Genetics is a branch of biology that sure is a must-know topic for NEET. The topic weights around 3 to 4 questions every year. The two chapters from class 11th and 12th that come under genetics are the Principle of inheritance & variation and the Molecular basis of inheritance. The principle of inheritance & variation is trickier and weighs slightly a bit more compared to the molecular basis of inheritance. 

• A slight knowledge about the history of genetics is also important while going through NEET Biology Syllabus. 

• Principle of inheritance deals with the events that took part from 1850 to 1900. The period is evaluated as Mendel period as he started his experiments in 1856. 

• The second chapter, the molecular basis of inheritance deals with the number of events that proved DNA to be genetic material. 

This chapter also looks upon further discoveries and concepts such as replication, transcription and translation which are important and need to be covered while preparing for NEET 2021. This article is to give brief detailed notes regarding chapters related to genetics. The detailed notes below are very useful during the last minute revision of genetic chapters. 

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What are the Principles of inheritance and variations?

 The simple and basic topics to know before starting of the chapter are: 

1. Heredity: Heredity is major process that transmits heritable traits or characters from parents to the offspring.

2. Genetics: A subsequent branch of biology that deals with principles as well as the mechanism of inheritance and variation.

3. Inheritance: It is the basis of heredity and by which traits are passed from parents to the offsprings. The regular continuity of the gene pool is supervised and controlled by the process of inheritance.

4. Genes: Genes localise themselves at the chromosomes and are known as basic unit of inheritance.

5. Variation: The process of variation is seen among individuals of one species. Variations are caused majorly due to crossing over, mutation, gene recombination and environmental effect on genes located on chromosomes.

Must Read: 

• NEET Study Notes for Human Health and Disease
• NEET Study Notes for Microbes in Human Welfare

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Mendel's law of inheritance

Gregor johann mendel, popularly known as the father of genetics was the one performed experiments on garden pea. 14 true-breeding pea plants that have 7 distinguishable characters with 2 different traits were used for the experiment. While performing the experiment, he quoted genes as ‘factors’ and genes that code for a couple of contrary traits as ‘alleles'. 

Why did Mendel choose pea plant to perform the experiment on? 

1. The life span of the pea plant is short compared to other plants.

2. Flowers of the pea plant are self-pollinated.

3. Pea flowers were hermaphrodite as well as bisexual in nature.

Sample Question

1) Which of the following traits of garden pea is studied by Mendel, was a recessive feature? 

a)Round seed shape

b) Axial flower position

c) Green seed colour 

d) Green pod colour

Answer: B

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Laws based on Mendel Experiment

1. Law of dominance: Only one allele of all other alleles is dominant and gets expressed in the case of heterozygous. To prove this, he also gave an experimented example, when homozygous tall(TT) and dwarf(tt) plants are crossed, the offsprings that are produced are all tall plants with the genotype of Tt. Here, tallness is the dominant trait expressed over dwarfism.

2. Law of segregation of genes: When the process of meiosis takes place at the time of gamete formation, each allele gets separated. No blending is noticed and characteristics are passed to different gametes. A basic point to remember here is, homozygotes produce only one kind of gametes while heterozygotes produce different kind of gametes.

3. Law of independent assortment: The law states that ‘alleles for different traits are inherited independently'. Mendel explained this law using a hybrid cross.

Sample Question

Question: Mendel's law of independent assortment can be demonstrated by?

a)Test cross

b) Back cross

c) Monohybrid cross

d) Dihybrid

Answer: C

Question: The law of segregation is based on the fact that, 

a)The alleles do not show any blending 

b) There is random fertilization 

c) Gametes are pure for the trait

d) The zygotes are diploid 

Answer: A

What is ‘test cross'? 

Test cross is a process that helps in finding out the genotype of plants showing the dominant traits. The two observations to make a note here are:

1. In offsprings, if the phenotypes show only the dominant trait, then the parent plant is believed to be homozygote to the dominant trait.

2. When both the phenotypes are noticed in the offsprings produced, then the parent plant is believed to be heterozygote to the dominant trait.

Incomplete dominance

• Incomplete dominance is noticed when the heterozygote/offspring doesn’t exactly resemble any of the parent but a mixture of two parent's traits. This happens when neither of two alleles are dominant as the phenotype of offspring has no resemblance of any parent.

