Mendelian Genetics: Principles, Patterns, and Human Applications

Mendelian Genetics

Introduction to Mendel's Experiments

Gregor Mendel's experiments with garden peas established the foundational principles of heredity. By using a scientific and quantitative approach, Mendel identified two key laws of inheritance that explain how traits are passed from one generation to the next.

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

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

• Advantages of Peas: Short generation time, large numbers of offspring, and controlled mating (self- or cross-pollination).

Mendel's Experimental Approach

Mendel tracked characters with two distinct forms and used true-breeding varieties. He performed hybridization by mating contrasting true-breeding parents (P generation), producing F1 hybrids, and then F2 generation through self- or cross-pollination of F1 individuals.

• P Generation: True-breeding parents.

• F1 Generation: Hybrid offspring of P generation.

• F2 Generation: Offspring from F1 self- or cross-pollination.

The Law of Segregation

Mendel observed a 3:1 ratio of purple to white flowers in the F2 generation, contradicting the blending hypothesis. He reasoned that traits are determined by discrete factors (now called genes), with dominant and recessive alleles.

• Dominant Trait: Trait that appears in F1 hybrids (e.g., purple flowers).

• Recessive Trait: Trait masked in F1 but reappears in F2 (e.g., white flowers).

• Gene: Heritable factor; now known as a segment of DNA at a specific locus.

Mendel's Model of Inheritance

Mendel's model explains the 3:1 ratio and consists of four concepts:

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

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

3. Dominance: If alleles differ, the dominant allele determines appearance; the recessive allele is masked.

4. Law of Segregation: The two alleles for a character segregate during gamete formation, ending up in different gametes.

Punnett Square: Used to predict allele combinations in offspring. Capital letters represent dominant alleles; lowercase represent recessive.

Genetic Vocabulary

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

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

• Phenotype: Physical appearance.

• Genotype: Genetic makeup.

Example: Both PP and Pp pea plants have purple flowers (same phenotype) but different genotypes.

The Testcross

A testcross determines the genotype of an individual with a dominant phenotype by crossing it with a homozygous recessive individual. If any offspring display the recessive phenotype, the mystery parent is heterozygous.

Monohybrid and Dihybrid Crosses

• Monohybrid Cross: Cross between heterozygotes for one character.

• Dihybrid Cross: Cross between heterozygotes for two characters.

The Law of Independent Assortment

Mendel's second law states that each pair of alleles segregates independently during gamete formation. This applies to genes on different chromosomes or far apart on the same chromosome.

• Genes close together: Tend to be inherited together (linked genes).

Probability in Genetics

Mendelian inheritance follows probability laws:

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

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

Example equation for probability of independent events:

$P(A \text{ and } B) = P(A) \times P(B)$

Example equation for probability of mutually exclusive events:

$P(A \text{ or } B) = P(A) + P(B)$

Complex Patterns of Inheritance

Inheritance patterns can deviate from simple Mendelian genetics in several ways:

• Incomplete Dominance: Heterozygote phenotype is intermediate between parental phenotypes.

• Codominance: Both alleles affect the phenotype in separate, distinguishable ways.

• Multiple Alleles: More than two alleles exist for a gene (e.g., ABO blood group).

• Pleiotropy: One gene affects multiple phenotypic traits.

• Epistasis: One gene affects the expression of another gene.

• Polygenic Inheritance: Multiple genes independently affect a single trait (e.g., height, skin color).

Table: ABO Blood Group Alleles and Phenotypes

Allele	Carbohydrate Added	Blood Type
IA	A	A
IB	B	B
i	None	O
IAIB	A and B	AB

Table: Epistasis in Labrador Retrievers

Genotype	Pigment Color	Deposition Gene	Coat Color
B_E_	Black	Color	Black
b_E_	Brown	Color	Chocolate
__ee	Any	No color	Yellow

Additional info: The above table summarizes how epistatic interactions modify expected Mendelian ratios.

Environmental Impact on Phenotype

Phenotype is influenced by both genotype and environment. Multifactorial traits depend on multiple genes and environmental factors, resulting in a broad phenotypic range.

Mendelian Genetics in Humans

Human genetics relies on pedigree analysis due to ethical and practical limitations. Pedigrees help predict inheritance patterns and probabilities for future offspring.

Recessively Inherited Disorders

• Carriers: Heterozygous individuals who carry the recessive allele but are phenotypically normal.

• Albinism: Recessive disorder causing lack of pigmentation.

• Cystic Fibrosis: Most common lethal genetic disease in the U.S.; caused by defective chloride channels.

• Sickle-Cell Disease: Caused by a single amino acid substitution in hemoglobin; heterozygotes have sickle-cell trait and are less susceptible to malaria.

Dominantly Inherited Disorders

• Achondroplasia: Form of dwarfism caused by a dominant allele.

• Huntington's Disease: Degenerative nervous system disorder; onset in mid-adulthood.

Multifactorial Disorders

Many diseases (e.g., heart disease, cancer) have both genetic and environmental components. Lifestyle choices significantly affect phenotype.

Genetic Testing and Counseling

• Pedigree Analysis: Used to assess risk for inherited diseases.

• Probability Calculation: For recessive diseases, risk is calculated using multiplication and addition rules.

Example: If both parents are carriers, the probability of a child with a recessive disease is:

$\frac{2}{3} \times \frac{2}{3} \times \frac{1}{4} = \frac{1}{9}$

Fetal and Newborn Testing

• Amniocentesis: Removal and testing of fetal liquid.

• Chorionic Villus Sampling (CVS): Removal and testing of placental tissue.

• Ultrasound: Visual assessment of fetal health.

• Newborn Screening: Routine tests for genetic disorders (e.g., phenylketonuria, PKU).

Additional info: Over 100 conditions can be screened in newborns, and the number is increasing with advances in genetic testing.