This article is basically on two topics. First of all, we will learn about the various types of Non-Mendelian Inheritance. Secondly, we will learn about the Chromosomal Theory of Inheritance. But before studying the Non-Mendelian Inheritance, we need to understand one important term related to it, i.e., “alleles” or “genes” or “factors“.
Concept of ‘Factors’
Based on these observations, Mendel proposed that something was being stable passed down, unchanged, from parent to offspring through the gametes, over successive generations. He called these things as ‘factors’. We now call these factors “genes”. Therefore, a gene is defined as the functional unit of inheritance which consist of the information required for expressing a particular trait in an organism. Chemically gene is a segment of DNA that has a particular function, the common being synthesis of the polypeptide.
Genes which code for a pair of contrasting traits are known as alleles. Alleles are slightly different forms of the same gene. Therefore, term gene can be used for any factor but term allele is used with reference to another allele. We use alphabetical symbols for each gene, the capital letter is used for the trait expressed at the F1 stage and small alphabet for the other trait. For example, if T is used for the ‘tall’ trait and t for ‘dwarf’ then T and t are alleles of each other. Therefore, in plants (Diploid) the pair of alleles for height would be TT. Tt or tt. We should not use T for tall and d for dwarf because we will find it difficult to remember whether T and d are alleles of the same character or not.
From the preceding paragraphs it is clear that though the genotypic ratios can be calculated using mathematical probability, by looking at the phenotype of a dominant trait, it is not possible to know the genotypic composition. For example, whether a tall plant from F1 or F2 generation has TT or Tt composition, cannot be predicted. Therefore, to determine the genotype of a tall plant at F2, Mendel crossed the tall plant from F2 with a dwarf plant. This is called a test cross. In a typical test cross, an organism showing a dominant phenotype is crossed with the recessive parent instead of self-pollination. The progenies of such a cross can be easily analysed to predict the genotype of the test organism.
E.g. If homozygous dominant F1 hybrid (TT) is crossed with the recessive parent, 100% heterozygous tall hybrids will be obtained. However, only 50% will be tall and the rest 50% will be dwarf, if heterozygous dominant F1 hybrid (Tt) is test crossed with the recessive parent.
Non-Mendelian Inheritance or Deviation from Mendelism
Here we will be discussing in details along with examples for better understanding the four different types of Non-Mendelian Inheritance. They are – Incomplete Dominance, Co-dominance, Multiple Alleles and Pleiotropy.
1. Incomplete Dominance- First Non-Mendelian Inheritance
The first Non-Mendelian Inheritance is Incomplete dominance. It refers to a genetic situation in which one allele does not completely dominate another allele and therefore results in a new phenotype. When experiments on peas were repeated using other traits in other plants, it was found that sometimes the F1 had a phenotype that did not resemble either of the two parents and was in between the two.
Incomplete dominance is seen in cross-pollination experiments between red and white Snapdragon (Antirrhinum majus) plants. The allele that produces the red colour (R) is not completely expressed over the recessive allele that produces the white colour (r). The resulting offspring are pink. The genotypes are (RR) Red, (rr) White, and (Rr) Pink in the ratio of 1:2:1.
In heterozygous condition (Rr), phenotypic effect of one allele is more pronounced than that of other and then mixing of both of the colours (red and white) occurs that results in the development of pink colour.
2. Co-Dominance- Second Non-Mendelian Inheritance
Another Non-Mendelian Inheritance is Co-dominance. In co-dominance, a hybrid organism shows a third phenotype — not the usual “dominant” one and not the “recessive” one … but a third, different phenotype. With incomplete dominance, we get that the third phenotype is something in the middle (red x white = pink) as a blending of the dominant and recessive traits takes place. In Co-dominance, the “recessive” and “dominant” traits appear together in the phenotype of hybrid organisms. The symbols used for co-dominant genes are different. One method is to show by their own capital alphabets.
E.g. R (for red hair in cattle) and W (for white hair in cattle). In another method, capital base symbols are employed for both the alleles with different superscripts, e.g., IA, IB.
