Mendelian genetics lays the foundation for understanding inheritance patterns in organisms. It explains how traits are passed from parents to offspring through genes and alleles, following principles like segregation and independent assortment.

Mendel's laws, discovered through pea plant experiments, form the basis of classical genetics. They describe how traits are inherited, including concepts like dominant and recessive alleles, and the use of tools like Punnett squares to predict offspring traits.

Mendel's laws of inheritance

• Gregor Mendel, known as the "father of modern genetics," discovered the fundamental laws of inheritance through experiments with pea plants
• Mendel's laws form the foundation of classical genetics and are essential for understanding how traits are passed from parents to offspring in sexually reproducing organisms

Law of segregation

• During the formation of gametes (egg and sperm cells), the two alleles for each gene separate, or segregate, from each other
• Each gamete carries only one allele for each gene
• When fertilization occurs, the offspring receives one allele from each parent, restoring the paired condition

Law of independent assortment

• The segregation of alleles for one gene occurs independently of the segregation of alleles for other genes during gamete formation
• This means that the inheritance of one trait does not influence the inheritance of another trait
• Allows for the production of offspring with various combinations of traits

Exceptions to Mendel's laws

• Mendel's laws apply to genes that are located on different chromosomes or far apart on the same chromosome
• Genes that are close together on the same chromosome tend to be inherited together (linkage) and do not follow independent assortment
• Other exceptions include gene interactions, such as epistasis, where one gene influences the expression of another gene

Monohybrid crosses

• A monohybrid cross involves the crossing of individuals that differ in a single trait controlled by one gene with two alleles
• Used to study the inheritance pattern of a single gene and to determine the genotypes of the parents and offspring

Punnett squares

• A Punnett square is a diagram used to predict the genotypes and phenotypes of offspring in a genetic cross
• For a monohybrid cross, a 2x2 Punnett square is used, with each parent contributing one allele to each box
• The genotypes and phenotypes of the offspring can be determined by combining the alleles in each box

Genotypic ratios

• In a monohybrid cross between two heterozygous individuals (Aa x Aa), the expected genotypic ratio of the offspring is 1:2:1
• This means that 25% of the offspring will be homozygous dominant (AA), 50% will be heterozygous (Aa), and 25% will be homozygous recessive (aa)

Phenotypic ratios

• The phenotypic ratio in a monohybrid cross between two heterozygous individuals (Aa x Aa) is 3:1
• This means that 75% of the offspring will display the dominant phenotype, while 25% will display the recessive phenotype
• The phenotypic ratio is determined by the genotypes of the offspring and the dominance relationship between the alleles

Dihybrid crosses

• A dihybrid cross involves the crossing of individuals that differ in two traits controlled by two separate genes
• Used to study the inheritance patterns of two genes simultaneously and to observe the independent assortment of alleles

Punnett squares for dihybrid crosses

• For a dihybrid cross, a 4x4 Punnett square is used, with each parent contributing two alleles (one for each gene) to each box
• The genotypes and phenotypes of the offspring can be determined by combining the alleles for both genes in each box

Genotypic ratios in dihybrid crosses

• In a dihybrid cross between two individuals heterozygous for both genes (AaBb x AaBb), the expected genotypic ratio of the offspring is 1:2:1:2:4:2:1:2:1
• This ratio represents the various combinations of genotypes for the two genes in the offspring

Phenotypic ratios in dihybrid crosses

• The phenotypic ratio in a dihybrid cross between two individuals heterozygous for both genes (AaBb x AaBb) is 9:3:3:1
• This means that 9/16 of the offspring will display both dominant phenotypes, 3/16 will display the dominant phenotype for one trait and the recessive phenotype for the other (for each gene), and 1/16 will display both recessive phenotypes

Genetic terminology

• Understanding the terminology used in genetics is crucial for communicating about inheritance patterns and describing the relationships between genes and traits

Alleles vs genes

• A gene is a segment of DNA that encodes a specific trait or characteristic
• Alleles are different versions of a gene that can result in variations in the trait controlled by that gene
• For example, the gene for flower color in pea plants has two alleles: one for purple flowers and one for white flowers

Homozygous vs heterozygous

• An individual is homozygous for a gene if they have two identical alleles for that gene (e.g., AA or aa)
• An individual is heterozygous for a gene if they have two different alleles for that gene (e.g., Aa)

Dominant vs recessive traits

• A dominant trait is expressed when an individual has at least one dominant allele (e.g., AA or Aa)
• A recessive trait is only expressed when an individual has two recessive alleles (e.g., aa)
• In a heterozygous individual (Aa), the dominant allele masks the expression of the recessive allele

