Arpita Srivastava Content Writer
Content Writer
Principles of Inheritance and Variation deals with the study of heredity and variation principles and their mechanisms. Gregor Johann Mendel is called The 'Father of Genetics'.
- Principles of Inheritance and Variation explain how offspring are identical to their parents.
- The genetic legacy passed down by our biological parents is referred to as heredity.
- It occurs when a trait is passed on from generation to generation.
- Genetics is a branch of biology concerned with the study of chromosomes, genetic differences, and heredity.
- Characters are passed down from parent to progeny by inheritance, which is the basis of heredity.
- The basic physical unit of inheritance is the gene.
- They are passed down from parents to offspring and include the information needed to determine traits.
- The degree to which progeny differ from their parents is referred to as variation.
- It is caused by crossing over, recombination, mutation, and environmental effects on genes.
Read More: Chromosome Theory of Inheritance
Key Terms: Principles of Inheritance and Variation, Genetics, Heredity, Inheritance, Gene, Variation, Hybridization, Cross-pollination, Pollination, Pollen Grains, Monohybrid
Mendel’s Law of Inheritance
[Click Here for Sample Questions]
Gregor Mendel, an Austrian monk, was a pioneer in the field of heredity research. Mendel was the first to accurately predict how traits are passed on from generation to generation.
- Mendel proposed the law of inheritance after studying hybridisation on the garden pea (Pisum sativum).
- He took 14 true-breeding pea plants with seven distinct characters, each has two opposing traits.
- He used available strategies to avoid cross-pollination by unwanted pollen grains.
- Mendel analyzed the outcome with the help of mathematics and statistics.
- He used true-breeding pea lines for artificial hybridization and cross-pollination.
- True breeding lines are those that have stable trait inheritance and self-pollinate.
- The pollen transfer and emasculation (removal of the anther) experiments are both part of the hybridization process (pollination).
Mendel proposed three law of inheritance based on his study which are as follows:
Law of Dominance
One of the alleles in a heterozygote is dominant and manifests itself in the phenotype. We get all tall plants with the genotype Tt in the offspring when we cross homozygous tall (TT) and dwarf (tt) plants, meaning that tallness is a dominant trait over dwarfness.
Law of Segregation
Alleles do not mix in the F2 generation, and both characters are recovered during gamete formation. During gamete formation, traits detach (separate) from one another and transfer to different gametes.
- Individuals that are homozygous develop similar gametes.
- Heterozygous individuals produce a number of gametes with different characteristics.
Law of Independent Assortment
The Law of Independent Assortment states that when two sets of traits form a hybrid form in combination, then one set is independent of the other set. It means alleles are independent of each other.
Read More:
| Chapter Related Concept | ||
|---|---|---|
| Pleiotropy | Linkage and crossing over | Mendelian Genetics |
| non mendelian inheritance | Chromosomes and Genes | Difference between Gene and DNA |
Inheritance of one Gene
[Click Here for Previous Year's Questions]
In inheritance of one gene, Mendel crossed two types of pea plants, tall and dwarf, and collected all the seeds of plants. He grew all of the seeds from the F1 generation, the first hybrid generation, to build plants.
- He discovered that all of the plants were tall.
- It was found that another pair of traits yielded a similar result.
- Mendel discovered that some of the F2 plants are dwarf as well after self-pollinating the F1 plants.
- Tall plants account for 3/4 of the total, while dwarf plants account for 1/4.
- An alphabetical symbol is used to represent each gene.
- A capital letter (TT) is used for genes expressed in the F1 generation, and a small letter (tt) for other genes.
- Mendel proposed that the allelic pair of genes for height in tall and dwarf varieties is homozygous in true breeding (TT or tt).
- The phenotype is tall or dwarf, and the genotype is TT, Tt, or tt.
- Alleles that show opposing traits are found in heterozygous hybrids (Tt).
- The monohybrid ratio of F2 hybrids is 3:1 (phenotypic) and 1:2:1. (genotypic).
- The inheritance of one gene is called a monohybrid cross.
Read More: Cross-Pollination Vs Self-Pollination
Test Cross
Test Cross is the method of determining the genotype of a plant that exhibits a dominant trait. It involves crossing the given plant with a recessive homozygote. The process is based on following two points which are as follows:
- The parent plant was homozygote if only the dominant trait was present in the offspring's phenotype.
- If the offspring have both phenotypes, the parent plant was heterozygote for the dominant trait.
Read More: Important Principles of Inheritance and Variation
Incomplete Dominance
[Click Here for Previous Year's Questions]
Incomplete Dominance happens when a dominant allele is not completely dominant over a recessive allele. In this case, the F1 hybrid formed is intermediate between the two parents.
