Mendelian Genetics: Terminology and Analysis Tools

Mendelian Genetics: Terminology and Analysis Tools

Introduction

This study guide covers foundational concepts in Mendelian genetics, including key terminology, Mendel's postulates, gene and allele definitions, genetic crosses, probability calculations, and statistical analysis tools. These principles are essential for understanding inheritance patterns and genetic analysis in college-level genetics.

Mendelian Principles

Mendel's Postulates

Mendel's experiments with pea plants led to the formulation of basic laws of inheritance. These postulates explain how traits are transmitted from parents to offspring.

• Traits are controlled by factors: These factors are now known as genes, which determine specific characteristics such as seed shape or flower color.

• Alternate forms of genes exist: These are called alleles (e.g., round vs. wrinkled seed shape).

• Individuals inherit one allele from each parent: Each parent contributes one allele for each gene, resulting in two alleles per gene in most organisms.

Key Mendelian Laws

• Law of Segregation: During meiosis, alleles for a trait separate so that each gamete carries only one allele for each gene.

• Law of Independent Assortment: Genes located on different chromosomes are inherited independently of each other.

Genetic Terminology

Genes and Alleles

A gene is a segment of DNA that contributes to genetic traits, characteristics, or functions. Genes are the fundamental units of heredity and determine specific traits of an organism.

• Allele: An alternate form of a gene. For example, the gene for flower color may have a purple allele and a white allele.

• Homozygous: An organism with two identical alleles for a gene (e.g., PP or pp).

• Heterozygous: An organism with two different alleles for a gene (e.g., Pp).

Key Differences Between Genes and Alleles

Gene	Allele
A sequence of DNA that codes for a specific protein or trait.	A specific version or variant of a gene.
Determines the general trait (e.g., eye color).	Determines the specific form of the trait (e.g., blue or brown eyes).
Each organism has two copies of each gene (one from each parent).	Each organism has two alleles for each gene, one from each parent.

Genotype and Phenotype

Definitions

• Genotype: The combination of alleles an organism inherits for a specific gene (e.g., PP, Pp, pp).

• Phenotype: The physical expression of the genotype—what the trait looks like (e.g., purple or white flowers).

Examples

• PP: Purple flower (homozygous dominant)

• Pp: Purple flower (heterozygous; dominant allele expressed)

• pp: White flower (homozygous recessive)

Test Crosses

Purpose and Method

A test cross is used to determine the genotype of an individual displaying a dominant phenotype. The individual is crossed with a homozygous recessive organism.

• If the unknown is homozygous dominant (AA): All offspring will show the dominant trait (Aa).

• If the unknown is heterozygous (Aa): Offspring will be in a 1:1 ratio of dominant (Aa) to recessive (aa) traits.

Probability in Genetics

Independent Events and Calculations

The probability of inheriting one trait is independent of the probability of inheriting another trait (if genes are on different chromosomes).

• Combined probability for two independent traits: Multiply the probabilities of each trait.

Example: If the probability of yellow seeds is $\frac{3}{4}$ and round seeds is $\frac{3}{4}$, then the probability of yellow and round seeds is:

$\frac{3}{4} \times \frac{3}{4} = \frac{9}{16}$

Punnett Squares

Purpose and Use

A Punnett square is a diagram used to predict the possible genotypes and phenotypes of offspring from a genetic cross.

• Monohybrid cross: Involves one trait; typical F2 generation ratio is 3:1 for phenotype, 1:2:1 for genotype.

• Dihybrid cross: Involves two traits; typical F2 generation ratio is 9:3:3:1 for phenotype.

Reciprocal Crosses

Definition and Application

A reciprocal cross switches the sexes of the parents in a genetic cross to determine if inheritance is affected by sex linkage.

• Autosomal reciprocal cross: No difference in offspring ratios between the two crosses; trait is autosomal.

• X-linked reciprocal cross: Offspring ratios differ depending on which parent carries the trait; indicates sex-linked inheritance.

Chi-Square Analysis

Purpose and Formula

The chi-square test is a statistical method used to compare observed results with expected results based on Mendelian ratios.

• Formula:

$\chi^2 = \sum \frac{(O - E)^2}{E}$

• O: Observed number for each category

• E: Expected number for each category

• Degrees of freedom (df): Number of categories minus one

• Critical value: For df = 1 and p = 0.05, the critical value is 3.84

Example Calculation

• Observed: 62 purple, 18 white (total 80 plants)

• Expected (3:1 ratio): 60 purple, 20 white

• Calculation:

For purple: $(62 - 60)^2 / 60 = 0.067$ For white: $(18 - 20)^2 / 20 = 0.2$ Total $\chi^2 = 0.267$

• Since $0.267 < 3.84$, the results fit Mendel's 3:1 ratio.

Case Study: Mendel’s White Flowers

Genetic Basis of Flower Color

The genetic mystery of Mendel’s white flowers in peas was solved by identifying a transcription factor gene responsible for regulating the anthocyanin gene (pigment production).

• Gene A encodes a transcription factor for anthocyanin expression.

• A single-nucleotide change in gene A can render the transcription factor inactive, resulting in white flowers.

• Plants with a normal copy of gene A express anthocyanin and have purple flowers.

• Plants with a mutant form of gene A do not produce anthocyanin and are white.

Summary Table: Key Genetic Terms

Term	Definition	Example
Gene	Segment of DNA coding for a trait	Flower color gene
Allele	Alternate form of a gene	Purple allele, white allele
Genotype	Allele combination	PP, Pp, pp
Phenotype	Physical trait expression	Purple flower, white flower
Test Cross	Cross to determine genotype	Unknown x aa
Punnett Square	Diagram for predicting offspring	Monohybrid, dihybrid crosses
Chi-Square Test	Statistical test for observed vs expected	$\chi^2 = \sum \frac{(O - E)^2}{E}$

Additional info: Some explanations and examples were expanded for clarity and completeness, including the summary tables and probability calculations.