Mendel and the Gene: Principles of Inheritance

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Chapter 14: Mendel and the Gene

Introduction

This chapter explores the foundational principles of genetics established by Gregor Mendel. It covers Mendel's experiments, the chromosomal basis of inheritance, extensions to Mendel's rules, and the application of these principles to human genetics.

I. Mendel’s Experimental System

A. Historical Context and Questions

Blending Inheritance: The hypothesis that parental traits blend together in offspring (e.g., black and white sheep produce gray offspring).
Inheritance of Acquired Characters: The idea that traits modified during an organism's life are passed to offspring (e.g., giraffes stretching their necks).
Mendel tested these hypotheses using Pisum sativum (garden pea).

B. Why Pea Plants?

Inexpensive, easy to grow, short generation time, and produce many seeds.
Model organisms allow generalization of findings to other species.

C. Mendel’s Experimental Approach

Self-fertilization: Pollen from a plant fertilizes its own ovules.
Cross-pollination: Pollen from one plant fertilizes another, controlled manually by Mendel.
Mendel studied seven traits, each with two distinct phenotypes (e.g., seed shape, color).
True-breeding (pure lines): Plants that consistently produce the same phenotype.
Hybrids: Offspring from parents differing in one or more traits.
Parental generation (P): Initial cross; F1 generation: First filial offspring.

II. Mendel’s Experiments with a Single Trait

A. Monohybrid Crosses and Particulate Inheritance

Monohybrid cross: Cross between parents differing in one trait.
F1 offspring showed only the dominant trait; recessive trait reappeared in F2 (3:1 ratio).
Dominant: Trait visible in hybrids; Recessive: Trait masked in hybrids.
Reciprocal crosses (switching male/female parents) yielded the same results, indicating inheritance is not sex-dependent for autosomal traits.

B. Genes, Alleles, and the Principle of Segregation

Gene: Hereditary determinant for a trait.
Allele: Different versions of a gene.
Genotype: Combination of alleles in an individual.
Phenotype: Observable trait.
Principle of Segregation: Alleles separate during gamete formation; each gamete receives one allele.
Homozygous: Two identical alleles; Heterozygous: Two different alleles.

C. Predicting Offspring: Punnett Squares

Visual tool to predict genotype and phenotype ratios.
For a cross between Rr and Rr: 1/4 RR, 1/2 Rr, 1/4 rr (3:1 phenotype ratio).

III. Mendel’s Experiments with Two Traits

A. Dihybrid Crosses and Independent Assortment

Dihybrid cross: Parents differ in two traits (e.g., RRYY × rryy).
F1 generation (RrYy) self-fertilization yields 9:3:3:1 phenotype ratio in F2.
Principle of Independent Assortment: Alleles of different genes assort independently during gamete formation.

B. Testcrosses

Crossing an individual with dominant phenotype to a homozygous recessive to determine genotype.
For RrYy × rryy, expect four phenotypes in 1:1:1:1 ratio if independent assortment occurs.

IV. Chromosome Theory of Inheritance

A. Chromosomal Basis

Genes are located on chromosomes; meiosis explains segregation and independent assortment.
Locus: Specific location of a gene on a chromosome.

B. Sex Linkage and Morgan’s Experiments

Thomas Hunt Morgan used Drosophila melanogaster to study sex-linked inheritance.
Genes on X or Y chromosomes show unique inheritance patterns (e.g., white-eyed flies).
Autosomal inheritance: Genes on non-sex chromosomes.

V. Extensions to Mendel’s Rules

A. Linkage and Recombination

Linkage: Genes on the same chromosome tend to be inherited together.
Crossing over: Exchange of genetic material during meiosis can separate linked genes, producing recombinants.
Genetic map: Diagram showing relative positions of genes based on recombination frequency.
centiMorgan (cM): Unit of genetic distance; 1 cM = 1% recombination frequency.

B. Multiple Allelism, Codominance, and Incomplete Dominance

Multiple allelism: More than two alleles exist for a gene (e.g., ABO blood groups).
Codominance: Heterozygotes express both phenotypes (e.g., AB blood type).
Incomplete dominance: Heterozygotes have intermediate phenotype (e.g., pink flowers from red × white parents).

C. Pleiotropy and Gene Interactions

Pleiotropy: One gene affects multiple traits (e.g., Marfan syndrome).
Epistasis: Expression of one gene is masked by another gene (e.g., comb shape in chickens).

D. Environmental Effects and Quantitative Traits

Phenotype can be influenced by environment (e.g., PKU can be managed by diet).
Quantitative traits: Traits with continuous variation (e.g., height), often polygenic and influenced by environment.
Polygenic inheritance: Multiple genes contribute to a single trait, resulting in a normal distribution of phenotypes.

VI. Applying Mendel’s Rules to Human Inheritance

A. Pedigree Analysis

Pedigrees are family trees tracking inheritance of traits.
Used to infer mode of inheritance (autosomal vs. sex-linked, dominant vs. recessive).

B. Patterns of Inheritance

Trait Type	Key Features	Example
Autosomal Recessive	Males and females equally affected Affected offspring often have unaffected (carrier) parents Trait can skip generations	Sickle-cell anemia
Autosomal Dominant	Males and females equally affected Affected offspring have at least one affected parent Trait does not skip generations	Huntington’s disease
X-linked Recessive	Males more frequently affected Trait often skips generations Carrier females can have affected sons	Hemophilia, red-green color blindness
X-linked Dominant	Males and females equally affected All daughters of affected males are affected Trait does not skip generations	Hypophosphatemia

VII. Key Genetics Terms

Allele: Variant form of a gene.
Autosome: Non-sex chromosome.
Carrier: Heterozygote for a recessive trait; does not express the trait but can pass it on.
Genotype: Genetic makeup of an organism.
Phenotype: Observable characteristics.
Punnett square: Diagram to predict genetic crosses.
Recombination: Exchange of genetic material between homologous chromosomes.
Testcross: Cross with a homozygous recessive to determine genotype.
X-linked/Y-linked: Genes located on the X or Y chromosome, respectively.

VIII. Key Ratios and Equations

Monohybrid F2 ratio: (dominant:recessive phenotype)
Dihybrid F2 ratio: (for two independently assorting genes)
Testcross ratio (dihybrid): (if independent assortment)
Genotype ratio (incomplete dominance):

IX. Example Punnett Square (Monohybrid Cross)

	R	r
R	RR	Rr
r	Rr	rr

Genotype ratio: 1 RR : 2 Rr : 1 rr Phenotype ratio: 3 round : 1 wrinkled

X. Summary Table: Extensions to Mendel’s Rules

Extension	Description	Example
Linkage	Genes on the same chromosome inherited together unless crossing over occurs	Body color and eye color in Drosophila
Multiple allelism	More than two alleles for a gene exist in a population	ABO blood groups
Codominance	Both alleles expressed in heterozygotes	AB blood type
Incomplete dominance	Heterozygotes have intermediate phenotype	Pink flowers in four o’clocks
Pleiotropy	One gene affects multiple traits	Marfan syndrome
Polygenic inheritance	Multiple genes contribute to a single trait	Height, skin color

XI. Key Equations (LaTeX Format)

Probability of genotype in monohybrid cross:
Recombination frequency:

Additional info: This summary includes inferred explanations and examples for clarity and completeness, as well as reconstructed tables based on standard genetics content.