- The snapdragon (Mirabilis jalapa) comes in two types of pure breeding plants: red-flowered and white-flowered.
- Pink flowers are formed in F1 plants created by crossing the two.
- The F2 generation has one red, two pink, and one white while selfing.
- The pink bloom is caused by incomplete domination.
Read More: Pollination & Fertilization
Co-Dominance
[Click Here for Sample Questions]
Codominance occurs when the coexistence of two alleles that don't have a dominant-recessive relationship but are both present in the organism take place. In humans, a gene I control ABO blood grouping.
- The gene has three alleles: IA, IB, and I.
- IA, IB, and I are the three alleles that are dominant over the others.
- Sugar polymers protrude from the surface of red blood cells' plasma membranes.
- The type of sugar is regulated by a gene.
- Both IA and IB demonstrate their characters' segregation when they are present together due to codominance.
Read More: Class 12 Biology Double Fertilisation in Angiosperms
Inheritance of two Genes
[Click Here for Previous Year's Questions]
Inheritance of two genes is a breeding experiment between two animals that are identical hybrids for two traits. It is also known as a dihybrid cross. A dihybrid cross is a cross between two animals that are heterozygous for two distinct traits.
- Individuals that are homozygous for a specific trait make up this sort of trait.
- These characteristics are determined by genes, which are DNA segments.
- Each phenotype is expressed by a pair of alleles the parents bear in a dihybrid cross.
- One parent carries the dominant allele, and the other carries the recessive allele.
- As a result of the crosses, the F1 generation's offspring are all heterozygous for specific traits.
Law of Independent Assortment
The rule notes that the segregation of one pair of characters is independent of the segregation of the other pair of characters. It occurs when two pairs of traits are combined in a hybrid.
- Two new combinations, round green & wrinkled yellow, are developed in dihybrid crosses.
- It separates assortments of traits for seed form, wrinkled, and seed colour, yellow and green.
- A combination sequence of three yellow: one green, with three round: one wrinkled, yields the 9:3:3:1 ratio.
The following is a representation of the derivation.
(3 Round : 1 Wrinkled) (3 Yellow : 1 Green) = 9 Round, Yellow : 3 Wrinkled, Yellow: 3 Round, Green : 1 Wrinkled, Green
Chromosomal Theory of Inheritance
Sutton and Boveri agreed with Mendel's findings, claiming that chromosomes serve as gene carriers. Chromosomes are found as a homologous pair, and the two alleles of a gene are found on the same chromosome.
- During meiosis, homologous chromosomes separate during gamete formation.
- Separation and chromosome assortment happen on their own.
- Morgan used fruit flies Drosophila melanogaster, to conduct comprehensive experiments.
- It creates scientific evidence to support the chromosomal theory of inheritance.
- It mixes gametes and developing offspring with the same diploid number of chromosomes as the parent.
Read More:
| Chapter Related Conncepts | ||
|---|---|---|
| Plant Breeding | Gibberellins | Epimorphosis |
| Tissue culture | Gemmule Formula | Fragmentation |
Linkage and Recombination
The physical association of genes on a chromosome is referred to as linkage. The parental combination is more common than non-parental combinations or recombinants in a dihybrid cross if the two genes are closely related or present on the same chromosome.
- The distance between two genes influences linkage and recombination.
- The longer the gap, the more likely recombination is.
Read More: Pollination by Birds
Sex Determination
[Click Here for Sample Questions]
Different sex determination mechanisms exist in different organisms. Henking was the first to discover the X chromosome, dubbed the X body by him. The chromosomes that control sexual character development are called sex chromosomes.
- The rest of the chromosomes are called autosomes.
- In humans, grasshoppers, and drosophila, male heterogamety occurs when a male produces two different forms of gametes.
- In birds, female heterogamety occurs when a female produces two different forms of gametes.
The process of Sex Determination for living organisms are listed below
| Name | Male | Female |
|---|---|---|
| Human | XY | XX |
| Drosophila | XY | XX |
| Grasshopper | XO | XX |
| Birds | ZZ | ZW |
Read More: Pollination by Insects
Mutation
[Click Here for Previous Year's Questions]
Mutations that are viable are passed on from generation to generation. The term "mutation" refers to a modification in DNA sequence. A mutation affects both the genotype and phenotype of an organism.
- Not all mutations are dangerous, despite the fact that they have been linked to a number of diseases.
- Changes such as deletion, addition, replication, substitution, and so on cause mutations.
- Mutation is the most common cause of cancer.
- UV rays are one of the agents that cause mutations (mutagens).
