reveal unknown genotypes.

Mendel’s Principle of Independent Assortment

Different genes assort independently because they are located on different chromosomes, which line up randomly during metaphase I of meiosis.

The Chromosome Theory of Inheritance

• Genes are located on chromosomes at specific loci.

• The physical separation of alleles during anaphase I explains segregation.

• The random alignment of homologous chromosomes during metaphase I explains independent assortment.

Testing the Chromosome Theory: Morgan’s Fruit Fly Experiments

• Thomas Hunt Morgan used Drosophila melanogaster (fruit flies) as a model organism.

• Wild type: Most common phenotype.

• Mutants: Individuals with traits caused by mutations.

• Discovery of sex-linked inheritance (e.g., white-eyed mutant gene is X-linked).

Sex Linkage and Reciprocal Crosses

• Genes on sex chromosomes (X or Y) show unique inheritance patterns.

• Reciprocal crosses can reveal sex linkage (e.g., white eyes in male flies only when the gene is X-linked).

Extending Mendel’s Rules

• Linkage: Genes located close together on the same chromosome tend to be inherited together unless crossing over occurs.

• Genetic Recombination: Crossing over during meiosis can separate linked genes, producing recombinants.

• Genetic Mapping: The frequency of recombination can be used to estimate the distance between genes on a chromosome.

Multiple Alleles, Codominance, and Incomplete Dominance

• Many genes have more than two alleles (e.g., ABO blood types in humans).

• Codominance: Heterozygotes express both alleles (e.g., AB blood type).

• Incomplete Dominance: Heterozygotes have an intermediate phenotype (e.g., pink flowers from red and white parents).

Pleiotropy and Polygenic Inheritance

• Pleiotropy: One gene influences multiple traits (e.g., Marfan syndrome).

• Polygenic Inheritance: Multiple genes contribute to a single trait, often resulting in continuous variation (e.g., height, skin color).

Gene-Environment Interaction

• The expression of many genes depends on environmental factors (e.g., phenylketonuria [PKU] can be managed with a special diet).

Summary Table: Key Concepts in Mendelian Genetics

Concept	Description	Example
Segregation	Alleles separate during gamete formation	Monohybrid cross
Independent Assortment	Genes on different chromosomes assort independently	Dihybrid cross
Linkage	Genes close together on a chromosome are inherited together	Body color and wing shape in fruit flies
Sex Linkage	Genes on sex chromosomes show unique inheritance	Color blindness in humans
Pleiotropy	One gene affects multiple traits	Marfan syndrome
Polygenic Inheritance	Multiple genes affect one trait	Human height

Additional info: These notes integrate and expand upon the provided slides and images, ensuring all major Mendelian genetics concepts are covered for exam preparation.