New Terminologies

. Gene 

. Allele 

. Heredity

. Inheritance

. Genotype 

. Phenotype

. Haploid

. Diploid 

. Autosomes 

. Sex chomosomes

. Homologous chromosomes

. Heterologous chromosomes

What is genetics?

The study of heredity, or how the characteristics of living things are transmitted from the generation to the next 

What is heredity?

 Heredity use a process of transmission of traits from parents to their offspring’s either via asexual reproduction or sexual reproduction. These characteristics or traits are located on the chromosomes in the form of genes.

What is Variations?

  1. Individual of same special have some differences is known as variations.


An autosome is a chromosome that is not an allosome (a sex chromosome)

. For examole :-  Humans have a diploid genome that usually contains 22 pairs of autosoms and one allosome pair (46 chormos total).

Sex chromosomes

  • It hepls to determine sex. In humans, only 1 pair of sex choromsomrs exists.

What is gene and allele?

Genes are found on structures called chromosomes, long pieces of DNA  wound up around protein.

Each chromosome contains many, many genes. And a specific gene, such as the gene for eye color, is at the same location on the same chromosomes in every person. The different possible versions of the genes are called alleles.

ALLELES:- an allele is one of the possible forms of a gene. Most genes have two alleles, a dominant allele and a recessive allele.

Homologous and Heterologous chromosomes

  1. A homologous pair of chromosomes  containing a maternal and paternal chromatid joined to together at the centromere. the have the exact same gene – although may have different alleles of these genes. 

    Heterologous chromosomes 

    • Differ in shape, size or function 
    • Do not belong to the same pair 


-Greor mendel [father of genetics] started studying inheritance in peas.

– Performed experiments with pea plants for 7 years ( 1856-1863).

-Proposed laws of inheritance

-Statistical analysis and mathematical logic used.

-He chose pea plant (pisus sativum). 

-He proposed the law of inheritance in living organism.

Why mendel selected pea plant for his experiment ?

. Pea plant (Pisum sativum) was easy to cultivate. It grew well in his garden. 

. Its flowers were hermaphrodite l.e. pea  plant have both male and female sexual organs.

. It is Self pollinated in nature.

. Cross pollination is easy to be done artificially.

. It completes its lifecycle in one season. 

. Pea had many sharply distinct. its  each trait had two clear cut alternative forms or varieties:

 E.g. seed shape had a round or wrinkled phenotype, plant height was either tall or short, seed color could be yellow or green etc. Mendel called them pair of contrasting traits. He focused on seven such contrasting pair of traits.



These devices are inserted by doctors in the uterus through vagina. 

There are three types of (IUDs) available . 

  1. Non mediated (IUDs) :- these increase phagocytosis  of sperm within the uterus 

For example – the lippes consists of a thin plastip (or polyethylene ) wire bent in a series of S- shapes . it needs to be straightened when it is being inserted into the uterus but resumes its shape once inside it .

  1. Copper releasing ( IUDs) :-  along with phagocytosis of sperm , the copper ions released suttress sperm notility and fertilizing capacity of sperms .

For example :- cu T , cu7 , multiload 375 

  1. Hormone releasing ( IUDs) :- these make the uterus unsuitable for iamplantation and the servix hostile to sperms . 

For example – progestassert , LNG – 20 .

Seven Different Traits

He traits that Mendel studied are listed below: 

. Form of ripe seed (R) – smooth or wrinkled 

. Color of seed albumen (Y) – yellow or green 

. Color of flower (P)- purple or white 

. foe\rm of ripe pods (I) – inflated or constricted 

. colour of unripe pods (G) – green or yellow 

. position of flowers (A) – axial or terminal 

. Length of stem (T) – tall or dwarf 

Dominant and Recessive Trait

The following table shows each of the traits and which traits are dominant and which recessive.


Dominant Expression 

Recessive Expression

Form of ripe seed (R)



Color of seed albumen (Y)



Color of flower (P)



Form of ripe pods (I)



Color of unripe pods (G)



Position of flowers (A)



Length of stem (T)



Mendel’s Observation

. Cross Pollination between tall and dwarf pea plant.

