Class 12 Biology Chapter 4 is “Principles of Inheritance and Variation”. Here, you will get chapter 4 study materials, notes along with important questions with answers. You can not only study this online but can get all of these study materials in PDF form for free download. This will help you to understand the chapter 4 more easily and learn quickly.
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Class 12 Biology Book pdf free download – https://ncert.nic.in/textbook.php?lebo1=1-13
Class 12 Biology Chapter 4 “Principles of Inheritance and Variation” notes
In this chapter, we delve into the important contributions of Gregor Mendel, who is known as the father of modern genetics. Mendel conducted groundbreaking experiments with pea plants, which allowed him to establish Mendel’s laws of inheritance. This chapter covers a wide range of topics, from Mendel’s discoveries to gene expression, genetic mutations, and the role of DNA in heredity. By studying inheritance patterns, genetic crosses, and molecular mechanisms, students can gain a deeper understanding of the factors that drive evolutionary change and the genetic diversity within populations.
Mendel’s Laws of Inheritance:
Gregor Mendel, an Austrian monk, is considered the father of modern genetics for his important research on inheritance in pea plants. Mendel’s experiments led to the creation of three key laws of inheritance, which formed the basis of our knowledge of genetics.
Law of Segregation:
Mendel’s first law explains that during the formation of gametes, alleles for each trait separate from each other so that each gamete carries only one allele for each trait. This means that each parent gives one allele for each trait to their offspring, and the alleles are randomly sorted into gametes during meiosis. This law clarifies why offspring inherit traits from their parents in predictable ratios, as seen in Mendel’s monohybrid crosses.
Law of Independent Assortment:
Mendel’s second law states that alleles for different traits assort independently during gamete formation. This means that the inheritance of one trait does not affect the inheritance of another trait. This law is applicable when genes for different traits are on separate chromosomes or are far apart on the same chromosome, allowing them to assort independently during meiosis. The law of independent assortment explains the inheritance patterns observed in Mendel’s dihybrid crosses, where two traits are considered at the same time.
Law of Dominance:
Although not one of Mendel’s original laws, the principle of dominance is often linked to his work. The law of dominance explains that in a heterozygous individual (having two different alleles for a trait), one allele (the dominant allele) masks the expression of the other allele (the recessive allele) in the phenotype. Only when an individual has two recessive alleles does the recessive trait show in the phenotype. For instance, in Mendel’s pea plant experiments, the dominant allele for yellow seed color covered up the expression of the recessive allele for green seed colour.
Types of Crosses:
Monohybrid Crosses: The study of inheritance patterns for a single trait is involved in monohybrid crosses. These crosses occur between individuals that differ in only one trait.
Dihybrid Crosses: Dihybrid crosses involve the study of inheritance patterns for two different traits simultaneously. Mendel’s experiments with pea plants revealed the principle of independent assortment through dihybrid crosses.
Test Cross: A test cross is used to determine the genotype of an organism with a dominant phenotype. It involves crossing the organism with a homozygous recessive individual.
Incomplete Dominance: Incomplete dominance occurs when the heterozygous phenotype is intermediate between the two homozygous phenotypes. For example, in snapdragons, crossing red flowers with white flowers produces pink flowers.
Co-dominance: Co-dominance occurs when both alleles for a gene are fully expressed in the phenotype. An example of co-dominance is seen in the blood groups of humans, where alleles for A and B blood types are co-dominant.
Multiple Alleles: Multiple alleles refer to the existence of more than two alternative form(alleles) for a gene in a population. The ABO blood group system in humans is controlled by three alleles: IA, IB, and i.
Pleiotropy: Pleiotropy occurs when a single gene affects multiple phenotypic traits. Sickle cell anemia is an example of pleiotropy, where a mutation in the hemoglobin gene affects multiple traits such as red blood cell shape and oxygen-carrying capacity.
Polygenic Inheritance: Polygenic inheritance occurs when a trait is controlled by multiple genes. This results in a continuous variation of phenotypes. Human height, skin colour, and eye colour are examples of traits influenced by multiple genes.
