Genetics: Principles of Heredity

The genetics, principles of heredity explain how traits are inherited from one generation to the next through genes. These principles explain patterns of inheritance, such as eye colour, height, and even genetic disorders. This topic is from the Class 12 chapter Principles of Inheritance and Variation in the subject of biology. It is important for examinations like NEET and AIIMS BSc Nursing, where a good understanding of genetic principles can help answer questions on heredity, variation, and function of genes.

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This Story also Contains

1. What is Heredity?
2. What is Genetics?
3. Variation
4. Mendel's Laws of Heredity
5. Genetic Disorders

What is Heredity?

Heredity is defined as the passing of genetic traits from parents to their offspring through genes. It means that characteristics, such as eye colour, texture of hair, and even some kind of disease, can be passed on from generation to generation. Genes are kept in a very small thread-like structure within each cell, called a chromosome. Thousands of genes are carried by the chromosome, referring to segments of DNA responsible for specific characteristics. In the process of reproduction, genetic material from both parents combines to create a unique genetic makeup in their offspring. This process helps in maintaining continuity in a species while also permitting individual differences and variations.

What is Genetics?

Genetics is the study of heredity and how traits are different among living things. It relates to understanding genes and how genes work when transmitted, expressed, and regulated. The basic units of heredity are genes, which in turn control specific characteristics through the synthesis of proteins and other molecules. There are branches of genetics, molecular genetics, population genetics, and evolutionary genetics. It has a huge impact on medicine, agriculture, and biotechnology, making it easier to understand human health, biodiversity, and genetic variety.

Also Read:

• MCQ Practice on Genetics
• Principles of Inheritance and Variation
• Mendelian Genetics
• Molecular Basis of Inheritance

Variation

Variation refers to the differences in traits among individuals in a group or population. Such differences can be in physical features, body shape, colour, or even internal traits like how the body works. Such visible or genetic differences are often described using the term polymorphism. Variation is important for evolution and gives rise to adaptation in species to change in their environment.

There are two primary types of variation:

Genotypic Variation

• This comes about as a result of differences in the genes, or alleles passed from parents to the offspring.
• It gives rise to variations in a population and allows species to thrive in various environments.
• Such traits include physical characteristics, body functions, as well as disease resistance and other protection mechanisms.

Somatic Variation

• These are variations in traits that occur during the lifetime of an organism but are not passed on to the next generation.
• They are caused by diet, environment, lifestyle, or exposure to harmful substances.
• Somatic variation caused by somatic mutation also brings about changes in body cells. Sometimes, it results in causing diseases like cancer.

Mendel's Laws of Heredity

Gregor Mendel, considered the father of modern genetics, established three basic principles of heredity from his experiments on pea plants, as follows:

• Law of Segregation: Two alleles of a gene segregate during meiosis from each other for gamete formation such that each gamete carries only one allele of each gene.

• Law of Independent Assortment: Genes for different traits segregate independently of one another in forming gametes and their inheritance is independent.

• Law of Dominance: Some alleles are dominant and mask the expression of recessive alleles in heterozygous individuals. Dominance relationships determine the phenotypic outcomes of genetic crosses.

Genetic Disorders

Genetic disorders are diseases resulting from abnormalities in an individual's genetic material. Broadly, there are three types of disorders based on the pattern of inheritance of a particular disorder:

Autosomal Recessive Disorders: Disorders occurring due to the inheritance of two recessive alleles for a gene. Examples include cystic fibrosis, sickle cell disease, and phenylketonuria.

Autosomal Dominant Disorders: It results from the inheritance of a single allele copy for a gene. Examples include Huntington's disease, Marfan syndrome, and neurofibromatosis.

X-linked Recessive Disorders: A result of a mutation in genes found in the X-chromosome, much of their effect falls upon males, who have only one X chromosome. Examples are haemophilia, Duchenne muscular dystrophy, and colour blindness.

Examples of Genetic Disorders: Genetic disorders manifest themselves in multiple ways and, therefore, impact many of the systems and organs within the body. Some of these disorders are:

• Cystic fibrosis affects the respiratory and gastrointestinal systems.

• Huntington's disease consists of involuntary movements and cognitive decline.

• Chromosomal conditions like Down syndrome, result in mental retardation along with developmental delays.

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