Inheritance of One Gene- Definition, Explanation & Significance

It is because of the inheritance of genes from the parents to offspring that we look similar to our parents. Gregor Mendel, the Father of Genetics, studied the pattern of inheritance of genes and later, Correns proposed the most famous three laws of inheritance from the conclusions of Mendel’s experiment. The first two laws, i.e., Law of Dominance and Law of Segregation, are derived from the results of monohybrid cross or inheritance of one gene and the third law, i.e. Law of Independent Assortment is derived from the results of dihybrid cross or inheritance of two genes. As per Mendel, each character is controlled by a gene that has at least two alleles.

Inheritance of One Gene: Definition

Inheritance of one gene is defined as the study of the inheritance of a single gene (one pair of alleles) controlling a single character. It is also called monohybrid cross or monogenic inheritance.

Terminologies Associated with Inheritance of One Gene

1. Inheritance: It is defined as the process of transfer of the genetic material from the parents to the offspring.
2. Gene: It is defined as the basic physical and functional unit of heredity that codes for certain proteins.
3. Alleles: It is defined as alternative forms of a gene that arise by mutation and are found at the same locus on a chromosome.
4. Homozygous condition: It is the condition where an organism has two identical alleles of a particular gene.
5. Heterozygous condition: It is defined as a condition where an organism has two different alleles of a particular gene.
6. Monohybrid cross: It is a cross carried out to study the inheritance of one gene between two organisms having different alleles of the same gene.

Inheritance of One Gene Explanation

Each organism has a large number of characters like height, skin colour, etc. Each character is represented in an individual by two-unit factors called alleles which occupy the same gene locus on the two homologous chromosomes. Mendel conducted the hybridisation experiments for seven years, and from his conclusions, Correns later raised the three laws of inheritance, i.e., Law of Dominance, Law of Segregation and Law of Independent Assortment.

Explanation of Monohybrid Cross

i. The cross that is carried out to study the inheritance of a single gene controlling a character is called a monohybrid cross.
ii. The law of dominance and law of segregation can be explained by the help of a monohybrid cross between a pure tall pea plant and a dwarf pea plant.
iii. The hybrids or plants of the \({{\rm{F}}_1}\) generation are all tall though they have also received the allele for dwarfness.
iv. It is because the allele for tallness is dominant while the allele for dwarfness is recessive.
v. If the hybrids are allowed to self-breed, the plants of the \({{\rm{F}}_2}\) generation appear to be both tall and dwarf in the phenotypic ratio of \(3:1.\)
vi. Further self-breeding of these plants shows that the dwarf plants breed true (tt), i.e., produce only dwarf plants.
vii. Amongst the tall plants, \(1/3\) breed true, i.e., yield only tall plants. The remaining \(2/3\) of the \({{\rm{F}}_2}\) tall plants or \(50\%\) of the total \({{\rm{F}}_2}\) plants behave as hybrid plants and produce both tall and dwarf plants in the ratio of \(3:1.\)
viii. Thus, the \({{\rm{F}}_2}\) phenotypic ratio of \(3:1\) is genotypically \(1\) pure tall: \(2\) hybrids tall: \(1\) dwarf. From the above cross, it is concluded that:

1. Although \({{\rm{F}}_1}\) plants show only one alternative or dominant trait of a character, it actually carries alleles of both the traits of the character as the second alternative, or recessive trait appears in the \({{\rm{F}}_2}\) generation. Therefore, \({{\rm{F}}_1}\) plants are genetically hybrid, i.e., Tt.
2. \({{\rm{F}}_1}\) plants are a product of the fusion of male and female gametes. As they carry the gene complement of Tt, the gametes formed must inherit only one allele each, i.e., T from TT and t from tt parent.
3. The \({{\rm{F}}_2}\) generation is produced by self-breeding of the \({{\rm{F}}_1}\) plants. \({{\rm{F}}_2}\) generation consists of three types of plants, i.e., pure tall, hybrid tall and dwarf. This is possible only when:

a. The two alleles present in the \({{\rm{F}}_1}\) plants segregate during the gamete formation.
b. Each gamete carries a single allele for a character.
c. The alleles get distributed randomly in the offspring due to random fusion of gametes during fertilisation.

Fig: Monohybrid Cross

Inheritance of One Gene Conclusions

Following are the conclusions derived from Mendel’s experiments on the study of the inheritance of one gene:

Law of Dominance

1. Mendel’s postulate of dominance was raised to the status of Law of Dominance by Correns \(\left( {1901} \right).\)
2. This law states that “In a heterozygote, one trait will conceal the presence of another trait for the same characteristic.” or “The allele that masks the expression of other allele and is always expressed in both homozygous or heterozygous form is called a dominant allele, while the allele whose expression is masked or can express itself only in homozygous form is called recessive allele.”
3. The law of dominance can be summarised as follows:
i. Each character is controlled by certain factors, now called genes, and each factor has discrete unit factors, now known as 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.
iv. The allele which expresses itself in the hybrid is called the dominant allele, and the other allele which is unable to express in the hybrid is called the recessive allele.
4. A capital letter is assigned to the dominant allele, and a corresponding small letter is assigned to the recessive allele, e.g., T (tallness) and t (dwarfness), respectively.
5. Mendel experimented with Pisum sativum for seven characters only. In each case, he found that one trait of the character, e.g., T is dominant over the other trait of the character, e.g., t.

Significance

1. It explains that characters are controlled by discrete unit factors called alleles.
2. Alleles occur in pairs.
3. It explains why individuals of the \({{\rm{F}}_1}\) generation express traits of only one parent.
4. It indicates why in a population, the number of heterozygotes are more and how the expression of the defective recessive alleles is masked.

Exceptions to Law of Dominance

Incomplete dominance and codominance are exceptions to the Law of Dominance.

Law of Segregation

1. The Law of Segregation states that “Allelic pairs separate (or segregate) during gamete formation, and the paired condition is restored by random fusion of gametes during fertilisation.”
2. Since one gamete or spore contains only one allele of each character, all gametes or spores are always pure.
3. Therefore, the Law of Segregation is also known as the Law of Purity of Gametes.
4. This law is based on the fact that the alleles do not show ant blending (mixing), and both the characters are recovered like in the \({{\rm{F}}_2}\) generation though one of these is not seen in the \({{\rm{F}}_1}\) generation.

Fig: Explanation of Law of Segregation

Significance

1. It is the most fundamental principle of heredity that has universal applications.
2. It also states that the gametes or spores are always pure for a character. Thus, this law is also known as the Law of Purity of Gametes or the Law of non-mixing of alleles.
3. It explains the occurrence of a \(3:1\) phenotypic ratio in \({{\rm{F}}_2}\) individuals during monohybrid cross or inheritance of one gene.
4. It explains that the alleles do not blend with each other.
5. This law predicted the occurrence of meiosis long before it was discovered.

Exceptions to the Law of Segregation

Non- disjunction is an exception to the Law of Segregation.

Summary

The inheritance of one gene is defined as the study of the inheritance of a single gene (one pair of alleles) of a character at a time (monohybrid cross). It is also known as monogenic inheritance. Gregor Mendel selected the garden pea plant (Pisum sativum) for studying the inheritance of one gene as well as the inheritance of two or more genes.

Correns studied the conclusions obtained from the experiments of Mendel and proposed the famous three laws of inheritance, i.e. Law of Dominance, Law of Segregation and Law of Independent Assortment. Law of dominance explains how the expression of recessive alleles is masked in heterozygotes, while Law of segregation explains that gametes are pure for a character and there is no blending of alleles in a heterozygous organism.

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