Mendel’s Laws of Inheritance: Can you name the process because of which you look like your parents? It’s because of inheritance in which a child receives genetic information from the parents. Gregor John Mendel, who is considered the Father of Genetics, had formulated certain laws to understand the pattern of inheritance which are known as Mendel’s Laws of Inheritance.

What are Mendel’s Laws of Inheritance?

Mendel’s Laws of Inheritance are certain laws or statements which describe the mechanism of transmission of certain characteristics from one generation to another in an organism.

Why did Mendel Select Pea Plant for His Experiments?

Gregor Mendel carried out his experiments on the garden pea plant, i.e., Pisum sativum L.because of the following reasons:

1. The plants are primarily self-pollinated. But cross pollination is also easily possible.
2. Pure varieties of pea were available.
3. Plants have several contrasting characters which are phenotypically detectable.
4. It is an annual plant with a short life-span and gives results within three to four months.
5. A large number of seeds are produced per plant.
6. The plant is grown easily and does not require after-care except at the time of pollination.
7. \({F_1}\) hybrids are fertile.
8. Intermediate characters are not formed easily.

Mendel selected seven pairs of contrasting traits which always appear in two opposing conditions, one dominant and the other recessive, which are as follows:

Fig: Table containing the seven pairs of contrasting characters in pea plant studied by Mendel

Mendel’s Experimental Process

Mendel involved three steps in his experimental technique which are as follows:

1. First he produced the parent generation as a true-breeding generation. Continuous self pollination (also called selfing) resulted into homozygous generation or true-breeding generation.
2. He followed standard hybridisation techniques of pure breeding parents, like emasculation, bagging, tagging, hybridisation.
3. He crosses the two pure-line parents differing for a single contrasting character. For example, a pure-line tall female plant was pollinated by pollen of a pure-line dwarf plant.
4. The seeds of the cross or crosses were collected and sown next year. The hybrid offspring including the seeds constitute the first generation.
5. The first filial generation (\({F_1}\)-hybrid) was observed and they were eventually selfed.
6. This created the second generation or \({F_2}\) which were noted for different characters.

Mendel’s Monohybrid Cross

Monohybrid cross is a cross involving two parents that differ in only one heritable character. This cross involves the study of simultaneous inheritance of a single pair of Mendelian factor. For this, Mendel selected the size of the stem (i.e., a cross between tall and dwarf plants) as shown in the figure below.

Results of Monohybrid Cross

1. Mendel proposed that the “factors” were passed from parents to offspring through the gametes, over successive generations.
2. \({F_1}\) plants were not intermediate between the two alternate traits of a character. Out of the two factors representing the alternate traits of a character, one is dominant that expresses itself in the hybrid or \({F_1}\) generation.
3. In \({F_2}\) generation, both the parental traits of the character are expressed.
4. At the time of gamete formation, the two factors separate or segregate and pass into different gametes. A gamete receives one factor of a pair. Thus, Mendel predicted the occurrence of meiosis long before it was discovered.
5. The gametes fuse randomly during fertilisation so that factors come together in the new generation and express themselvesly.
6. The phenotypic ratio of \({F_2}\) is given as \(A\_ = aa = 3:1\) The genotypic ratio of \({F_2}\) is given as \({\rm{AA}}:{\rm{Aa:aa}} = 1:2:1,\) (where, \(A = \) dominant allele and \(a = \) recessive allele).

So, from the results of monohybrid cross, Mendel proposed two laws of inheritance, i.e., Law of Dominance and Law of Segregation.

Fig: Monohybrid Cross

Mendel’s Dihybrid Cross

Dihybrid cross is a cross involving two inheritable characters at the same time. A dihybrid cross is a cross made to study inheritance of two pairs of Mendelian factors (or genes). For this, Mendel selected the shape of the seeds (i.e., round and wrinkled) and colour of the seeds (i.e., yellow and green), which is shown in the figure below.

Results of Dihybrid Cross

1. Four types of plants were produced in the \({F_2}\) generation, out of which two new combinations of traits, i.e., round green and wrinkled yellow had appeared.
2. The formation of four types of individuals in \({F_2}\) generation shows that the factors of the two characters assort independently which leads to the formation of independent assortment or selection of factors.
3. The phenotypic ratio of \({F_2}\) generation is given as \(A\_B:A\_bb:aaB\_:aabb = 9:3:3:1\) (Where, \(A\) and \(B\) are the dominant genes and \(a\) and \(b\) are the recessive alleles for the two genes, each controlling one character). Thus, for the above example, we have Round Yellow \(\left({R\_Y\_} \right):\) Round Green \(\left( {R\_yy} \right):\) Wrinkled Yellow \(\left({rrY\_} \right)\) Wrinkled Green \(\left({rryy} \right) = 9:3:3:1\)

So, from the results of dihybrid cross, Mendel had proposed the third law of inheritance, i.e., Law of Independent Assortment.

