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

The three laws of inheritance, discovered by Gregor Mendel, explain how traits are passed from parents to offspring. 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. This topic is from the Class 12 chapter Principles of Inheritance and Variation in Biology.

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1. Mendel’s Laws
2. Mendel's Experiments
3. The Law of Segregation
4. Law of Dominance
5. Law of Independent Assortment
6. Application of Mendelian Laws

Mendel’s Laws

Genetics deals with heredity, and that is why it explains how characters are passed from parents to offspring. At the core of genetic inheritance are laws of Mendel, unearthed by Gregor Mendel in his groundbreaking experiments with the garden pea in the mid-nineteenth century. These laws give us the fundamental principles underlying our 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, emanate from things in agriculture to medicine.

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 traits.

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 speculative laws.

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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 dame 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, 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 combine according to a 3:1 phenotypic ratio of yellow: green seeds, thus showing the segregation of alleles according to Mendel's First Law.

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 instance, 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 for 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.

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 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 transported to most other organisms, including humans. In humans, most of the traits, such as eye colour or hair texture, not to mention their susceptibility to certain genetic disorders, show a pattern of Mendelian inheritance. 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 livestock. 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.

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