Mutations can be divided into two categories:
- Point Mutation
- Frameshift
Read More: Mendelian Inheritance
Genetic Disorders
[Click Here for Sample Questions]
The two most common types of genetic disorders are mendelian disorders and chromosomal disorders.
Mendelian Conditions
Mendelian Conditions arise due to single gene mutation and cause these diseases. It has the same inheritance pattern as it does according to Mendel's law. Some common diseases are as follows:
| Name | Genetic Trait | Cause | Effects | Inheritance Pattern |
|---|---|---|---|---|
| Colour blindness | X-linked recessive | Defect in the green or red cone of the eye | Unable to discriminate between red and green color | A daughter will be color blind only if the father is color blind |
| Haemophilia | X-linked recessive | Defect in one protein involved in the clotting of blood | Continuous bleeding from wounds | There is a 50 percent probability of a carrier female to transfer the disease to sons |
| Sickle cell anemia | Autosome-linked recessive | A single point mutation in the beta-globin chain of hemoglobin | Anemia | Offsprings may get the disease when both the parents are a carrier (heterozygote) |
| Thalassemia | Autosome-linked recessive | Mutation in the genes HBA1 and HBA2 present on chromosome 16 | Formation of abnormal hemoglobin molecule resulting in anemia | Offsprings may get the disease when both the parents are a carrier (heterozygote) |
| Phenylketonuria | Autosome-linked recessive | Lack of an enzyme that converts phenylalanine to tyrosine | Mental retardation. Accumulation and excretion of phenylalanine and its derivatives in urine | Offsprings may get the disease when both the parents are a carrier (heterozygote) |
Read More: Difference Between Carpel and Pistil
Chromosomal Diseases
Chromosomal Diseases are caused by an abundance of, a lack of, or an abnormal arrangement of chromosomes. There are two types of chromosomal disorders: aneuploidy and polyploidy.
- The benefit or loss of one or more chromosomes is known as aneuploidy.
- During anaphase of meiosis, chromatid segregation failure causes it.
- Polyploidy in plants is a natural phenomenon.
- This is due to an increase in the body's chromosome count.
- When cytokinesis fails, polyploidy results.
Some examples of chromosomal abnormalities are as follows:
Down's Syndrome
The trisomy of chromosome 21 causes Down's syndrome. Symptoms include mental retardation, short stature, a furrowed tongue, and a slightly open mouth.
Klinefelter's Syndrome
A total of 47 chromosomes are present in Klinefelter's syndrome, one of which is an extra X chromosome (XXY). They have feminine characteristics such as breast growth and are sterile, tall, and masculine in appearance (gynecomastia)
Turner's Syndrome
Turner's syndrome is described by the presence of 45 chromosomes. XO, one of the X chromosomes, is missing. Females are sterile, have a short stature, and have underdeveloped sexual characteristics.
Read More:
| Class 12 Biology Related Conncepts | ||
|---|---|---|
| Hormones in Animals | Biogenetic Law | Pollination by Water |
| Heterotrophic Nutrition | Tap Root System | Chromoplasts |
Things to Remember
- Principles of Inheritance and Variation explain the transfer process of genes from one generation to another.
- Genetics help study about the functioning of inheritance from parents to offspring.
- Mendel proposed three laws of inheritance after an experiment on peas.
- Mutation specifies the modification of DNA sequence within the living organism.
- Down syndrome and Turner syndrome are genetic disorders caused due to Chromosomal Diseases.
Read More: Vestigial Organs
Previous years questions
- Persons who are colourblind cannot distinguish
- A child of blood group O cannot have parents of blood groups
- An example for codominance
- Cri-du-Chat (cry of the cat) syndrome occurs in human babies due to
- Find out the correct statement
- In Morgan's experiments on linkage, the percentage of white eyed, miniature winged recombinants in F generation is
- In XO type of sex determination
- Which of the following is not a Mendelian disorder ?
- Which of the following is incorrectly paired ?
- Chromosome complement with 2n−1 is called
- Functional unit of mutation is:
Sample Questions
Ques: What is Pedigree Analysis? (3 Marks)
Ans.Pedigree analysis is a process that is used to determine whether the mode of inheritance is dominant, recessive, partial dominant, autosomal, mitochondrial or sex-linked.
- It observes the inheritance pattern of a particular trait present in various generations of an individual.
- It evaluates the probability of a genetic disorder in offspring.
- Pedigree analysis is like a chart representing the family tree and their genetic traits.
- Genes are responsible for determining the characteristics from parents to offspring.
