【Mendel's laws and their importance】 | ADNTRO

Mendel's laws of inheritance, the father of genetics

Mendel's laws, also known as Mendelian genetics, are the set of basic rules about genetic inheritance. In other words, how the different possibilities (alleles) that they exist for a specific position (locus) of a gene is inherited. These laws were postulated by an Austrian Augustinian monk called Gregor Mendel .

Mendel´s Experiments

Mendel´s law were postulated as a conclusion of a laborious work carried out with pure lines of 33 varieties of the pea plant . To be a pure line, the individual should be homozygous for all its characters, meaning that the two possibilities (alleles) that we could have for the same gene should be the same. In case the individual present two different alleles, the individual will be heterozygous .

Mendel, to ensure that he was working with pure lines , subjected all his pea varieties to selfing for two years (two successive generations). The different plant varieties 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 color: 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? Could be a good reason, but it wasn´t for that reason. Peas are cheap, it doesn´t require much space, abundant offspring and they can be obtained in a short period of time, there is genetic variability (multiple varieties with different features) and it is a selfing plant. Also, if we want to do crosses bewteen varieties, it can be easily controlled.

He started with simple crosses , he observed the offspring (F1) resulting from two parents that differed only in one of the features mentioned above. Once he knew the transmission of each character separately (explained in "Mendelian inheritance patterns"), he began to make crosses between parents that differed in two different features .

Mendel´s patterns of inheritance

Mendelian inheritance patterns explain how a character is inherited and what determines individuals phenotype (what is shown). As we will see in the section on “exceptions to Mendel's laws”, these patterns are not applicable for all loci in the genome.

1. Autosomal dominant inheritance (AD): this type of inheritance affects autosomal chromosomes, that is, 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 other whether we have one copy (heterozygous) or two (homozygous dominant). An example of AD inheritance is achondroplasia, the most common form of dwarfism (Figure 1).

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 this case, we will need two copies of the allele associated with the disease for the individual to present a disease phenotype. An example of a disease that follow this pattern is sickle cell anemia - alteration of red blood cells (Figure 2).

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 only through the X chromosome. Women have two copies of the X chromosome (XX), while men only have one (XY). Within the inheritance linked to the X chromosome we can find two different patterns: the dominant (a single copy is enough to suffer the disease) or recessive (we need two copies of the defective allele in the case of women while in the case of men a single copy would be enough, since men have just one X chromosome and there is no other chromosome to "compensates" the defective allele). Let's look at the example of color blindness (X-linked recessive inheritance, Figure 3).


Figure 3. Crossing with an X-linked recessive inheritance pattern . Color blindness is transmitted on the X chromosome (X d ). Neither individual in the offspring will be color blind, but both women 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, he postulated three important laws that help us understand genetics . However, this recognition was obtained long after publishing his work in 1858 since, despite the fact that he made handwritten copies for all the recognized scientists in the area, no one showcase his laws and Mendel, the father of genetics , died without knowing the great contribution he had made to science in general and genetics in particular. It was in 1900 when his work was rediscovered and his laws were put into value:

1.Mendel's First Law or principle of uniformity : if two pure lines are crossed, the descendants of the first generation will be equal to each other both at the phenotypic level (appearance) and at the genotypic level (alleles) . Also, all 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 represented in uppercase and the recessive in lowercase. Let's see an example for the color character (A> a; allele “A” (yellow) dominates over allele “a” (green)). In figure 4 we can see the explanation represented visually.


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 or principle of segregation : defends that alleles of the same locus segregate (separate) giving rise to two classes of gametes in equal proportion, half of the gametes with the dominant allele (A) and half with recessive allele (a). This conclusion was obtained by selfing the F1 (heterozygous) from the crossing of two parents of pure lines that differ in one feature (Figure 1) and obtaining a second generation of descendants (F2) of which ¾ of the phenotypes were equal to the phenotype of the dominant homozygous parent (yellow) and ¼ was equal to the phenotype of the recessive homozygous 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.


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 or principle of independent combination: this law was proposed by making crosses between parents that deferred in two features. Mendel concluded that different traits are inherited independently of each other, there is no relationship between them, which 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 performed backcrosses or test crosses. This type of crosses are make by crossing the F1 heterozygotes (AaBb) with the recessive parent (aabb). With backcrosses, the type and proportion of gametes produced by the heterozygotes can be verified, since the phenotype of the descendants of this cross is equal to the gametes produced by the F1 heterozygote, since the recessive parent only produces recessive-type gametes. Let's visualize this law by crossing plants that differ in color (A = yellow; a = green) and pea shape (B = smooth; b = rough) (Figure 6).


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.

MENDEL'S LAWS EXCEPTIONS

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 the features corresponding to the two alleles are mixed. An example of intermediate dominance is carnation color. When we cross a red carnation (C R ) with a white carnation (C W ) (Illustration 2).

Illustration 2. Intermediate dominance

2. Codominance : in the heterozygous state there is no recessive allele, but both behave as dominant, such as in intermediate inheritance, but unlike the latter, both features are manifested without mixing . An example of codominance is the color of begonias (Illustration 3) or the ABO system. People with blood group AB present antigens A and B simultaneously. Both alleles are being expressed in the heterozygote individual.


Illustration 3. Codominance

3. New character : it is possible that F1 individuals present a new phenotype that is not the result of an intermediate character between both parents. In this case we are talking about the appearance of a new character, as occurs for example in the coleus plant.

4. New mutations with a dominant effect : sometimes a new allele with a dominant effect appears, breaking the dominance pattern that was known until now. Suppose we have an A (dominant) allele and a (recessive) allele for one locus. It is possible that at a certain moment in history a mutation de novo appears, causing a new allele a 'that dominates the allele A (previously dominant).

5. Epistasia : phenomenon that involves the interaction between different genes when expressing a certain phenotypic character. In other words, the expression of one or more genes depends on the expression of another gene. There is a gene - gene interaction when determining the phenotype of the individual. For example, animals that have the gene to produce brown pigmentation will only be expressed (brown phenotype), if there is lack of the albinism mutation. In case they present the albinism gene mutation, the pigmentation gene will be masked by the albinism gene.

6. Pleiotropy : 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 still applying . If we go to the field of health, Mendel's laws are fulfilled in what we know as monogenic or Mendelian diseases - cystic fibrosis, color blindness.

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

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