Mendelian Genetics and Single-Gene Inheritance: Study Notes

Genetics: Mendelian Principles and Patterns

Introduction to Genetics

Genetics is the study of heredity and variation in living organisms. Mendelian genetics, based on the pioneering work of Gregor Mendel, explains how traits are inherited from one generation to the next through discrete units called genes.

• Alleles: Different forms of a gene, inherited from each parent.

• Genotype: The genetic makeup of an individual (the alleles present).

• Phenotype: The observable characteristics or traits of an individual.

Mendelian Genetics

Gregor Johann Mendel and His Experiments

Gregor Mendel (1822–1884) conducted experiments with pea plants to uncover the basic principles of inheritance. He chose pea plants because they have several distinct characters, each with two possible traits.

• Character: A heritable feature (e.g., seed shape, flower color).

• Trait: A variant of a character (e.g., round or wrinkled seeds).

Pea Plant Traits Studied by Mendel

Trait	Dominant Phenotype	Recessive Phenotype	Ratio
Seed shape	Round	Wrinkled	2.96:1
Seed color	Yellow	Green	3.01:1
Pod shape	Inflated	Constricted	2.83:1
Pod color	Green	Yellow	3.13:1
Flower color	Purple	White	3.15:1
Flower position	Axial	Terminal	3.14:1
Stem length	Tall	Dwarf	2.84:1

Experimental Methods: Self- and Cross-Fertilization

• Self-fertilization: Pollen from a flower fertilizes eggs of the same flower.

• Cross-fertilization: Pollen from one plant is transferred to another plant whose male organs have been removed.

Historical Hypotheses of Inheritance

• Blending hypothesis: Offspring are a blend of parental traits.

• Acquired traits hypothesis: Offspring inherit traits acquired by parents during their lifetime.

• Mendel disproved these with his experiments.

Mendel’s Experiments and Conclusions

Monohybrid Crosses

Mendel crossed pure-breeding plants with contrasting traits (e.g., tall vs. short). All F1 offspring resembled one parent (dominant trait).

• Monohybrid cross: A cross between two individuals differing in one trait.

• F1 generation: All showed the dominant trait.

• F2 generation: Traits segregated in a 3:1 ratio (dominant:recessive).

Mendel’s Conclusions

• Each individual has two hereditary determinants (alleles) for each character.

• If the two determinants differ, one is dominant and the other is recessive.

• Principle of Segregation: The two alleles for a character separate during gamete formation, so each gamete receives one allele.

Genetic Terminology

• Genotype: The alleles present in an individual (e.g., RR, Rr, rr).

• Phenotype: The outward appearance (e.g., round or wrinkled seeds).

• Dominant allele: Expressed in the phenotype if present.

• Recessive allele: Masked in the phenotype if a dominant allele is present.

• Homozygote: Two identical alleles (e.g., RR or rr).

• Heterozygote: Two different alleles (e.g., Rr).

Molecular Basis of Seed Shape

Seed Shape Gene and Amylopectin Synthesis

The seed shape gene (R) codes for an enzyme that synthesizes amylopectin, a type of starch. The dominant allele (R) produces a functional enzyme, resulting in round seeds. The recessive allele (r) contains a mutation (e.g., early stop codon), producing a non-functional enzyme and wrinkled seeds.

• Amylose: Linear starch molecule (1,4 linkages).

• Amylopectin: Branched starch molecule (1,4 and 1,6 linkages).

• Mutation in r allele: Leads to non-functional enzyme and wrinkled seeds.

Predicting Genetic Crosses

Monohybrid Cross Example

Crossing two heterozygotes (Rr x Rr) for seed shape:

• Possible gametes: R or r from each parent.

• Punnett square shows possible offspring genotypes: RR, Rr, rR, rr.

• Genotypic ratio: 1 RR : 2 Rr : 1 rr

• Phenotypic ratio: 3 round : 1 wrinkled

Probability Theory in Genetics

• Product Rule: Probability that two independent events both occur = product of their individual probabilities.

• Sum Rule: Probability that one of two mutually exclusive events occurs = sum of their individual probabilities.

Example calculations for Rr x Rr cross:

• Probability of RR zygote: $\frac{1}{2} \times \frac{1}{2} = \frac{1}{4}$

• Probability of rr zygote: $\frac{1}{2} \times \frac{1}{2} = \frac{1}{4}$

• Probability of Rr or rR zygote: $\frac{1}{2} \times \frac{1}{2} + \frac{1}{2} \times \frac{1}{2} = \frac{1}{2}$

• Probability of round seeds (RR + Rr): $\frac{1}{4} + \frac{1}{2} = \frac{3}{4}$

• Probability of wrinkled seeds (rr): $\frac{1}{4}$

Summary Table: Mendelian Ratios in Monohybrid Crosses

Genotype	Probability	Phenotype
RR	1/4	Round
Rr	1/2	Round
rr	1/4	Wrinkled

Additional info:

• Crossing over during meiosis can affect the assortment of alleles, but for genes on different chromosomes or far apart on the same chromosome, Mendel’s laws apply.

• These principles form the foundation for understanding more complex inheritance patterns and human genetics.