Mendelian Genetics: Principles, Laws, and Patterns of Inheritance

Mendelian Genetics (Chapter 14)

Introduction

Mendelian genetics explores the fundamental principles of heredity as discovered by Gregor Mendel through his experiments with garden peas. This chapter covers Mendel's scientific approach, the laws of inheritance, probability in genetics, and patterns that extend beyond simple Mendelian inheritance.

Explanations of Heredity

Historical Hypotheses

• Blending Hypothesis: The idea that genetic material from two parents blends together, much like mixing blue and yellow paint to make green. This hypothesis could not explain the reappearance of traits after several generations.

• Particulate Hypothesis: The concept that parents pass on discrete heritable units, now known as genes. This hypothesis accounts for the reappearance of traits and forms the basis of modern genetics.

Mendel’s Experiments

Scientific Approach and Laws

• Mendel used a scientific approach, carefully planning experiments with Pisum sativum (garden peas).

• He discovered two fundamental laws of inheritance:

  • Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation.

  • Law of Independent Assortment: Genes for different traits can segregate independently during the formation of gametes.

Experimental Approach

Advantages of Pea Plants

• Many varieties with distinct heritable features (characters), such as flower color.

• Character variants are called traits (e.g., purple or white flowers).

• Short generation time and large number of offspring per mating.

• Controlled mating: Each flower has sperm-producing organs (stamens) and egg-producing organs (carpels). Cross-pollination is possible by transferring pollen between plants.

Key Factors in Mendel’s Experiments

• Mendel tracked characters with two distinct alternative forms.

• Used true-breeding varieties (plants that produce offspring identical to themselves when self-pollinated).

• Hybridization: Mating two contrasting, true-breeding varieties, resulting in a monohybrid cross (one character studied).

Monohybrid Cross: Flower Color Example

Experimental Results

• P Generation: True-breeding parents (purple × white flowers).

• F1 Generation: All hybrids had purple flowers.

• F2 Generation: 705 purple-flowered and 224 white-flowered plants (approximate 3:1 ratio).

Conclusions from Mendel’s Experiments

• Only the purple flower factor affected F1 hybrids.

• Purple flower color is a dominant trait; white is recessive.

• The white factor was not destroyed or diluted, as it reappeared in F2 generation.

Table: Results of Mendel’s F1 Crosses for Seven Characters in Pea Plants

Character	Dominant Trait	Recessive Trait	F2 Count (Dominant:Recessive)	Ratio
Flower color	Purple	White	705:224	3.15:1
Seed color	Yellow	Green	6,022:2,001	3.01:1
Seed shape	Round	Wrinkled	5,474:1,850	2.96:1
Pod shape	Inflated	Constricted	882:299	2.95:1
Pod color	Green	Yellow	428:152	2.82:1
Flower position	Axial	Terminal	651:207	3.14:1
Stem length	Tall	Dwarf	787:277	2.84:1

Mendel’s Model of Inheritance

Key Principles

• Alleles: Alternative versions of genes account for variations in inherited characters.

• Each gene is located at a specific locus on a chromosome.

• For each character, an organism inherits two alleles, one from each parent. These may be identical (homozygous) or different (heterozygous).

• If the alleles differ, the dominant allele determines the organism's appearance; the recessive allele has no noticeable effect.

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

Genotype and Phenotype Ratios

Monohybrid Cross Example

• P Generation: Homozygous dominant (PP) × homozygous recessive (pp).

• F1 Generation: All heterozygous (Pp), phenotype is purple.

• F2 Generation: Genotypes: 1 PP : 2 Pp : 1 pp; Phenotypes: 3 purple : 1 white.

Phenotype	Genotype	Ratio
Purple	PP (homozygous)	1
Purple	Pp (heterozygous)	2
White	pp (homozygous)	1

Punnett Squares

Predicting Offspring Genotypes and Phenotypes

• Punnett Square: A diagram used to predict the outcome of a genetic cross.

• Example: Cross of F1 hybrids (Tt × Tt) for plant height.

• Genotype ratio: 1 TT : 2 Tt : 1 tt

• Phenotype ratio: 3 tall : 1 dwarf

The Testcross

Determining Unknown Genotypes

• Cross an individual with dominant phenotype (unknown genotype) with a homozygous recessive individual.

• If all offspring show the dominant phenotype, the parent is homozygous dominant; if offspring are mixed, the parent is heterozygous.

Law of Independent Assortment

Dihybrid Crosses

• Dihybrid Cross: A cross between F1 individuals that are heterozygous for two characters (e.g., YyRr × YyRr).

• Genes for different traits assort independently if they are on different chromosomes.

• Phenotypic ratio for dihybrid cross: 9:3:3:1

Laws of Probability in Genetics

Rules and Applications

• Multiplication Rule: Probability that two independent events will occur together is the product of their individual probabilities. Example: Probability of YYRR in YyRr × YyRr cross: $\frac{1}{4} \times \frac{1}{4} = \frac{1}{16}$

• Addition Rule: Probability that any one of two or more mutually exclusive events will occur is the sum of their individual probabilities. Example: Probability of heterozygous offspring (Rr) in a monohybrid cross: $\frac{1}{4} + \frac{1}{4} = \frac{1}{2}$

Solving Complex Genetics Problems

Multiple Characters

• Apply multiplication and addition rules to predict outcomes of crosses involving multiple genes.

• Each gene is considered independently if genes are on different chromosomes.

• Example: Probability of ppyyrr in PpYyRr × Ppyyrr cross: $\frac{1}{4} \times \frac{1}{2} \times \frac{1}{2} = \frac{1}{16}$

Non-Mendelian Genetics

Complex Patterns of Inheritance

• Many traits are not determined by a single gene with two alleles.

• Basic principles of segregation and independent assortment still apply, but inheritance patterns can be more complex.

Degree of Dominance

Types of Dominance

• Complete Dominance: Heterozygote and dominant homozygote have identical phenotypes.

• Incomplete Dominance: Heterozygote phenotype is intermediate between two parental varieties (e.g., red × white snapdragons produce pink F1).

• Codominance: Both alleles are expressed in the phenotype in distinguishable ways (e.g., MN blood group).

Genotype	Phenotype	Antigen Present
LMLM	M	M
LMLN	MN	M and N
LNLN	N	N

Relationship Between Dominance and Phenotype

• Dominant alleles do not subdue recessive alleles; alleles are variations in a gene's nucleotide sequence.

• Dominance relationships depend on the level at which the phenotype is examined (organismal, biochemical, molecular).

Additional info:

• Pleiotropy: Most genes have multiple phenotypic effects. For example, pleiotropic alleles are responsible for multiple symptoms in diseases such as cystic fibrosis and sickle-cell disease.