Table of Contents

Mendel’s Laws of Inheritance

Incomplete Dominance

Chromosomal Theory of Inheritance

Sex Determination

Mutation

Genetic Disorders

| Name of the NEET Sub-Section | Topic | Notes Helpful For |

| Biology | Principles of Inheritance and Variation | NEET Exams |

Principles of Inheritance and Variation - Important Points, Summary, Revision, Highlights

Principles of Inheritance and Variation

Heredity is the process by which traits are passed down from parent to offspring.

Genetics is the branch of biology that focuses on the principles and mechanisms of inheritance and variation.

The process of Inheritance is essential for the preservation of the genetic pool, as it allows for the transfer of traits from parents to their offspring. This process is the foundation of heredity.

Genes are the basic unit of inheritance and are located on chromosomes.

There is a range of differences between individuals of the same species. This variation is caused by the process of crossing over, recombination, mutation, and the influence of the environment on the activity of genes located on chromosomes.

| Don’t miss: All NEET Question Papers |

Mendel’s Laws of Inheritance

Gregor Johann Mendel is known as the “Father of Genetics”.

Mendel performed experiments on Garden peas, taking 14 true-breeding plants with seven distinguishable characters that had two opposite traits.

He referred to genes as “factors”, which are inherited from parents to their offspring.

Genes that code for a pair of opposite traits are called “alleles”.

Gregor Mendel gave three laws of inheritance based on his observation:

1. This is a statement.

2. Law of Dominance: When two different alleles are present in a genotype, one of the alleles is dominant and will be expressed in the phenotype. For example, when homozygous tall (TT) and homozygous dwarf (tt) plants are crossed, all the offspring will be tall (Tt) and tallness is dominant over dwarfness.

3. Law of Segregation of Genes: During meiosis, each allele separates and is passed to different gametes, resulting in no blending of characters. Homozygotes produce only one type of gamete, while heterozygotes produce different types of gametes.

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1. Law of Independent Assortment: He demonstrated this law through a dihybrid cross, which states that alleles for different traits are inherited independently.

Test Cross: It is to find out the genotype of the plant showing dominant trait. To do this, the given plant is crossed with the recessive homozygote. The two observations are:

If the phenotype of the offspring only displays the dominant trait, then the parent plant was homozygous for the dominant trait.

If the offspring produced are of both the dominant and recessive phenotypes, then the parent plant was heterozygous for the dominant trait.

Incomplete Dominance

When neither of the two alleles is dominant and the phenotype of the heterozygote does not resemble any of the parents, the heterozygote expresses intermediate or a mixture of two parents’ traits.

The flower colour inheritance of snapdragon (dog flower) can be seen when crossing true breeding red (RR) and white (rr) flowers. The F1 generation will have all pink colour flowers, and self-pollination will result in a ratio of 1:2:1 for red: pink: white flowers in the F2 generation.

Co-dominance

When both alleles are expressed in an individual, they are referred to as co-dominant.

The ABO blood group in humans is controlled by the gene I which has three allelic forms, IA, IB and i. IA and IB code for different kinds of sugar polymers present on the surface of RBC and ‘i’ does not produce any sugar. IA and IB are dominant over ‘i’, but IA and IB are co-dominant and express themselves together.

Chromosomal Theory of Inheritance

Sutton and Boveri supported Mendel’s observations and stated that chromosomes are the carriers of genes.

Chromosomes occur in homologous pairs and the two alleles of a gene can be found at the same locus on each of the homologous chromosomes.

Homologous chromosomes separate during meiosis in the process of gamete formation.

Chromosomes segregate and assort independently

During fertilization, gametes combine to create offspring with a diploid number of chromosomes, similar to the parents.

Morgan extensively worked on fruit flies, Drosophila melanogaster, and provided experimental evidence to support the chromosomal theory of inheritance.

Linkage and Recombination

The physical association of genes located on a chromosome is known as linkage.

In a dihybrid cross, if the two genes are tightly linked or present on the same chromosome, then the parental combination is more frequent than non-parental combinations or recombinants.

The greater the distance between a pair of genes, the higher the likelihood of linkage and recombination.

Multiple Alleles: When a trait is determined by more than two alleles, such as the ABO blood group, it is referred to as multiple alleles.

Polygenic Inheritance: When a trait is determined by multiple genes that have either a similar or cumulative effect on the trait, it is known as polygenic inheritance. Examples of this include eye color, skin pigmentation, height, and hair color.

Polygenic inheritance can be influenced by environmental factors as well.

Pleiotropy: When a single gene influences multiple phenotypic traits, it is referred to as a pleiotropic gene. These different phenotypic expressions are often caused by the gene’s impact on metabolic pathways.

A single gene mutation in the gene coding for the enzyme Phenylalanine Hydroxylase can lead to Phenylketonuria, a disease characterized by mental retardation, reduced hair and skin pigmentation.

Learn About Gene Interaction Here

Sex Determination

Different organisms have different systems of sex determination.

Henking was the first to observe the X chromosome and he named it the “X body”.

The chromosomes that determine the development of sexual characters are known as sex chromosomes, while the rest of the chromosomes are known as autosomes.

