Mendel and the Gene Idea

Gregor Mendel and His Experiments

Gregor Mendel, a friar and teacher with a background in gardening and natural sciences, conducted pioneering experiments with pea plants between 1856 and 1863. His work established the foundational rules of heredity, though it remained largely unknown until the 20th century.

• Mendel’s Experimental Plant: Pisum sativum (garden pea) was chosen for its simple inheritance patterns and ease of manipulation.

• Simple Inheritance: Many traits in peas are controlled by single genes, making them ideal for genetic studies.

• Manipulation of Flowers: Mendel could control which plants acted as male or female and prevent self-pollination to perform cross-breeding experiments.

Section 14.1: Mendel’s Scientific Approach and Laws of Inheritance

Mendel used a scientific approach to identify two fundamental laws of inheritance: the Law of Segregation and the Law of Independent Assortment.

• Heredity: The transmission of traits from one generation to the next.

• Genetics: The scientific study of heredity, revolutionized by modern technology (microscopy, DNA sequencing, PCR, gene-editing).

• Pedigrees: Family trees showing inheritance of traits.

• Selective Breeding: Choosing parents with desired traits to reproduce.

Key Vocabulary

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

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

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

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

• Dominant Allele: The allele whose trait is visible in the phenotype.

• Recessive Allele: The allele whose trait is only visible if there is no dominant allele present.

• True-breeding: Parents produce offspring identical to themselves (homozygous).

• Hybridization: Crossing two true-breeding varieties to produce offspring with both traits.

Mendel’s Laws

• Law of Segregation: During meiosis, alleles for a gene separate so each gamete receives only one allele.

• Law of Independent Assortment: Alleles for different genes separate independently during gamete formation, especially if genes are on different chromosomes or far apart on the same chromosome.

Mendel’s Pea Experiments and Genetic Crosses

Mendel performed genetic crosses to study inheritance patterns, using monohybrid and dihybrid crosses.

• P Generation: Parent generation.

• F1 Generation: First filial generation (children).

• F2 Generation: Second filial generation (grandchildren).

• Monohybrid Cross: Cross examining one trait.

• Dihybrid Cross: Cross examining two traits.

Punnett Square

The Punnett Square is a tool used to predict the possible genotypes and phenotypes of offspring from a genetic cross.

• Example: Crossing a true-breeding purple flower (PP) with a true-breeding white flower (pp) produces all heterozygous (Pp) offspring in the F1 generation, which display the purple phenotype.

Test Cross

A test cross is used to determine the genotype of an organism showing the dominant phenotype by crossing it with a homozygous recessive organism.

• Prediction: If the dominant organism is homozygous (PP), all offspring will show the dominant trait. If heterozygous (Pp), offspring will be split between dominant and recessive phenotypes.

Dihybrid Cross and the Law of Independent Assortment

Dihybrid crosses examine the inheritance of two traits simultaneously. Mendel’s experiments showed that traits are inherited independently, leading to the classic 9:3:3:1 phenotypic ratio in the offspring of two heterozygous parents.

Section 14.2: Probability Laws in Mendelian Inheritance

Probability and Mendel’s Genetics

Probability laws govern the inheritance of traits. Each allele option is considered an independent event, and the likelihood of a particular outcome can be calculated using probability rules.

• Multiplication Rule: The probability of two independent events both occurring is the product of their individual probabilities.

• Addition Rule: The probability of either of two mutually exclusive events occurring is the sum of their individual probabilities.

Example: The probability of an AaBb offspring from a cross where each allele has a 1/2 chance is .

Section 14.3: Complex Patterns of Inheritance

Complete Dominance

In complete dominance, the dominant allele completely masks the effect of the recessive allele in the phenotype.

Incomplete Dominance

Neither allele is completely dominant. The heterozygous condition results in a phenotype that is intermediate between the two homozygous phenotypes.

Codominance

Both alleles are fully and distinctly expressed in the heterozygous phenotype, rather than blending.

Multiple Alleles

Some genes have more than two alleles. The ABO blood group system is a classic example, where A and B are codominant and O is recessive.

Pleiotropy

Pleiotropy occurs when one gene influences multiple, seemingly unrelated traits. Examples include albinism and sickle-cell disease.

Epistasis

Epistasis is when one gene controls or masks the expression of another gene. In Labrador Retrievers, the E gene controls pigment deposition, while the B gene determines pigment color.

Polygenic Inheritance

Polygenic inheritance occurs when multiple genes contribute to a single trait, such as human skin color.

Environmental Influences and Epigenetics

Environmental factors can affect gene expression, leading to changes in phenotype without altering the DNA sequence. This is known as epigenetics.

Section 14.4: Human Inheritance Patterns and Pedigrees

Pedigree Charts

Pedigree charts are used to study genetic traits and heredity in families, especially before DNA-based genetic data was available. Roman numerals indicate generations, and individual numbers identify specific people.

Dominant and Recessive Pedigrees

Pedigrees can show dominant and recessive inheritance patterns. The Punnett Square predicts genotypes, while the pedigree shows phenotypes.

Lethal Alleles

• Recessive Lethal: Homozygous recessive condition is fatal, skewing ratios of affected and unaffected individuals. Examples: cystic fibrosis, sickle cell anemia, achondroplasia.

• Dominant Lethal: Homozygous dominant condition is lethal, but late onset allows reproduction. Example: Huntington’s disease.

Incomplete Dominance in Human Traits

Hair texture is an example of incomplete dominance, where straight, wavy, and curly hair result from different allele combinations.

Multiple Alleles – Blood Types

The ABO blood type system demonstrates multiple alleles and codominance. Genotypes IAIB result in AB phenotype because both alleles are codominant.

Inbreeding and Homozygosity

Close relative marriages increase the chance of inheriting two copies of harmful recessive mutations, leading to genetic disorders. The inbreeding coefficient (F-statistic) measures the degree of homozygosity.

Example: Charles II of Spain suffered from multiple health issues due to high homozygosity from repeated close-relative marriages.

Additional info: Probability rules, epigenetics, and inbreeding coefficients were expanded for academic completeness.