Patterns of Inheritance: Mendelian and Non-Mendelian Genetics

Patterns of Inheritance

Introduction

Patterns of inheritance describe how genetic traits are transmitted from one generation to the next. This chapter explores both Mendelian and Non-Mendelian genetics, including their application to human traits and genetic disorders.

Mendelian Genetics

Basic Principles

• Mendelian genetics refers to predictable inheritance patterns based on the laws of segregation and independent assortment, first described by Gregor Mendel.

• Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation so that each gamete carries only one allele for each gene.

• Law of Independent Assortment: Genes for different traits assort independently of one another during gamete formation.

Key Terminology

• Character: A heritable feature (e.g., flower color, eye color).

• Trait: A variant of a character (e.g., purple flowers, brown eyes).

• Allele: Alternative versions of a gene.

• Genotype: The genetic makeup (combination of alleles) of an individual.

• Phenotype: The observable physical traits of an individual.

• Homozygous: Having two identical alleles for a gene (e.g., AA or aa).

• Heterozygous: Having two different alleles for a gene (e.g., Aa).

• Dominant allele: The allele that determines the phenotype in a heterozygote.

• Recessive allele: The allele whose effect is masked in a heterozygote.

Common Human Mendelian Traits

• Achoo Syndrome: Sneezing in response to bright light.

• Tongue Rolling: Ability is dominant; inability is recessive.

• Hand Clasping: Left over right is dominant; right over left is recessive.

• Face Shape: Oval is dominant; square is recessive.

• Hair Texture: Curly is dominant, wavy is heterozygous, straight is recessive.

Genetic Crosses and Punnett Squares

• Genetic Cross: An intentional mating between two individuals to study inheritance patterns.

• Punnett Square: A diagram used to predict the genotypes and phenotypes of offspring from a genetic cross.

Monohybrid Cross Example

• Crossing two pea plants with different flower colors (purple and white):

• Purple is dominant (F), white is recessive (f).

• P generation: FF (purple) × ff (white)

• F1 generation: All Ff (purple)

• F2 generation: Ff × Ff yields genotypes FF, Ff, ff (phenotypic ratio 3:1, genotypic ratio 1:2:1).

Testcross

• Used to determine whether an individual with a dominant phenotype is homozygous dominant or heterozygous.

• Cross the individual with a homozygous recessive (e.g., FF or Ff × ff).

• If any offspring display the recessive phenotype, the tested individual is heterozygous.

Dihybrid Crosses

• Track inheritance of two traits simultaneously (e.g., seed shape and color).

• Demonstrates the law of independent assortment.

• Typical phenotypic ratio for a dihybrid cross (AaBb × AaBb) is 9:3:3:1.

Non-Mendelian Genetics

Environmental Influence on Phenotype

• Phenotype can be influenced by environmental factors, not just genotype.

• Phenotypic plasticity: The degree to which phenotype is determined by genotype versus environment.

• Examples: Flamingo color (diet-dependent), hydrangea flower color (soil chemistry).

Types of Non-Mendelian Inheritance

• Co-Dominance: Both alleles are fully expressed in the heterozygote (e.g., human blood types IAIB = AB type).

• Incomplete Dominance: Heterozygote phenotype is intermediate between the two homozygotes (e.g., red × white flowers = pink flowers).

• Pleiotropy: One gene influences multiple traits (e.g., sickle cell anemia affects blood, kidney, and other organs).

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

Co-Dominance Example: Human Blood Types

Genotype	Phenotype
IAIA, IAi	Type A
IBIB, IBi	Type B
IAIB	Type AB
ii	Type O

Incomplete Dominance Example

• Red snapdragon × white snapdragon = pink snapdragon (heterozygote).

• Roan horses (Rr) from chestnut (RR) and white (rr) parents show a blend of red and white hairs.

Pleiotropy Example

• Sickle Cell Anemia: One gene mutation affects hemoglobin, causing multiple symptoms (anemia, organ damage, malaria resistance).

• Marfan Syndrome: Mutation in a gene for connective tissue protein affects skeleton, heart, eyes, and more.

Polygenic Inheritance Example

• Traits like skin color, height, and eye color are controlled by multiple genes, resulting in a range of phenotypes.

Human Genetics: Disease Patterns

Autosomal Genetic Disorders

• Autosomal Recessive: Sickle cell anemia, albinism, cystic fibrosis.

• Autosomal Dominant: Polycystic kidney disease, polydactyly, Huntington disease.

Sex-Linked Genetic Disorders

• Genes located on the X chromosome (pair 23) are more likely to affect males (XY) than females (XX).

• Examples: Hemophilia, Duchenne muscular dystrophy, red-green color blindness.

Physical Chromosomal Abnormalities

• Aneuploidy: Abnormal number of chromosomes due to nondisjunction during meiosis.

• Examples: Down syndrome (trisomy 21), Turner syndrome (XO), Klinefelter syndrome (XXY).

• Karyotype: A display of chromosomes used to diagnose chromosomal abnormalities.

Pedigree Analysis

• Pedigrees are diagrams that show the inheritance of a trait through several generations of a family.

• Useful for tracking genetic disorders and predicting inheritance patterns.

Practical Applications of Genetics

• Genetically modified crops and livestock.

• Gene therapy in humans and other organisms.