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

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What is genetics?

Introduction to Genetics:

The subject known as genetics involves the study of genes, their mechanisms, regulation, and their pattern of expression. We must know about the genes to study the subject. Specific sequences of the nucleotides are known as the genes. They express a particular phenotype. These entities are also known as the functional units of heredity (transmission of genes from one generation to the next). We all resemble our parents, yet look different. The physical, mental and behavioral characteristics get passed on from the parents to the children, through gametes. In eukaryotes, the genetic material is present in a membrane-bound nucleus within the cells.

Differences between organisms are due to differences in their genetic makeup. These differences occurred due to events such as mutation, recombination, and selection. Mutation leads to a change in the genetic material. The exchange of genetic material between chromosomes is known as recombination. A particular combination of genes chosen in a given environment is known as selection. Mutations may either occur spontaneously or occur due to mutagens. They get permanently incorporated in the genetic code.

Image 1: Introduction to genetics

Basic Terms in Genetics

We must know the basic terms used in genetics.

Cells: The basic functional units in the body, with a full-fledged genetic and biochemical, machinery, are known as cells. They synthesize certain molecules necessary for the body, carry out regulation mechanisms, participate in signaling, and undergo a cascade of events. Right from the synthesis of energy to the functioning of every body part requires the pre-planned efforts of the cells.

Nucleus: Inside the cell, there is a nucleus. It is the main store of genetic machinery.

Chromosomes: The thread-like structures present in the nucleus are known as chromosomes. The chromosomes have different shapes.

Base Pairs: There are four bases in DNA such as Adenine, Guanine, Thymine, and Cytosine. In RNA, uracil is an alternative to the thymine. The sequence of these bases within the strand determines the genetic information.

DNA: Deoxyribonucleic acid or DNA is the hereditary material present in the nucleus of the cell. It is also known as a blueprint for life. It is in the form of a double helix with base pairs attached to the sugar-phosphate backbone.

RNA: The ribonucleic acid is involved in protein synthesis.

In more sophisticated terms, a gene is a sequence that codes for a functional molecule. Gene expression is an important process in all the organisms.

Phenotypic traits: These are the specific characteristics passed on from the parent to offspring. Examples include eye color, skin type, the shape of the nose, etc.

Genotype: Different DNA sequences constitute a genotype. Thus, genotypes combine with the environmental factors and determine the trait or phenotype.

Homozygous organism: An organism having a pair of identical alleles is said to be homozygous.
Heterozygous organism: An organism having two different alleles is said to be heterozygous.

The Discovery of Genetics

Mendelian Genetics (1856-1866):

Gregor Mendel was an Austrian monk in the 19th century. He was the pioneer in genetics. His experiments on pea plant led to the discovery of the basic mechanisms of the heredity. Mendel established the basic principles of heredity. This study is known as Mendelian genetics. Later on, various researchers came up with interesting concepts in the field of genetics.

Gregor Mendel is known as the father of modern genetics. He worked on pea plants.

Image 2: Mendel and his study on the pea plants

The DNA era (1944-1972):

It started with Avery's experiment which demonstrated isolation of DNA as a genetic material (also known as transforming principle). Transposons were discovered later on by other scientists.

Discovery of Genomic science (1972- 2016):

The genomics era started with Walter Fiers's discovery of the genetic sequence for a bacteriophage. Likewise, Sanger, Maxam, and Gilbert sequenced the DNA for the first time. The computer stores important information in databases. There are many genetic databases available on the internet. One of the examples of genetic databases includes the National Center for Biotechnology Information (NCBI). A genetic database consists of a store of researched documents, computational biology data, genomic data software and other useful information. Gene maps show the location of genes on chromosomes. The position of a gene on the chromosome is known as gene locus. The unit of genetic distance is known as the map unit. Genetic maps are used to study the organization of genes on the chromosomes. They are used to obtain complete genome sequences.

Many discoveries led to advances in genomic science such as:

·        The DNA sequencing

·        Nucleic acid labeling

·        Mapping the structure of DNA

·        Gene Cloning

·        Transposon-mediated mutation and chromosome breakage studies

·        DNA fingerprinting

·        CFTR protein sequencing

·        Identification of BRCA gene

·        Cloning of Dolly sheep

·        Genome sequencing of Drosophila

·        Development of Human Genome Project

·        Genetic Databases and Maps

Branches of Genetics

Four main branches of genetics are:

Human genetics: It is the study of inheritance in humans.

Plant genetics: It is the study of genes and inheritance in plants.

Animal genetics: It involves the study of genes in animals. Genetic engineering is used to breed animals with a specific trait. Animal geneticists develop genetically modified animals.

Microbial genetics: Various genetic mechanisms also occur in microbes such as bacteria, fungi, and viruses. Microbial genetics is the study of genetic mechanisms in microbes.

Image 3: Branches of genetics

Based on the above categories, genetics can be further studied as follows:

Classical genetics: It is the oldest branch of genetics that depends only on the physical characteristics of an organism. It involves the study of Mendelian Inheritance.

Molecular genetics: It involves the study of the structure and function of the genes at molecular levels.

Cytogenetics: It mainly deals with chromosomes and cellular behavior.

Biochemical genetics: It involves the study of biochemical processes, metabolic disorders and the role of genetic machinery.

Medical genetics: It helps in diagnosing genetic disorders.

Epigenetics: It is the study of heritable changes in gene function under the influence of certain environmental factors. It covers important aspects such as DNA methylation and histone modification.

Developmental genetics: It is the study of genes and their way of controlling the growth and development of an organism.

Behavioral genetics: It involves genetic science to understand the behavior of an individual or an organism.

Population genetics: It is a study based on evolutionary biology and Mendelian inheritance. Population genetics involves the study of particular traits, changes in alleles and genotypes in a population.

Ecological genetics: It involves a combination of genetics and ecological sciences. In layman terms, ecological genetics is the study of natural populations.

Genetic engineering: It is a direct way of manipulating the gene of an organism to get the desired trait.

Genetics of intelligence: It uses genetics to determine the I.Q. of an individual.

Genomics: It is a study of structure, function, evolution, and mapping of the genomes.

Importance of Genetics in Medicine

A large number of diseases have a genetic background. That is why genetics plays a crucial role in medical science. Genetics may be used to avoid disorders related to genes and chromosomes. With the latest genetic techniques such as prenatal diagnosis, it is easy to detect the chances of abnormalities in the fetus.

   

Role of Medical Genetics

Diagnosing genetic disorders Assessing the risk of occurrence of the disease Genetic counseling Avoiding repeated spontaneous abortions Assisting in cousins marriages and disputed paternity Management of genetic disorders Pharmacogenomics and personalized medicine Deciding dietary patterns

Table: Role of Genetics in medicine and healthcare

Genetic Engineering

Genetic engineering involves modification or cloning a specific gene preferably involving a vector. Many drugs and hormones such as insulin, somatostatin, blood clotting factors and growth hormones are synthesized using recombinant DNA technology. Synthetic vaccines such as anti-rabies, anti-malaria, anti-hepatitis vaccines are produced using genetic engineering.

Genetic alterations to the plants may improve the quality of the crop. Such crops may be able to satisfy human needs. For example, disease resistance to plants can be achieved using gene alteration. Other desirable traits include stress resistance, drought resistance and tolerance to extreme conditions, salinity and temperatures.

References:

[1] Genetics, Daniel Hartl, 2011, Preview
[2] IGenetics, a molecular approach, Peter Russel, second edition

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