When a male produces two distinct types of gametes, it is known as male heterogamety, for example in humans, grasshoppers, drosophila, etc.

When the female produces two different kinds of gametes, it is known as female heterogamety, e.g. birds.

| Sex Determination in Different Animals |

Name	Male	Female

Human	XY	XX
John	54	22

| Drosophila | XY | XX |

| Grasshopper (insects) | XO | XX |

Birds	ZZ	ZW
Sparrow	32	12
Hawk	34	10

Sex Determination in the Honey Bee:

Haplo-diploid Sex-Determination System

Female (queen or worker) is formed by the fusion of an egg and sperm and has diploid (2n) number of chromosomes.

• Male (drone) is formed from an unfertilized egg through parthenogenesis and has haploid (16) number of chromosomes. Mitosis is responsible for the production of sperms.

Mutation

Any changes in the sequence of DNA is called a mutation. Viable mutations can be passed down from one generation to the next. A mutation can alter both the genotype and phenotype of an organism.

It is linked to various diseases, but not all mutations are detrimental

Mutations, which can be caused by changes such as deletion, insertion, duplication, and substitution, are the major cause of cancer. There are many agents, known as mutagens, which can induce mutations, such as UV rays.

There are two types of genetic mutation:

1. Point Mutation: In the case of Sickle Cell Anemia, there is a substitution of a single base pair in the DNA. This results in the 6th codon of the gene coding for the 𝛃-globin chain of Haemoglobin changing from GAG to GUG, which causes a substitution of Glutamic Acid with Valine.

2. Frameshift mutation: It results from the insertion or deletion of one or more base pairs in DNA. This changes the reading frame of triplet codons, which code for certain amino acids of the protein.

Genetic Disorders

There are many disorders in humans that are inherited, caused by mutation in genes or alteration in chromosomes.

Pedigree Analysis can be used to assess the likelihood of a genetic disorder being passed down through multiple generations of a family by examining the pattern of inheritance of a particular trait.

Genetic disorders can be categorized into two types:

1. Mendelian Disorders

These disorders are caused by changes in a single gene

Mendel’s law states that it follows the same inheritance pattern.

Pedigree analysis can help trace the inheritance pattern and also ascertain whether the trait is dominant or recessive.

| Mendelian Disorders |

| 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 colour | A daughter will be colour blind only if the father is colour blind |— title: “Important Notes For Neet Principles Of Inheritance And Variation” link: “/important-notes-for-neet-principles-of-inheritance-and-variation” draft: false

Table of Contents:

Mendel’s Laws of Inheritance

Incomplete Dominance

Chromosomal Theory of Inheritance

Sex Determination

Mutation

Genetic Disorders

Name of the NEET Sub-Section	Topic	Notes Helpful For

| Biology | Principles of Inheritance and Variation | NEET Exam |

Principles of Inheritance and Variation - Important Points, Summary, Revision, Highlights

Principles of Inheritance and Variation

Heredity is the process through which traits are passed down from parents to their offspring.

Genetics is the branch of biology that focuses on the principles and mechanisms of inheritance and variation.

The process of Inheritance is essential for the continuity of the gene pool, as it allows for traits to be passed from parents to their offspring. This process of heredity is the basis of the gene pool.

Genes are the basic unit of inheritance and are located on chromosomes.

Variation among individuals of one species is due to crossing over, recombination, mutation and environmental effects on the expression of genes present on chromosomes.

| Don’t miss: All NEET Question Papers|

Mendel’s Laws of Inheritance

Gregor Johann Mendel is referred to as the “Father of Genetics”.

Mendel performed experiments on Garden pea, taking 14 true-breeding plants with seven distinguishable characters, each having two opposite traits.

He referred to genes as “factors”, which are inherited from parents to their offspring.

“Alleles” are genes that code for a pair of opposite traits.

Gregor Mendel gave three laws of inheritance based on his observation:

1. One

2. Law of Dominance: When two different alleles (T and t) are present in a heterozygote (Tt), one allele (T) is dominant and its trait (tallness) is expressed in the phenotype. For example, when homozygous tall (TT) and dwarf (tt) plants are crossed, all the offspring will be tall (Tt) and tallness is a dominant trait over dwarfness.

3. Law of Segregation of Genes: During meiosis, each allele separates and passes to different gametes, resulting in no blending of characters. Homozygotes produce only one type of gamete, while heterozygotes produce different types of gametes.

You should check out: NEET Key Answer 2022

1. Law of Independent Assortment: He demonstrated this law through a dihybrid cross, which states that alleles for different traits are inherited independently.

Test Cross: It is used to determine the genotype of a plant exhibiting a dominant trait. The given plant is crossed with a homozygous recessive plant. The two observations are:

If the phenotype of the offspring only shows the dominant trait, then the parent plant was homozygous for the dominant trait.

If the offspring produced are of both the dominant and recessive phenotypes, then the parent plant was heterozygous for the dominant trait.

Incomplete Dominance

When neither of the two alleles is dominant and the phenotype of the heterozygote does not resemble any of the parents, the heterozygote expresses intermediate or a mixture of two parents’ traits.

