Non-Mendelian inheritance: what it is, examples and genetic mechanisms

Mendel's research only accounted for some forms of genetic inheritance, not all.

Gregor Mendel established laws of genetics that he determined based on his famous experiments with the pea plant.

These laws worked very well to explain how peas could be yellow and smooth if they inherited genes with dominant alleles or green and rough if they inherited only recessive alleles.

The problem is that in nature not everything is a matter of dominance. There are heritable traits that manifest themselves in an intermediate form or that depend on more than one gene. This has been called non-Mendelian inheritance.and we are going to see it next.

What is non-Mendelian inheritance?

Gregor Mendel made a significant contribution to the study of heredity when, back in the 19th century, he discovered how the color and texture of peas were inherited. how the color and texture of peas were inherited.. Through his research, he discovered that yellow color and smooth texture were characteristics that prevailed over green color and rough texture.

On this basis, he established the famous Mendel's laws which, in essence, indicate that if a dominant purebred individual is combined with a recessive purebred individual, the first generation of descendants of these individuals will be born, the first generation of offspring of these individuals will be genotypically hybridizedbut phenotypically the dominant traits will be displayed. For example, when a yellow pea plant (AA) is combined with a green pea plant (aa), the offspring peas will be yellow (Aa) but will have the alleles coding for green and yellow.

Mendel only studied traits that depended on a single gene (although at that time neither he nor other scientists knew of the existence of genes per se). Depending on whether a variant or allele of the color gene ('A' dominant and 'a' recessive) was inherited, the plant would produce yellow or green peas, and depending on whether an allele of the texture gene ('R' dominant and 'r' recessive) was inherited, the peas would be smooth or rough.

The problem is that in other aspects of nature this is not so straightforward. Traits need not depend on a single gene with two alleles.. For example, the color of human eyes, while limited, there is some degree of variety. This variety could not be explained in simple terms of dominance and recessivity, since it would imply that there are only two types of iris color, not the various shades of brown, blue, green and gray that we know.

The following we will see in more detail the different types of non-Mendelian inheritance mechanisms that exist, as well as their differences with respect to the laws proposed by Mendel.We will also highlight their differences with respect to the laws proposed by Mendel.

1. Codominance

Mendel saw with his pea experiments a mechanism of inheritance of traits that depended on whether the inherited allele was dominant or recessive. Dominant means that, either by inheriting two genes with the same allele or by inheriting one gene with the dominant allele and another with the recessive allele, the individual will show a phenotype determined by the dominant allele. This is the case of yellow peas which, despite being the offspring of green peas and yellow peas, resemble the latter..

This does not occur in codominance. There is no situation in which one allele prevails over the other, but rather both are expressed equally in the phenotype of the individual, whose phenotype will be shown as a combination of both alleles. To try to better understand this idea, let's take the following example with black hens and white hens

Certain types of hens have a gene whose allele determines the color of their feathers. They can inherit an allele that makes the feathers black (N), and they can receive an allele that makes the feathers white (B)..

Both alleles are equally dominant, there is not one that is recessive to the other, so the question is, what happens if an individual is genotypically hybrid (BN), i.e., the offspring of white hen (BB) and black rooster (NN)? What happens is that it will be neither completely black nor completely white, but a combination of both alleles. It will have white feathers and black feathers.

If the plumage color of hens depended on dominance and not co-dominance and, let's say that black is the dominant allele, a hybrid individual would have black feathers, regardless of whether it is the offspring of a white hen.

2. Incomplete dominance

Incomplete dominance would be halfway between the dominance seen by Mendel and the codominance described in the previous section. This type of non-Mendelian inheritance mechanism implies that the phenotype of an individual is halfway between the phenotypes of the parents. That is, it is as if it were a mixture between the characteristics presented by the parents.

The clearest example of this type of dominance is the case of the snapdragon flower. This type of flower can be presented in three colors: red (RR), white (BB) and pink (RB). Purebred red individuals, when mated with purebred white individuals, their first generation of offspring, which will be hybrids, will be neither red nor white, but pink. The red allele and the white allele have the same strength in determining the color of the petals, causing them to mix as if they were red and white.The red allele and the white allele have the same strength in determining the color of the petals, causing them to mix as if we were mixing those colors on a palette.

In turn, if the hybrid individuals are crossed with each other (RB x RB), their descendants can be red (RR), white (BB) and pink (RB), fulfilling Mendel's laws, although not in the way the Benedictine monk exemplified with his case of peas.

3. Multiple alleles

Mendel worked with genes that only had two alleles, one being dominant and the other recessive. But the fact is that it is possible for a gene to have more than two alleles.The fact that these alleles function in terms of incomplete dominance, Mendelian dominance or co-dominance, which makes the diversity in phenotypes even greater.

An example of a gene with more than two alleles is the rabbit coat. This gene can come in four common alleles, being 'C' the dominant allele that gives a dark shade to the fur, while the other three are recessive: allele 'c^ch', known as chinchilla, allele 'c^h', known as himalaya and allele 'c', known as albino. In order to have a black rabbit it is only necessary for it to have a gene with the 'C' allele, and it can be a hybrid, but in order to be one of the other three variants it must be a pure breed for one of these alleles.

Another example is the Blood group in humans.. The vast majority of people have one of the following four groups: 0, A, B or AB. Depending on which blood group one belongs to, some molecules, called antigens, will be present or not on the surface of the red blood cells, and there may be type A, type B, both types, or simply no antigens.

