Genetics & Inheritance

Introduction & Key Terminology

Genetics is the scientific 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.

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

• Chromosome: A structure composed of DNA and proteins that contains many genes.

• Allele: Different versions of a gene found at the same locus on homologous chromosomes.

• Genome: The complete set of genes or genetic material in an organism.

Additional info: The human genome consists of approximately 20,000-25,000 genes distributed across 23 pairs of chromosomes.

DNA Organization

Chromosomes and Karyotype

Human DNA is organized into 23 pairs of chromosomes:

• 22 pairs of autosomes: Identical in length and shape, but may have slight DNA sequence differences.

• 1 pair of sex chromosomes: X and Y chromosomes differ in length and shape; the Y chromosome is smaller.

A karyotype is a visual display of all 23 pairs of chromosomes in a cell.

Homologous Chromosomes & Alleles

Definitions and Examples

• Homologous chromosomes: Chromosomes that are similar in shape and size and carry the same genes at the same loci. Example: autosomes and the two X chromosomes in females.

• Alleles: Variations of a gene that may result in different traits due to differences in DNA sequence. These differences can alter protein structure and function, leading to phenotypic variation.

Types of Alleles

Homozygous vs. Heterozygous

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

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

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

• Allelic variation often arises from mutations in gamete-forming cells.

Genotype vs. Phenotype

Definitions and Examples

• Genotype: The genetic makeup of an individual, including all alleles present. It is not always directly observable.

• Phenotype: The observable physical and functional traits of an individual, such as eye color, height, or blood type. Phenotype is influenced by genotype and environmental factors.

Dominant vs. Recessive Alleles

Patterns of Expression

• Dominant allele: Expresses its phenotype even if only one copy is present (represented by a capital letter, e.g., A).

• Recessive allele: Expresses its phenotype only when two copies are present (represented by a lowercase letter, e.g., a).

• Dominance does not indicate frequency in the population. For example, polydactylism (extra fingers/toes) is dominant but rare.

• Recessive alleles may persist in populations, especially if they are not harmful or are masked 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.

• Monohybrid crosses (single trait) consistently produced 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.

Punnett Squares

Predicting Inheritance Patterns

• Punnett squares are tools to predict the probability of offspring genotypes and phenotypes.

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

• Monohybrid cross example (Aa x Aa):

• Genotype ratio: 1 AA : 2 Aa : 1 aa

• Phenotype ratio (if A is dominant): 3 dominant : 1 recessive

Dihybrid Crosses & Mendel’s Second Law

Two-Trait Crosses

When following two genes, each with two alleles, the inheritance patterns can be predicted using larger Punnett squares or by multiplying probabilities from individual monohybrid crosses.

• Example traits: Widow’s peak (dominant) and free-hanging earlobes (dominant).

• Crossing two heterozygotes (AaBb x AaBb) yields a 9:3:3:1 phenotypic ratio.

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 genotype results in a phenotype intermediate between the two homozygous phenotypes.

• Example: Curly hair (cc) x straight hair (ss) yields wavy hair (cs).

Co-dominance

• Both alleles in a heterozygote are fully expressed.

• Example: Blood types A and B are co-dominant; AB individuals express both antigens.

• O is recessive to both A and B.

Polygenic Inheritance

• Multiple genes contribute to a single trait, resulting in continuous variation.

• Examples: Eye color, height, and body shape are polygenic traits.

• Polygenic traits often show a bell-shaped distribution in populations.

Environmental Influences & Epigenetics

Gene-Environment Interactions

• Environmental factors such as diet and exercise can influence phenotypic traits like height and weight.

• Some diseases have both genetic and environmental components (e.g., skin cancer risk, cholesterol levels).

• The "nature vs. nurture" debate recognizes that both genes and environment shape traits.

Epigenetics

• Epigenetics studies heritable changes in gene expression that do not involve changes to the DNA sequence.

• Examples include DNA methylation and histone modification, which can be influenced by environmental factors.

• Epigenetic changes can explain differences between identical twins as they age.

Linked Alleles

Genetic Linkage

• Genes located close together on the same chromosome tend to be inherited together (linked alleles).

• Crossing-over during meiosis can separate linked alleles, especially if they are far apart on the chromosome.

• The closer two genes are, the less likely they are to be separated by crossing-over.

Genetic Pedigrees

Pedigree Charts

• Pedigree charts are used to track inheritance patterns of traits across generations.

• Standardized symbols represent individuals, relationships, and trait expression.

• Pedigrees help determine the mode of inheritance (dominant, recessive, sex-linked, etc.).

• Example: If diabetes is a recessive trait, individuals with genotype dd are diabetic, Dd are carriers, and DD are normal.

Summary Table: Key Genetic Terms

Additional info: Practice problems involving monohybrid and dihybrid crosses, as well as pedigree analysis, are recommended for mastering these concepts.