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Genetics

Last Updated : 22 Nov, 2023
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Genetics is the study of genes, genetic variation, and transmission of traits in organisms. In the 19th century, Gregor Mendel was the first to study genetics scientifically. Mendel observed pea plants to study trait inheritance and gave three fundamental laws of inheritance: the Law of Dominance, the Law of Segregation, and the Law of Independent Assortment. Genetics helps us to diagnose and treat various genetic diseases, improve crop yields, and contribute to our understanding of evolution.

Genetics

What is Genetics?

Genetics is a branch of biology that studies how traits and characteristics are passed from parents to offspring. The concept of genetics is based on inheritance, which is defined as the transmission of characters from one generation to another over a period of time. Gregor Johann Mendel is regarded as the “Father of Genetics” for his study on “trait inheritance” and discoveries on the basic “principles of heredity”.

As genes are transmitted from parents to offspring some variations occur due to mutations, and recombination of genes. It plays an important role in evolution and individuals with advantageous variations in their traits easily adapt to changing environments and pass on their genes to the next generation. Genes contribute to the diversity and adaptation of species over time.

Law of Inheritance by Gregor Mendel

Gregor Johann Mendel conducted his experiments on the Garden Pea. Mendel crossbred pea plant with seven different traits, such as flower color, seed texture, and plant height, for over seven long years. He used true breeding lines. After observing the, he discovered the basic principles of heredity, also known as Mendel’s Laws of Inheritance. His experiments with pea plants and proposed laws forms the basis of modern genetics and demonstrates how traits or characters are passed down from one generation to the next. He also gave the concept of dominant and recessive traits.

Principles of Inheritance

Mendel experiment started with a monohybrid cross. He introduced the two principles of inheritance after observing monohybrid cross that are as follows:

  • Law of Dominance
  • Law of Segregation

Law of Dominance

The law of dominance states that in a heterozygous condition where an individual has two different alleles for a trait, the dominant allele’s characteristic will be expressed unless the individual possesses two identical recessive alleles (homozygous condition), in which case the recessive trait is expressed.It is the first law of inheritance and has the following features:

  • Traits are controlled by separate units called as ‘factors,'(now called as genes).
  • Genes are present in pairs.
  • In heterozygous pairs, one gene of the pair will show dominance over the other.

The law explains that in a monohybrid cross, only one of the parental characters or trait is expressed in F1 generation and in the F2 generation both the characters are expressed. In the F1 generation the trait which is expresed is called the dominant trait, and the trait which is suppressed is called the recessive trait. 

Law of Segregation

The Law of Segregation, the second law of inheritance, states that genes are present in pairs, and during the gamete formation, these pairs of gene separate from each other. This separation ensures that each gamete carries only one allele for each gene. It shows following features:

  • In the first generation (F1), there is no blending of alleles; both traits are expressed.
  • Heterozygous parents produce two types of gametes, each containing one allele in equal proportions, while homozygous parents yield uniform gametes.

Incomplete Dominance

Incomplete dominance, also called partial dominance or intermediate inheritance, describes a situation where a heterozygous individual does not display the dominant allele’s phenotype but rather show a trait that falls between the characteristics associated with the dominant and recessive alleles. For instance, in four o’clock plants, two pure breeding types exist: one with red flowers and the other with white flowers. When these two are crossed, the resulting F1 plants show pink flowers, a demonstartion of incomplete dominance.

Codominance

Codominance, is defined as the phenomenon where both alleles having contrasting traits are expressed simultaneously in the phenotype, without one being dominant over the other. Each allele express its own trait without blending or mixing with the other trait. For example, in certain blood types, the A and B alleles show codominance resulting in individuals having both A and B antigens present on their red blood cells.

Law of Independent Assortment

Law of independent assortment states that during gamete formation different genes segregate independently resulting in various combinations of alleles in the offspring. It is the third law of inheritance and have the following features:

  • It refers to the inheritance of alleles (gene variants) for different traits, not for alleles of the same trait.
  • The law describes how, during the formation of gametes (sperm and egg cells), alleles for different traits segregate independently of each other.
  • This independent segregation results in various combinations of alleles in offspring, leading to genetic diversity.

