Mendelian genetics lays the foundation for understanding inheritance patterns. It explains how traits are passed down through generations, introducing key concepts like segregation and independent assortment. These principles help us predict offspring characteristics and understand genetic diversity.

Genetic interactions add complexity to inheritance patterns. Gene linkage, recombination, and epistasis influence how traits are expressed and inherited. These phenomena explain deviations from expected Mendelian ratios and contribute to the vast diversity of life we observe.

Mendelian Genetics

Laws of segregation and assortment

• Law of segregation states that alleles for a gene segregate during gamete formation resulting in each gamete receiving only one allele for each gene
  • Relates to the separation of homologous chromosomes during meiosis I ensuring that each gamete contains only one allele for each gene
• Law of independent assortment posits that alleles for different genes assort independently during gamete formation meaning the inheritance of one trait does not influence the inheritance of another trait
  • Relates to the random alignment of homologous chromosome pairs during metaphase I of meiosis allowing for independent assortment of genes on different chromosomes

Probability in genetic crosses

• Forked-line method used to predict the probability of genotypes and phenotypes in a cross by creating a diagram with lines representing the possible gametes from each parent (Punnett square)
  • Lines are combined to determine the possible genotypes and phenotypes of the offspring providing a visual representation of the expected ratios
• Probability rules applied in genetic crosses to calculate the likelihood of specific genotypes or phenotypes
  • Multiplication rule used when calculating the probability of two or more independent events occurring together $P(A and B) = P(A) × P(B)$
    • Example: probability of an offspring inheriting both a dominant allele from the mother and a recessive allele from the father
  • Addition rule used when calculating the probability of two or more mutually exclusive events occurring $P(A or B) = P(A) + P(B)$
    • Example: probability of an offspring being either homozygous dominant or heterozygous for a trait

Genetic terminology and concepts

• Allele: Alternative forms of a gene that can occur at a specific locus
• Genotype: The genetic makeup of an organism, represented by the combination of alleles it possesses
• Phenotype: The observable physical or biochemical characteristics of an organism, determined by its genotype and environmental factors
• Homozygous: Having identical alleles for a particular gene (e.g., AA or aa)
• Heterozygous: Having different alleles for a particular gene (e.g., Aa)
• Dominance: The relationship between alleles where one allele (dominant) masks the expression of another allele (recessive) in the heterozygous condition

Genetic Interactions

Gene linkage and recombination

• Gene linkage occurs when genes are located close together on the same chromosome causing them to be inherited together and violating the law of independent assortment
  • Linked genes tend to be passed down as a unit from parent to offspring (red hair and freckles)
• Recombination is the exchange of genetic material between homologous chromosomes during meiosis specifically occurring during prophase I (crossing over)
  • Recombination frequency depends on the distance between genes on a chromosome with genes farther apart having a higher chance of recombination and genes closer together having a lower chance
• Impact on gamete formation
  1. Linked genes that do not undergo recombination will be inherited together in gametes maintaining their association in the offspring
  2. Recombination can lead to the formation of gametes with new combinations of alleles resulting in offspring with traits that differ from the parents (genetic diversity)

Epistasis in phenotypic expression

• Epistasis is a form of gene interaction where one gene (epistatic gene) influences the expression of another gene (hypostatic gene) with the phenotype being determined by the epistatic gene and masking the effects of the hypostatic gene
  • Example: coat color in Labrador retrievers where the epistatic gene for the black/brown coat color masks the expression of the hypostatic gene for the yellow coat color
• Types of epistasis
  • Dominant epistasis occurs when the presence of a dominant allele at one gene masks the expression of alleles at another gene (fruit color in summer squash)
  • Recessive epistasis occurs when the presence of recessive alleles at one gene masks the expression of alleles at another gene (albinism in humans)
• Impact on phenotypic expression
  • Expected phenotypic ratios in a cross may deviate from Mendelian ratios due to epistatic interactions leading to modified phenotypic ratios
  • Presence of an epistatic gene can alter the phenotypic expression of the hypostatic gene resulting in unexpected phenotypes in the offspring (9:3:4 ratio in fruit color inheritance)