6.2 Mendelian Genetics

4 min read•august 22, 2024

Mendelian genetics forms the foundation of heredity, explaining how traits are passed from parents to offspring. It introduces key concepts like dominant and recessive alleles, which determine how genes are expressed in an organism's physical characteristics.

Understanding Mendelian inheritance patterns is crucial for predicting offspring traits in genetic crosses. This knowledge helps us grasp more complex genetic phenomena, like incomplete dominance and codominance, which expand our understanding of heredity beyond simple dominant-recessive relationships.

Mendelian Inheritance and Genetic Crosses

Mendel's Laws of Inheritance

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  • Mendel's law of segregation states that each individual possesses two alleles for a trait, and these alleles segregate (separate) during gamete formation so that each gamete carries only one allele for each gene
  • Mendel's law of independent assortment states that the inheritance of one trait is independent of the inheritance of other traits, meaning that alleles of different genes assort independently during gamete formation
  • These laws form the basis for predicting the outcomes of genetic crosses, as they describe how alleles are transmitted from parents to offspring
  • Mendel's laws apply to genes located on different chromosomes or far apart on the same chromosome, allowing them to segregate and assort independently

Deviations from Mendel's Laws

  • Deviations from Mendel's laws can occur due to genetic linkage, where genes located close together on the same chromosome tend to be inherited together, limiting independent assortment
  • Genetic linkage can lead to the observation of higher frequencies of certain allele combinations than expected by independent assortment alone
  • Crossing over during meiosis can break genetic linkage and allow for the recombination of alleles, resulting in new combinations of traits in offspring
  • The principles of segregation and independent assortment can be applied to predict the genotypic and phenotypic ratios of offspring resulting from various types of genetic crosses, such as monohybrid and dihybrid crosses

Punnett Squares for Monohybrid and Dihybrid Crosses

Monohybrid Crosses

  • A monohybrid cross involves the crossing of individuals that differ in a single trait (controlled by one gene)
  • In a monohybrid cross, a Punnett square is constructed by placing the possible gametes from each parent on the top and left side of the square and then filling in the genotypes of the offspring in each cell
  • For example, in a cross between two heterozygous individuals (Aa x Aa), the possible gametes are A and a from each parent, and the resulting Punnett square will show the genotypic ratio of 1 AA : 2 Aa : 1 aa in the offspring
  • The phenotypic ratio of the offspring depends on the dominance relationship between the alleles (complete dominance, incomplete dominance, or codominance)

Dihybrid Crosses

  • A dihybrid cross involves individuals that differ in two traits (controlled by two genes)
  • For a dihybrid cross, a larger Punnett square is used, with the possible gametes from each parent (considering both genes) placed on the top and left side, and the genotypes of the offspring determined by combining the alleles from each parent
  • In a dihybrid cross between two individuals heterozygous for both traits (AaBb x AaBb), the possible gametes are AB, Ab, aB, and ab from each parent, resulting in a 16-cell Punnett square
  • The genotypic ratio of the offspring in this example is 1 AABB : 2 AABb : 2 AaBB : 4 AaBb : 1 AAbb : 2 Aabb : 1 aaBB : 2 aaBb : 1 aabb, while the phenotypic ratio depends on the dominance relationships of the alleles for each gene
  • When solving problems involving crosses, it is essential to consider the dominance relationships between alleles, as well as the number of genes involved and their modes of inheritance (e.g., autosomal or sex-linked)

Genotype vs Phenotype: The Relationship

Genotype and Phenotype Definitions

  • An individual's genotype is their genetic makeup, consisting of the specific alleles they possess for a given trait or set of traits, determined by the alleles inherited from the parents
  • The phenotype is the observable physical or biochemical characteristics of an individual, which result from the expression of the genotype and its interaction with the environment
  • The relationship between genotype and phenotype is not always straightforward, as different genotypes can produce the same phenotype (due to dominance relationships or environmental influences), and the same genotype can result in different phenotypes (due to environmental factors or incomplete penetrance)

Dominance Relationships and Non-Mendelian Inheritance

  • In complete dominance, the presence of one dominant allele masks the effect of the recessive allele, resulting in the dominant phenotype (individuals with genotypes AA and Aa will have the same dominant phenotype, while only individuals with the aa genotype will display the recessive phenotype)
  • Incomplete dominance and codominance are examples of non-Mendelian inheritance patterns where the relationship between genotype and phenotype differs from complete dominance
  • In incomplete dominance, heterozygous individuals display an intermediate phenotype (e.g., pink flowers in snapdragons when crossing red and white flowered plants)
  • In codominance, both alleles are expressed in the phenotype of heterozygous individuals (e.g., AB blood type in humans, where both A and B antigens are expressed on the surface of red blood cells)
  • Environmental factors can influence the expression of genotype, leading to phenotypic plasticity, where a single genotype can produce different phenotypes depending on the environment (e.g., the effect of temperature on the coat color of Siamese cats)
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