• A perfect example taken here is The flower colour inheritance of snapdragon or dog flower. When two true-breeding flowers are crossed, we get all the offsprings of the colour pink in the F1 generation. When these offsprings are self-pollinated, they give red:pink:white flowers at the ratio of 1:2:1 during F2 generation.

Co-dominance

• In an individual, when both the alleles express themselves together, that is when they are said to be co-dominant.

• A perfect example for this situation is the inheritance of ABO blood group in humans that is being controlled by gene I. The gene I is said to be having three allelic forms namely, IA, IB and I. In an individual any two of the three allelic forms are said to be present. IA and IB are said to be dominant over i whereas IA and IB are said to be co-dominant over each other.

Sample Question

Question: Possible blood group of children born to parents having A and B blood groups are

a) O and A

b) A, B and AB

c) O, A, B

d) O, A, B and AB

Answer: B

Question: Incase of incomplete dominance, F2 generation has

a)Genotypic ratio equal to phenotypic ratio

b) Genotypic ratio is 3:1

c) Phenotypic ratio is 3:1

d) None of the above 

Answer: A

Must Read: 

• NEET Study Notes for Molecular Basis of Inheritance
• NEET Study Notes for Biological Classification
• NEET Study Notes for Reproduction

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Chromosomal theory of inheritance

1. Mendel's observations were supported by Sutton and Boveri leading them to state that chromosomes are the carriers of the genes.

2. Here, chromosomes arise as a homologous pair and at the same region, the two alleles of a gene are located on the homologous pair of chromosomes.

3. When meiosis takes place during the process of gamete formation, the homologous chromosomes gets separated.

4. Chromosomes are segregated and assorted independently.

5. When fertilization happens, gametes get combined and offsprings with a diploid number of chromosomes are produced which is similar to that of parents'.

6. Morgan periodically worked on fruit flies Known as Drosophila melanogaster that widely provided evidence to prove and state chromosomal theory of inheritance.

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Linkage and recombination

What is linkage? 

• Linkage are physical association of genes located on the chromosomes.

• During a dihybrid cross, when the two genes are tightly linked or found on the same chromosome rather than the parental combination, it is said to be more prevalent than non-parental combinations or recombinants.

• The linkage and recombination are directly dependent or proportional to that of the distance between a pair of genes. Hence, a statement ‘more the distance, greater is the probability of recombination’ is stated. 

What are multiple alleles? 

• When a trait/character is regulated by more than two alleles, that is known as multiple alleles.

• What is polygenic inheritance – When a trait is judged by multiple numbers of independent genes of those having a similar or additive effect on the trait. That is known as polygenic inheritance. It is also affected by environmental conditions.

• In a situation where a single gene controls many phenotypic traits, that is known as pleiotropy. The different phenotypic expressions are vastly a result of the effect that is due to a gene on the metabolic pathway.

Sex determination

There are noticed several systems of sex determination is varied organisms.

1. The first person to notice and identify the chromosome was Mr.Henking. He observed X-chromosome and named it as X-body.

2. Sex chromosomes are nothing but the chromosomes that determine the development of sexual characters while the rest of the chromosomes which do not determine are known as autosomes.

3. If a male individual produces two varied types of chromosomes, that situation/condition is known as male heterogamy. Few animals that are lf this type are humans, grasshopper, drosophila and etc. 

4. If a female individual is said to produce two varied kinds of chromosomes, that condition is known as female heterogamy. Example of such condition are birds.

Sex determination of honey bees in detail: The determination system that is being followed is a haplo-diploid system.

How are female honey bees produced? 

Female honey bees are known as Queen or worker bees. These are formed by the fusion of egg and sperm. The number of chromosomes present are diploid(32).

How are male honey bees formed? 

Male honey bees are also known as drones. These are formed by unfertilized eggs by a process called parthenogenesis. The number chromosome present are haploid(16) and the sperms are produced by the mitosis process. 

Sample Question

Which of the following statements are correct regarding chromosomal basis of sex determination?

