Gregor Mendel was an Austrian monk and scientist who conducted experiments on pea plants during the mid-19th century. Mendel's experiments focused on the inheritance of certain traits, such as seed color, pod shape, and flower color.
- Mendel’s laws of inheritance explain how traits are passed from parents to offspring, resulting in similarities between them.
- This law of inheritance depends on three other laws, including the law of dominance, the law of segregation, and the law of independent assortment.
An allele that expresses itself in the presence of its contrasting allele is called a dominant allele. A recessivegene is one whose expression is suppressed in the presence of a dominant gene; e.g., in a hybrid (Tt) tall plant, the allele for dwarfness (t) is recessive, and the T gene for tallness is dominant, and T is dominant over t.
Why Was the Pea Plant Selected for Mendel’s Experiments?
Mendel selected the pea plant (Pisum sativum) for the following reasons:
- Many varieties were available with observable alternative forms for a trait or characteristic.
- Peas are normally self-pollinated, as their corolla completely encloses the reproductive organs until pollination is completed. But cross-pollination can also be done.
- Pea plants are easy to grow and maintain.
- Pea plants show several easily observable contrasting traits such as seed shape, seed color, flower color, pod shape, pod color, flower position, and plant height.
- Its life cycle was short and produced a large number of offspring.
- The plant is an easily grown annual plant and does not require care except at the time of pollination.
Characteristics Studied by Mendel
Mendel explains the concept of dominant and recessive alleles. The following table shows each of the traits and which traits are dominant and which are recessive.
Crosses Done by Mendel
Mendel's experiments focused on the inheritance of certain traits, such as seed color, pod shape, and flower color, and he discovered that these traits are passed down predictably.
Monohybrid Cross
Mendel crossed two homozygous plants differing in one trait. It is a single cross between two organisms of a species that is made to study the inheritance of single pairs of genes or factors. A monohybrid cross helps to study the principle of dominance given by Mendel, as shown below:
Features of the monohybrid cross are given below:
- It is a cross in which only one character is considered at a time, e.g., in a cross between a tall and a dwarf plant, the size of the stem is considered.
- Mendel made a cross between a pure tall (TT) and a pure dwarf (tt) pea plant.
- He obtained all the tall (hybrid) plants in the F1 generation.
- When self-pollinated, these plants produced tall and dwarf plants in the ratio 3:1. The genotypic ratio of 1:2:1 and the phenotypic ratio of 3:1 are termed the monohybrid ratio.
Dihybrid Cross
Mendel crossed plants with round yellow seeds and wrinkled green seeds, which is a dihybrid cross that is shown below:
Features of the dihybrid cross are given below:
- It is a cross between two individuals, taking two contrasting traits at a time.
- It helps to study the inheritance of two pairs of alleles.
- The genotypic ratio in the F2 generation is 1:2:1:2:4:2:1:2:1, and the phenotypic ratio in the F2 generation is 9:3:3:1.
- This cross helps to study the principle of independent assortment given by Mendel.
Conclusion of Mendel's Experiments
After multiple crosses, Mendel concludes the following points:
- Genes are transferred from parent to new generation in pairs known as alleles.
- The genetic composition is known as the genotype, and the physical appearance of any organism is known as the phenotype.
- Alleles segregate independently during gamete formation
- Genes occur in pairs called alleles. If both of them are the same, known as "homozygous," and if both alleles are different, the organism is called "heterozygous."
Mendel's Laws
Mendel had given three laws of inheritance after observing his experiments. These are:
- Law of Dominance
- Law of Independent Assortment
- Law of Segregation
1. Law of Dominance
The law of dominance states that the expression of only one of the forms of the parental trait occurs in the F1 hybrid. In a heterozygous condition, i.e., different alleles, the dominant allele gets expressed. In a heterozygous condition, only the dominant allele is expressed. The F1 generation expresses dominant alleles. The suppressed allele is known as the recessive allele or trait.
TT × tt (parents) ------> Tt; F1 generation
2. Law of Independent Assortment
The law of independent assortment is also the second law of Mendel's. It states that completely different pairs of alleles are passed on to the offspring independently of each other; that is, during gamete formation, two genes segregate independently of each other as well as of the other trait. The inheritance of one gene does not affect the inheritance of any other gene.
3. Law of Segregation
The law of segregation is the third law of Mendel. The law of segregation states that for any trait, each pair of alleles of a gene segregates, and one gene passes from each parent to an offspring. The two alleles remain distinct and separate during gamete formation.
Key Points
- Mendel proposed the three laws of inheritance after conducting observations from its different crosses on the pea plant.
- Mendel's third law, i.e., the Law of Segregation, states that at the time of gametogenesis, both alleles separate during gamete formation so that the offspring get one copy of each gene from both parents.
- Mendel's Law of Independent Assortment states that at the time of gamete segregation, alleles assort independently during gamete formation.
Modern Applications
Below are the modern applications and examples of Mendel's Laws of Inheritance: Farmers and breeders use Mendelian principles to selectively breed plants and animals with desired traits. This has led to the development of crops with improved yield, resistance to diseases, and other desirable characteristics.
- Medical genetics: It helps in predicting the likelihood of genetic disorders and diseases in individuals based on their family history. Genetic counseling often involves explaining Mendelian patterns to individuals or families at risk.
- Genetic engineering: Mendel's laws guide the understanding of how genes segregate and assort, providing a basis for the design of genetically modified organisms (GMOs).
- Pharmacogenetics: Researchers study how genetic variations influence an individual's response to drugs. This information is used to tailor drug treatments based on a person's genetic makeup.
- Population genetics: explores how gene frequencies change over time in populations. This has applications in evolutionary biology and understanding the genetic diversity within species.
- Forensic genetics: DNA analysis is used to identify individuals based on their genetic profiles. Understanding inheritance patterns is essential for interpreting genetic data in forensic investigations.
- Cancer genetics: Mendelian principles are used to understand the inheritance of genetic mutations that may predispose individuals to certain types of cancer. This knowledge informs cancer risk assessments and preventive measures.