Mendelian Genetics

Introduction to Mendelian Genetics

Mendelian genetics is the foundation of classical genetics, describing how traits are inherited from one generation to the next. Gregor Mendel's experiments with pea plants established the basic principles of heredity, including the concepts of alleles, genotype, phenotype, and the laws of segregation and independent assortment.

• Alleles: Alternative forms of a gene that lead to different traits.

• Genotype: The genetic composition of an individual (e.g., AA, Aa, or aa).

• Phenotype: The observable characteristics or traits of an individual (e.g., flower color).

Alleles and Chromosomes

Alleles are located on homologous chromosomes at specific gene loci. Each individual inherits one allele from each parent, resulting in a pair of alleles for each trait.

• Homologous chromosomes: Chromosome pairs that carry the same genes at the same loci, but may have different alleles.

• Gene locus: The physical location of a gene on a chromosome.

Genotype and Phenotype

The genotype determines the phenotype, but dominance relationships between alleles affect which traits are expressed.

• Homozygous dominant: Both alleles are dominant (e.g., AA).

• Heterozygous: One dominant and one recessive allele (e.g., Aa).

• Homozygous recessive: Both alleles are recessive (e.g., aa).

• Dominant allele: Masks the effect of the recessive allele in heterozygotes.

• Recessive allele: Only expressed when both alleles are recessive.

Mendel’s Laws of Inheritance

Law of Segregation

Mendel’s first law states that each individual has two alleles for each trait, which segregate during gamete formation. Each gamete receives only one allele from each pair, and fertilization restores the pair in offspring.

• Occurs during Anaphase I of Meiosis.

• Explains why offspring inherit one allele from each parent.

Law of Independent Assortment

Mendel’s second law states that alleles of different genes assort independently during gamete formation, provided the genes are on different chromosomes.

• Occurs during Metaphase I of Meiosis.

• Explains genetic variation in offspring.

Punnett Squares and Genetic Ratios

Punnett squares are used to predict the genotypic and phenotypic ratios of offspring from genetic crosses.

• For a monohybrid cross (Aa x Aa):

• For a dihybrid cross (AaBb x AaBb):

Test Crosses

A test cross is used to determine the genotype of an individual with a dominant phenotype by crossing it with a homozygous recessive individual.

• If all offspring show the dominant phenotype, the parent is homozygous dominant.

• If offspring show a 1:1 ratio of dominant to recessive phenotypes, the parent is heterozygous.

Pedigrees and Autosomal Recessive Traits

Pedigrees are diagrams that track the inheritance of traits across generations. Autosomal recessive traits require two recessive alleles to be expressed.

• Autosome: Any chromosome that is not a sex chromosome (X or Y).

• Individuals with genotype bb express the recessive trait; BB and Bb are unaffected.

Genetic Probability and Multiple Traits

The probability of independent events occurring together is the product of their individual probabilities. This principle is used to solve genetics problems involving multiple traits.

• Example: Probability of a child being a carrier and male = Probability of carrier × Probability of male.

Gene Expression and Mutation

Gene expression links genotype to phenotype through the synthesis of proteins. Mutations in DNA can alter protein function, resulting in changes in phenotype.

• Transcription: DNA is transcribed to mRNA.

• Translation: mRNA is translated to protein.

• Mutations can disrupt protein synthesis, leading to altered traits.

Summary Table: Key Terms

Practice Problems

• Predict the genotypic and phenotypic ratios for a cross between two heterozygous parents.

• Use a test cross to determine the genotype of a parent with a dominant phenotype.

• Apply probability rules to calculate the chance of multiple traits appearing together.

Equations

• Probability of independent events:

• Genotypic ratio for monohybrid cross:

• Phenotypic ratio for monohybrid cross:

• Phenotypic ratio for dihybrid cross:

Conclusion

Mendelian genetics provides the basic framework for understanding inheritance. By mastering the concepts of alleles, genotype, phenotype, and Mendel’s laws, students can solve genetic problems and explain how traits are passed from one generation to the next.