Mendel's Second Law emerged in the continuity of Gregor Mendel's studies. This Law studies, simultaneously, the manifestation of two or more characteristics. Mendel noted that these characteristics, also called phenotypes, were independent.
The independence of the factors was confirmed by crossing smooth yellow peas with rough green peas. In which, Mendel noticed that these characteristics alternated in the second generation.
Practice your knowledge on the subject with the 10 exercises below.
1) What was the approximate proportion found by Mendel when developing the Second Law?
a) 9:3:3:1
b) 9:3:2:1
c) 1: 3
d) 3: 3: 3: 1
e) 9: 2: 2: 2
Right answer: letter a - 9: 3: 3: 1.
When crossing smooth yellow peas, dominant genotypes, with wrinkled green peas (recessive genotypes), he observed the following proportion:
- 9 Yellow and smooth seeds;
- 3 Yellow, wrinkled seeds:
- 3 green and smooth seeds;
- 1 green and wrinkled seed.
What he understood was that there is a pattern of distribution of alleles and that these, the alleles, are independent, that is, they can confer isolated characteristics. As was the case with green and smooth seeds (vvRR).
2) Mendel's Second Law is also known as:
a) Monohybridism
b) Law of dependent segregation (monohybridism)
c) Law of independent segregation (dihybridism)
d) Variety of factors
e) Combining factors
Right answer: Letter C - Law of independent segregation (dihybridism).
Mendel realized that the alleles (factors) that conferred a certain characteristic (phenotype) were independent. Sometimes, a yellow seed appeared wrinkled, another smooth yellow, that is, the two characteristics were independent of each other.
To achieve this, Mendel worked with more than one characteristic and with dihybrid beings, that is, those that contained alleles that expressed two or more distinct phenotypes.
3) In a cross of dihybrid organisms with black, long fur (ppll) and white, short fur (PPLL), the first generation (F1) of 100% of individuals with white, short fur was obtained.
In the second generation, what will be the proportion of individuals with short black fur?
a) 25%
b) 18.75%
c) 20%
d) 50%
e) 75%
Right answer: letter B - 18,75%.
By crossing the second generation (PpLl) with each other, the following is obtained:
| PL | Pl | pL | pl | |
| PL | PPLL | PPLl | PpLL | PpLl |
| Pl | PPLl | PPll | PpLl | ppll |
| pL | PpLL | PpLl | ppLL | ppLl |
| pl | PpLl | ppll | ppLl | ppll |
The result is 3/16, which when performing the division gives the result 0.1875. In percentage 18,75.
Therefore, the correct value is 18.75%.
4) What is the main difference between Mendel's first and second laws?
a) There is no difference, both deal with heredity
b) The first deals with independent segregation, the second with dependent segregation
c) In the first there is the expression of just one characteristic (monohybridism), in the second two or more (dihybridism)
d) The first investigates the color, the second only the texture of the peas
e) The first was created by Gregor Mendel, the second by his brother Ernest Mendel.
Right answer: letter C - In the first there is the expression of just one characteristic (monohybridism), in the second two or more (dihybridism).
In developing the first Law, Mendel observed a single trait (phenotype) manifesting itself in peas, this trait was color.
He managed to map the mechanism of expression of something that he called, at the time, a factor. However, he expanded his research by observing two phenotypes simultaneously, which allowed him to glimpse that they, the phenotypes, occurred independently.
Sometimes the seed was yellow and smooth, sometimes green and smooth, sometimes yellow and wrinkled, and sometimes green and wrinkled. This made him conclude that these factors were independent of each other.
Mendel's first law is known, for this reason, monohybridism, while Mendel's second law of dihybridism.
5) Tall tomato plants are produced by the action of the dominant allele A and dwarf plants due to their recessive allele The. The hairy stems are produced by the dominant gene N and the hairless stems are produced by its recessive allele n.
The genes that determine these two characteristics segregate independently.
5.1 What is the phenotypic proportion expected from the cross, between dihybrids, in which 256 individuals were born?
5.2 What is the expected genotypic proportion of dihybrid individuals among the 256 offspring?
The) 5.1 = 144, 48, 48, 16 - 5.2 = 64
B) 5.1 = 200, 50, 22, 10 - 5.2 = 72
B) 5.1 = 9/16, 3/16, 3/16, 1/16 - 5.2 = 1/2
w) 5.1 = 144, 48, 32, 10 - 5.2 = 25%
d) 5.1 = 9/16, 3/16, 3/16, 1/16 - 5.2 = 50%
It is) 5.1 = 144, 48, 48, 16 - 5.2 = 72
Right answer: letter a - 5.1 = 144, 48, 48, 16 - 5.2 = 64.
Knowing that the final proportion of a cross between dihybrids results in 9: 3: 3: 1, we have:
-
tall, with hair (of the total of 256, 144 have this phenotype);
-
tall, hairless (of the total of 256, 48 have this phenotype);
-
-
dwarfs, hairless (of the total of 256, 16 have this phenotype).
To respond to the item 5.2 it is not necessary to cross with 16 houses, as the question wants to know the genotypic proportion of dihybrid individuals, that is, NnAa. Therefore, doing the separate crossing we obtain:
| N | n | |
| N | NN | Nn |
| n | Nn | nn |
| A | The | |
| A | AA | Aa |
| The | Aa | aa |
The genotypic proportion, independently segregated, is:
NN = ; Nn =
; n =
AA = ; Aa =
; yy =
Using Aa and Nn we have:
=
which is equal to 25%
25% of 256 is equal to 64 dihybrid individuals in the cross.
