Mendel's Laws of Inheritance: A Guide

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Mendel's laws of inheritance, also known as the mendelian genetics, are the set of basic rules on the Genetic heritage.

They explain the different possibilities (alleles) that exist for a specific position (locus) of a gene.

They were postulated by an Austrian Augustinian monk named Gregor Mendel born in the 19th century.

Mendel's laws allow us to understand how character is inherited and what determines the phenotype that different individuals acquire. By doing this, it greatly contributes to our understanding of genetics.

What were Mendel's Experiments?

Mendel's experiments were a series of experiments carried out by Mendel on 33 varieties of pea plant. They involved lots of laborious work, but ultimately helped Mendel form his laws of inheritance.

An inbred line (of anything, including peas) is one that is homozygous for all of its traits .For the same gene we have two possibilities (alleles). If these alleles are the same in the same person or plant we will say that this individual is homozygous. In cases where two different allelesare present, the individual is heterozygous.

To make sure he was working with clean lines, Mendel subjected all pea varieties to selfing for two years (two successive generations).

The various plant variants differed in the following features (Illustration 1):

Illustration 1. Characteristics studied by Mendel
  1. Seed shape: smooth vs rough
  2. Seed color: green vs yellow
  3. Flower tone: white vs purple
  4. Sheath color: green vs yellow
  5. Pod shape: full vs constricted
  6. Pod location: axial vs terminal
  7. Plant size: normal vs small

And why peas? Were peas his favourite food?

This would be an interesting reason, but it isn't the case.

The real reason is that peas are cheap, they don't require much space, they have abundant offspring that can be obtained in a short period of time, they have genetic variability (multiple varieties with different features) and it is a selfing plant.

Also, doing crosses between varieties of peas can be easily controlled.

It started with single crosses . Mendel observed that the offspring (F1) resulting from two parents that differed only in one of the features mentioned earlier.

Once he knew how the transmission of each character worked separately (explained in "Mendelian inheritance patterns"), he began to make crosses between parents that differed in two features.

Mendel's patterns of inheritance

Mendelian inheritance patterns explain how character is inherited and what determines the phenotype they acquire.

As we will see in the section on “exceptions to Mendel's laws”, these patterns do not apply to all loci in the genome.

1. Autosomal Dominant Inheritance (AD)

This type of inheritance affects autosomal chromosomes. These are all those that are neither the X chromosome nor the Y chromosome.

The phenotype will be determined by the dominant allele. A dominant allele is one that predominates over the rest of the alleles, whether we have one copy (heterozygous) or two (homozygous dominant).

An example of AD inheritance is achondroplasia (dwarfism). In fact, this is the most common form of AD . (Figure 1).

Autosomal Dominant Inheritance (AD)
Figure 1. Crossing with an AD inheritance pattern . Allele A (dwarfism) dominates over allele a (non-dwarfism), so as long as there is a copy of allele A, the individual will present a dwarf phenotype. Only individuals that have two copies of the recessive allele won't suffer the disease.

2. Autosomal recessive inheritance (AR)

This type of inheritance affects autosomal chromosomes. In these cases, there need to be two copies of the allele associated with the disease for it to manifest in the individual.

An example of a disease that follows this pattern is sickle cell anemia – alteration of red blood cells (Figure 2).

Autosomal Recessive Inheritance (AR)
Figure 2. Crossing with an AR inheritance pattern . The A allele (normal erythrocytes) predominates over the a allele (sickle cell anemia) which means that individual with two copies of the a allele would suffer the disease. In this example, none of the descendants are sick, but all individuals are carriers of the disease and will be able to transmit it to their descendants.

3. X-linked inheritance

This type of inheritance is a bit more complex. Alleles are inherited solely through the X chromosome.

Women have two copies of the X chromosome (XX), while men have only one (XY).
The inheritance linked to the X chromosome will be dominant (a single copy is enough) or recessive (we need two copies of the defective allele).

For this last type of inheritance linked to the X chromosome, we will find differences according to gender. In the case of women, we will need two copies of the recessive allele for them to manifest the disease.

However, for men a single copy is enough to manifest the disease since they only have one X chromosome and there is no other area that "compensates" for that defective allele). Let's look at the example of colour blindness (X-linked recessive inheritance, Figure 3).

Sex-linked inheritance (X chromosome)

Figure 3. Crossing with an X-linked recessive inheritance pattern . Color blindness is transmitted on the X chromosome (X d ). Neither offspring will be color blind, but both will be carriers of color blindness and may have color blind offspring (25% chance).

Mendel's laws

Thanks to all the information he obtained from his experiments, Mendel postulated three important laws that help us understand genetics.

However, this recognition was obtained long after he published his work in 1858. This is despite the fact that he made handwritten copies for all the recognized scientists in the area.

Unfortunately, nobody understood the value his laws and Mendel, the father of genetics, died without knowing the great contribution he had made to science in general and to genetics in particular.

It was in 1900 when his work was rediscovered and his laws were finally valued.

Mendel's laws of inheritance are:

1. Mendel's First Law: The law of equal segregation

If two inbred lines are crossed, the offspring of the first generation will be equal to each other both at the phenotypic level (appearance) and at the genotypic level (alleles).

Also, the offspring will be the same in appearance (phenotype) as one of the parents. The phenotype will be determined by the dominant allele.

The dominant allele is capitalized and the recessive allele is lowercase. Let's see an example for the color character (A > a; the allele “A” (yellow) dominates over the allele “a” (green)). In figure 4 we can see the explanation represented visually.

