Patterns of Inheritance: Mendelian and Non-Mendelian Traits

Patterns of Inheritance: Mendelian, Non-Mendelian, Sex-Linked, Autosomal, and Polygenic

Mendelian Inheritance

Gregor Mendel, in his groundbreaking studies on pea plants, identified four main patterns of inheritance: autosomal dominant, autosomal recessive, X-linked dominant, and X-linked recessive. In Mendelian inheritance, a trait is controlled by a single gene and follows predictable patterns based on the inheritance of alleles.

Autosomal Dominant

A single mutated gene copy is enough to determine the presence of a trait. Both parents have a chance to pass on the mutated gene. Common example: Huntington's disease.

Autosomal Recessive

Both parents must pass on the mutated gene for the trait to manifest. Parents are usually unaffected and carrying the mutated gene without symptoms. Common example: Cystic Fibrosis.

X-Linked Dominant

One mutated gene copy is enough for the trait to manifest in all affected individuals. Women are more commonly affected since they have two copies of the X chromosome compared to men who have only one. Examples include hypophosphatemic rickets (vitamin D-resistant rickets) and ornithine transcarbamylase deficiency.

X-Linked Recessive

Both copies of the mutated gene must be present for the trait to manifest. Affected males inherit the condition from their mothers since fathers can only pass on Y chromosomes to male offspring. Common example: Hemophilia A.

Non-Mendelian Inheritance

Some genetic conditions do not follow Mendel's laws due to various factors such as multiple alleles, incomplete dominance, co-dominance, pleiotropy, lethal alleles, sex linkage, and polygenic traits.

Multiple Alleles

Real populations often have more than two versions of a given gene. For example, ABO blood group system has three different alleles for each location at a locus.

Incomplete Dominance

Two alleles may produce an intermediate phenotype when both are present. Example: Snapdragon flowers where a cross between white and red homozygotes produces wavy, intermediate-colored flowers.

Co-Dominance

Neither allele is dominant over the other; both contribute to the phenotype. Example: Duodenal alkaline phosphatase enzyme activity follows this pattern.

Pleiotropy

Single genes affect multiple traits. Example: Sickle cell disease affects hemoglobin production and oxygen transport in addition to causing sickle cells.

Lethal Alles

Can never survive until developmentally advanced stages. For example, some mutations in the EGFR gene cause embryonic lethality.

Sex Linkage

Traits linked with the sex chromosomes (X and Y). Mutations in a single copy of the gene can lead to the disorder in females but not males. X-linked diseases are usually recessive but sometimes appear to be dominant because males are affected while females are carriers.

Polygenic Inheritance

Involves many genes contributing to a particular trait rather than just one gene or a pair of opposing alleles. This type of inheritance makes it difficult to predict outcomes based on individual parental genotypes alone.

A complex organism like humans exhibits continuous variation like height, weight, etc., which can't be explained by simple Mendelian patterns. Instead, these traits involve multiple genes and environmental interactions.