6) (UFES) In a given species of parrot, there are four varieties: green, blue, yellow and white. Green parrots are the only ones normally found in the wild. Blue ones lack yellow pigment; yellow ones lack melanin granules, and white ones have neither blue melanin nor yellow pigment in their feathers. When wild green parrots are crossed with white parrots, 100% green parrots are generated in the first generation (F1). By crossing F1 with each other, generating the second generation (F2), the four types of colors are generated.
Considering that the genes for melanin and the yellow pigment are found on different chromosomes, the expected frequency of each of the F2 parrot types is:
a) 9 white people; 3 green; 3 yellow; 1 blue
b) 4 yellow; 2 green; 1 blue; 1 white;
c) 9 green; 3 yellow; 3 blue; 1 white
d) 1 green; 1 yellow; 1 blue; 2 white
e) 9 blue; 4 yellow; 4 white; 1 green
Right answer: letter C - 9 green; 3 yellow; 3 blue; 1 white.
Whereas green parrots, dihybrids, have the MMAA genotype. In which MM for the presence of melanin, and AA for the presence of yellow pigment, the issue can be understood.
To continue, an important fact of the question is:
- Blue parrots do not have yellow pigmentation (M-aa), that is, they are recessive for this phenotype;
- Yellow parrots do not have melanin (mmA-), that is, they are recessive for this phenotype.
Now let's proceed. By crossing green and white parrots, that is, MMAA x mmaa, there are 100% green parrots in the first generation (MmAa).
By crossing the F1 generation with each other, we obtain:
| BAD | Bad | bad | bad | |
| BAD | MMAA | MMAa | MmAA | MmAa |
| Bad | MMAa | MMaa | MmAa | MMaa |
| bad | MmAA | MmAa | mmAA | mmAa |
| bad | MmAa | Mmaa | mmAa | mmaa |
Those who have genotypes: MMAA; MMAa; MmAA; MmAa are green parrots, as there are dominant genes for melanin It is yellow pigment.
Those who have genotypes: MMaa; Mmaa are blue, as there are only dominant genes for melanin.
Those who have genotypes: mmAa; mmAA are yellow parrots, as there is only a dominant gene for the yellow pigment.
Those who have genotype mmaa are white parrots, as there are no dominant genes for melanin and yellow pigment.
Therefore, the proportion is: 9: 3: 3: 1. 9 green parrots, 3 yellow, 3 blue and 1 white.
7) A pea plant produced 208 seeds. Knowing that it is a dihybrid species and double heterozygous for color and texture, how many wrinkled green seeds were produced?
a) 14
b) 15
c) 25
d) 60
e) 13
Right answer: letter e - 13.
By squaring 16 crosses, we obtain the result of .
This is the ratio of phenotypic to wrinkled green seeds in the cross. This way, you can convert the value to a percentage, which is equivalent to 6.25%.
When in doubt, use the following algebraic expression:
0.13 x 100 (percentage) = 13 green, wrinkled seeds.
Or simply get the result of 6.25% of 208, which is equal to 13.
8) The Law of independent segregation of factors occurs in:
a) different chromosomes
b) identical chromosomes
c) equational cell division
d) crossing over
It is) Linkage
Right answer: letter a - Different chromosomes.
In Mendel's second law, two or more non-allele genes segregate independently as long as they are located on different chromosomes.
9) Mendel, in continuation of his studies on the Second Law, extended what he called polyhybridism to 3 characteristics. What is the phenotypic ratio for studying three phenotypes?
a) 30:9:3:3:1
b) 27:9:3:3:1
c) 30: 3: 3: 3: 1
d) 27:3:3:3:1
e) 27: 9: 9: 9: 3: 3: 3: 1
Right answer: letter e- 27: 9: 9: 9: 3: 3: 3: 1.
There is an equivalence and proportionality in the increase in the study of characteristics. If with two (dihybridism) we have the proportion 9: 3: 3: 1, studying three (polyhybridism) we have 27: 9: 9: 9: 3: 3: 3: 1.
10) Is Mendel's Second Law always obeyed in the process of producing physical characteristics?
ah yes! This is how phenotypes are formed.
b) No! When genes are present on the same chromosome, this occurs Linkage
c) Yes! Only on identical chromosomes
d) No! Just on different chromosomes.
and yes! This occurs through equational cell division.
Right answer: Letter B -No! When genes are present on the same chromosome, this occurs Linkage.
Mendel stated that genes related to two or more characteristics always showed independent segregation. If this were true, there would be one chromosome for each gene, or each chromosome would only have one gene. This is inconceivable, as there would be a disproportionate number of chromosomes to meet the phenotypic demands of organisms. In this way, T. H. Morgan and his collaborators worked on the genre fly Drosophila sp. to understand their phenotypic mechanisms and realized that phenotypes did not always occur in the known proportion of Mendel's Second Law (9: 3: 3: 1). This clarified and demonstrated the Linkage, because the factors (genes) were found on the same chromosome.
Bibliographic references
UZUNIAN, A.; BIRNER, E. Biology: single volume. 3rd ed. São Paulo: Harbra, 2008.
