Gregor Mendel established some laws about 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 only inherited recessive alleles.

The problem is that in nature not everything is a question of dominance. There are inheritable 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’ll look at it below.

What is non-Mendelian heritage?

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

On this basis he established Mendel’s famous laws which, in essence, indicate that if a dominant pure breed individual is combined with a recessive pure breed individual, the first generation of descendants of these individuals will be genotypically hybrid , but phenotypically show the dominant traits. For example, when you put a yellow pea plant (AA) together with a green pea plant (aa), the offspring will be yellow (Aa) but will have the alleles that code for green and yellow.

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

The problem is that in other aspects of nature this does not happen so easily. Traits do not have to depend on a single gene with two alleles . For example, the colour of human eyes, although limited, there is a certain degree of variety. This variety could not be explained in simple terms of dominance and recessive, 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.

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

1. Co-dominance

Mendel saw in 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 one with the recessive allele, the individual will show a phenotype determined by the dominant allele. This is the case previously exposed of yellow peas that, despite being children of green peas and yellow peas, resemble the latter .

In codominance this does not happen. There is no situation where one allele prevails over the other, but both are expressed equally in the phenotype of the individual, whose phenotype will be shown as a combination of both alleles. To try to understand this idea better, let’s use 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, no one is recessive to the other, so the question is, what if an individual is a genotypically hybrid (BN), i.e. the son of a white hen (BB) and a black cock (NN)? What happens is that it will be neither completely black nor white, but a combination of both alleles. It will have white feathers and black feathers.

If the color of the hens’ plumage depended on dominance and not co-dominance and, say, black is the dominant allele, a hybrid individual would have black feathers, regardless of whether it was the child of a white hen.

2. Incomplete dominance

Incomplete dominance would be halfway between the dominance seen by Mendel and the co-dominance we have discussed 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 dragon’s mouth flower. This type of flower can come in three colors: red (RR), white (BB) and pink (RB). Purebred red individuals, when paired with purebred white individuals, their first generation 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 colour of the petals , making them blend together as if mixing those colours in a palette.

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

3. Multiple alleles

Mendel worked with genes that only occurred on two alleles, one 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 co-dominance, 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, ‘C’ being the dominant allele which gives a dark shade to the coat, while the other three 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 allele ‘c’,being able to be a hybrid,but to be one of the other three variants it must be a pure breed for one of these alleles.

Another example is 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 or will not be some molecules, called antigens, and there may be type A, type B, both types, or simply none at all.

The alleles that determine whether or not there are these antigens are called ‘I^A’, ‘I^B’ and ‘i’. The first two are dominant over the third, and co-dominant with each other. Thus, the blood type of the individual, shown as a phenotype, will be determined according to the following genotypes.

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

4. Polygenic characteristics

Mendel researched characteristics that depended on a single gene. However, in nature it is normal for a characteristic, such as intelligence, skin colour, height or organ presentation, to depend on the coding of more than one gene, i.e. they are polygenic characteristics.

Genes that are responsible for the same characteristic can belong to the same chromosome, or be found on several scattered chromosomes. If they are on the same chromosome, it is most likely that they are inherited together , although it may be the case that, during the crossbreeding that occurs during meiosis, they are separated. This is one of the reasons why polygenic inheritance is so complicated.

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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, those characteristics are always inherited together.

An example of this is Marfan syndrome , a medical problem in which the affected person has several symptoms, such as unusually tall height, long fingers and toes, heart problems, and dislocation of the lens. All of these characteristics, which may appear to be unrelated in some way, are always inherited together, since they originate from a single gene mutation.

  • 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 an individual’s survival. 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 dark forest. The plumage of this bird would make it stand out from the dark branches and foliage of the forest, making it very vulnerable to predators.

However, there are genes whose alleles are directly lethal, that is, they make the individual already have problems surviving as soon as he is conceived . A classic example is the case of the lethal yellow allele, a totally spontaneous mutation that occurs in rodents, a mutation that makes their fur yellow and they die soon after birth. In that particular case, the lethal allele is dominant, but there are other cases of lethal alleles that can be recessive, co-dominant, 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 from the environment can significantly determine the characteristics of the individual

For this reason, although height is largely determined by genetics, living in a place with poor nutrition and a sedentary lifestyle can make an individual short. Another example is that people of Caucasian descent who live in tropical places end up developing a brown skin tone because of prolonged exposure to sunlight.

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

8. Sex-linked inheritance

There are characteristics that depend on genes found exclusively on the sex chromosomes , that is, the X and the Y, which will make a sex have little or no chance of manifesting a particular trait.

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

Hemophilia

Hemophilia is a genetic disease that prevents the blood from clotting properly. This means that if you are injured, you tend to bleed, and depending on how large the injury is, the risk to your life is greater. Individuals with the disease lack a gene that causes the clotting factor (X’) to be produced .

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This disease, historically, was lethal to 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 that codes for clotting factor is located on the X chromosome and is dominant. If a female (X’X) has one chromosome with the gene and the other with the absence of the gene, she will produce the clotting 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 suffer the same fate, because, as it is not on the Y chromosome, he will not have the gene that coagulates the factor and, therefore, will have hemophilia (X’Y).

This is why more men than women have the disease, since women must have been unlucky enough to inherit two defective X chromosomes in order to have the disease.

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 hereditary, sex-dependent disease , associated with a differentiated 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, yes or no condition will occur.

In contrast, in women, because 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. Wessler; R. C. Lewontin & S. B. Carrol (2008). Introduction to genetic analysis. 9th edition. McGraw-Hill Interamerican.
  • Albert, Bray, Hopkin, Johnson, Lewis, Raff, Roberts, Walter. Introduction to Cellular Biology. Editorial Médica Panamericana.