Mendelian Genetics

Introduction to Mendelian Genetics

Mendelian genetics is the study of how traits are inherited from one generation to the next, based on the principles established by Gregor Mendel through his experiments with pea plants. These principles explain the transmission of genetic information and the resulting variation in populations.

• Alleles are alternative forms of a gene that lead to different traits.

• Genotype refers to the genetic composition of an individual.

• Phenotype is the observable appearance or trait of an individual.

Alleles and Chromosomes

Genes are located on chromosomes, and each individual has two alleles for each gene, one inherited from each parent.

• Alleles can be dominant or recessive.

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

Genotype and Phenotype

The genotype determines the phenotype, but the relationship depends on dominance.

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

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

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

• Dominant alleles mask the effect of recessive alleles in heterozygotes.

Mendel’s Experiments and Laws

Gregor Mendel, through quantitative experiments with pea plants, established the basic laws of inheritance.

• Mendel’s First Law: Principle of Segregation

  • Each individual has two factors (alleles) for each trait.

  • The factors segregate during gamete formation.

  • Each gamete contains only one factor from each pair.

  • Fertilization restores the pair in offspring.

• Mendel’s Second Law: Independent Assortment

  • Alleles of different genes assort independently during gamete formation.

  • This law applies when genes are on different chromosomes.

Punnett Squares and Ratios

Punnett squares are used to predict the outcome of genetic crosses.

• For a monohybrid cross (Aa x Aa):

  • Genotypic ratio: 1 AA : 2 Aa : 1 aa

  • Phenotypic ratio: 3 dominant : 1 recessive

• For a dihybrid cross (AaBb x AaBb):

  • Phenotypic ratio: 9:3:3:1

Example: Monohybrid Cross

• Parents: Aa x Aa

• Possible genotypes: AA, Aa, aa

• Possible phenotypes: Dominant (A_), Recessive (aa)

Example: Dihybrid Cross

• Parents: AaBb x AaBb

• Possible genotypes: A_B_, A_bb, aaB_, aabb

• Phenotypic ratio: 9:3:3:1

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 Traits

Pedigrees are diagrams that show the inheritance of traits across generations.

• Autosomal recessive traits require two recessive alleles to be expressed.

• Pedigrees help track genotypes and phenotypes in families.

Probability in Genetics

The probability of independent events occurring together is the product of their individual probabilities.

• For example, the probability of a child being a carrier and a boy is calculated by multiplying the probability of each event.

Gene Expression and Mutation

Gene expression links genotype to phenotype through the synthesis of proteins. Mutations can alter gene function and result in changes in phenotype.

• Transcription and translation produce proteins from DNA.

• A mutation in the gene can prevent protein synthesis, altering the phenotype.

Summary Table: Genotype vs. Phenotype

Key Terms

• Allele: Alternative form of a gene

• Genotype: Genetic makeup of an organism

• Phenotype: Observable trait

• Homozygous: Two identical alleles

• Heterozygous: Two different alleles

• Dominant: Allele that masks the effect of another

• Recessive: Allele whose effect is masked

Important Equations

• Probability of independent events:

• Monohybrid cross genotypic ratio:

• Monohybrid cross phenotypic ratio:

• Dihybrid cross phenotypic ratio:

Applications and Practice

• Use Punnett squares to solve single and multiple trait inheritance problems.

• Apply probability laws to predict genetic outcomes.

• Interpret pedigrees to determine genotypes and phenotypes across generations.

Summary

Mendelian genetics provides the foundation for understanding inheritance, genetic variation, and the relationship between genotype and phenotype. Mastery of these principles is essential for solving genetics problems and understanding biological diversity.