Linkage:
Linkage is when genes that are close together on the same chromosome tend to be inherited together. Even though genes on the same chromosome can separate if they are far apart, genes that are closely linked usually stay together during meiosis. The strength of the linkage between genes depends on how close they are to each other on the chromosome: the closer they are, the stronger the linkage.
Crossing Over:
During prophase I of meiosis I, crossing over takes place. This process involves the swapping of genetic material between homologous chromosomes, which leads to the mixing of alleles. Crossing over results in new combinations of alleles on chromosomes, which brings about genetic diversity in offspring. Chiasmata, the locations where crossing over happens, physically link the homologous chromosomes and aid in the exchange of genetic material. By breaking the physical linkage between genes on the same chromosome, crossing over allows for the independent assortment of alleles during meiosis.
Sex Determination:
Sex determination is the process in which an organism develops as either a male or a female. This process is found in many species, including humans. In humans, sex determination is mainly influenced by the presence or absence of specific sex chromosomes, namely X and Y. Females typically have two X chromosomes (XX), while males have one X and one Y chromosome (XY). When the Y chromosome is present, it triggers the development of male characteristics during embryonic development. On the other hand, the absence of the Y chromosome leads to the development of female characteristics.
Sex-linked Inheritance:
Sex-linked inheritance refers to the way genes are inherited on the sex chromosomes (X or Y). In humans, most of the genes that are sex-linked are located on the X chromosome, while fewer genes are found on the Y chromosome. Since males have only one X chromosome (XY), they will express any recessive allele present on the X chromosome. This is because there is no corresponding allele on the Y chromosome to mask its expression. Some common examples of sex-linked traits in humans include color blindness, hemophilia, and Duchenne muscular dystrophy. Females can be carriers of sex-linked recessive traits if they inherit one normal allele and one affected allele on their two X chromosomes. They will only express the trait if they inherit two affected alleles.
Mutation:
Mutation are changes in an organism’s DNA sequence. They can happen on their own or be caused by things like radiation, chemicals, or viruses.
Different Types of Mutation:
- Point Mutation: involve altering a single nucleotide base pair. This can be through substitutions, insertions, or deletions.
- Chromosomal Mutation: involve changes in the structure or number of chromosomes. Examples include deletions, duplications, inversions, and translocations.
Mechanism of Mutation:
- Spontaneous Mutation: occur naturally during DNA replication or repair. This can be due to replication errors, changes in nucleotide bases, or DNA damage from metabolic by-products.
- Induced Mutation: caused by external factors like radiation, chemicals, or mutagenic compounds. These can damage DNA directly or disrupt replication and repair processes.
Applications of Mutation:
- Genetic Research: Mutations are important for studying gene function, genetic diseases, and evolution.
- Biotechnology: Mutations are used in genetic engineering to create GMOs, develop new traits in crops, and produce valuable proteins and drugs.
- Medical Diagnosis and Treatment: Mutations help diagnose genetic disorders, predict disease risk, and create personalized therapies.
Chromosomal disorders:
Chromosomal disorders are genetic conditions that happen when there are problems with the structure or number of chromosomes. These disorders can occur because of mistakes during cell division, like nondisjunction, translocation, deletion, or duplication of chromosomes. Here are some important things to know about chromosomal disorders:
Different Types of Chromosomal Disorders:
- Aneuploidy: This is when someone has an abnormal number of chromosomes, either too many (trisomy) or too few (monosomy).
- Polyploidy: This happens when someone has extra sets of chromosomes, like triploidy (three sets) or tetraploidy (four sets).
- Structural Chromosomal Abnormalities: These occur when there are changes in the structure of chromosomes, like deletions, duplications, inversions, or translocations.
Examples of Chromosomal Disorders:
- Down Syndrome (Trisomy 21): Caused by an extra copy of chromosome 21, leading to intellectual disability, characteristic facial features, and increased risk of certain health problems.
- Turner Syndrome (Monosomy X): Found in females with only one X chromosome, resulting in short stature, infertility, and specific physical features.
