Extensions to Mendelian Genetics

Incomplete Dominance

Incomplete dominance occurs when the phenotype of heterozygotes is intermediate between the phenotypes of individuals homozygous for either allele. This is a deviation from Mendel's principle of dominance, where one allele completely masks the effect of the other.

• Definition: In incomplete dominance, neither allele is completely dominant over the other, resulting in a blended phenotype.

• Example: In snapdragons (Antirrhinum majus), crossing a red-flowered plant (RR) with a white-flowered plant (rr) produces pink-flowered offspring (Rr).

• Genotypic and Phenotypic Ratios: The F2 generation from two pink-flowered plants (Rr x Rr) yields a 1:2:1 ratio for both genotype and phenotype: 1 red (RR), 2 pink (Rr), 1 white (rr).

Equation:

(pink)

Codominance and Multiple Alleles: ABO Blood Groups

Codominance occurs when both alleles in a heterozygote are fully expressed, resulting in offspring with a phenotype that is neither dominant nor recessive. Multiple alleles refer to the presence of more than two alleles for a genetic trait within a population.

• Definition: In codominance, both alleles contribute equally and visibly to the phenotype.

• Example: The ABO blood group system in humans is determined by three alleles: IA, IB, and i.

• Phenotypes: Four blood types: A, B, AB, and O.

• Genotypes:

  • Type A: IAIA or IAi

  • Type B: IBIB or IBi

  • Type AB: IAIB (codominant expression)

  • Type O: ii

Table: ABO Blood Groups and Antibodies

Additional info: The table above summarizes the main features of the ABO blood group system, including genotypes, antigens, antibodies, and compatible blood types for transfusion.

Pleiotropy: Sickle-Cell Disease

Pleiotropy occurs when a single gene influences multiple phenotypic traits. Sickle-cell disease is a classic example in humans.

• Definition: A single gene affects more than one characteristic.

• Example: The sickle-cell allele (HbS) causes abnormal hemoglobin, which leads to sickle-shaped red blood cells. This results in multiple symptoms, including anemia, pain, organ damage, and paralysis.

• Genotypes and Phenotypes:

  • HbAHbA: Normal hemoglobin

  • HbAHbS: Carrier, usually healthy but may show symptoms under stress

  • HbSHbS: Sickle-cell disease

Equation:

Example: Homozygotes for the sickle-cell allele have abnormal hemoglobin, leading to a cascade of symptoms affecting many organs.

Polygenic Inheritance

Polygenic inheritance refers to traits that are controlled by two or more genes, often resulting in continuous variation within a population. Many human traits, such as height, skin color, and intelligence, are polygenic.

• Definition: Multiple genes (polygenes) contribute additively to a single trait.

• Example: Human height is influenced by several genes, each with a small effect.

• Phenotypic Distribution: Polygenic traits often show a bell-shaped (normal) distribution in the population.

Equation:

Example: In a hypothetical cross involving three genes (A, B, C), each with two alleles, the number of dominant alleles determines the degree of the trait (e.g., height or skin color).

Environmental Influence on Phenotype

Not all traits are determined solely by genes; the environment can also play a significant role in the expression of certain characteristics.

• Definition: The phenotype is the result of the interaction between genotype and environment.

• Example: Nutrition can affect height, and sunlight exposure can influence skin color.

• Application: Even genetically identical individuals (such as identical twins) can show differences in traits due to environmental factors.

Summary Table: Extensions of Mendelian Genetics