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 draw a Punnett square for monohybrid crosses?

• How to calculate fractions for Punnett squares?

• Probability Rules in genetics

• What are environmental factors and how do they work?

• Compare the difference between the 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

Basic Genetic Vocabulary

Understanding the terminology is essential for studying inheritance patterns.

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

• Trait: Each variant for a character, such as purple or white color for flowers.

• Gene: A unit of heredity that is transferred from a parent to offspring and determines some characteristic.

• Allele: Alternative versions of a gene that account for variations in inherited characters.

Why Peas Are Ideal for Genetic Testing

Pisum sativum (pea plants) were chosen by Mendel for their suitability in genetic experiments.

• Short generation time

• Large numbers of offspring per generation

• Controlled mating: plants can self-pollinate or be cross-pollinated

• Distinct, easily observable traits

Mendel’s Experiments and Laws

Mendel’s Experimental Design

Mendel tracked only those characters that occurred in two distinct forms and used true-breeding varieties.

• True-breeding: Plants that produce offspring of the same variety when they self-pollinate.

• Hybridization: The crossing of two true-breeding varieties.

• P Generation: Parental generation (true-breeding parents).

• F1 Generation: First filial generation, offspring of the P generation.

• F2 Generation: Second filial generation, offspring of F1 individuals.

Example: Crossing purple-flowered and white-flowered pea plants resulted in all purple F1 offspring, but the F2 generation showed a 3:1 ratio of purple to white flowers.

The Law of Segregation

This law states that two alleles for a heritable character separate (segregate) 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 carries only one allele for each gene.

• Offspring inherit one allele from each parent.

Example: In flower color, the allele for purple is dominant, and the allele for white is recessive. F1 hybrids (Pp) produce gametes with either P or p.

Punnett Squares and Genotype Ratios

Punnett squares are used to predict the possible combinations of alleles in offspring.

• Dominant allele: Represented by a capital letter (e.g., P).

• Recessive allele: Represented by a lowercase letter (e.g., p).

• Genotype: The genetic makeup (e.g., PP, Pp, pp).

• Phenotype: The observable trait (e.g., purple or white flowers).

Example: Crossing two heterozygotes (Pp x Pp) yields a 3:1 phenotypic ratio in the F2 generation.

Testcross

A testcross is used to determine the genotype of an individual with a dominant phenotype.

• Breed the individual with a homozygous recessive (pp).

• If any offspring display the recessive phenotype, the parent must be heterozygous.

The Law of Independent Assortment

This law states that each pair of alleles segregates independently of other pairs during gamete formation.

• Applies to genes on different chromosomes or far apart on the same chromosome.

• Genes located near each other on the same chromosome tend to be inherited together.

Example: Dihybrid cross (YyRr x YyRr) can produce offspring with combinations of traits in a 9:3:3:1 ratio.

Probability in Genetics

Probability Rules

Genetic outcomes can be predicted using probability rules.

• Multiplication Rule: The probability that two independent events will occur together is the product of their individual probabilities.

• Addition Rule: The probability that any one of two or more mutually exclusive events will occur is calculated by adding their individual probabilities.

Example:

• Probability of YYRR = (YY) x (RR) =

• Probability of YyRR = (Yy) x (RR) =

Complex Patterns of Inheritance

Degrees of Dominance

Not all alleles show complete dominance.

• Complete Dominance: Heterozygote and dominant homozygote are indistinguishable in phenotype.

• Incomplete Dominance: F1 hybrids have a phenotype intermediate between the two parents.

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

Multiple Alleles

Some genes exist in more than two allelic forms.

• Example: ABO blood group in humans is determined by three alleles: IA, IB, and i.

Pleiotropy

One gene can affect multiple phenotypic traits.

• Example: Sickle-cell disease and cystic fibrosis are caused by pleiotropic alleles.

Epistasis

One gene affects the expression of another gene.

• Example: In Labrador retrievers, one gene determines pigment color (B for black, b for brown), another gene (E/e) determines whether pigment is deposited in the hair.

Polygenic Inheritance

Multiple genes contribute to a single phenotype, often resulting in continuous variation.

• Example: Human height and skin color are controlled by many genes.

Environmental Impact on Phenotype

Phenotype can be influenced by environmental factors as well as genotype.

• Multifactorial traits: Traits that depend on multiple genes and environmental influences.

• Example: Heart disease, cancer, and mental illnesses are influenced by both genetic and environmental factors.

Human Genetics and Pedigree Analysis

Pedigree Analysis

Pedigrees are family trees that describe the inheritance of traits across generations.

• Used to predict the probability of specific phenotypes in offspring.

• Multiplication and addition rules are applied to calculate probabilities.

Inherited Disorders

Recessively Inherited Disorders

• Appear only in individuals homozygous for the recessive allele.

• Carriers are heterozygous and phenotypically normal.

• Consanguineous matings increase the chance of both parents carrying the same rare allele.

Dominantly Inherited Disorders

• Caused by dominant alleles.

• Dominant alleles causing lethal diseases are rare and often arise by mutation.

Multifactorial Disorders

• Diseases with both genetic and environmental components.

• Examples include heart disease, cancer, alcoholism, and mental illnesses.

Genetic Testing and Counseling

Genetic counselors help prospective parents assess risks for inherited diseases.

• Carrier identification tests are available for many diseases.

• Techniques such as amniocentesis and chorionic villus sampling (CVS) can detect genetic disorders in fetuses.

• Ultrasound allows visual assessment of fetal health.

Example: If both parents are carriers for a recessive disease, the probability of having an affected child is . *Additional info: Probability calculations and pedigree analysis are essential tools in genetic counseling and risk assessment for inherited diseases.*