The flower colour inheritance of Snapdragon (dog flower) when a true breeding Red (RR) and White flower (rr) are crossed, results in all Pink coloured flowers in the F1 generation. On self-pollination, the F2 generation will have Red: Pink: White flowers in the ratio of 1:2:1.

Co-dominance

When both alleles are expressed in an individual, they are referred to as co-dominant.

The ABO blood group in humans is controlled by the gene I which has three allelic forms - IA, IB and i. IA and IB code for different kinds of sugar polymers present on the surface of RBC while ‘i’ does not produce any sugar. IA and IB are dominant over ‘i’, but they are co-dominant and hence express themselves together.

Chromosomal Theory of Inheritance

Sutton and Boveri supported Mendel’s observations and stated that chromosomes are the carriers of genes.

Chromosomes occur in homologous pairs, and the two alleles of a gene are located on the corresponding chromosomes of the pair at the same locus.

Homologous chromosomes separate during meiosis in the process of gamete formation

Chromosomes segregate and assort independently

During fertilization, gametes combine to create offspring with a diploid number of chromosomes, which is similar to that of the parent.

Morgan extensively worked on fruit flies, Drosophila melanogaster, and provided experimental evidence to support the chromosomal theory of inheritance.

Linkage and Recombination

**Physical association of genes located on a chromosome is known as **Linkage.

In a dihybrid cross, if the two genes are tightly linked or present on the same chromosome, the parental combination is more likely to appear than non-parental combinations or recombinants.

The farther apart a pair of genes are, the higher the probability of linkage and recombination.

Multiple alleles: When a trait is governed by more than two alleles, such as the ABO blood group, it is known as a multiple allele system.

Polygenic Inheritance: When a trait is determined by multiple, independent genes that have a cumulative or additive effect on the trait, it is known as polygenic inheritance. Examples include eye color, skin pigmentation, height, and hair color.

Polygenic inheritance can be further modified by environmental factors.

Pleiotropy: When a single gene has an effect on multiple phenotypic traits, it is known as a pleiotropic gene. These various phenotypic expressions are usually the result of the gene’s influence on metabolic pathways.

A single mutation in the gene that codes for the enzyme Phenylalanine Hydroxylase can lead to the disease Phenylketonuria, which is characterized by mental retardation, and reduced hair and skin pigmentation.

Learn More About Gene Interaction Here

Sex Determination

Different organisms have different systems of sex determination.

Henking was the first to observe the X chromosome and he named it the “X body”.

The chromosomes that determine the development of sexual characters are known as sex chromosomes, whereas the rest of the chromosomes are known as autosomes.

When the male produces two different types of gametes, it is known as male heterogamety, for example, humans, grasshoppers, drosophila, etc.

When the female produces two different kinds of gametes, it is known as female heterogamety, e.g. birds.

| Sex Determination in Different Animals |

Name	Male	Female

Human	XY	XX

Drosophila	XY	XX

| Grasshopper (insects) | XO | XX |

Birds	ZZ	ZW

Sex Determination in Honey Bees:

Haplo-diploid Sex-Determination System

Female (queen or worker) is formed by the fusion of an egg and sperm and has diploid (2n) number of chromosomes.

• The male (drone) is formed from an unfertilized egg through parthenogenesis and has haploid (16) chromosomes. Mitosis is the process by which sperm is produced.

Mutation

Any changes in the sequence of DNA is called a mutation. Viable mutations can be passed down from one generation to the next. A mutation alters both the genotype and phenotype of an organism.

It is linked to various diseases, but not all mutations are detrimental

Mutations, caused by changes such as deletion, insertion, duplication, and substitution, are the major cause of cancer. Mutagens, such as UV rays, are agents that can induce mutations.

There are two types of genetic mutation:

1. Point Mutation: In the case of Sickle Cell Anemia, there is a substitution in a single base pair of DNA, resulting in the 6th codon of the gene coding for the 𝛃-globin chain of Haemoglobin changing from GAG to GUG, which leads to the substitution of Glutamic Acid by Valine.

2. Frameshift mutation: It results from the insertion or deletion of one or more base pairs in DNA. This change alters the reading frame of triplet codons, which code for certain amino acids of the protein.

Genetic Disorders

There are many inherited disorders in humans caused by mutations in genes or alterations in chromosomes.

Pedigree Analysis can be used to assess the likelihood of a genetic disorder appearing in the progeny of an individual by analyzing the transmission of a certain trait across multiple generations.

Genetic disorders can be categorized into two types:

1. Mendelian Disorders

These disorders are caused by a change in a single gene

Mendel’s law states that it follows the same inheritance pattern.

Pedigree analysis can help trace the inheritance pattern and also determine whether the trait is dominant or recessive.

| Mendelian Disorders |

| Name | Genetic Trait | Cause | Effects | Inheritance Pattern |

Condition	Mode of Inheritance	Description	Symptoms	Inheritance Pattern
Colour Blindness	X-linked Recessive	Defect in the green or red cone of the eye	Unable to discriminate between red and green colour	A daughter will be colour blind only if the father is colour blind