- Klinefelter Syndrome (XXY): Affects males with an extra X chromosome, leading to infertility, reduced testosterone production, and developmental delays.
- Cri du Chat Syndrome: Caused by a deletion on chromosome 5, resulting in intellectual disability, unique facial features, and a high-pitched cry similar to a cat’s cry.
- Philadelphia Chromosome: Arises from a translocation between chromosomes 9 and 22, leading to chronic myeloid leukemia (CML) due to the fusion of the BCR and ABL genes.
Genetic disorders:
Genetic disorders are conditions that occur when there are changes or mistakes in a person’s DNA. These disorders can be passed down from parents or can happen spontaneously.
Types of genetic disorders are as follows:
Single-gene disorders: It occurs when there are mutations in a single gene. Some examples of single-gene disorders are cystic fibrosis, sickle cell anemia, Huntington’s disease, and Duchenne muscular dystrophy.
Chromosomal disorders: It occur when there are abnormalities in the number or structure of chromosomes. Down syndrome (trisomy 21), Turner syndrome (monosomy X), and Klinefelter syndrome (XXY) are examples of chromosomal disorders.
Multifactorial disorders: They are a result of a combination of genetic and environmental factors. Some examples of multifactorial disorders are diabetes, heart disease, and certain types of cancer.
Mitochondrial disorders: They are caused by mutations in the DNA of mitochondria, which are inherited from the mother. Leigh syndrome and mitochondrial myopathy are examples of mitochondrial disorders.
Class 12 Biology Chapter 4 “Principles of Inheritance and Variation” notes PDF Free Download
For your better understanding of the chapter 4, the notes is given in the PDF format below. Hope, the notes given in these PDF help you to better understand the chapter and to solve the extra questions based on the chapter. Notes of the chapter is given in the PDF form. You can either read it online or download in PDF for free and study offline.
Class 12 chapter 1 notes –
Class 12 Biology chapter 4 “Principles of Inheritance and Variation” important questions with answers are also given below. You can directly download it, in the PDF form. It will help you to understand the chapter better.
Chapter 4 “Principles of Inheritance and Variation” Important Questions with Answers
For your better understanding of the chapter 4, students must study the question based on the chapter. It will give you the idea to better understand the chapter. Here’s given some important very short, short and long types of questions with answers. Click the link below to download the PDF directly. You can also visit the CBSE 12 Board Previous questions papers page to directly download 5-10 years papers with answer. It will help you to understand the types of questions come in the CBSE 12 Board exam. For your better preparation for the exam, don’t skip to practise previous years papers along with sample papers.
Click in the link to directly download the important questions with answer –
Short types questions with answers
Long types questions with answers
Study Materials for remaining Class 12 Biology chapters:
Unit 6: REPRODUCTION
Chapter 2 – Human Reproduction
Chapter 3 – Reproductive Health
Unit 7: GENETICS AND EVOLUTION
Chapter 4 – Principles of Inheritance and Variation
Chapter 5 – Molecular Basis of Inheritance
Chapter 6 – Evolution
Unit 8: BIOLOGY IN HUMAN WELFARE
Chapter 7 – Human Health and Disease
Chapter 8 – Microbes in Human Welfare
Unit 9: BIOTECHNOLOGY
Chapter 9 – Biotechnology : Principles and Processes
Chapter 10 – Biotechnology and Its Applications
Unit 10: ECOLOGY
Chapter 11 – Organisms and Populations
Chapter 12 – Ecosystem
Chapter 13 – Biodiversity and Conservation
Students can get all the study materials that are required for class 12 Biology in Your Mentor Guru website. So, click in the links above to get all the chapters study materials including note and important questions with answer of class 12 Biology in PDF format to study offline. It is free PDF download. And don’t forget to check out previous years question papers of CBSE 12 Board exam.
Also check out –
- Class 12 Chemistry Study Materials
- Class 12 Physics Study Materials
- Class 12 English Study Materials
- Class 12 Mathematics Study Materials