Mendelian genetics lays the foundation for understanding inheritance patterns. It explains how traits pass from parents to offspring through genes and alleles, introducing key concepts like dominant and recessive traits, genotypes, and phenotypes.

Punnett squares and probability calculations help predict offspring characteristics. Advanced techniques like test crosses and backcrosses, along with complex inheritance patterns like codominance and polygenic traits, show the nuances of genetic inheritance beyond simple dominant-recessive relationships.

Mendelian genetics principles

Fundamental concepts of heredity

• Genes function as discrete units of heredity located on chromosomes encoding specific traits or characteristics
• Alleles represent alternative forms of a gene occupying the same locus on homologous chromosomes
• Genotype describes the genetic makeup of an organism while phenotype refers to the observable expression of that genetic makeup
• Homozygous individuals possess two identical alleles for a particular gene whereas heterozygous individuals have two different alleles
• Inheritance patterns explain how traits pass from parents to offspring through the transmission of genetic material

Mendel's principles of inheritance

• Principle of segregation dictates that during gamete formation the two alleles for each trait separate so each gamete receives only one allele
• Principle of independent assortment states that alleles of different genes segregate independently during gamete formation
• These principles form the foundation for predicting inheritance patterns and offspring genotypes
• Exceptions to Mendelian inheritance (incomplete dominance, codominance) can affect predicted ratios of offspring phenotypes

Predicting genetic crosses

Punnett squares and probability

• Punnett squares provide visual representations to predict possible genotypes and phenotypes of offspring from a genetic cross
• Monohybrid crosses involve inheritance of a single gene with two alleles while dihybrid crosses involve two genes with two alleles each
• Law of segregation predicts a 3:1 phenotypic ratio in F2 generation of monohybrid cross between two heterozygous parents
• Law of independent assortment predicts a 9:3:3:1 phenotypic ratio in F2 generation of dihybrid cross between two heterozygous parents
• Probability calculations determine likelihood of specific genotypes and phenotypes in offspring
  • Multiply probabilities for independent events
  • Add probabilities for mutually exclusive events

Advanced crossing techniques

• Test crosses determine genotype of an individual with dominant phenotype by crossing with homozygous recessive individual
  • If all offspring show recessive phenotype, parent was homozygous recessive
  • If 50% of offspring show dominant phenotype, parent was heterozygous
• Backcrosses involve crossing an individual with one of its parents or an individual with identical genotype
  • Used to introduce desired traits into breeding lines
• Reciprocal crosses compare results of crosses where male and female parental roles are reversed
  • Help identify sex-linked or maternal effect traits

Allele types and phenotypes

Dominance relationships

• Dominant alleles mask expression of recessive alleles when present in heterozygous individuals
• Recessive alleles only express in phenotype when present in homozygous form
• Codominant alleles both express in phenotype of heterozygous individuals resulting in blending or mixture of traits (ABO blood types)
• Incomplete dominance occurs when one allele is not completely dominant over the other producing intermediate phenotype in heterozygous individuals (pink flowers in snapdragons)

Complex inheritance patterns

• Multiple alleles for a single gene can exist in a population leading to more complex inheritance patterns and phenotypic variations (fur color in rabbits)
• Pleiotropy describes how a single gene can influence multiple phenotypic traits (Marfan syndrome affecting connective tissue, eyes, and cardiovascular system)
• Epistasis occurs when expression of one gene is modified by presence or absence of one or more other genes (coat color in Labrador retrievers)
• Polygenic inheritance involves multiple genes contributing to a single trait resulting in continuous variation (human height, skin color)

Pedigree analysis for inheritance patterns

Pedigree chart interpretation

• Pedigree charts use standardized symbols to represent individuals their relationships and presence or absence of specific traits or disorders across generations
• Key symbols include squares for males circles for females shaded symbols for affected individuals and lines connecting parents to offspring
• Analyzing pedigrees reveals carrier status predicts risk for future offspring and aids in genetic counseling and diagnosis of inherited disorders

Autosomal inheritance patterns

• Autosomal dominant inheritance characterizes by trait appearing in every generation and affecting both males and females equally (Huntington's disease)
• Autosomal recessive inheritance typically shows trait skipping generations and requiring both parents to be carriers for affected offspring (cystic fibrosis)
• Pedigree analysis for autosomal traits focuses on vertical transmission and equal distribution between sexes

Sex-linked and other inheritance patterns

• X-linked inheritance patterns differ between males and females with males more commonly affected by recessive X-linked traits (hemophilia, color blindness)
• Y-linked inheritance only affects males and passes directly from father to son (certain forms of male infertility)
• Mitochondrial inheritance passes exclusively from mother to all offspring and affects both sexes (Leber hereditary optic neuropathy)
• Pedigree analysis for sex-linked traits examines differences in transmission and expression between males and females