The Chromosomal Theory of Inheritance

The Chromosomal Theory of Inheritance explains how the principles of inheritance proposed by Gregor Mendel are connected to the behavior of chromosomes during cell division and reproduction. It states that genes are located on the chromosomes and are segments of DNA that carry the information for specific traits. Offspring inherit two sets of chromosomes, one from each parent. These homologous chromosome pairs carry alleles for the same genes. The process of meiosis, which reduces the chromosome number by half, ensures the segregation of alleles (Law of Segregation) and independent assortment of genes (Law of Independent Assortment)takes place during the formation of gametes.

Sex Determination

In humans there are two types of sex chromosomes, represented as X and Y. Female have two pair of X chromosomes (XX), while male have one X and one Y chromosome (XY). Therefore the sex of the individual is determined by the presence or absence of the Y chromosome. The chromosomes other than the sex chromosomes( X and Y) are called as autosomes. In some species, environmental factors, such as temperature, can also determine the sex of individual.

Mutations in DNA can lead to variations in phenotype as well as genotype of organism that contributes to genetic diversity and evolution.

Also Read: Sex Determination

Genetic Disorders

Genetic disorders are caused due to mutation or abnormalities in the genes of individual. These disorders can be inherited from parents or arise as new mutations. Some examples of disorders of Mendelian nature are as follows:

  • Hemophilia: Haemophilia is a X linked recessive disoder caused due to mutation in F8 gene for hemophilia A and the F9 gene for hemophilia B. It mainly affects blood clotting.
  • Sickle Cell Anemia: It is an autosomal recessive disorder caused due to mutation in the beta globin gene that leads to faulty hemoglobin protein, called hemoglobin S. The RBC become sickel shape and cause pain and anaemia.
  • Phenylketonuria (PKU): It is an autosomal recessive disorder caused due to mutations in the gene that helps make an enzyme called phenylalanine hydroxylase. Individuals with PKU lack the enzyme necessary to metabolize the amino acid phenylalanine. Accumulation of phenylalanine in the body can lead to intellectual and developmental disabilities.

Chromosomal Disorders includes:

  • Down Syndrome (Trisomy 21): It is caused due to extra copy of chromosome 21 and results in physical and intellectual disabilities.
  • Klinefelter Syndrome (47,XXY): It is caused by the presence of one or more extra X chromosomes in males. The most common form is 47,XXY, which means the individual has two X chromosomes and one Y chromosome.
  • Turner Syndrome (45,X): It is caused due to absence or partial absence of one of the X chromosomes in females. Most individuals with Turner syndrome have a single X chromosome (45,X).

Significance of Genetics

The significance of the genetics are as follows:

  • It help us in understanding hereditary that is the transmission of the traits or genetic disorders from one generation to another.
  • Genetics plays an importanat role in diagnosing, treating, and preventing genetic diseases.
  • Genetics is used in forensic science to solve crimes by analyzing DNA evidence.
  • It is used in agriculture for selective breeding of crops and livestock for desirable traits, improving food production.
  • It help in studying the evolutionary history of living organisms.
  • Genetic information helps in the preservation of endangered species.

Also Read:

FAQs on Genetics Introduction, Laws

1. What are the Laws of Genetics?

George Mendel conducted experiments on pea plant and proposed three law of genetics that includes: The law of dominance, the law of segregation, and law of independent assortment.

2. What is the Introduction of Genetics?

Genetics is a branch of biology that studies genes, genetic variations and hereditary. It studies the traits or characteristics that are passed from parents to offspring.

3. What is the First Law if Genetics?

The first law of genetics, also known as Mendel’s First Law or the Law of Segregation, states that each individual has two alleles for each gene, one inherited from each parent. During gamete formation, these alleles separate, from each other that ensures that each gamete carries only one allele for a given trait.

4. What are the 3 Types of Genetics?

The three types of genetics includes: Mendelian genetics, it focuses on the inheritance of single genes, Molecular genetics and population genetics that provide information about evolution and genetic diversity within and between species.

5. Give Some Examples of the Genetic Disorder.

Some example of genetic disorder that are of Mendelian nature or are single gene disorder are Haemophilia, Sickle Cell Anaemia, Phenylketonuria. Gentic disoder caused due to chromosomal disorder are Down’s syndrome, Klinefelter’s Syndrome,Turners Syndrome.



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