Genetic Concepts

• Genetics is the study of heredity and variation in living organisms

Fundamental Genetic Concepts

• A gene is a segment of DNA that codes for a protein and determines a trait
• The locus is the specific location of a gene on a chromosome
• A chromosome is a structure composed of DNA and proteins that carries genetic information
• Homologous chromosomes are a pair of chromosomes (one from each parent) that carry the same genes but may have different alleles
• An allele is a variant form of a gene
• Homozygous means having two identical alleles for a trait, either dominant (AA) or recessive (aa)
• Heterozygous means having two different alleles for a trait (Aa)

Genetic Traits and Inheritance

• The genotype is the genetic makeup of an organism (e.g., AA, Aa, aa)
• The phenotype is the physical expression of a genotype
• A dominant trait is expressed when at least one dominant allele is present
• A recessive trait is only expressed when both alleles are recessive
• A monohybrid cross involves one trait
• A dihybrid cross involves two traits
• "True-breeding" or "pure-bred" organisms produce offspring with the same traits over generations
• Hybrids are offspring resulting from the cross of two genetically different parents
• Incomplete dominance is a condition where heterozygous individuals show a blend of both traits (e.g., red + white = pink)
• Codominance is a condition where both alleles are fully expressed (e.g., AB blood type)
• Multiple alleles refers to when there are more than two possible alleles for a gene (e.g., blood type: A, B, O alleles)
• Polygenic inheritance is when a trait is controlled by multiple genes (e.g., skin color, height)
• Sex-linked traits are controlled by genes on sex chromosomes (e.g., color blindness, hemophilia)

Cell Division and Chromosomal Behavior

• A tetrad is a group of four chromatids formed during meiosis
• Sister chromatids are identical copies of a chromosome attached at the centromere
• Haploid (n) cells have one set of chromosomes (gametes: sperm and egg)
• Diploid (2n) cells have two sets of chromosomes (somatic cells)
• A gamete is a reproductive cell (sperm or egg) that carries half the genetic material
• A somatic cell is any body cell except for gametes
• Autosomes are non-sex chromosomes
• Sex chromosomes determine sex (XX for females, XY for males)
• Mitosis is cell division resulting in two identical diploid daughter cells
• Meiosis is cell division that reduces chromosome number by half, producing four genetically diverse haploid cells

Laws of Inheritance

• The Law of Independent Assortment describes how genes for different traits are inherited independently
• The Law of Segregation describes how each gamete receives only one allele for each gene

Genetic Variation

• Genetic variation refers to differences in DNA sequences that contribute to diversity
• Crossing over is the exchange of genetic material between homologous chromosomes during meiosis
• Random fertilization describes how any sperm can fertilize any egg, increasing genetic variation

Mendelian Genetics and Experiments

• The P generation (Parental Generation) is the first set of parents crossed in a genetic experiment
• The F1 generation is the first generation of offspring from the P generation
• The F2 generation is the second generation of offspring from the F1 generation
• A 3:1 phenotypic ratio is a common ratio observed in Mendelian monohybrid crosses

Problem-Solving Strategies

• Punnett squares can be used to predict the genotypes and phenotypes of offspring in genetic crosses

Monohybrid Cross

• An example would be a cross between a heterozygous (Aa) and a homozygous recessive (aa) individual
• The expected ratio is 1:1 (Aa:aa)

Dihybrid Cross

• An example would be a cross between two heterozygous individuals (AaBb × AaBb)
• The expected ratio is 9:3:3:1 (dominant-dominant: dominant-recessive: recessive-dominant: recessive-recessive)

Pedigree Analysis

• Dominant traits typically appear in every generation
• Recessive traits may skip generations
• Sex-linked traits are more common in males if X-linked recessive

Blood Type Inheritance

• In the ABO blood group system, A and B are codominant; O is recessive
• Possible blood types are A, B, AB, O
• For the Rh factor, Rh+ is dominant; Rh- is recessive
• An example cross would be Heterozygous Rh+ (Rh+/Rh-) × Rh- (Rh-/Rh-)
• The expected offspring ratio is 50% Rh+, 50% Rh-

Meiosis and Genetic Disorders

• Diploid cells have two sets of chromosomes (e.g., humans: 46)
• Haploid cells have one set of chromosomes (e.g., humans: 23 in gametes)
• Nondisjunction can lead to chromosomal disorders
• Trisomy 21 (Down syndrome) results from an extra chromosome 21 due to nondisjunction