Introduction
The principle of inheritance and variation in the class 12 biology syllabus is a very important topic with a good weightage in the competitive exams. This chapter explains the basic concepts of inheritance and theories put forward by biologists. In this chapter, students will learn the laws of heredity, incomplete dominance, the chromosomal theory of inheritance, Sex determination, mutation, Mendel’s law of inheritance, and much more. The idea of inheritance pattern was developed after Mendel’s work and his contributions in the field of genetics. This chapter provides an insight into the essence of modern biology - which is genetics.
Let’s have a look at the notes of chapter principles of inheritance and variation in this article.
An elephant gives birth to only an elephant baby, a mango seed only gives rise to a mango tree, human beings give birth to only young human beings. This kind of continuation of species by giving birth to individuals with similar heredity or traits is because of inheritance. Thus, heredity is a process of transmitting heritable traits to young ones by their parents. The process of heredity can easily be described as the process of transferring characteristics or traits from parents to offspring or their unique individuals. This transfer can be conducted through either asexual reproduction or sexual reproduction. The traits that continue the legacy are found in genes on the chromosomes within the body of an individual. Genes are chemicals that carry genetic information about how to code proteins. Genes are the carriers of instructions that could have a similarity to one of their two parents.
Sir Gregor Mendel performed various experiments to study and know more about inheritance, heredity, genetics, and variation. Also known as the Father of Genetics was the first one in his field to conduct experiments on Inheritance, Genetics, Heredity, and Variations.
What is Variation?
The difference in DNA among individuals is called a variation in genetics. Mutation, genetic recombination, crossing over, environmental changes, and other such ways lead to genetic variation. It is the impact of these on the way genes are expressed.
Mendel’s Law of Inheritance
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Mendel performed experiments on Garden pea in the back of his garden to establish the principle of heredity. For this, he took 14 true-breeding pea plants with seven distinguishable characters that had two opposite traits.
Mendel called genes “factors” that pass to the offspring from the parents.
And the factors (genes) that coded for a pair of opposite traits were called “alleles”.
Based on his observation, Mendel gave three laws of inheritance.
1. Law of Dominance
In the case of the formation of the heterozygote, one of the alleles is dominant. And, the dominant allele is expressed in the phenotype. So, when homozygous tall with (TT) alleles are crossed with dwarf (tt) plants, all the resulting offspring plants are tall and have the tall dominant trait with genotype Tt.
2. Law of Segregation of Genes
At the time of gamete formation during meiosis, each allele separates. So, the characters are passed to gametes without blending. Also, heterozygotes produce different kinds of gametes while homozygous only have one type.
3. Law of Independent Assortment
Mendel proposed another set of rules which was based on the observations he made on dihybrid crosses which are known as Mendel’s Law of Independent Assortment. This particular law of Mendel states that “when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters”. In simple words, this law explains the independent separation of genes. It means that alleles inherited by progeny from different traits are acquired independently. A simple way to understand this law is with the help of a Punette square when carried out with two pairs of genes during the stage of meiosis and egg-production and pollen in the F1 generation of the RrYy plant.
principles of inheritance and variation, mendel’s law of inheritance, variation, mendelian disorders, mutation
Mutation
The process of Mutation is a result of alteration in DNA sequences which then results in the changes in the phenotype and genotype of an entity. The mutation also brings variations in DNA along with the recombination. The chromosomal alteration has its negative effects and causes aberrations or abnormalities. These aberrations or abnormalities are usually observed in the cancer cells, according to the studies. Another type of mutation that takes place as a result of changes in a single base pair of DN is called a point mutation. For example insertions and deletions of base pairs of DNA along with sickle cell anaemia leads to frameshift mutations.
Mendelian Disorders
Mentioned below are some of Mendelian Disorders:-
Colour blindness
Cystic fibrosis
Thalassemia
Phenylketonuria
Sickle cell anaemia
Haemophilia
FAQs on Principles of Inheritance and Variation
1. What are the foundational principles of inheritance and variation?
The two foundational principles are inheritance and variation. Inheritance is the process by which genetic information and traits are passed from parents to their offspring. Variation refers to the differences observed among individuals within a species, including offspring from the same parents. These principles explain why we resemble our parents yet are also unique individuals.
