THE Mendel's second law, also known as independent segregation law, establishes that each pair of alleles segregates independently from other pairs of alleles during the formation of gametes. It was formulated based on inheritance analyzes of two or more features tracked at the same time. Next, we will better understand this law and the experiments carried out by the monk Gregor Mendel and which were fundamental for him to arrive at these ideas.
Heads up: To better understand Mendel's second law, it is essential to know Mendel's first law. We suggest that you read the text before: Mendel's First Law. |
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Mendel's experiment
How we know, Gregor Mendel (1822-1884) was a monk and biologist, born in region of Austria, that stands out for its studies on theheredity. His experiments were started around 1857 and were based on the study of the cross of peas. Based on these studies, Mendel reached important conclusions, which became known as Mendel's First Law and Second Law.
The first conclusions, which gave rise to the call Mendel's first law, were based on the analysis of the heredity process of only a characteristic of peas. Mendel then continued his work and carried out analyzes of two or more features at the same time. It was these analyzes that gave rise to the independent segregation law, More known as Mendel's second law.
To better understand these experiments, we will use the example of the crossing of individuals that present smooth and yellow seed (RRVV) with individuals who have the rough and green seed (rrvv). Based on his previous studies, Mendel already knew that yellow seeds were dominant over green ones, and smooth seeds were dominant over wrinkled ones.
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In his experiment, Mendel always used as parental generation pure parents, that is, that, after several generations of self-pollination, they generate offspring with the same characteristic. From this cross, Mendel obtained 100% of peas with smooth and yellow seeds (F1 generation). The plants of this generation are di-hybrid, Yes, they are heterozygotes for both characteristics (RrVv).
Mendel then crossed between F1 generation individuals, obtaining his F2 generation. In this generation, the biologist obtained four phenotypic categories with a 9:3:3:1 ratio (nine smooth yellow seeds, for three smooth green, for three wrinkled yellow, for one wrinkled green).
Mendel then analyzed the different characteristics of peas by combining them in a di-hybrid way. Your results have always shown the same phenotypic proportion: 9:3:3:1.
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Mendel's Conclusions
When performing his experiments, Mendel sought to answer a question:
Are the factors for a given trait always together, or are the factors for different traits inherited independently?
To answer these questions, the scientist analyzed the results of F1 and F2.
If the alleles were always transmitted together, the individuals of the F1 generation would only have to produce two types of gametes: RV and RV. This way of separating the factors would form an F2 generation with a 3:1 ratio, however, what can be observed was a 9:3:3:1 ratio.
With the result obtained, we can conclude that the F1 generation produced four types of gametes different (RV, Rv, rV and rv) and that, consequently, each allele is transmitted in a different way. independent of the other. Furthermore, when fertilization occurs between F1 individuals, we have four different types of female gametes and four different types of male gametes, which will combine in 16 different ways (see figure Following). Therefore, the alleles are distributed independently and at fertilization they combine randomly.
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Statement of Mendel's Second Law or Independent Segregation Law
Mendel's second law, or law of independent segregation, can be stated as follows:
Factor pairs for two or more traits segregate independently in the formation of gametes. |
Exercise solved on Mendel's second law
See an exercise that addresses Mendel's second law:
(Udesc) If an individual of the AaBb genotype is self-fertilized, the number of different gametes produced by it and the proportion of individuals with the aabb genotype in its offspring will be, respectively:
a) 2 and 1/16
b) 2 and 1/4
c) 4 and 1/16
d) 1 and 1/16
e) 4 and 1/4
Resolution: The correct answer is the letter C. As the individual has the AaBb genotype, he can generate the gametes: AB, Ab, aB and ab. By performing self-fertilization, we will have:
AB |
Ab |
aB |
ab |
|
AB |
AABB |
AABb |
AaBB |
AaBb |
Ab |
AABb |
AAbb |
AaBb |
Aabb |
aB |
AaBB |
AaBb |
yyyy |
yyyy |
ab |
AaBb |
Aabb |
yyyy |
aab |
Thus, we have a probability of 1/16 for the generation of an individual aabb.
By Ma. Vanessa Sardinha dos Santos
Source: Brazil School - https://brasilescola.uol.com.br/biologia/segunda-mendel.htm