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Types of non-mendelian inheritance

Non-Mendelian genetics refers to inheritance patterns that do not follow Mendel's first and second laws of inheritance.

Phenotypes that do not appear in ratios predicted by Mendelian genetics are the most common indicators of non-Mendelian inheritance.

The phenotypes of a non-Mendelian dihybrid cross do not have the 9:3:3:1 ratio.

The phenomena which result in non-Mendelian inheritance include:

  • Epistasis: when one gene affects the expression of another.
  • Linkage: when two genes are situated together on a chromosome so they are not inherited independently.
The colour pattern of this cat is an example of non-Mendelian inheritance. Only female cats can have this coat pattern. The distribution of the colour patches is unique to every cat and is not genetically determined.
The colour pattern of this cat is an example of non-Mendelian inheritance. Only female cats can have this coat pattern. The distribution of the colour patches is unique to every cat and is not genetically determined.

Epistasis is the phenomenon where one trait is controlled by the interaction of two genes.

In epistasis, one gene will usually prevent the expression of another.

In this case, the gene that is "dominant" is epistatic, while the gene that is inhibited is hypostatic.

The gene that causes albinism will inhibit all the genes coding for pigments.

As one gene affects another, they are not independent and the ratio of phenotypes in crosses will deviate from the expected ratios.

The phenotypic ratio here is 9:3:4. This is an example of epistasis. The allele for albinism is epistatic.
The phenotypic ratio here is 9:3:4. This is an example of epistasis. The allele for albinism is epistatic.

Two genes are linked if they are located close together on the same chromosome - their loci are similar. Linked genes are more likely to be inherited together, as chromosomal crossover is less likely to separate them.

It is possible to tell whether two genes are linked by studying the genotypic frequencies from a dihybrid cross.

If genes are not linked, the probabilities of all allele combinations are equal. If the genes are linked, then the alleles that are passed on will be strongly correlated.

The genes for flower colour and pollen grain shape (PpLl) are linked. If one chromosome of a plant carries PL alleles and the other carries pl alleles, the majority of the gametes would contain either PL or pl alleles. There would not be an equal distribution of the four combinations.

The genetic link of colour and grain size was performed in 1905. The results are shown below.

Phenotype Observed number Observed ratio Expected number Expected ratio
Purple, long 1528 14.4 1199 9
Purple, round 106 1.0 400 3
Red, long 117 1.1 400 3
Red, round 881 3.6 133 1

Chromosomal crossover occurs during meiosis. It is the exchange of equivalent sections between a pair of homologous chromosomes.

The two chromosomes twist together and exchange alleles at a point called the chiasma. This can happen multiple times on the same chromosome at several chiasmata.

Crossing over is a source of genetic variation.

Genes that are located further away from each other on a chromosome are more likely to be separated than those that lie together. Genes that lie close together and are rarely separated are said to be linked.

The distance between genes on a chromosome can be estimated by recording how often they are separated through crossing over.

An adapted version of Thomas Morgan's original illustration of chromosomal crossover. The area in the red circle is the chiasma.
An adapted version of Thomas Morgan's original illustration of chromosomal crossover. The area in the red circle is the chiasma.