2. What are Mendel's three laws of inheritance?
Gregor Mendel, based on his experiments with pea plants, proposed three fundamental laws of inheritance:
- Law of Dominance: In a heterozygous individual, one allele (the dominant one) will mask the effect of the other allele (the recessive one).
- Law of Segregation: During gamete formation, the two alleles for a heritable character separate (segregate) from each other, so that each gamete ends up with only one allele.
- Law of Independent Assortment: Alleles for different traits are sorted into gametes independently of one another. The inheritance of one trait does not affect the inheritance of another.
3. How is codominance different from incomplete dominance? Provide an example for each.
The key difference lies in how alleles are expressed in a heterozygote. In codominance, both alleles are fully and simultaneously expressed. A classic example is the ABO blood group system in humans, where a person with the genotype IAIB has AB blood type, expressing both A and B antigens. In incomplete dominance, the heterozygote exhibits a phenotype that is an intermediate blend of the two parental phenotypes. For instance, crossing a red snapdragon (RR) with a white one (ww) results in pink offspring (Rw).
4. What is the Chromosomal Theory of Inheritance and who proposed it?
The Chromosomal Theory of Inheritance was proposed by Walter Sutton and Theodor Boveri in the early 1900s. It states that genes, the units of heredity, are located on chromosomes. The behaviour of chromosomes during meiosis, specifically their segregation and independent assortment, provides the physical basis for Mendel's laws of inheritance. This theory connected the fields of cytology (study of cells) and genetics.
5. Explain the mechanism of sex determination in humans.
Sex determination in humans follows the XX-XY system. Females possess two X chromosomes (XX), making them homogametic, meaning all their eggs contain a single X chromosome. Males possess one X and one Y chromosome (XY), making them heterogametic. They produce two types of sperm: 50% carry an X chromosome and 50% carry a Y chromosome. The sex of the offspring is determined by the sperm that fertilizes the egg: if an X-sperm fertilizes the egg, the child will be female (XX); if a Y-sperm fertilizes it, the child will be male (XY).
6. What is a genetic mutation, and what are its main types?
A genetic mutation is a permanent alteration in the DNA sequence that makes up a gene. This change can affect a single DNA building block or a large segment of a chromosome. Key types include:
- Point Mutation: A change in a single nucleotide base pair. An example is Sickle-cell anaemia, where a single substitution occurs in the beta-globin gene.
- Frameshift Mutation: Caused by the insertion or deletion of one or more nucleotides, which shifts the reading frame of the genetic code, often resulting in a non-functional protein.
7. How does a single point mutation, as seen in sickle-cell anaemia, cause a genetic disorder?
In sickle-cell anaemia, a point mutation in the HBB gene changes a single DNA base (GAG to GTG). This alters the corresponding mRNA codon, causing the amino acid glutamic acid to be replaced by valine in the haemoglobin protein. This seemingly small change drastically alters the protein's structure, causing haemoglobin molecules to clump together under low-oxygen conditions. This deforms red blood cells into a sickle shape, leading to blockages in blood vessels and the wide-ranging symptoms of the disorder.
8. What is the significance of the Law of Independent Assortment in creating genetic variation?
The Law of Independent Assortment is a primary driver of genetic variation in sexually reproducing organisms. It ensures that the alleles for different traits are inherited independently. During meiosis, the alignment of one homologous chromosome pair at the metaphase plate does not influence how other pairs align. This random orientation shuffles maternal and paternal chromosomes, creating new combinations of alleles in the gametes that were not present in the parents, thereby increasing the genetic diversity of the offspring.
9. What distinguishes Mendelian disorders from chromosomal disorders?
The primary distinction is the scale of the genetic error. Mendelian disorders are caused by mutations in a single gene. They follow predictable inheritance patterns (dominant, recessive, etc.) and include conditions like Cystic Fibrosis and Haemophilia. In contrast, chromosomal disorders are caused by an error in the number or structure of entire chromosomes, such as having an extra chromosome (Trisomy 21 or Down's Syndrome) or a missing one (Turner's Syndrome). They affect a much larger amount of genetic material.