Fig: Dihybrid Cross

What are Mendel’s 3 Laws of Inheritance?

From the results of his two experiments, i.e., monohybrid and dihybrid cross experiments, Mendel had proposed the following three laws of inheritance:

1. Law of Dominance
   The statement of Law of Dominance states that:
   “When two homozygous individuals with one or more sets of contrasting characters are crossed, the alleles (characters) that appear in the \({F_1}\) are dominant and those which do not appear in the \({F_1}\) are recessive”.
   According to the law of dominance:
   I. The characters are controlled by discrete units called factors, which are now called alleles.
   II. Alleles occur in pairs.
   III. Out of two alleles, only one allele expresses itself in the hybrid and prevents the expression of the other allele. The allele which expresses itself in the hybrid is called dominant allele and the other allele which is unable to express in the hybrid is called recessive allele.
2. Law of Segregation
   According to the law of segregation, two alleles of a gene controlling each character stay together in the individual but during gamete formation by meiosis, the alleles of a pair segregate or separate from each other so that a gamete carries only one allele of a character. Or, in other words “When hybrid \(\left({{F_1}} \right)\) forms gametes, the alleles segregate from each other and enter in different gametes”.
   Law of Segregation is also known as Law of Purity of Gametes because one gamete contains only one allele from each parent, so all gametes are always pure.
3. Law of Independent Assortment
   According to the law of independent assortment, the alleles of two pairs of traits separate independently of each other during gamete or spore formation and get randomly rearranged in the offspring at the time of fertilisation, producing both parental and new combination of traits.
   Or in other words  “When a hybrid possessing two (or more) pairs of contrasting factors (alleles) forms gametes, the factors in each pair segregate independently of the other pair”.

Importance of Mendel’s Laws of Inheritance

Following are the importance of Mendel’s Laws of Inheritance:
1. It helps us to determine the new combinations in the progeny of hybrids and to predict their frequency.
2. The law of independent assortment helps to confirm the concept of genes.
3. The appearance of new traits can be easily explained by Mendelian laws.
4. The traits can be carefully selected for producing new varieties of plants with desirable characteristics.
5. It helps the plant and animal breeders to produce better breeds.

Mendel’s Laws of Inheritance Exceptions

1. Law of dominance is not seen if one of the alleles is not able to completely mask the effect of another allele. Codominance and incomplete dominance are deviations from this law.
2. The Law of Segregation does not hold good when there is non-disjunction of chromosomes, leading to aneuploidy. This is a type of numerical change in the chromosome (mutation).
3. The Law of Independent Assortment is deviated if there is linkage between the two genes, which is possible when the genes are present on the same chromosome.
4. His concept of one character being controlled by one ‘factor’ does not hold good in several cases, like polygenic inheritance, multiple allelism, etc.

Summary

G. J. Mendel is known as the father of genetics. He performed several experiments on garden pea plants (Pisum sativum \(\left(L \right)\) for several contrasting characters. Based on his results, he formulated the laws of inheritance. He primarily proposed three laws,
(i) the Law of Dominance,
(ii) Law of Segregation, and
(iii) Law of Independent Assortment.

Frequently Asked Questions (FAQs) on Mendel’s Laws of Inheritance

Q.1. What is the importance of Mendel’s Laws of Inheritance?
Ans: 1. It helps us to determine the new combinations in the progeny of hybrids and to predict their frequency.
2. The law of independent assortment helps to confirm the concept of genes.
3. The appearance of new traits can be easily explained by Mendelian laws.

Q.2. Why did Mendel select pea plant for his experiments?
Ans: Mendel had selected pea plant for his experiments because of the following reasons:
1. Pure varieties of pea were available.
2. Pea plants showed a number of easily detectable contrasting characters.
3. The flower of the pea plant is so structured that it allowed self -pollination only, but pea flowers could be easily cross-pollinated or hybridised by hand.
4. It is an annual plant with a short life-span and gives results within three months.

Q.3. What are Mendel’s Laws of Inheritance?
Ans: Mendel’s Laws of Inheritance are certain laws or statements which describe the mechanism of transmission of certain characteristics from one generation to another in an organism.

Q.4. What are Mendel’s three important conclusions?
Ans: By analysing the results of his experiments, Mendel proposed the following three laws:
1. Law of Dominance
2. Law of Segregation
3. Law of Independent Assortment

Q.5. Which law of Mendel is most important and why?
Ans: Out of the three laws proposed by Mendel, the Law of Segregation is the most important and acceptable law as it has no exceptions. This law states that each trait consists of two alleles that segregate or separate during the formation of gametes and one allele from each parent combines during fertilisation.

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