Genetics I: Mendel and the Gene Idea – Study Notes

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Genetics I: Mendel and the Gene Idea

Introduction

This lesson introduces the foundational principles of genetics as established by Gregor Mendel. It covers the distinction between genotype and phenotype, Mendel's laws, the prediction of genetic outcomes, and the complexity of inheritance patterns in humans and other organisms.

Distinguishing Dominant and Recessive Traits

Genotype vs. Phenotype

Genotype: The genetic makeup of an organism; the specific alleles present at a gene locus.
Phenotype: The observable physical or physiological traits of an organism, determined by its genotype and environment.
Dominant Trait: A trait that is expressed in the phenotype even when only one allele is present (heterozygous condition).
Recessive Trait: A trait that is only expressed when two copies of the allele are present (homozygous condition).
Example: In pea plants, purple flower color (P) is dominant over white (p). A plant with genotype Pp or PP will have purple flowers, while only pp will have white flowers.

Mendel’s Laws of Inheritance

Law of Segregation

Each individual has two alleles for each gene, which segregate (separate) during gamete formation, so each gamete receives only one allele.
Example: A plant with genotype Pp produces gametes with either P or p alleles.

Law of Independent Assortment

Alleles of different genes assort independently of one another during gamete formation.
Example: The inheritance of seed shape (round or wrinkled) is independent of seed color (yellow or green) in pea plants.

Predicting Genetic Cross Outcomes

Punnett Square: A diagram used to predict the genotypes and phenotypes of offspring from a genetic cross.
Multiplication Rule: The probability of two independent events occurring together is the product of their individual probabilities.
Addition Rule: The probability of either of two mutually exclusive events occurring is the sum of their individual probabilities.
Formula Example:

Complex Inheritance Patterns

Beyond Simple Dominance

Incomplete Dominance: The heterozygote has a phenotype intermediate between the two homozygotes (e.g., red and white flowers produce pink offspring).
Codominance: Both alleles are fully expressed in the heterozygote (e.g., AB blood type in humans).
Pleiotropy: A single gene affects multiple traits (e.g., sickle-cell gene affects red blood cell shape and other symptoms).
Epistasis: One gene affects the expression of another gene.
Polygenic Inheritance: Multiple genes contribute to a single trait (e.g., skin color, height).
Multifactorial Characters: Traits influenced by both genetic and environmental factors.

Human Genetics and Genetic Counseling

Pedigree Analysis and Genetic Testing

Pedigree Analysis: A diagram showing the inheritance of a trait over several generations in a family, used to determine inheritance patterns.
Recessively Inherited Disorders: Disorders that appear only when an individual has two copies of the mutant allele (e.g., cystic fibrosis, sickle-cell anemia).
Dominantly Inherited Disorders: Disorders that appear when only one copy of the mutant allele is present (e.g., Huntington’s disease).
Genetic Testing and Counseling: Procedures and advice provided to individuals or families about the risks of inherited conditions, including techniques like amniocentesis and chorionic villus sampling.

Key Terms and Definitions

Term	Definition
Character	A heritable feature that varies among individuals (e.g., flower color).
Trait	A specific variant of a character (e.g., purple or white flowers).
Allele	Alternative versions of a gene.
Genotype	The genetic makeup of an organism.
Phenotype	The observable traits of an organism.
Homozygous	Having two identical alleles for a gene.
Heterozygous	Having two different alleles for a gene.
Punnett Square	A tool to predict genetic cross outcomes.
Law of Segregation	Alleles separate during gamete formation.
Law of Independent Assortment	Genes for different traits assort independently.
Dominant allele	Expressed in the phenotype when present.
Recessive allele	Expressed only when homozygous.
Codominance	Both alleles are fully expressed.
Incomplete dominance	Heterozygote phenotype is intermediate.
Pleiotropy	One gene affects multiple traits.
Epistasis	One gene affects another gene's expression.
Polygenic	Trait controlled by multiple genes.
Multifactorial	Trait influenced by genes and environment.
Pedigree	Family tree showing trait inheritance.
Cystic fibrosis	Recessive genetic disorder affecting lungs and pancreas.
Sickle-cell anemia	Recessive disorder causing abnormal hemoglobin.
Huntington’s disease	Dominant neurodegenerative disorder.
Amniocentesis	Prenatal test sampling amniotic fluid.
Chorionic villus sampling	Prenatal test sampling placental tissue.

Additional Resources

Pearson Modified Mastering Lesson 14 Assignment
Chapter 14, Figure 14.4: Walk Through: Alleles, Alternative Versions of Genes
Chapter 14 Animation: Mendel’s Cross of Two Characters: Seed Shape and Color