What Are The Three Laws Of Inheritance?: Segregation & Dominance

The Three Laws of Inheritance are given by Gregor Mendel—the Father of Genetics. He explained how traits are passed from parents to offspring based on his experiments. His experiments were based on pea plants. These laws laid the basis for the Principles of Inheritance and Variation.

These are known as Mendel’s Laws of Inheritance and include the Law of Dominance, the Law of Segregation, and the Law of Independent Assortment. Mendel's Laws of Inheritance show how traits are passed from parents to children. Behavioral genetics uses Mendelian laws to study how behaviour is inherited. These laws form the basis of Mendelian Genetics, helping in understanding heredity.

This Story also Contains

1. Mendel’s Laws
2. Mendel's Experiments
3. Law of Dominance
4. The Law of Segregation
5. Law of Independent Assortment
6. Application of Mendelian Laws
7. Recommended Video on "What are the Three Laws of Inheritance"
8. MCQs on Laws of Inheritance

Mendel’s Laws

Genetics is the study of heredity, and it explains how characteristics are passed from parents to offspring. These laws of inheritance were given by Gregor Mendel in his groundbreaking experiments with the garden pea in the mid-nineteenth century. These laws give us the fundamental principles underlying knowledge of how genetic traits are transferred and expressed from one generation to another. Knowledge of Mendelian genetics is, therefore, the key to knowing inheritance patterns, and the applications, therefore, stem from things in agriculture to medicine. The Three Laws of Inheritance are:

1. Law of Dominance

2. Law of Segregation

3. Law of Independent Assortment

Mendel's Experiments

Gregor Mendel's founding work was the nucleus of modern genetics science. This he did through carefully planned experiments on pea plants that he carefully controlled, crossing independent varieties that displayed fixed characteristics such as the colour of flowers, the shape of seeds, and the length of plants. Making sense of the patterns of inheritance in the generations of pea plants, Mendel deduced fundamental laws that govern the transmission of genetic principles of heredity.

However, what Mendel did was manipulate the breeding of pea plants to discover something about their particular traits using several generations of controlled experiments. He carefully recorded the trait inheritance from crosses and numerically documented the ratios of progeny with contrasting traits. This empirical approach allowed him to develop his laws based on numbers rather than merely theoretical laws.

Law of Dominance

The law of dominance states that when there are two different alleles for a particular character in the contrasting pair in a heterozygous condition, one will dominate the other in its effect on the phenotype of the organism. Then, the dominant masks the recessive. It explains why some of the traits are expressed in the offspring and others are not.

For example, taking a cross between a monohybrid cross of a homozygous dominant tall pea plant with a homozygous recessive short pea plant, all F1 offspring would be tall. This is because the tall phenotype masks the expression of the recessive allele coding for short stature. Only in the F2 generation, after the segregation of alleles according to Mendel's Laws, is the recessive trait allowed to reappear in a 3:1 ratio.

The Law of Segregation

The law of Segregation says that there are two alleles for each trait and that each passes into gametes separately so that each gamete has one allele. This ensures that the offspring will have a mix of sire and dam genetic material. This is how the re-emergence of characteristics in later generations is explained and forms the basis of understanding how genetic traits are inherited.

For instance, if one parent plant of a monohybrid cross is homozygous dominant (YY) for yellow seeds and the other parent is homozygous recessive (yy) for green seeds, the first generation receives one chromosome with the dominant allele and another with the recessive allele. A second cross, known as the F2 generation, will then express a combination according to a 3:1 phenotypic ratio of yellow: green seeds, thus showing the segregation of alleles.

Law of Independent Assortment

The law of independent assortment, or Mendel's third law, states that if genes that control separate characters are present on different chromosomes, then during a dihybrid cross, these genes will be separated freely from one another in the F2 generation. Since characters are present on different chromosomes, this will lead to their independent assortment.

In genetic crosses that involve two different traits (dihybrid crosses), alleles for each of the traits assort independently. This will result in new combinations of traits that did not exist in either parent but are possible because of an independent assortment of alleles. These new combinations of traits and phenotypic ratios in the F2 generation will be in a ratio of 9:3:3:1.

Application of Mendelian Laws

Mendelian genetics principles do not hold for pea plants alone but can be applied to most other organisms, including humans. In humans, most of the traits, such as eye color or hair texture, not to mention their susceptibility to certain genetic disorders. Genetics helps in understanding how traits and Mendelian disorders are inherited in families. In light of Mendel's laws of inheritance, these diseases, such as cystic fibrosis and sickle cell anaemia, can be made understandable to shed light on genetic counselling and medical genetics.

These laws also underlie an understanding of agricultural practices as breeders apply selective breeding to bring out desirable traits in crops and reproduction in animals. A breeder, applying Mendelian genetics, can know in advance what any crossbreeding will bring forward and then design a program of selective breeding for many important traits such as resistance to diseases, yield, or nutritional value.

Recommended Video on "What are the Three Laws of Inheritance"

MCQs on Laws of Inheritance

Question: It is said that Mendel proposed that the factor controlling any character is discrete and independent. His proposition was based on the

1. Results of the F3 generation of a cross.

2. Observations that the offspring of a cross made between the plants having two contrasting characters show only one character without any blending.

3. Self-pollination of F1 offspring

4. Cross-pollination of the F1 generation with the recessive parent

Answer: Mendel discovered that distinct, non-mixing components (genes) govern characteristics. The F₁ generation of his pea plant studies had a single characteristic (tall plants, not a combination of tall and small plants, for example). This demonstrated the dominance of one element over the other. In the F₂ generation, the hidden feature resurfaced, demonstrating that the factors remain constant. Mendel's research clarified how characteristics are inherited from parents.

Hence, the correct answer is option 2) Observations that the offspring of a cross made between the plants having two contrasting characters show only one character without any blending.

Question: Read the following statements:

1. The heterozygotes and homozygotes in a monohybrid cross have the same phenotype, explained in light of the law of dominance of the recessive allele.

2. The allele expressing itself in a heterozygous condition of a gene is called the dominant allele.

3. The effect of deleterious recessive alleles is masked due to the law of dominance.

1. 1, 2, and 3

2. 2 and 3

3. 1 and 3

4. None of these

Answer: 1. The statement is incorrect. In a monohybrid cross, individuals with the same phenotype may not necessarily have the same genotype. According to Mendel's Law of Dominance, the dominant allele (A) overpowers the recessive allele (a) in a heterozygous condition (Aa), resulting in both homozygous dominant (AA) and heterozygous (Aa) individuals exhibiting the dominant phenotype.

2. Correct. A dominant allele is the one that is expressed in the phenotype when present in a heterozygous state. For example, in a genotype of Aa, the dominant allele 'A' determines the observable trait.

3. Correct. The law of dominance explains that in a heterozygous condition, the effect of a deleterious or harmful recessive allele (a) is obscured by the presence of a dominant allele (A), which is the one that manifests in the organism's phenotype. This is a crucial principle in the study of genetics and inheritance patterns.

Hence, the correct answer is option 2) 2 and 3

Question: According to Mendel, .........................................segregate and................. assort independently.

1. Alleles of a gene; alleles of different genes

2. Alleles of different genes; alleles of a gene

3. Dominant traits, recessive traits

4. Recessive traits; recessive traits

Answer: As per Mendel’s law of segregation, parents contain two alleles during gamete formation, alleles of a pair segregate from each other, and gametes receive only one of the two alleles of a gene.

As per the law of independent assortment, the inheritance of one pair of alleles of a gene is independent of the inheritance of another pair of another gene.

Hence, the correct answer is option 1) alleles of a gene; alleles of different genes.