Chpt 3, 9/8. 9/12

Introduction to Mendelian Genetics

Genetics is the study of heredity and variation in living organisms. The foundational work of Gregor Mendel in the 19th century established the basic principles of inheritance using the garden pea plant. Mendel's experiments led to the discovery of how traits are transmitted from parents to offspring through discrete units called genes.

Key Terms and Definitions

• Transmission genetics: The study of how genes are passed from one generation to the next.

• Trait: A specific observable feature or characteristic of an organism.

• Monohybrid cross: A genetic cross involving a single pair of contrasting traits.

• P generation: Parental generation in a genetic cross.

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

• F2 generation: Second filial generation, offspring of the F1 generation.

Mendel's Experiments and Principles

Mendel's Laws or Principles of Inheritance

Mendel's experiments with pea plants led to the formulation of several key principles:

• Law of Unit Factors in Pairs: Each individual possesses two unit factors (genes) for each trait, one inherited from each parent.

• Law of Dominance and Recessiveness: When two different unit factors are present, one may mask the expression of the other (dominant vs. recessive).

• Law of Segregation: During gamete formation, the two alleles for a trait separate so that each gamete receives only one allele.

• Law of Independent Assortment: Genes for different traits assort independently of one another during gamete formation.

Genotype and Phenotype

• Genotype: The genetic constitution of an organism (e.g., DD, Dd, or dd).

• Phenotype: The observable physical or physiological traits of an organism.

• Homozygous: Both alleles for a gene are the same (e.g., DD or dd).

• Heterozygous: The two alleles for a gene are different (e.g., Dd).

Punnett Squares

Punnett squares are diagrams used to predict the genotypic and phenotypic outcomes of genetic crosses.

Monohybrid and Dihybrid Crosses

Monohybrid Cross

A monohybrid cross examines the inheritance of a single trait. For example, crossing two heterozygous individuals (F1 generation) for a trait with dominant and recessive alleles yields a 3:1 phenotypic ratio in the F2 generation.

• Example: Crossing pea plants with round (dominant) and wrinkled (recessive) seeds.

Dihybrid Cross

A dihybrid cross examines the inheritance of two different traits simultaneously. Mendel's experiments with seed color and shape demonstrated the law of independent assortment, resulting in a 9:3:3:1 phenotypic ratio in the F2 generation.

• Example: Crossing plants with yellow round seeds (YYRR) and green wrinkled seeds (yyrr).

Sample Dihybrid Cross Table

Genotype	Phenotype	Ratio
YYRR	Yellow, Round	9/16
YYrr, Yyrr	Yellow, Wrinkled	3/16
yyRR, yyRr	Green, Round	3/16
yyrr	Green, Wrinkled	1/16

Sexual Life Cycle and Gamete Formation

• Zygote: The first stage of a new organism, formed by the fusion of gametes.

• Haploid cells: Cells with a single set of chromosomes (e.g., sperm and egg).

• Meiosis: The process by which haploid gametes are produced from diploid cells.

Meiosis in Males vs. Females

• Males: Meiosis occurs in a small time frame during adulthood, producing sperm.

• Females: Meiosis starts prenatally, pauses, and resumes at puberty, producing eggs. The process is completed only if fertilization occurs.

Testcross and Pedigree Analysis

Testcross

A testcross is used to determine the genotype of an individual expressing a dominant phenotype by crossing it with a homozygous recessive individual.

• If all offspring display the dominant trait, the tested individual is likely homozygous dominant.

• If offspring show a 1:1 ratio of dominant to recessive traits, the tested individual is heterozygous.

Pedigree Analysis

Pedigrees are diagrams that show the inheritance patterns of traits in families across generations. They use standardized symbols to represent individuals and relationships.

Symbol	Meaning
■, ●	Affected individuals (male, female)
□, ○	Unaffected individuals (male, female)
—	Mating line
│	Offspring line
◇	Multiple individuals (amalgamated)
⧫	Proband (first affected family member studied)

Probability in Genetics

Product Rule and Sum Law

• Product Rule: The probability of two or more independent events occurring together is the product of their individual probabilities.

• Sum Law: The probability of any one of two or more mutually exclusive events occurring is the sum of their individual probabilities.

Chi-Square Analysis

Chi-square analysis is used to evaluate whether observed genetic data fit expected ratios. It tests the null hypothesis that there is no significant difference between observed and expected results.

• Formula:

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

• O = Observed frequency

• E = Expected frequency

Degrees of freedom (df) is calculated as the number of categories minus one: $df = n - 1$.

• If p < 0.05, reject the null hypothesis (significant deviation).

• If p ≥ 0.05, fail to reject the null hypothesis (no significant deviation).

Chromosomal Theory of Inheritance

The chromosomal theory of inheritance states that genes are located on chromosomes, which are the carriers of genetic information in living organisms.

Summary Table: Mendelian Ratios

Cross Type	Genotypic Ratio	Phenotypic Ratio
Monohybrid (heterozygous x heterozygous)	1:2:1	3:1
Dihybrid (heterozygous x heterozygous)	9:3:3:1	9:3:3:1

Additional info:

• Some explanations and terminology have been expanded for clarity and completeness.

• Tables have been reconstructed and symbols clarified for self-contained study.