Genetics: Mendel and the Gene – Study Notes

Genetics: Mendel and the Gene

Introduction

Genetics is the study of heredity and the variation of inherited characteristics. This unit focuses on Mendelian genetics, patterns of inheritance, and the molecular basis of genes, as outlined in Chapter 14: Mendel and the Gene.

Key Concepts in Genetics

Dominant and Recessive Traits

• Dominant allele: An allele that expresses its phenotype even in the presence of a recessive allele.

• Recessive allele: An allele whose phenotype is masked by a dominant allele; only expressed when two copies are present.

• Example: In pea plants, the allele for purple flowers (P) is dominant over the allele for white flowers (p).

Homozygous and Heterozygous

• Homozygous: Having two identical alleles for a particular gene (e.g., PP or pp).

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

Genotype and Phenotype

• Genotype: The genetic makeup of an organism (e.g., Pp).

• Phenotype: The observable physical or physiological traits of an organism (e.g., purple flowers).

Punnett Squares and Probability

Using Punnett Squares

Punnett squares are diagrams used to predict the genotypes and phenotypes of offspring from genetic crosses.

• Monohybrid cross: Examines the inheritance of one trait.

• Dihybrid cross: Examines the inheritance of two traits simultaneously.

• Probability: The likelihood of a particular genotype or phenotype appearing in the offspring can be calculated using the Punnett square.

Example: Crossing two heterozygous pea plants (Pp x Pp):

• Genotypic ratio: 1 PP : 2 Pp : 1 pp

• Phenotypic ratio: 3 purple : 1 white

Patterns of Inheritance

Autosomal vs. Sex-Linked Inheritance

• Autosomal inheritance: Traits determined by genes located on non-sex chromosomes (autosomes).

• Sex-linked inheritance: Traits determined by genes located on sex chromosomes (X or Y).

• Example: Red-green color blindness is an X-linked recessive trait.

Complex Patterns of Inheritance

• Codominance: Both alleles are fully expressed in the phenotype (e.g., AB blood type).

• Multiple alleles: More than two possible alleles exist for a gene (e.g., IA, IB, i for human ABO blood group).

• Sex-linked genes: Genes located on the sex chromosomes, often showing different inheritance patterns in males and females.

Human ABO Blood System

The ABO blood group is determined by three alleles: IA, IB, and i. IA and IB are codominant, while i is recessive.

Genotype	Phenotype (Blood Type)
IAIA or IAi	A
IBIB or IBi	B
IAIB	AB
ii	O

Key Terms and Definitions

• Allele: Different forms of a gene.

• Locus: The specific location of a gene on a chromosome.

• Monohybrid cross: A cross between individuals heterozygous for a single trait.

• Dihybrid cross: A cross between individuals heterozygous for two traits.

Summary Table: Patterns of Inheritance

Pattern	Description	Example
Complete Dominance	One allele completely masks the other	Purple vs. white flowers in peas
Codominance	Both alleles are fully expressed	AB blood type
Multiple Alleles	More than two alleles exist for a gene	ABO blood group
Sex-linked	Gene located on a sex chromosome	Color blindness

Key Equations

• Probability of independent events:

$P(A \text{ and } B) = P(A) \times P(B)$

• Genotypic ratio (monohybrid cross):

$1 : 2 : 1$

• Phenotypic ratio (monohybrid cross):

$3 : 1$

Additional info: These notes expand on the syllabus outline by providing definitions, examples, and tables for clarity and exam preparation.