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

Gregor Mendel established some laws on 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 wrinkled if they only inherited recessive alleles.

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

• Related article: "Mendel's 3 laws and peas: this is what they teach us"

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. Through his research, he discovered that the yellow color and smooth texture were characteristics that prevailed over the green color and rough texture.

Based on this, he established Mendel's famous laws which, in essence, indicate that if a dominant purebred individual is combined with a recessive purebred individual,

the first generation of offspring from these individuals will be genotypically hybrid, but phenotypically the dominant traits will be displayed. For example, when putting together a yellow pea plant (AA) with one that has green peas (aa), the daughter peas will be yellow (Aa) but will have the alleles that code for the color green and the color 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 was inherited ('A' dominant and 'a' recessive), the plant would produce yellow peas. or green, and depending on whether you inherited an allele of the texture gene ('R' dominant and 'r' recessive), the peas would be smooth or rough.

The problem is that in other aspects of nature this does not happen in such a simple way. Traits do not have to 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 recessiveness, since it would imply that there were only two types of iris color, not the various shades of brown, blue, green, and gray that we know.

Next We will see in more detail the different types of non-Mendelian inheritance mechanisms that exist., in addition to highlighting their differences with respect to the laws proposed by Mendel.

1. codominance

Mendel saw with his experiments with the pea 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 allele dominant. This is the previously mentioned case of yellow peas which, despite being children of green peas and yellow peas, they resemble the latter.

In codominance this does not happen. There is not a situation in which one allele prevails over the other, but rather both are expressed equally in the individual phenotype, whose phenotype will show up as a combination of both alleles. To try to better understand this idea, we are going to put the following example with black chickens and white chickens

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

Both alleles are equally dominant, there is no one that is recessive with respect to the other, so the question is, What happens if an individual is genotypically hybrid (BN), that is, the son of a white hen (BB) and a black rooster (NN)? What happens is that he will not be completely black or completely white, but a combination of both alleles. He will have white feathers and black feathers.

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

2. incomplete dominance

Incomplete dominance would be halfway between the dominance seen by Mendel and the codominance that we have exposed in the previous section. This type of non-Mendelian inheritance mechanism implies that an individual's phenotype falls halfway between the parents' phenotypes. 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 appear in three colors: red (RR), white (BB) and pink (RB). The red purebreds, when mated with the white purebreds, 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 blend together as if we were mixing those colors on a palette.

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

3. multiple alleles

Mendel worked with genes that only occurred in two alleles, one allele being dominant and the other recessive. But the truth is that it may be the case that a gene has more than two alleles, and that these alleles function in terms of incomplete dominance, Mendelian dominance or codominance, which makes the diversity in phenotypes even greater.

An example of a gene with more than two alleles is found in the fur of rabbits. This gene can come in four common alleles, with 'C' being the dominant allele that gives a dark hue to the coat, while the three others are recessive: allele ‘c^ch’, known as chinchilla, allele ‘c^h’, known as himalaya, and allele ‘c’, known as albino. To have a black rabbit, it is enough to have a gene with the 'C' allele, and it can be a hybrid, but to be one of the other three variants it must be a purebred for one of those alleles.

We have another example with 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 you belong to, on the surface of the red blood cells there will be, or not, some molecules, called antigens, and there may be type A, type B, both types, or simply not have them

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

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

4. polygenic characteristics

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

The genes that are in charge of the same characteristic can belong to the same chromosome, or they can be found in several distributed chromosomes. If they are on the same chromosome, they are most likely to be inherited together., although it may be the case that, during the crossing over that occurs during meiosis, they separate. This is one of the reasons why polygenic inheritance is so complicated.

• You may be interested in: "Differences between DNA and RNA"

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. It is the situation that occurs when the same gene codes for more than one characteristic and, therefore, these characteristics are always inherited jointly.

An example of this is the case of Marfan syndrome., a medical problem in which the affected person presents various symptoms, such as an unusually tall stature, long fingers and toes, heart problems and a dislocated lens. All these characteristics, which may seem to be unrelated in any way, are always inherited together, since their origin is a mutation in a single gene.

• You may be interested in: "Hereditary diseases: what they are, types, characteristics and examples"

6. lethal alleles

Inheriting one type of gene or another 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 is going to have problems. An example of this would be to be a bird with white plumage in a forest with dark tones. The plumage of this bird would make it stand out against the branches and dark foliage of the forest, making it very vulnerable to predators.

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

7. environment effects

Genes determine many characteristics of the individual and, without a doubt, they are behind many traits that are manifested in the form of their phenotype. However, they are not the only factor that can cause the living being in question to be one way or another. Factors such as sunlight, diet, access to water, radiation, and other aspects from the environment can significantly determine the characteristics of the individual

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

Giving 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., that is, the X and the Y, which will mean that a sex has little or no chance of manifesting a specific 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 the sex chromosomes.

Hemophilia

Hemophilia is a genetic disease that prevents blood from clotting properly. This means that, in case of suffering an injury, there is a tendency to suffer bleeding and, depending on how big the injury is, the risk to life is greater. Individuals with the disease lack a gene that makes clotting factor (X').

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

The gene that codes for the coagulation factor is located on the X chromosome and it is dominant. If a woman (X'X) has one chromosome with the gene and the other without it, she will produce the clotting factor and will not have the disease, although she will be a carrier.

The man who inherits an X chromosome with the absence of the gene does not have the same luck., since, since it is not on the Y chromosome, it will not have the gene that clots the factor and, therefore, will have hemophilia (X'Y).

It is for this reason that there are more men than women who present the disease, given that for women to Women presenting it must have been unlucky enough to have inherited two defective X chromosomes.

colour blindness

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

Color blindness is also a hereditary disease dependent on sex, 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, given that in the case of men there is only one X chromosome, and if this is defective, yes or yes it will present the condition.

On the other hand, in women, since there are two X's, if only one of them is defective, the healthy chromosome 'counteracts' the defect of the other.

Bibliographic references:

• Griffiths, A. J. F.; S. R. Wesler; R. c. Lewontin & S. b. Carroll (2008). Introduction to genetic analysis. 9th edition. McGraw-Hill Interamericana.
• Albert, Bray, Hopkin, Johnson, Lewis, Raff, Roberts, Walter. Introduction to Cell Biology. Panamerican Medical Editorial.