Ques: What is Frameshift Mutation? (3 marks)
Ans.Frameshift Mutation happens when one or more pairs of bases in DNA are added or removed. It alters the reading frame of triplet codons, which code for particular protein amino acids. The mutation is caused by the insertion and deletion of DNA.
- This will shift the reading of the existing DNA sequence.
- The addition and subtraction take place due to the presence of nucleotides.
- It is also known as reading frame shift or framing error.
- It analyses the function of proteins in all organisms.
Ques: Is Polygenic inheritance affected by environmental conditions? (2 marks)
Ans. Yes. Polygenic inheritance is affected by environmental conditions. The genotype is responsible for setting the traits of organisms, but environmental factors are responsible for deciding the phenotype within its genetic limits. The environmental controls the activity of genes by setting their state to ON and OFF. The required range of phenotype under a different set of environmental conditions is called the norm of reaction.
Ques: What is the use of Pedigree Analysis? (1 mark)
Ans. Pedigree Analysis is used to track down the origins of a specific trait, abnormality, or disease.
Ques: What are the main reasons why Mendel selected the Pea plant for his experiments? (4 marks)
Ans. The main reasons why Mendel selected the pea plant for his experiment are as follows:
- Pea has a lot of different personalities.
- Flowers with reproductive whorls enclosed by the corolla display self-pollination.
- Pea plants have a short lifespan.
- Artificial cross-pollination of pea flowers is simple.
- The resulting hybrids were fertile.
Ques: What is the difference between homozygous and heterozygous? (4 marks)
Ans. The difference between homozygous and heterozygous are as follows:
| Homozygous | Heterozygous |
|---|---|
| Homozygous have two copies of same alleles for a particular set of traits. | Heterozygous have two copies of different alleles for a particular set of traits. |
| It produce only one set of gamete. | It produce two sets of gamete. |
| The process is either homozygous dominant or recessive. | The process is complete heterozygous recessive and dominant. |
| It consists of only one type of allele. | It consists of different type of allele. |
Ques: State three law of Mendels? (3 marks)
Ans. Mendels proposed three law of inheritance which are as follows:
- Law of Dominance: The law of Dominance states that when two parents with pure and contrasting traits are crossed together, one trait is visible in the next generation.
- Law of Segregation: The law of Segregation states that when the production of gametes takes place, two copies of heredity factors are segregated so that the gamete has the property of one of the parents.
- Law of independent assortment: The law of independent assortment states that alleles are inherited by the parent-offspring, with different traits independent of each other.
Ques: What is the difference between codominace and incomplete dominace? (4 marks)
Ans. The difference between codominace and incomplete dominance are as follows:
| Codominance | Incomplete Dominance |
|---|---|
| Codominance is a process when two parent phenotypes are expressed together. | Incomplete Dominace is a process when two parents phenotype are blended together. |
| The two alleles are neither recessive nor dominant in nature. | The one alllele is either dominant or recessive in nature. |
| The hybridisation will not form a new phenotype. | The hybridisation will form a new phenotype. |
| For example: Individuals having ABO blood group. | For example: Snapdragon |
Ques: Explain the type of mutation? (2 marks)
Ans. Mutation is divided into two categories which are as follows:
- Germline Mutation: Germline Mutation is a type of mutation that occur in gametes. These type of mutation is transmitted to offspring with the help of mutation.
- Somantic Mutation: Somantic Mutation is a type of mutations that occur in the different cells of the body. These type of mutation is transferred to offspring. The process of mutation will effect the organism.
Ques: Name some diseases that is caused by Mendalian Condition? (4 marks)
Ans. The diseases that is caused by Mendalian conditions are as follows:
- Colour blindness: In this daughter will be color blind only if the father is color blind. It is caused by defect in the green or red cone of the eye.
- Sickle cell anemia: In this offsprings may get the disease when both the parents are a carrier. It is caused by single point mutation in the beta-globin chain of hemoglobin.
- Haemophilia: In this there are 50 percent probability of a carrier female to transfer the disease to sons. It is caused by defect in one protein involved in the clotting of blood.
- Phenylketonuria: In this problem like mental retardation. Accumulation and excretion of phenylalanine and its derivatives in urine is experienced. It is caused by lack of an enzyme that converts phenylalanine to tyrosine.
Ques: Explain the terms aneuploidy and Polyploidy? (2 marks)
Ans. Aneuploidy: Aneuploidy is a type of chromosome abnormalities in extra amount of chromosomes are present in the haploid cells. They are of three types namely monosomy, disomy, trisomy, and tetrasomy.
Polyploidy: Polyploidy is a type of chromosome abnormalities extra amount of chromosomes are formed during cell division specially in plants, They are of two types namely are autopolyploidy and allopolyploidy.
Read More:
Comments