Genetics & Inheritance

Introduction & Terminology

Genetics is the study of genes and their transmission from one generation to the next. Inheritance refers to the process by which genetic information is passed from parents to offspring. Each individual inherits one complete set of genes from each parent, resulting in genetic diversity among individuals.

• Genes: Units of heredity that encode information for specific traits.

• Inheritance: The process of receiving genetic material from ancestors.

• Diversity: Slight differences in inherited genes account for variation among individuals.

DNA Organization

Human DNA is organized into 23 pairs of chromosomes:

• Autosomes: 22 pairs, identical in length and shape, but may have slight DNA sequence differences.

• Sex Chromosomes: 1 pair (X and Y); X and Y differ in length and shape, with the Y chromosome being smaller.

Homologous Chromosomes: Chromosomes that look alike and carry the same genes at the same loci (locations). Examples include autosomes and the two X chromosomes in females.

Alleles: Different versions of a gene found at the same locus. Alleles arise from mutations and can result in different protein structures and functions, leading to varied traits.

Types of Alleles

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

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

• Multiple Alleles: Some genes have more than two possible alleles (e.g., blood types A, B, O), but each individual inherits only two alleles per gene.

Allelic variation is the result of mutations in gamete-forming cells.

Genotype vs. Phenotype

• Genotype: The complete set of genes and alleles (genome) inherited from parents. The exact genotype is not always observable without genetic testing.

• Phenotype: Observable physical and functional traits (e.g., eye color, height, blood type). Phenotype is determined by genotype and can be influenced by environmental factors.

Dominant vs. Recessive Alleles

• Dominant Allele: Produces the same phenotype whether paired with an identical or different allele. Indicated by uppercase letters (e.g., AA or Aa).

• Recessive Allele: Expressed only when paired with an identical allele. Indicated by lowercase letters (e.g., aa).

Note: Dominance refers to allele behavior in heterozygotes, not frequency in the population. For example, polydactylism (extra fingers/toes) is dominant but rare.

Recessive alleles may persist in populations if they are harmless or masked by dominant alleles in heterozygotes.

Mendelian Genetics

Gregor Mendel and His Experiments

Gregor Mendel, known as the "Father of Genetics," conducted experiments with pea plants in the 1850s. He studied traits such as seed shape, color, flower position, and stem length, observing consistent inheritance patterns.

• Monohybrid Cross: Cross involving one trait. Mendel observed a 3:1 ratio of dominant to recessive phenotypes in the F2 generation.

Mendel’s First Law: Law of Segregation

During gamete formation, alleles for each gene separate so that each gamete receives only one allele. This process occurs during meiosis.

Punnett Squares

Punnett squares are tools used to predict inheritance patterns and probabilities of genotypes and phenotypes in offspring.

• Probabilities can be expressed as ratios (e.g., 3:1), fractions (e.g., 3/4), or percentages (e.g., 75%).

• Monohybrid crosses involve one gene; dihybrid crosses involve two genes.

Example: Crossing two heterozygous individuals (Aa x Aa) yields:

Phenotypic ratio: 3 dominant : 1 recessive

Dihybrid Crosses & Pedigree Charts

Two-Trait (Dihybrid) Crosses

Dihybrid crosses track the inheritance of two genes simultaneously. For example, hairline pattern (widow’s peak) and earlobe attachment (free hanging) can be studied together.

• Crossing two homozygous parents (dominant for both traits x recessive for both traits) yields offspring that are all heterozygous for both traits.

• Crossing two heterozygous individuals results in a variety of genotype and phenotype combinations.

Results can be predicted using large Punnett squares or by multiplying probabilities from individual monohybrid crosses.

Mendel’s Second Law: Law of Independent Assortment

Alleles of different genes are distributed independently to gametes during meiosis. This law applies strictly to genes on different chromosomes; genes on the same chromosome may be inherited together (linked).

Other Inheritance Patterns

• Incomplete Dominance: Heterozygous phenotype is intermediate between the two homozygous phenotypes (e.g., wavy hair from curly and straight hair alleles).

• Co-dominance: Both alleles are expressed equally in the phenotype (e.g., blood type AB expresses both A and B antigens).

• Polygenic Inheritance: Multiple genes influence a single trait, resulting in continuous variation (e.g., eye color, height).

Example Table: Blood Type Inheritance

Environmental Influences & Epigenetics

• Environmental Factors: Diet, exercise, and lifestyle can influence phenotypic expression (e.g., height, risk for diseases).

• Epigenetics: Study of heritable changes in gene expression that do not involve changes to the DNA sequence. Examples include DNA methylation and histone modification.

Epigenetic changes can explain differences in identical twins as they age.

Linked Alleles

Genes located close together on the same chromosome tend to be inherited together (linked). However, crossing-over during meiosis can separate linked alleles, especially if they are far apart on the chromosome.

• Linked Genes: Inherited together if close on the same chromosome.

• Unlinked Genes: On different chromosomes or far apart on the same chromosome; assort independently.

Genetic Pedigrees

Pedigree charts are used to track inheritance patterns across generations. Standardized symbols represent individuals, relationships, and traits. Pedigrees help determine the probability of inheriting specific traits, especially for genetic disorders.

• Example: Tracking a recessive trait such as diabetes (DD = normal, Dd = carrier, dd = diabetic).

Summary Table: Key Genetic Terms

Key Equations and Probability

• Probability of a genotype in a monohybrid cross:

• Probability in dihybrid crosses (if genes assort independently):

Practice and Application

• Practice monohybrid and dihybrid crosses using Punnett squares.

• Analyze pedigree charts to determine inheritance patterns.

• Consider environmental and epigenetic factors in trait expression.

Additional info: This guide expands on the provided lecture notes by including definitions, tables, and equations for clarity and completeness. For further practice, students are encouraged to solve additional genetic cross problems and analyze sample pedigrees.