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.

• Gene: A segment of DNA that codes for a specific protein or trait.

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

• Diversity: Genetic variation among individuals due to differences in inherited genes.

DNA Organization

Chromosomes and Karyotype

Human DNA is organized into 23 pairs of chromosomes, including 22 pairs of autosomes and 1 pair of sex chromosomes. Autosomes are identical in length and shape, while sex chromosomes (X and Y) differ in size and structure.

• Autosomes: Chromosomes not involved in determining sex (pairs 1-22).

• Sex Chromosomes: X and Y chromosomes; females have XX, males have XY.

• Karyotype: The complete set of chromosomes in an individual, arranged in pairs.

Homologous Chromosomes & Alleles

Definitions and Examples

• Homologous Chromosomes: Chromosome pairs that are similar in shape and size and carry the same genes at corresponding loci.

• Alleles: Different versions of a gene found at the same locus on homologous chromosomes. Allelic differences arise from mutations.

• Example: Blood type alleles (A, B, O) are different forms of the gene for blood type.

Types of Alleles

Homozygous vs. Heterozygous

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

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

• Multiple Alleles: Some genes have more than two possible alleles in the population, but each individual can only inherit two (e.g., blood types).

Genotype vs. Phenotype

Definitions and Examples

• Genotype: The genetic makeup of an individual, including all alleles (e.g., AA, Aa, or aa).

• Phenotype: The observable traits or characteristics of an individual, influenced by genotype and environment (e.g., eye color, height).

• Example: Blood type is determined by genotype but is easily measured as a phenotype.

Dominant vs. Recessive Alleles

Patterns of Inheritance

• Dominant Allele: Expressed in the phenotype even if only one copy is present (e.g., AA or Aa).

• Recessive Allele: Expressed only when two copies are present (e.g., aa).

• Notation: Dominant alleles are represented by uppercase letters; recessive alleles by lowercase.

• Example: Polydactylism (extra fingers/toes) is dominant but rare in the population.

Mendelian Genetics

Gregor Mendel's Experiments

• Monohybrid Cross: Cross between individuals differing in one trait.

• Results: Mendel observed a 3:1 ratio of dominant to recessive phenotypes in the F2 generation.

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

Punnett Squares

Predicting Inheritance Patterns

• Punnett Square: A grid used to predict the genotypes and phenotypes of offspring from a genetic cross.

• Reporting Outcomes: Results can be expressed as ratios (3:1), fractions (3/4), or percentages (75%).

• Example: Crossing two heterozygotes (Aa x Aa) yields 1 AA : 2 Aa : 1 aa.

Dihybrid Crosses & Mendel's Second Law

Two-Trait Crosses and Independent Assortment

• Dihybrid Cross: Cross involving two different traits (e.g., hairline and earlobe type).

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

• Predicting Outcomes: Use large Punnett squares or multiply probabilities from individual monohybrid crosses.

Other Inheritance Patterns

Non-Mendelian Genetics

• Incomplete Dominance: Heterozygotes have a phenotype intermediate between the two homozygotes (e.g., wavy hair from curly and straight hair alleles).

• Co-dominance: Both alleles are fully expressed in the phenotype (e.g., AB blood type).

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

Environmental Influences & Epigenetics

Gene-Environment Interactions

• Environmental Effects: Diet, exercise, and lifestyle can influence phenotypes such as height and disease risk.

• Epigenetics: Study of heritable changes in gene expression that do not involve changes to the DNA sequence (e.g., DNA methylation, histone modification).

• Example: Identical twins may become more different over time due to epigenetic changes.

Linked Alleles

Genetic Linkage

• Linked Genes: Genes located close together on the same chromosome tend to be inherited together.

• Crossing Over: During meiosis I, linked genes can be separated if crossing over occurs between them.

• Likelihood of Linkage: The closer two genes are, the less likely they are to be separated by crossing over.

Genetic Pedigrees

Tracing Inheritance in Families

• Pedigree Chart: Diagram showing inheritance patterns of traits across generations using standardized symbols.

• Application: Used to track genetic disorders and predict inheritance risks (e.g., diabetes as a recessive trait).

• Example: If diabetes is recessive, DD = normal, Dd = carrier, dd = diabetic.

Key Equations and Probability

Genetic Probability Calculations

• Probability of a genotype:

• Example (monohybrid cross): Probability of homozygous recessive (aa) in Aa x Aa cross:

• Example (dihybrid cross): Probability of AABB in AaBb x AaBb cross:

Summary Table: Mendelian vs. Non-Mendelian Inheritance

Additional info: Some content, such as the specific pedigree chart and practice sheet, was referenced but not fully included in the source. Standardized pedigree symbols and probability equations were added for completeness.