Chapter 9: Patterns of Inheritance

Genetics and Heredity

Introduction to Heredity and Genetics

Genetics is the scientific study of heredity, which is the transmission of traits from one generation to the next. Gregor Mendel, working in the 1860s, established foundational principles by studying how parents pass discrete genes (heritable factors) to their offspring. These genes are responsible for inherited traits and retain their individual identities across generations.

• Heredity: Transmission of traits from parents to offspring.

• Genetics: Study of heredity and variation in organisms.

• Genes: Units of heredity that maintain identity through generations.

Mendel’s Experiments in an Abbey Garden

Experimental Model: Garden Peas

Mendel chose garden peas for his experiments due to their distinct traits and the ability to control their reproduction. He defined a character as a heritable feature that varies among individuals, and a trait as a variant of a character.

• Character: Heritable feature (e.g., flower color).

• Trait: Variant of a character (e.g., purple or white flowers).

• Experimental control: Mendel could manually pollinate plants to ensure parentage.

Hybridization and Generations

Mendel created purebred varieties and crossed them to produce hybrids. The parental plants are the P generation, their hybrid offspring are the F1 generation, and a cross of F1 plants forms the F2 generation.

• Hybrid: Offspring of two different purebred varieties.

• Genetic cross: Cross-fertilization between different varieties.

• P, F1, F2 generations: Parental, first filial, and second filial generations.

Mendel’s Law of Segregation

Inheritance of a Single Character

Mendel tracked inheritance of characters such as flower color, leading to several hypotheses:

1. Alternative versions of genes (alleles) account for variations in inherited characters.

2. Each organism inherits two alleles for each character, one from each parent.

   • Homozygous: Two identical alleles for a gene.

   • Heterozygous: Two different alleles for a gene.

3. If two alleles differ, the dominant allele determines appearance; the recessive allele has no noticeable effect.

   • Dominant alleles: Uppercase italic letters (e.g., P).

   • Recessive alleles: Lowercase italic letters (e.g., p).

4. Law of segregation: During gamete formation, allele pairs separate so each gamete carries only one allele for each character. Fertilization restores paired condition.

Punnett Squares and Ratios

Punnett squares illustrate possible combinations of gametes and resulting offspring. They help distinguish between an organism’s phenotype (physical appearance) and genotype (genetic makeup).

• Phenotypic ratio: Ratio of observable traits (e.g., 3 purple:1 white).

• Genotypic ratio: Ratio of genetic combinations (e.g., 1 PP:2 Pp:1 pp).

The Seven Characters of Pea Plants Studied by Mendel

Character	Dominant Trait	Recessive Trait
Flower color	Purple	White
Flower position	Axial	Terminal
Seed color	Yellow	Green
Seed shape	Round	Wrinkled
Pod shape	Inflated	Constricted
Pod color	Green	Yellow
Stem length	Tall	Dwarf

Relationship Between Alleles and Homologous Chromosomes

Gene Locus and Chromosome Structure

A gene locus is a specific location of a gene on a chromosome. Alleles reside at the same locus on homologous chromosomes, which may bear identical or different alleles.

• Homologous chromosomes: Chromosome pairs with the same gene loci.

• Genotype examples:

  • PP: Homozygous dominant

  • aa: Homozygous recessive

  • Bb: Heterozygous

Mendel’s Law of Independent Assortment

Monohybrid and Dihybrid Crosses

A monohybrid cross involves individuals heterozygous for one character. A dihybrid cross involves individuals heterozygous for two characters. Mendel’s experiments showed that allele pairs segregate independently during gamete formation, resulting in a 9:3:3:1 phenotypic ratio in dihybrid crosses.

• Law of independent assortment: Inheritance of one character does not affect inheritance of another.

• Example: Labrador retrievers inherit coat color and texture independently.

Using a Testcross to Determine an Unknown Genotype

Testcross Method

A testcross is a mating between an individual of dominant phenotype (unknown genotype) and a homozygous recessive individual. The offspring’s phenotypes reveal the unknown genotype.

• Application: Used to determine if an organism is homozygous or heterozygous for a dominant trait.

The Rules of Probability in Genetics

Probability and Genetic Crosses

Genetic crosses obey the rules of probability. The rule of multiplication states that the probability of a compound event is the product of the probabilities of independent events.

• Example: Probability of two independent traits appearing together is the product of their individual probabilities.

Family Pedigrees

Pedigree Analysis

Geneticists use pedigrees to analyze inheritance patterns in families. Pedigrees help deduce genotypes and track traits such as freckles across generations.

• Dominant trait: Heterozygous genotype results in dominant phenotype.

• Wild-type trait: Most common in nature, not necessarily dominant.

Human Traits Controlled by a Single Gene

Single-Gene Disorders

Some human traits and disorders are controlled by a single gene and follow Mendelian inheritance patterns. Examples include albinism, cystic fibrosis, and sickle-cell disease.

Disorder	Major Symptoms
Albinism	Lack of pigment in skin, hair, and eyes
Cystic fibrosis	Excess mucus, increased infection risk
Phenylketonuria (PKU)	Accumulation of phenylalanine, intellectual disability
Sickle-cell disease	Misshapen red blood cells, tissue damage
Achondroplasia	Dwarfism
Alzheimer's disease (some types)	Mental deterioration
Huntington's disease	Uncontrollable movements, cognitive impairment
Hypercholesterolemia	Excess cholesterol, heart disease

Key Equations

Probability in Genetics

• Rule of multiplication: $P(A ext{ and } B) = P(A) imes P(B)$

Punnett Square Ratios

• Monohybrid cross: $1:2:1$ genotypic ratio, $3:1$ phenotypic ratio

• Dihybrid cross: $9:3:3:1$ phenotypic ratio

Additional info:

• These notes expand on the provided slides with definitions, examples, and tables for clarity and completeness.