Mendelian Genetics: Principles of Heredity and Patterns of Inheritance

Mendelian Genetics

Introduction to Heredity

Genetics is the scientific study of heredity and variation. Mendelian genetics refers to the foundational pr

inciples of inheritance first described by Gregor Mendel in the 19th century. Mendel's experiments with garden peas led to the discovery of predictable patterns in the transmission of traits from parents to offspring.

Drawing from the Deck of Genes

Historical Hypotheses of Inheritance

• Blending Hypothesis: Proposed that genetic material from two parents blends together in offspring (e.g., blue and yellow paint make green).

• Particulate Hypothesis: Mendel's alternative, stating that parents pass on discrete heritable units (genes) that retain their identity in offspring.

• Mendel's Contribution: Through experiments with garden peas, Mendel documented the particulate mechanism of inheritance.

Mendel's Experimental, Quantitative Approach

Key Terms and Experimental Design

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

• Trait: Each variant for a character (e.g., purple or white flowers).

• Advantages of Peas:

  • Short generation time

  • Large numbers of offspring

  • Controlled mating (self-pollination or cross-pollination)

• True-breeding: Varieties that produce offspring of the same variety when self-pollinated.

• Hybridization: Mating of two contrasting, true-breeding varieties.

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

• F1 Generation: First filial generation (hybrid offspring of P generation).

• F2 Generation: Offspring from self- or cross-pollination of F1 hybrids.

The Law of Segregation

Mendel's Observations and Conclusions

• Crossing true-breeding purple and white flowered peas produced all purple F1 hybrids.

• Self-pollination of F1 hybrids yielded F2 plants with a 3:1 ratio of purple to white flowers.

• Dominant Trait: Purple flower color (expressed in F1 and majority of F2).

• Recessive Trait: White flower color (masked in F1, reappears in F2).

• Mendel observed similar patterns in six other pea plant characters, each with two traits.

Table: Results of Mendel's F2 Crosses for Seven Characters in Pea Plants

Character	Dominant Trait	Recessive Trait	F2 Generation Dominant:Recessive	Ratio
Flower color	Purple	White	705:224	3.15:1
Seed color	Yellow	Green	6,022:2,001	3.01:1
Seed shape	Round	Wrinkled	5,474:1,850	2.96:1
Pod shape	Inflated	Constricted	882:299	2.95:1
Pod color	Green	Yellow	428:152	2.82:1
Flower position	Axial	Terminal	651:207	3.14:1
Stem length	Tall	Dwarf	787:277	2.84:1

Mendel's Model

Four Concepts Explaining Inheritance Patterns

1. Alternative versions of genes (alleles) account for variations in inherited characters.

   • Each gene resides at a specific locus on a specific chromosome.

2. For each character, an organism inherits two alleles, one from each parent.

   • Alleles may be identical (true-breeding) or different (hybrids).

3. If the two alleles at a locus differ, the dominant allele determines the organism's appearance; the recessive allele has no noticeable effect.

4. Law of Segregation: The two alleles for a heritable character separate during gamete formation and end up in different gametes.

   • This corresponds to the distribution of homologous chromosomes during meiosis.

Genetic Vocabulary

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

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

• Phenotype: Physical appearance (e.g., purple flowers).

• Genotype: Genetic makeup (e.g., PP, Pp, or pp).

Table: Relationship Between Phenotype and Genotype

Phenotype	Genotype	Number
Purple	PP (homozygous)	1
Purple	Pp (heterozygous)	2
White	pp (homozygous)	1

Phenotypic ratio: 3:1; Genotypic ratio: 1:2:1

The Testcross

Determining Unknown Genotypes

• To determine if an individual with a dominant phenotype is homozygous or heterozygous, cross with a homozygous recessive individual.

• If any offspring display the recessive phenotype, the mystery parent is heterozygous.

The Law of Independent Assortment

Inheritance of Multiple Characters

• Mendel derived the law of segregation by following a single character (monohybrid cross).

• By following two characters (dihybrid cross), Mendel found that alleles of different genes assort independently during gamete formation.

• This law applies to genes on different, nonhomologous chromosomes or those far apart on the same chromosome.

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

Table: Dihybrid Cross Results

Phenotype	Number	Ratio
Yellow round	315	9
Green round	108	3
Yellow wrinkled	101	3
Green wrinkled	32	1

Phenotypic ratio: 9:3:3:1

Probability Laws in Mendelian Inheritance

Rules of Probability

• Multiplication Rule: Probability that two or more independent events will occur together is the product of their individual probabilities. Example: Probability of YYRR = Probability of YY × Probability of RR

• Addition Rule: Probability that any one of two or more mutually exclusive events will occur is the sum of their individual probabilities.

• These rules allow prediction of genotype and phenotype ratios in genetic crosses.

Key Equations

• Probability of two independent events (A and B): $P(A \text{ and } B) = P(A) \times P(B)$

• Probability of either of two mutually exclusive events (A or B): $P(A \text{ or } B) = P(A) + P(B)$

Summary

• Mendel's principles of segregation and independent assortment form the foundation of classical genetics.

• Inheritance patterns can be predicted using probability laws and Punnett squares.

• These principles apply to many, but not all, genetic traits in living organisms.

Additional info: This summary covers the core concepts of Mendelian genetics, including experimental design, vocabulary, and the laws of segregation and independent assortment, as well as the application of probability to genetic crosses.