The alleles that determine whether or not these antigens are present are called 'I^A', 'I^B' and 'i'. The first two are dominant over the third, and co-dominant between them. Thus, the blood type of the individual, shown as phenotype, will be determined according to the following genotypes.

• Blood type A: purebred A (I^A) or hybrid A0 (I^Ai).
• Type B blood: purebred B (I^B) or hybrid B0 (I^Bi).
• Type AB blood: AB hybrid (I^AI^B).
• Blood type 0: purebred 0 (ii).

4. Polygenic characteristics

Mendel investigated traits that depended on a single gene. However, in nature it is normal that a characteristic, such as intelligence, skin color, height or the presence of an organ, depends on the coding of more than one gene, i.e. they are polygenic characteristics.

The genes that are responsible for the same characteristic can belong to the same chromosome, or they can be found on several chromosomes spread over several chromosomes. If they are on the same chromosome, they are most likely to be inherited together.However, it may be the case that, during the cross-linking that occurs during meiosis, they become separated. This is one of the reasons why polygenic inheritance is so complicated.

5. Pleiotropy

If polygenic characteristics are the case where a trait is determined by more than one gene, pleiotropy would be the case but in reverse. This is the situation that occurs when the same gene codes for more than one characteristic and, therefore, these characteristics are always inherited together.

An example of this is the case of Marfan syndrome, a medical condition in which the affected person has several symptoms, such as short stature and short stature.syndrome, a medical condition in which the affected person has several symptoms, such as unusually tall stature, long fingers and toes, heart problems and dislocation of the crystalline lens. All these characteristics, which may not seem to be related in any way, are always inherited together, since their origin is a mutation in a single gene.

6. Lethal alleles

Inheriting one or another type of gene can contribute significantly to the survival of the individual. If the individual has inherited a gene that codes for a phenotype that is not adaptive to the environment in which it is found, the individual will have problems. An example of this would be a bird with white plumage in a forest with dark shades. The plumage of this bird would make it stand out very much in the dark branches and foliage of the forest, making it very vulnerable to predators.

However, there are genes whose alleles are directly lethal, i.e., they cause the individual to have trouble surviving as soon as it is conceived.. A classic example is the case of the lethal yellow allele, a totally spontaneous mutation that occurs in rodents, a mutation that causes their fur to be yellow and they die shortly after birth. In this particular case, the lethal allele is dominant, but there are other cases of lethal alleles that can be recessive, codominant, function polygenically...

7. Effects of the environment

Genes determine many characteristics of the individual and are undoubtedly behind many traits that manifest themselves in the form of its phenotype. However, they are not the only factor that can make the living being in question one way or another. Factors such as sunlight, diet, access to water, radiation and other aspects of the environment can significantly determine the characteristics of the living being in question. can significantly determine the characteristics of the individual.

It is for this reason that, although height is largely determined by genetics, having lived in a place with poor nutrition and having a sedentary lifestyle can cause an individual to be short in stature. Another example is that of people of Caucasian descent who live in tropical locations end up developing a tanned skin tone due to prolonged exposure to sunlight.

Taking an example from the plant world, we have the case of hydrangeas. These plants will have petals of one color or another depending on the pH of the soil, making them blue or pink depending on their basicity.

8. Sex-linked inheritance

There are characteristics that depend on genes that are found exclusively on the sex chromosomes, i.e., the X and X and Y chromosomes.This means that one sex will have little or no chance of manifesting a particular trait.

The vast majority of women have two X chromosomes (XX) and most men have one X and one Y chromosome (XY). Here are two diseases that depend on sex chromosomes.

Hemophilia

Hemophilia is a genetic disease that prevents blood from clotting properly. This means that, in the event of an injury, bleeding tends to occur and, depending on the size of the injury, the risk to life is greater. Individuals with the disease lack a gene that causes coagulation factor (X') to be produced..

This disease, historically, was lethal for women because of menstruation. In the case of men, they had better survival, although it was rare for them to live more than 20 years. Today things have changed thanks to the existence of blood transfusions, even though the disease is considered serious and very limiting.

The gene coding for the coagulation factor is located on the X chromosome and is dominant. If a woman (X'X) has one chromosome with the gene and the other with the absence of it, she will produce the coagulation factor and will not have the disease, although she will be a carrier.

A man who inherits an X chromosome with the absence of the gene does not have the same fate.Since it is not found on the Y chromosome, it will not have the gene that coagulates the factor and, therefore, it will present hemophilia (X'Y).

It is for this reason that there are more men than women who present the disease, since women must have had the misfortune of having inherited two defective X chromosomes in order to present it.

Color blindness

Color blindness involves blindness to a certain basic color (red, green or blue), or two of them. The most common of these blindnesses is the inability to distinguish between green and red.

Color blindness is also a sex-dependent hereditary disease, associated with a distinct segment in the eye.associated with a distinct segment on the X chromosome.

This means that, as with hemophilia, there are more color-blind men than color-blind women, since in the case of men there is only one X chromosome, and if this is defective, the condition is bound to occur.

In contrast, in females, since there are two Xs, if only one of them is defective, the healthy chromosome 'counteracts' the defect of the other.

Bibliographic references:

• Griffiths, A. J. F.; S. R. Wessler; R. C. Lewontin & S. B. Carrol (2008). Introduction to genetic analysis. 9th edition. McGraw-Hill Interamericana.
• Albert, Bray, Hopkin, Johnson, Lewis, Raff, Roberts, Walter. Introduction to Cell Biology. Editorial Médica Panamericana.