Mendelian Genetics: Foundations of Inheritance

Introduction to Mendelian Genetics

Mendelian genetics forms the basis of classical genetics, describing how traits are inherited from one generation to the next. Gregor Mendel's experiments with pea plants led to the discovery of fundamental laws of inheritance, including the law of segregation and the law of independent assortment.

Key Terms and Concepts

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

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

• Gene: A heritable factor that determines a character; located at a specific locus on a chromosome.

• Allele: Alternative versions of a gene.

• Homozygous: Having two identical alleles for a gene.

• Heterozygous: Having two different alleles for a gene.

• Phenotype: The physical appearance or observable traits of an organism.

• Genotype: The genetic makeup of an organism.

Mendel's Experimental Approach

Mendel used pea plants for their short generation time, large number of offspring, and the ability to control mating. He began with true-breeding varieties and performed hybridization experiments to track inheritance patterns.

The Law of Segregation

Mendel observed that when crossing true-breeding purple-flowered and white-flowered pea plants, all F1 offspring were purple. However, self-pollination of F1 plants produced both purple and white flowers in a 3:1 ratio in the F2 generation. This led to the law of segregation: the two alleles for a heritable character separate during gamete formation and end up in different gametes.

Punnett Squares and Genetic Vocabulary

Punnett squares are used to predict the possible combinations of alleles in offspring. A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele. Homozygotes have identical alleles (e.g., PP or pp), while heterozygotes have different alleles (e.g., Pp).

Testcross

A testcross is used to determine the genotype of an individual with a dominant phenotype by crossing it with a homozygous recessive individual. If any offspring display the recessive phenotype, the unknown parent is heterozygous.

The Law of Independent Assortment

Mendel's law of independent assortment states that each pair of alleles segregates independently of other pairs during gamete formation. This law applies to genes on different chromosomes or those far apart on the same chromosome.

Probability and Mendelian Inheritance

Probability Rules in Genetics

The rules of probability, such as the multiplication and addition rules, are used to predict the outcomes of genetic crosses. The multiplication rule applies to independent events ("and"), while the addition rule applies to mutually exclusive events ("or").

Extensions of Mendelian Genetics

Degrees of Dominance

• Complete Dominance: The phenotype of the heterozygote is identical to the dominant homozygote.

• Incomplete Dominance: The phenotype of F1 hybrids is intermediate between the two parental varieties.

• Codominance: Both alleles affect the phenotype in separate, distinguishable ways.

Multiple Alleles and Codominance

Many genes exist in more than two allelic forms. For example, the ABO blood group in humans is determined by three alleles (IA, IB, i), and exhibits codominance between IA and IB.

Pleiotropy

Pleiotropy occurs when one gene influences multiple phenotypic traits. Examples include cystic fibrosis and sickle-cell disease, where a single gene affects multiple symptoms.

Epistasis

Epistasis is when the expression of one gene affects the phenotypic expression of another gene. For example, in Labrador retrievers, one gene determines pigment color and another gene determines whether pigment is deposited in the hair.

Polygenic Inheritance

Polygenic inheritance occurs when multiple genes independently affect a single trait, resulting in quantitative characters that vary along a continuum (e.g., human height, skin color).

Environmental Impact on Phenotype

Many traits are influenced by both genetic and environmental factors, leading to multifactorial inheritance. The phenotypic range is broadest for polygenic characters.

Human Genetics and Pedigree Analysis

Pedigree Analysis

Pedigrees are family trees that describe the inheritance of traits across generations. They are used to analyze human genetic disorders and predict the probability of future offspring inheriting certain traits.

Recessively and Dominantly Inherited Disorders

• Recessive Disorders: Only appear in individuals homozygous for the allele (e.g., cystic fibrosis, sickle-cell disease).

• Dominant Disorders: Caused by dominant alleles; often rare and may have late onset (e.g., Huntington's disease, achondroplasia).

Genetic Testing and Counseling

Genetic counselors use Mendelian genetics and probability rules to assess the risk of inherited disorders. Techniques such as amniocentesis and chorionic villus sampling (CVS) are used for fetal testing, while newborn screening detects genetic disorders at birth.

Biochemical Pathways and Genetics

Some genetic traits are determined by the presence or absence of specific enzymes in biochemical pathways. For example, the color of apple skin depends on the effectiveness of enzyme X in converting compound C (yellow) to compound D (red).

Additional info: These notes cover the core concepts of Mendelian genetics, probability in inheritance, and extensions such as incomplete dominance, codominance, pleiotropy, epistasis, polygenic inheritance, and the impact of environment on phenotype. Human genetics applications, including pedigree analysis and genetic counseling, are also discussed.