3. Multiple Alleles – Third Non-Mendelian Inheritance
Multiple alleles is a type of non-Mendelian inheritance which involves more than just the typical two alleles that usually code for a certain characteristic in a species. With multiple alleles, that means there are more than two phenotypes available depending on the dominant or recessive alleles that are available in the trait and the dominance pattern the individual alleles follow when combined together.
A good example is the different types of red blood cells that determine ABO blood grouping in human beings. The gene I controls the ABO blood groups. The plasma membrane of the red blood cells has sugar polymers that protrude from its surface and the kind of sugar is controlled by the gene. The gene (I) has three alleles IA, IB and i/Io. Despite the presence of three alleles of the same gene in a population, an individual (2n) can have only two alleles. Therefore, multiple alleles can be detected only in a population.
Since there are three different alleles. Therefore, six different genotypes are possible for this character (IAIA, IAIO; IBIB, IBIO; IAIB; IOIO or ii). Now to know, how many phenotypes are possible, we have to see the detailed behaviour of alleles. Thus, six genotypes and four phenotypes are possible.
4. Pleiotropy – Fourth Non-Mendelian Inheritance
Another Non-Mendelian Inheritance is Pleiotropy. It means that ‘One Gene Can Affect Multiple Traits’. During his study of inheritance in pea plants, Gregor Mendel made several interesting observations regarding the colour of various plant components. Specifically, Mendel noticed that plants with coloured seed coats always had coloured flowers and coloured leaf axils. Mendel also observed that pea plants with colourless seed coats always had white flowers and no pigmentation on their axils. In other words, in Mendel’s pea plants, seed coat colour was always associated with specific flower and axil colours.
Mendel’s observations were the result of the phenomenon in which a single gene contributes to multiple phenotypic traits (or pleiotropy). In this case, the seed coat colour gene denoted a, was not only responsible for seed coat colour, but also for flower and axil pigmentation. Examples include:
a. In phenylketonuria, mutation of a gene that codes for the enzymes phenylalanine hydroxylase results in a phenotypic expression characterized by mental retardation and a reduction in hair and skin pigmentation.
b. In Drosophila, white eye mutation leads to depigmentation in many other parts of the body, giving a pleiotropic effect.
c. Sickle cell anaemia is a form of pleiotropy, caused by a distinctive mutation in one gene which leads to a host of symptoms.
Chromosomal Theory of Inheritance
The experiments by Gregor Mendel with pea plants established many of the rules of heredity and they were widely accepted. However, it was not known what the mechanism of heredity could be – the function of DNA was unknown. As such, though Mendel started his work on pea in 1856 and published it in 1865, his work did not receive any recognition, it deserved, till 1900. Mendel work remained unnoticed and unappreciated. Later, due to advancements in microscopy that were taking place, scientists were able to carefully observe cell division. This led to the discovery of structures in the nucleus that appeared to double and divide just before each cell division.
Observations of Chromosomal Theory Of Inheritance
As was proposed independently by Sutton and Boveri, ‘The Chromosomal Theory of Inheritance’ was consistent with Mendel’s laws and was supported by the following observations:
- The chromosomes retain their number, structure and individuality throughout the life of an organism and from generation to generation, just like the hereditary traits. The two neither get lost nor mixed up. They behave as units.
- The chromosomes occur in pairs in the somatic or diploid cells. Similar is the case with genes. The two alleles of a gene pair are located on homologous sites on homologous chromosomes. Both chromosomes and genes segregate at the time of gamete formation such that only one of each pair is transmitted to a gamete.
- A gamete contains only one type of chromosome and only one of the two alleles of a trait.
- The paired condition of both chromosomes, as well as Mendelian factors, is restored during fertilization.
However, critics pointed out that individuals had far more independently segregating traits than they had chromosomes. Thomas Hunt Morgan provided experimental evidence to support the Chromosomal Theory of Inheritance by carrying out crosses with the fruit fly, Drosophila melanogaster for several years.