Genotype vs phenotype

• The genotype is the genetic makeup of an individual, representing the alleles they possess for a specific gene or set of genes
• The phenotype is the observable physical or biochemical characteristics of an individual, which result from the interaction of their genotype with the environment
• For example, in pea plants, the genotype Aa results in the purple flower phenotype, while the genotype aa results in the white flower phenotype

Probability in Mendelian genetics

• Probability is used to predict the likelihood of specific genotypes and phenotypes occurring in the offspring of a genetic cross
• Understanding probability is essential for solving complex genetic problems and predicting the outcomes of crosses

Product rule

• The product rule states that the probability of two independent events occurring together is equal to the product of their individual probabilities
• In genetics, this rule is used to calculate the probability of an offspring having a specific genotype or phenotype when considering multiple genes or traits
• For example, if the probability of an offspring being tall is 3/4 and the probability of having blue eyes is 1/2, the probability of an offspring being tall with blue eyes is 3/4 × 1/2 = 3/8

Sum rule

• The sum rule states that the probability of an event A or event B occurring is equal to the sum of their individual probabilities, minus the probability of both events occurring together
• In genetics, this rule is used to calculate the probability of an offspring having at least one of two specific traits
• For example, if the probability of an offspring having brown hair is 3/4 and the probability of having freckles is 1/2, the probability of an offspring having either brown hair or freckles (or both) is 3/4 + 1/2 - (3/4 × 1/2) = 7/8

Pedigree analysis

• A pedigree is a diagram that shows the inheritance pattern of a specific trait within a family across multiple generations
• Pedigree analysis involves using the principles of Mendelian genetics and probability to determine the genotypes of individuals in the pedigree and to predict the likelihood of future offspring having the trait
• Pedigrees can be used to study the inheritance of dominant, recessive, autosomal, and sex-linked traits

Extensions of Mendelian genetics

• While Mendel's laws form the foundation of inheritance, there are several extensions and modifications to these laws that explain more complex patterns of inheritance

Incomplete dominance

• Incomplete dominance occurs when the phenotype of the heterozygous individual is intermediate between the two homozygous phenotypes
• For example, in snapdragons, a cross between a red-flowered plant (RR) and a white-flowered plant (WW) results in pink-flowered offspring (RW)
• The genotypic ratio in the F2 generation is 1:2:1 (RR:RW:WW), and the phenotypic ratio is also 1:2:1 (red:pink:white)

Codominance

• Codominance occurs when both alleles in a heterozygous individual are fully expressed in the phenotype
• For example, in human blood types, the alleles IA and IB are codominant, and a heterozygous individual (IAIB) will have blood type AB, expressing both A and B antigens on their red blood cells
• The genotypic and phenotypic ratios in the offspring of two heterozygous individuals (IAIB × IAIB) are both 1:2:1 (IAIA:IAIB:IBIB and A:AB:B)

Multiple alleles

• Some genes have more than two alleles, resulting in multiple possible phenotypes
• For example, the gene for coat color in rabbits has four alleles: C (full color), cch (chinchilla), ch (himalayan), and c (albino)
• The dominance hierarchy is C > cch > ch > c, with each allele being dominant to those that follow it in the series

Polygenic inheritance

• Polygenic inheritance involves the cumulative effects of multiple genes on a single trait
• Each gene contributes a small effect to the phenotype, and the final phenotype is determined by the combined action of all the genes involved
• Examples of polygenic traits include human skin color, height, and intelligence

Chromosomal basis of inheritance

• Genes are located on chromosomes, and the arrangement and behavior of chromosomes during meiosis and fertilization influence the inheritance of traits

Linkage of genes

• Genes that are located close together on the same chromosome are said to be linked
• Linked genes tend to be inherited together more often than would be expected by chance, violating the law of independent assortment
• The closer the genes are on the chromosome, the higher the degree of linkage

Recombination of genes

• Recombination is the process by which linked genes can be separated during meiosis through crossing over between homologous chromosomes
• Crossing over involves the exchange of genetic material between non-sister chromatids, resulting in new combinations of alleles on the chromosomes
• The frequency of recombination between two genes depends on the distance between them on the chromosome

Genetic mapping

• Genetic mapping is the process of determining the relative positions and distances between genes on a chromosome based on the frequency of recombination between them
• The farther apart two genes are on a chromosome, the more likely they are to be separated by recombination during meiosis
• Genetic maps are constructed using recombination frequencies and can be used to predict the likelihood of inheriting specific combinations of traits and to locate genes responsible for genetic disorders