MENDEL'S LAW OF GENETICS

MENDEL'S LAW OF GENETICS

2.  MENDEL'S LAW OF GENETICS

2.1    Mendel's First Law of Genetics (Law of Segregation) 
During gametes formation, one allele of a gene gets separated from another allele of the same gene. When two gametes (one from each parent) fuse to produce a zygote, the allele of the male parent unites with another allele of the same gene from the female parent to produce the genotype of the offspring. Thus, Mendel’s F1 plants inherited an allele R from the round-seeded plant and an allele r from the wrinkled-seeded plant.
The two alleles of an individual plant separate with equal probability into the gametes. When plants of the (F1) Filial generation (with genotype Rr) produced gametes, half of the gametes received the allele R for round seeds and half received the allele r for wrinkled seeds. The gametes then paired randomly to produce the following genotypes in equal proportions among the filial generation II (F2): genotype RR, genotype Rr,  genotype rR, genotype rr. Because round (allele R) is dominant over wrinkled (allele r), there were three round progeny in the F2 (genotype RR, genotype Rr, genotype Rr) and one wrinkled progeny (rr) in the filial generation II (F2). 


This3:1 ratio of round to wrinkled progeny that Mendel observed in the filial generation II (F2) could occur only if the two alleles of a genotype separated into the gametes with equal probability.  This is known as the principle of segregation.


2.2    Mendel's Second Law of Genetics (Law of Dominance) 
One allele of a gene cancels the effect of another allele of the same gene. That means the recessive alleles expression is masked by dominant alleles. Therefore, a cross between a homozygous dominant and a homozygous recessive will always express the dominant phenotype, while still having a heterozygous genotype. Law of Dominance can be explained easily with the help of a monohybrid cross experiment. Dominant or recessive are not the properties of alleles or gene. It is a relationship between two alleles that are observed in phenotype.
2.3.    Monohybrid cross
Mendel crossed homozygous tall allele (T) with homozygous dwarf allele (t) of a pea plant. This first generation of a cross is the P (parental) generation. He called the offspring obtained from P generation as F1 (first filial) generation. When Mendel examined the F1 generation of this cross, he found that it expressed only one of the phenotypes present in the parental generation. The F1 generation failed to express the phenotype of both the parents. All the F1 generation plants were tall, not even a single dwarf plant was observed. He planted the F1 generation seeds, cultivated the plants that germinated from them, and allowed the plants to self-fertilize, producing a second generation called F2 generation. Both of the traits from the P generation emerged in the F2 generation. He observed that the parental character was again expressed in the F2 generation. However, the ration of the trait was not equal. He noticed that the number of the tall and dwarf plants constituted approximately a 3:1 ratio. Mendel conducted monohybrid crosses for all seven contrasting characters of pea plants, and in all of the crosses, he got the same result.  All of the F1 generations resembled only one of the two parents, but both parental traits emerged in the F2 generation in approximately a 3:1 ratio.


Molecular understanding of Phenotypic Ratio in the F2 Generation
Mendel crossed a pea plant homozygous for the round allele with homozygous wrinkled allele. This first generation of a cross was the P (parental) generation. The RFLP (Restriction Fragment Length Polymorphism) pattern also supported this. The RFLP pattern is different if a plant is heterozygous for an allele. The RFLP is obtained when the restriction site is changed in the genotype. The F1 generation obtained by the cross was heterozygous in nature and RFLP pattern was different from its parents. RFLP analysis showed that they were actually heterozygous even though they expressed a trait of a particular parent. That’s means in F1 generation trait of one parent was dominant over the trait of another parent. Here the allele r was hidden with respect to the morphological phenotype because allele r was recessive to allele R. Allele R masked the effect of allele r because allele R was dominant over allele r.


Nevertheless, the wrinkled seed phenotype reappeared in the F2 generation obtain after selfing in the F1 generation. RFLP analysis showed in F2 generation some plant were dominant homozygous (RR), some were heterozygous (Rr) and some were recessive homozygous (rr). 
We can postulate various result out of these crosses
•     The F1 generation, obtained from cross-fertilization between two homozygous parents, expressed only the dominant trait  because the F1 progeny were heterozygous—for example, Rr progeny)
In F2 generation some plant were dominant homozygous (RR genotype), some were heterozygous (rr genotype) and some were recessive homozygous. 
•     In the F2 generation, the phenotypic ratio of dominant and recessive plants was 3:1 and the genotypic ratio was 1:2:1.


2.3.1.    The significance of Monohybrid Crosses:-
Each plant must, therefore, possess two genetic factors coding for a particular trait i.e. The presence of both round and wrinkled seeds in the F2 could be explained only if the F1 plants possessed both round and wrinkled genetic factors that they inherited from the P generation.
2.4.    Mendel's third Law of Genetics (Law of Independent Assortment) 
One allele of a gene get segregate or assort independently from another allele of another gene.  That means genes coding for different characteristics separate independently of one another when gametes are formed, owing to the independent separation of homologous pairs of chromosomes during meiosis. Genes that show independent assortment are said to be unlinked that means they are present on a different chromosome. The hypothesis of independent assortment can be applicable for dihybrid and onwards crosses. This is not applicable for monohybrid crosses. 
Mendel crossed yellow, round seeds parent is crossed with green, wrinkled seeds parent. In a dihybrid cross, two characters are crossed at a single time is called as Dihybrid cross
The F1 generation seeds were all yellow and round. This indicates the alleles for these two traits were dominant. The selfing in F1 generation plant produces four phenotypic classes in the F2 generation represented all possible combinations of the color and texture traits but not in equal proportion.
Two classes—yellow, round seeds and green, wrinkled seeds—resembled the parental strains. The other two—green, round seeds and yellow, wrinkled seeds—showed new combinations of traits. The ratio of the dihybrid cross is 9:3:3:1.
•    The 9 represents the proportion of individuals displaying both dominant traits i.e. round and yellow seeds.
•    The first 3 represents the individuals displaying the first dominant trait and the second recessive trait i.e. round green seeds.
•    The second 3 represents those displaying the first recessive trait and second dominant trait i.e. wrinkled and yellow seeds. 
•    The 1 represents the homozygote, displaying both recessive traits i.e. wrinkled and green seeds.


The Dihybrid cross can be considered as two monohybrid crosses also. The predicted F2 generation results of the first cross are 3/4 yellow and 1/4 green. Similarly, the second cross yield, 3/4 round and 1/4 wrinkled.  The dihybrid ratio of 9:3:3:1 can also be predicted by probability method.
In the dihybrid cross, 12 out of every 16 F2 generation plants are yellow, while four out of every sixteen are green. This gives the expected 3:1 (3/4:1/4) ratio in F2 generation. Similarly, 12 out of every 16 of all F2 plants have round seeds, and four out of every sixteen have wrinkled seeds, again revealing the same 3:1 ratio. Thus the dihybrid ratio can be considered as two monohybrid crosses. These numbers prove that the two pairs of contrasting character are inherited independently. 
Now we can easily predict the probability of an F2 generation plant having yellow and round seeds is (3/4)(3/4), or 9/16. The probability of the F2 generation plant having yellow (3/4 ) and wrinkled seeds (1/4)  is 3/16. The probability of the F2 generation plant having green (1/4) and round seeds (3/4)  is 3/16. Thus the ratio of dihybrid cross is 9:3:3:1.


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