Chapter 14: Mendel and the Gene Idea

Key Questions and Concepts

• What is the difference between a character and a trait?

• What makes peas ideal for genetic testing?

• What are the two laws of Mendelian genetics? What do each mean?

• How to use a Punnett square for monohybrid crosses?

• How to calculate fractions and probabilities for Punnett squares?

• What are environmental factors and how do they work?

• What are the differences between types of dominance?

• How to analyze a pedigree?

• Compare and contrast dominant versus recessive inherited disorders.

• What are multifactorial disorders and how do they differ from other diseases?

Introduction to Mendelian Genetics

Characters and Traits

In genetics, a character is a heritable feature that varies among individuals, such as flower color. A trait is each variant for a character, such as purple or white flowers.

• Character: General heritable feature (e.g., seed shape, flower color).

• Trait: Specific form of a character (e.g., round seeds, purple flowers).

• Example: In pea plants, flower color is a character; purple and white are traits.

Why Peas Are Ideal for Genetic Testing

Pisum sativum (pea plants) were chosen by Mendel for genetic studies due to several advantages:

• Short generation time: Rapid life cycle allows for quick observation of inheritance.

• Large numbers of offspring: Enables statistical analysis of inheritance patterns.

• Controlled mating: Plants can self-pollinate or be cross-pollinated manually.

Mendel’s Experiments and Laws

Mendel’s Experimental Approach

Mendel tracked only those characters that occurred in two distinct forms (e.g., purple vs. white flowers). He used true-breeding varieties and performed hybridization to study inheritance.

• P Generation: True-breeding parents (e.g., purple and white flowers).

• F1 Generation: First filial generation, all had purple flowers.

• F2 Generation: Second filial generation, showed a 3:1 ratio of purple to white flowers.

• Hybridization: Crossing two true-breeding varieties to produce hybrids.

Law of Segregation

The law of segregation states that two alleles for a heritable character separate during gamete formation and end up in different gametes.

• Each parent has two alleles for each character.

• During gamete formation, alleles segregate so each gamete receives only one allele.

• Offspring inherit one allele from each parent.

Example: In pea plants, the allele for purple flowers (P) is dominant over the allele for white flowers (p). A cross between two heterozygotes (Pp x Pp) produces a 3:1 ratio of purple to white flowers in the F2 generation.

Punnett Squares and Genetic Vocabulary

A Punnett square is a diagram used to predict the outcome of a genetic cross.

• Homozygote: Organism with two identical alleles for a gene (e.g., PP or pp).

• Heterozygote: Organism with two different alleles for a gene (e.g., Pp).

• Phenotype: Observable traits (e.g., purple flowers).

• Genotype: Genetic makeup (e.g., PP, Pp, or pp).

Testcross: Breeding an individual with a dominant phenotype with a homozygous recessive individual to determine genotype.

Law of Independent Assortment

The law of independent assortment states that each pair of alleles segregates independently of other pairs during gamete formation. This applies to genes on different chromosomes or far apart on the same chromosome.

• Monohybrid cross: Cross between heterozygotes for one character.

• Dihybrid cross: Cross between heterozygotes for two characters.

Probability in Genetics

Probability Rules

Genetic outcomes can be predicted using probability rules:

• 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: Probability of getting genotype YYRR in a cross is:

Complex Patterns of Inheritance

Degrees of Dominance

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

• Incomplete dominance: Heterozygote phenotype is intermediate between the two homozygotes.

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

Example: ABO blood group in humans shows codominance and multiple alleles.

Multiple Alleles and Pleiotropy

• Multiple alleles: Most genes exist in populations in more than two allelic forms (e.g., ABO blood group: IA, IB, i).

• Pleiotropy: One gene affects multiple phenotypic traits (e.g., sickle-cell disease).

Epistasis and Polygenic Inheritance

• Epistasis: One gene affects the expression of another gene (e.g., coat color in Labrador retrievers).

• Polygenic inheritance: Multiple genes contribute to a single phenotype (e.g., human height, skin color).

Environmental Impact on Phenotype

Phenotype can be influenced by both genetic and environmental factors. Traits affected by multiple genes and environmental influences are called multifactorial.

Human Genetics and Pedigree Analysis

Pedigree Analysis

A pedigree is a family tree that describes the inheritance of a trait across generations. Pedigrees help predict the probability of specific phenotypes in offspring.

Inherited Disorders

• Recessively inherited disorders: Disorders appear only in individuals homozygous for the recessive allele. Carriers are heterozygous and phenotypically normal.

• Dominantly inherited disorders: Disorders caused by dominant alleles are rare and often arise by mutation.

Multifactorial Disorders

Many diseases, such as heart disease and cancer, have both genetic and environmental components. Lifestyle choices can significantly affect phenotype regardless of genotype.

Genetic Testing and Counseling

• Carrier identification: Tests are available to identify carriers and assess risk of having affected children.

• Fetal testing: Techniques such as amniocentesis and chorionic villus sampling (CVS) can detect genetic disorders before birth.

Summary Table: Types of Dominance

Summary Table: Genetic Testing Methods

Additional info: These notes expand on the brief points and diagrams in the original slides, providing definitions, examples, and context for key concepts in Mendelian genetics and inheritance patterns.