Non-Mendelian inheritance

Non-Mendelian inheritance helps in knowing more about the extranuclear genes and their pattern of inheritance. A typical feature of non-Mendelian inheritance includes the absence of meiosis based segregation pattern or Mendelian inheritance pattern. Hence, this type of inheritance does not include the ratios depicting the Mendelian type of inheritance. The results site the differences between the crosses involving the non-mendelian genes in comparison with the crosses involving the nuclear genes. The inheritance pattern of the extranuclear genes differs a lot. The Non-Mendelian pattern of segregation mainly affects the results of the reciprocal crosses. The maternal, paternal or uniparental pattern of inheritance usually gets involved in the Non-Mendelian inheritance. Most of the extranuclear genes such as mitochondria and chloroplast genes follow the maternal pattern of inheritance or the cytoplasmic pattern of inheritance. From generations to generations, these organelles follow this type of inheritance since they belong to the cytoplasmic content. The concept of cytoplasmic inheritance is very simple. Upon the fusion of the male and the female gametes, the zygote receives the cytoplasm from the oocyte or the female gamete.

Examples of non-Mendelian inheritance:

Image 1: Mirabilis jalapa

·        Shoot variegation in four o'clock plant:

The common name of Mirabilis Jalapa plant is also known as four o'clock plant. It possesses a variegated phenotype of the shoot belonging to the albomaculata strain. It consists of the shoots with variegated leaves. These leaves exhibit yellowish-white colored patches. The shoots also show the presence of complete green leaves or yellowish-white leaves. These leaves have no patches. Various crosses between the shoot bearing male and the female parents revealed the striking phenotypes. The crosses between the white female plants with any of the male plants (green, white or variegated) gave rise to the progeny plants consisting of the white phenotype. The crosses between the green female plant with white, green or variegated male plants gave rise to the progeny having a green phenotype. The crosses between the variegated female plants and white male plants resulted in three types of progeny such as variegated, white or green. Similarly, the crosses with the other two combinations of parents gave rise to the progeny with variegated, green or white phenotypes, respectively. The white coloration indicates the lack of chlorophyll pigment.

Such plants do not carry out photosynthesis and die early. Hence, the phenotype of progeny depended only on the type of the maternal phenotype. It led to the neglection of the paternal phenotype. The abnormal coloration or white coloration due to leukoplasts ( those lacking chlorophyll pigment) occurs as a result of cpDNA mutations. Two types of organelles exist in these plants such as the chloroplast and leukoplasts. Chloroplasts get involved in the process of photosynthesis. Colorless chloroplasts are known as leukoplasts. During organelle segregation, some zygotes receive chloroplasts. The others receive only leukoplasts. The remaining zygotes receive a combination of chloroplasts and leukoplasts. Hence, some of the plants show variegated phenotypes.

·        Poky mutants of Neurospora:

The fungi known as Neurospora require aerobic respiration. However, the mutant strains carry out the defective type of aerobic respiration. The Neurospora mutant strains are also known as poky mutants. The poky mutants show the presence of mutations in the mitochondrial DNA. Poky female and the wild-type male parents give rise to the poky mutants. Wild-type female and the poky male parents give rise to the progeny with wild-type phenotype. Hence, the progeny had maternal phenotypes. The tetrads analysis involving a cross between the poky female and the wild-type male gave rise to all poky spores. The tetrads analysis involving a cross between the normal female and poky male gave rise to all normal spores. The cytochrome deficiency in the Neurospora leads to the poky strains. It involves a four base pair deletion mutation in the promoter region of the 19s rRNA of the small mitochondrial ribosome.

·        Yeast petite mutants:

The small colonies are known as petite mutants. The large wild-type colonies are known as grandes. There are two mating types in yeast such as "a" and "alpha" respectively. Crosses involving the petite and the wild-type mutants produced wild-type colonies. Nuclear petite occur less than the extranuclear petites. The cross between the neutral petite and wild-type petite produce wild-type colonies. It follows uniparental inheritance since all the progeny have the phenotype of one parent. The cross between the suppressive Petites with wild-type strains give rise to diploid with respiratory properties between the petite and the normal properties. After meiosis, the tetrads produces four petite colonies.

·        Non-Mendelian inheritance in Chlamydomonas:

It has two mating strains such as mt+ and mt- strains. A chloroplast trait known as erythromycin resistance gets inherited through a non-Mendelian pattern of inheritance. A cross between the erythromycin resistant mt + and erythromycin sensitive mt - strain gives rise to all erythromycin-resistant progeny. A cross between the mt - erythromycin-resistant strain and mt + erythromycin sensitive strain gives rise to all the progeny showing erythromycin sensitivity. Cross between both the mating types results in syngamous mating. The diploid zygote after meiosis gives rise to tetrads segregating in different strains.

Image 2: Non-Mendelian inheritance in Chlamydomonas

Many human genetic diseases arise due to mtDNA mutations. Leber's hereditary optic neuropathy, Kearns Sayre syndrome and Myoclonic epilepsy arise due to mutations in the mitochondrial inheritance. The corn plants show cytoplasmic male sterility and hybrid seed production.

·        Maternal effects:

The maternal nuclear genome also affects the phenotype of the progeny. It is known as a maternal effect. Example of maternal effect includes the inheritance of the coiling direction in a snail known as Limnaea peregra. A cross between the dextral coiling female and the sinistral coiling male gives rise to dextral coiling progeny in the generation. Selfing the F1 progeny leads to a phenotypic ratio of 1:2:1. Most of them are dextral. A cross between the sinistral coiling females and dextral coiling males gave rise to F1 snails with sinistral phenotype. Selfing of the F1 gave rise to all dextral phenotype. Hence, the coiling phenotype follows the nuclear genotype of the mother. Thus, it is an example of the maternal effect.

References:

[1] Essentials of Genetics, Pragya Khanna
[2] Genetics, 9th Edition (Multicolour Edition), Verma P.S. & Agarwal V.K.
[3] Cytology genetics and molecular genetics, Pandey

© Copyright, 2018 All Rights Reserved.