Non-Mendelian Genetics

Non-Mendelian Genetics

  • Non-Mendelian genetics explores the inheritance of traits that do not follow the laws of segregation and independent assortment, as defined by Gregor Mendel.

Types of Non-Mendelian Inheritance

  • These patterns include complete dominance, codominance, incomplete dominance, multiple alleles, polygenic traits, and pleiotropy.

Complete Dominance

  • Complete dominance occurs when a dominant allele masks the effect of a recessive allele. In this scenario, the phenotype of the dominant allele is expressed, and the recessive allele is hidden.

Codominance

  • In codominance, both alleles are expressed equally, resulting in a phenotype that displays both traits simultaneously. An example of this can be seen in the AB blood type in humans, where both the A and B alleles are expressed.

Incomplete Dominance

  • Incomplete dominance is a genetic situation in which one allele does not completely dominate another allele, and therefore results in a new phenotype. This is seen, for example, when a red flower and white flower mate to produce pink offspring.

Multiple Alleles

  • Some traits are determined by more than two alleles, forming a series known as multiple alleles. An example of this is the ABO blood group system in humans, consisting of three alleles: IA, IB and i.

Polygenic Traits

  • Polygenic traits result from the interaction of multiple genes, rather than a single gene. Examples of polygenic traits include height, skin colour and eye colour in humans.

Pleiotropy

  • Pleiotropy occurs when one gene influences more than one phenotypic trait. A classic example of pleiotropy is the single gene in cats that affects fur colour, eye colour, and hearing.

Sex-Linked Traits

  • Sex-linked traits are those traits whose genes are located on sex chromosomes. In humans, the X chromosome carries a significantly larger number of genes, including those for colour blindness and haemophilia, making these conditions much more common in males.

Genetic Linkage and Recombination

  • Genetic linkage refers to the phenomenon that genes located physically close to each other on a chromosome are often inherited together.

  • Recombination can separate these linked genes through the exchange of DNA during meiosis, but the frequency of recombination is much lower than that of independent assortment. Recombination frequency can be used to map the relative positions of genes on the chromosome.

Genetic Disorders

  • Some genetic disorders, such as cystic fibrosis and sickle cell anaemia, are inherited in a Mendelian manner despite being detailed beyond Mendel’s simple inheritance model. On the other hand, disorders like autism and schizophrenia follow complex inheritance patterns that involve multiple genes and environmental factors.

Environmental Impact on Phenotype

  • Environmental factors can influence the expression of certain traits, a phenomenon called phenotypic plasticity. For instance, nutrition can affect height, and sun exposure can affect skin pigmentation.

  • Genetically identical individuals can express different phenotypes under different environmental conditions, illustrating that phenotype is the result of both genotype and environment.