1. The mechanism of sec determination in insects is XO type.

2. XX insect is female while XO insect is male.

3. The X chromosome is called sex chromosome because it was involved in sex determination.

4. Grasshopper has XY type of sex determination 

5. Man has XX type of sex determination 

a) 4 and 5

b) 1,2,3,4 and 5

c) 1,2 and 3

d) 1, 4 and 5

Answer: C

Must Read: 

• NEET Study Notes for Biomolecules
• NEET Study Notes for Ecosystems

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What is Mutation and its tyes?

Any noticeable changes occurring in the sequence of DNA is known as mutation. The viable mutation has the ability to inherited from generation to generation. When a mutation occurs, it has the ability to cause changes in the genotype as well as the phenotype of an organism. The mutation is linked to various diseases but not all mutations cause disease. The mutation is said to occur while changes like deletion, insertion, duplication and substitution occur and it is known to be a major cause of cancer. UV rays are said to one of the mutation induction agents/mutagens. 

There are two types of mutations: 

1. Point mutation: In this type of mutation, there is a substitution in single base pair of DNA.

2. Frameshift mutation: This type of mutation is a result of insertion or deletion of one or more pairs of bases in DNA.

Sample Question

Question: Ultimate source of variation.

e) Fertilization

f) Mitosis

g) Meiosis

h) Mutation

Answer: D

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Genetic disorders and their types

Several disorders are noticed in human beings that are inherited and cause due to mutation occurring in genes or alterations of the chromosomes. 

Pedigree analysis is a form of analysis that helps in identifying and determining the risk of getting the genetic disorder in the offsprings by studying and analysing the pattern of inheritance of a particular trait existing in various generation of an individual. 

Genetic disorders can be put into two various groups:

1. Mendelian disorder: These are the disorders that are caused due to alteration in a single gene and follows the same inheritance pattern similar to Mendel’s law, hence the name. The pedigree analysis helps in tracing the inheritance pattern and to determine if the trait is dominant of recessive. Diseases that are classified under Mendelian disorders are colour blindness, haemophilia, sickle cell anemia, thalassemia and phenylketonuria

2. Chromosomal disorders: Diseases classified under chromosomal disorders are caused due to excess/abnormal/absent of chromosomes. 

They are further of two types: 

1) Aneuploidy: This type of chromosomal disorder is caused when there’s loss or gain of only one chromosome caused due to failure of segregation of chromatids during anaphase of the meiosis.

2) Polyploidy: This type of chromosomal disorder if often seen in plants that occurs due to an increase in the full set of chromosomes. 

Few chromosomal disorders are down's syndrome, Klinefelter syndrome and turner's syndrome. 

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Previous Years Solved Sample Questions

Question: In Mendelian dihybrid cross, when heterozygous round yellow are self crossed, round green offsprings are produced which are represented by?

 a) RrYy, RrYY, RRYy

 b) RrYY, RRyy, rryy

 c) rrYy, rrYY

 d) RRyy, Rryy

Answer: D

Question: A person with turner's syndrome has;

a) 45 autosomes and X sex chromosomes

b) 44 autosomes and XYY sex chromosomes

c) 45 autosomes and XYY sex chromosomes

d) 44 autosomes and X sex chromosomes

Answer: A

Question: A pure tall plant is crossed to dwarf plant F1 generation consists of only tall plant while F2 generation has both tall and dwarf in the ratio of 3:1. The phenomenon is due to?

a) Dominance 

b) Co-dominance

c) Incomplete dominance 

d) Heredity

Answer: A

Question: Colour blindness is an _________ linked recessive trait

a) X-chromosoms

b) Y-chromosome

c) Z-chromosome

d) None of the above.

Answer: A

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Keys to overcome the troubles faced while solving the questions

• Start with well-acknowledged questions since that reduces the fear of failing and develops confidence.

• Read out the question and options mentioned more than just once to get a clear cut idea.

• Solving previous question papers help a lot.

• Don't try to attempt questions that you're not 100% sure with since when gone wrong, marks are been detected.

There are a few example questions that candidates can expect in NEET. Students are advised to be well prepared and to have thorough knowledge about every chapter and topic in NCERT is much necessary. The notes of various chapters and topics can be used while revision during the last few days with examination coming up. All the very best.

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