Mendel's First Law

Figure 4. Crossing of pure lines allows to postulate the principle of uniformity . By crossing a homozygous dominant parent (yellow pea plant) with a recessive homozygous (green pea plant) we obtain a homogeneous first generation (F1). All offspring are heterozygous and have the dominant phenotype (yellow). In the gamete line we can see the different alleles presented by the parents. By combining them we obtain heterozygous genotypes.

2. Mendel's second law: The law of independent assortment

Mendel's second law states that the alleles of the same locus segregate (separate), giving rise to two classes of gametes in equal proportion. Half of the gametes will have the dominant allele (A) and half will have the recessive allele (a).

This conclusion was obtained by self-pollinating the F1 (heterozygotes) from the crossing of two parents of pure lines that differ in one character (Figure 1).

Next, a second generation of descendants (F2) was obtained, of which ¾ of the phenotypes coincide with the phenotype of the homozygous dominant parent (yellow) and ¼ with the phenotype of the homozygous recessive parent (green).

The segregation of alleles in the production of gametes ensures genetic variation in the offspring.

In figure 5 we can see the explanation represented visually.

Mendel's Second Law

Figure 5. Crossing between the first generation allows us to postulate the principle of segregation . By crossing a heterozygous parent (yellow pea plant) with another heterozygous (yellow pea plant) we obtain a heterogeneous second generation (F2). ¾ of the offspring have the dominant phenotype (yellow) and ¼ of the offspring have the recessive phenotype (green). In the gamete line we can see the different alleles presented by the parents. When combined, we obtain ¼ of homozygous dominant, ½ of heterozygous and ¼ of homozygous recessive.

3. Mendel's Third Law: The law of dominance

Mendel's third law was formulated by making crosses between parents that differed in two characters.

Mendel concluded that different traits are inherited independently of each other, there is no relationship between them.

This means that the inheritance pattern of one trait will not affect the inheritance pattern of another (as long as the genes are not linked).

To verify the principle of segregation, he carried out backcrosses or test crosses. This consists of crossing the F1 heterozygotes (AaBb) with the recessive parent (aabb).

Through this crossing of lines, the type and proportion of gametes produced by the heterozygotes can be verified. This is because the phenotype of the descendants of this cross coincides with the gametes produced by the F1 heterozygote. The recessive parent only produces gametes of the recessive type.

Let's visualize this law by crossing plants that differ in color (A = yellow; a = green) and pea shape (B = smooth; b = wrinkled) (Figure 6).

Mendel's Third Law

Figure 6. Crossing pure lines that differ by two characters allows to postulate the principle of the independent combination . By crossing a dominant homozygous parent (smooth yellow pea plant) with a recessive homozygous (rough green pea plant) we obtain a homogeneous first generation (F1). All offspring are heterozygous and have the dominant phenotype (plain yellow). In the gamete line we can see the different alleles presented by the parents. By combining them we obtain heterozygous genotypes.

6 Exceptions to Mendel's laws

The first exceptions to Mendel's Laws were described in the early 20th century. Today many phenomena are known that are not governed by Mendel's laws. Among them we want to highlight:

1- Intermediate dominance

There is no dominant allele or recessive allele. In heterozygous individuals features are mixed corresponding to the two alleles.

An example of intermediate dominance is that of the carnation. This is evident when we cross a red carnation (CR) with a white carnation (CW) (Illustration 2).

Illustration 2. Intermediate dominance

2- Codominance

In the heterozygous state there is no recessive allele, but both behave as dominant, just as in intermediate inheritance, but unlike the latter, both characteristics are manifested without mixing.

An example of codominance is the color of begonias (Photo 1) or the ABO system. People with blood group AB simultaneously present the A and B antigens. This means that both alleles are being expressed in the heterozygote (Illustration 3).

Illustration 3. Codominance

3- New character

It is possible that F1 individuals have a new phenotype which is not the result of an intermediate character between both parents. In these cases, we are talking about the appearance of a new character. This occurs, for example, in the plant coleus.

4- New mutations with dominant effect

Sometimes a new allele with a dominant effect appears, breaking the dominance pattern known up until that point.

Suppose we have an A (dominant) allele and an a (recessive) allele for a locus. It is possible that at a certain moment in history a mutation de novo will originate a new allele a' that dominates over allele A (previously dominant)

5- Epistasis

Epistasis is the interaction between different genes that express a certain phenotypic character.

In other words, when the expression of one or more genes is dependent on the expression of another gene. There exists a gene-gene interaction when determining the phenotype of the individual.

For example, animals that possess the gene to produce brown pigmentation will only express the brown fur phenotype if they lack the albinism mutation. If they have the albinism gene mutation, the brown pigmentation gene will be masked by the albinism gene.

6- Pleiotropy

Pleiotropy is when a single gene is responsible for various phenotypes that are not related to each other. For example, the gene mutation that causes sickle cell anemia affects erythrocytes, or red blood cells, and also confers some resistance to malaria.

Nevertheless, Mendel's laws are very important for genetics and are still relevant and applicable today.

In field of health, Mendel's laws are visible in what we know as monogenic or Mendelian diseases - cystic fibrosis, color blindness, etc.

However, a large percentage of diseases are influenced by many genes and environmental factors.

These group of diseases are known as complex or multifactorial diseases. Thanks to a type of genetic study ( GWAS ; “genome-wide association studies”) it is possible to associate genetic variants with specific diseases and know the genetic predisposition that you present for complex diseases among many other traits (sport, nutrition, behavior, etc.)

Get your ADNTRO kit and discover your genetic predisposition to suffer from more than 100 diseases, to suffer genetically based intolerances and allergies, to different types of sports (strength, endurance ...), your ethnic origins, your behavior according to your genetics and much more!

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