Basics of Genetics: Mendel and the Gene Idea

Basics of Genetics: Mendel and the Gene Idea

Introduction to Mendelian Genetics

Genetics is the study of heredity and variation in living organisms. Gregor Mendel, known as the father of genetics, established foundational principles through his experiments with garden peas (Pisum sativum), which revealed how traits are inherited across generations.

• Blending Hypothesis: Proposed that parental traits mix in offspring, but failed to explain reappearance of traits after skipping generations.

• Particulate Hypothesis (Gene Idea): Mendel suggested that parents pass on discrete heritable units—genes—that retain their identities in offspring.

Gregor Mendel and His Experiments

Mendel conducted experiments using true-breeding pea plants, focusing on single traits to understand inheritance patterns.

• True Breeding: Plants that consistently produce offspring with the same trait over generations.

• Cross-Pollination: Mendel crossed purple and white flowered plants to observe inheritance.

• Character vs. Trait: Character is a heritable feature (e.g., flower color); trait is a variant of the character (e.g., purple, white).

Generational Terminology

• P Generation: Parental generation.

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

• F2 Generation: Second filial generation, produced by self-pollination of F1.

Mendel's Experimental Results

Crossing purple and white flowers yielded all purple F1 offspring. Self-pollination of F1 produced F2 with a 3:1 ratio of purple to white flowers, demonstrating dominance and segregation of traits.

Mendel's Laws of Inheritance

• Law of Segregation: Two alleles for a heritable character segregate during gamete formation.

• Law of Dominance: When two alleles differ, the dominant allele determines the phenotype; the recessive allele is masked.

• Law of Independent Assortment: Alleles of different genes assort independently during gamete formation.

Genotype and Phenotype

• Genotype: Genetic constitution of an organism (e.g., BB, Bb, bb).

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

• Homozygous: Two identical alleles for a gene.

• Heterozygous: Two different alleles for a gene.

Symbols in Genetic Analysis

• Symbols represent alleles and are used to predict outcomes of genetic crosses.

• Genetic analysis uses mathematical models to predict inheritance patterns.

Probability in Genetics

• Multiplication Rule: Probability of two independent events occurring together is the product of their probabilities.

• Addition Rule: Probability of either of two mutually exclusive events is the sum of their probabilities.

• Example: Probability of F2 heterozygote = $\frac{1}{4} + \frac{1}{4} = \frac{1}{2}$

Non-Mendelian Inheritance Patterns

Incomplete Dominance

Occurs when the heterozygote phenotype is intermediate between both homozygotes.

• Example: Snapdragon flower color—WW (red), Ww (pink), ww (white).

Phenotype	Genotype	Amount of Gene Product
Red	WW	2x
Pink	Ww	1x
White	ww	0

Codominance

Both alleles in a heterozygote are fully expressed.

• Example: Human blood types (M and N antigens).

• Genotypes: $L^M L^M$ (M), $L^N L^N$ (N), $L^M L^N$ (MN).

Genotype	Blood Type	Anti-M Serum	Anti-N Serum
$L^M L^M$	M	+	-
$L^N L^N$	N	-	+
$L^M L^N$	MN	+	+

Epistasis

Interaction between non-allelic genes where one gene masks the expression of another.

• Epistatic gene: Masks the effect of another gene.

• Hypostatic gene: Is masked by another gene.

• Example: Fruit color in summer squash—white, yellow, green.

Genotype	Phenotype
C- G-	White
C- gg	White
cc G-	Yellow
cc gg	Green

Additional Concepts

• Pleiotropy: One gene affects multiple phenotypic traits (e.g., phenylketonuria affects brain and hair color).

• Polygenic Inheritance: Multiple genes contribute to a single trait, resulting in a spectrum of phenotypes (e.g., skin color).

• Environmental Impact: Genotype can produce a range of phenotypes depending on environmental conditions (norm of reaction).

Genetic Disorders and Testing

• Recessive Disorders: Caused by homozygous recessive alleles; carriers are heterozygous.

• Dominant Disorders: Caused by dominant alleles (e.g., achondroplasia, Huntington's disease).

• Genetic Testing: Includes amniocentesis, chorionic villus sampling, newborn screening, and non-invasive prenatal testing (NIPT).

Non-Invasive Prenatal Testing (NIPT)

• Analyzes cell-free fetal DNA in maternal blood.

• Detects chromosomal disorders (e.g., Down syndrome, trisomy 18, trisomy 13).

• Advantages: Noninvasive, early detection, can also identify maternal genetic conditions.

Additional info: These notes are based on lecture slides referencing Campbell Biology, Ch. 14, and cover core topics in Mendelian and non-Mendelian genetics, suitable for General Biology college students.