Mendel’s Laws are a set of foundations that explain the mechanism of hereditary transmission over generations.
The monk Gregor Mendel’s studies were the basis for explaining the mechanisms of heredity. Even today, they are recognized as one of the greatest discoveries in Biology. This made Mendel considered the “Father of Genetics”.
Mendel’s experiments
To conduct his experiments, Mendel chose sweet peas ( Pisum sativum ). This plant is easy to grow, self-fertilizes, has a short reproductive cycle and is highly productive.
Mendel’s methodology consisted of carrying out crosses between several lines of peas considered “pure”. The plant was considered pure by Mendel when after six generations it still had the same characteristics.
After finding the purebred lines, Mendel began to perform cross-pollination crosses . The procedure consisted, for example, of removing pollen from a plant with yellow seeds and depositing it under the stigma of a plant with green seeds.
The characteristics observed by Mendel were seven: flower color, flower position on the stem, seed color, seed texture, pod shape, pod color and plant height.
Over time, Mendel carried out various types of crosses in order to verify how traits were inherited over generations.
With that, he established his Laws, which also became known as Mendelian Genetics .
Leis de Mendel
Mendel’s First Law
Mendel’s First Law is also known as the Law of Segregation of Factors or Mohybridism . It has the following statement:
“ Each character is determined by a pair of factors that separate in the formation of gametes, one factor of the pair going to each gamete, which is therefore pure ”.
This Law determines that each characteristic is determined by two factors, which are separated in the formation of gametes.
Mendel arrived at this conclusion when he realized that different strains, with the different attributes chosen, always generate pure seeds and without changes over the generations. That is, plants with yellow seeds always produced 100% of their offspring with yellow seeds.
Thus, the descendants of the first generation, called the F 1 generation , were 100% pure.
As all the seeds generated were yellow, Mendel carried out the self-fertilization between them. In the new lineage, generation F 2 , yellow and green seeds appeared in the ratio 3:1 (yellow:green).
Mendel’s First Law Crossings
With this, Mendel concluded that the color of the seeds was determined by two factors. One factor was dominant and causes yellow seeds, the other was recessive and causes green seeds.
Learn more about Dominant and Recessive Genes .
Mendel’s First Law applies to the study of a single trait. However, Mendel was still interested in how two or more traits were transmitted simultaneously.
Mendel’s Second Law
Mendel’s Second Law is also called the Law of Independent Assortment of Genes or Dihybridism . It has the following statement:
“ differences in one trait are inherited independently of differences in other traits ”.
In this case, Mendel also crossed plants with different characteristics. He crossed plants with yellow, smooth seeds with plants with green, wrinkled seeds.
Mendel already expected that the F 1 generation would be composed of 100% yellow and smooth seeds, as these characteristics are dominant.
That’s why he crossed this generation, because he imagined that green and wrinkled seeds would appear, and he was right.
The crossed genotypes and phenotypes were as follows:
- V_: Dominant (Yellow color)
- R_: Dominant (smooth form)
- vv: Recessive (Green color)
- rr: Recessive (Rough form)
Crossings of Mendel’s Second Law
Mendel discovered different phenotypes in the F² generation, in the following proportions: 9 yellow and smooth; 3 yellow and wrinkled; 3 green and smooth; 1 green and wrinkled.
Also read about Genotypes and Phenotypes .
Biography of Gregor Mendel
Born in 1822, in Heinzendorf bei Odrau, Austria, Gregor Mendel was the son of small and poor farmers. For this reason, he entered the Augustinian monastery in Brünn as a novice in 1843, where he was ordained a monk.
Subsequently, he entered the University of Vienna in 1847. There, he studied mathematics and science, conducting meteorological studies on the lives of bees and the cultivation of plants.
From 1856, he began his experiment trying to explain hereditary characteristics.
His study was presented to the “Natural History Society of Brünn”, in 1865. However, the results were not understood by the intellectual society of the time.
Mendel died in Brünn, in 1884, embittered by not obtaining academic recognition for his work, which was only valued decades later.
Want to learn more about Genetics? Also read Introduction to Genetics .
Exercises
- (UNIFESP-2008) A plant A and another B, with yellow peas and unknown genotypes, were crossed with C plants that produce green peas. Crossing A x C produced 100% plants with yellow peas and crossing B x C produced 50% plants with yellow peas and 50% green peas. The genotypes of plants A, B and C are, respectively:
a) Vv, vv, VV.
b) Vv, vv, vv.
c) Vv, vv, vv.
d) vv, vv, vv.
e) vv, vv, vv.
See Answer
- (Fuvest-2003) In pea plants, self-fertilization normally occurs. To study the mechanisms of inheritance, Mendel performed cross-fertilization, removing the anthers of the flower of a tall homozygous plant and placing pollen collected from the flower of a short homozygous plant on its stigma. With this procedure, the researcher
a) prevented the maturation of female gametes.
b) brought female gametes with alleles for short stature.
c) brought male gametes with alleles for short stature.
d) promoted the encounter of gametes with the same alleles for height.
e) prevented the encounter of gametes with different alleles for height.
See Answer
- (Mack-2007) Suppose that, in a plant, the genes that determine smooth edges of leaves and flowers with smooth petals are dominant in relation to their alleles that condition, respectively, serrated edges and spotted petals. A dihybrid plant was crossed with one with serrated leaves and smooth petals, heterozygous for this characteristic. 320 seeds were obtained. Assuming that all germinate, the number of plants, with both dominant characters, will be:
a) 120.
b) 160.
c) 320.
d) 80.
e) 200.
See Answer
- (UEL-2003) In the human species, myopia and left hand ability are characters conditioned by recessive genes that segregate independently. A right-handed, sighted man whose father was nearsighted and left-handed marries a right-handed, nearsighted woman whose mother was left-handed. What is the probability that this couple will have a child with the same phenotype as the father?
a) 1/2
b) 1/4
c) 1/8
d) 3/4
e) 3/8