. IN F1 generation all the off springs were tall. 

. Phenotypic ratio of pea plant is tall.

. Genotype of offsprings was different.

Reason for Mendal’s success

  1. Mendal applied statistical method and mathematical logic for analyzing his results.
  2. He kept accurate records of his experiment.
  3. Mendel experimented on a number of plants for the same trait and obtained hundreds of offspring.
  4. He tried to formulate theoretical explanation for the observed result.

Punnett Square

. It was developed by British geneticist Reginald punnett.


. A punnett square is a graphical representation of the possible genotypes of an offspring arising from a particular cross or breeding event.


. Some terms are often used in the study of genetics and these are particularly useful in understanding the function of punnett square. Among these is the term ‘allele’ and is used to denote a variant of a gene.


. For example, a pea plant can have red or white flowers and the gene dariants coding for each of these is called an allele. 

Test Cross :-

. It is the cross that is made to know whether a trait is homozygous (TT) or heterozygous (Tt) dominant by its cross with recessive trait.

. In this cross, the organism with unknown dominant genotype is crossed with the recessive parent. 

This cross finds out the homozygous and heterozygous nature of the genotype.

Example :-

Q1) :- If we want to determine the genotype of violet flowered pea plant.Then it is crossed with the recessive parent (white flowered pea plant ) instead of self  crossing.

Q2) :- A cross is carried out between two pea plant slowing the contrasting traits of height of  the  plant. The result of the cross showed 50% of parental characters. Work out the Cross with the help of punnett square.

Q3) :- Work out the cross to find out the genotype of pea plant.

Mendel’s Principle of inheritance:-

.Law of Dominance

.Law of segregation

Law of Dominance:-

. According to this law. A pair of allele  present in an individual do not get mixwd up. They get segregate or regret from each other at the time of gametogenesis and expreses its presence in the next generation. They still have distinct identity of their own.

.This law is universally applicable.

. For example:- In Tt allele, both will express in f2 generation when f1is selfing.

Incomplete Dominance

. A few cases were observed where f1 is intermediate of both Dominant and recessive phenotype.


. In incomplete dominance non of the allele whether it is dominant recessive is able to express itself completely when present in heterozygous condition.


. For example – Flower in snapdragon ( dog flower or antirrhinum majus) or mirabillis jalapa (4. 0 clock plant ) where red colour is due to gene RR white colour is due to gene rr and pink colour is gene Rr.

Multiple Allelism

 Mendel proposed that each gene has two contrasting forms i,e, Alleles but there are some genes which are having more than two alternative forms .  Presence of more than two alleles for a gene is known as multiple allelism and each are able to express itself . Example – Different types of red blood cells that determine ABO blood grouping in human beings. ABO Blood groups are controlling by gene .The gene I has three Alleles.


. In co-dominance both the alleles of hetrozygous condition are able to express itself independently.

. There is no dominant and recessive relation ship between both the allele in heterozygous condition.

Example :- AB blood group in human being both express their own types of sugars , this is known as co- dominance. 

    Where both the allele I  , I  is able to express itself independently.

Different Types Of Crosses

Monohybrid cross – 

When a cross is made between one character is called monohybrid cross.

i,e , cross between TT and tt

Dihybrid Cross –  

A cross that is made between two characters a called dihybrid cross.

Back Cross / Test Cross :- 

A cross is made between an offspring and either of the parent is called back cross.

Reciprocal cross :- 

A cross is made between dominant character of male and recessive characters of female or vice versa. 

 Example :- If a pollen (male) from tall plants is transferred to the stigma (female) of dwarf plants in one cross would use the pollen of dwarf plants to pollinate the stigma of tall plants. 

Inheritance Of Two Genes :-

. Mendel also crossed pea plants differing in two character (dihybrid cross) to verify the result of monohybrid cross. 

. A cross was made between a pure round yellow seeded plant (RRYY) with wrinkled green seeded plant (rryy).

. Yellow colour is dominant over green and round shape is dominant over wrinkled. 

Independent Assortment

. This law states that ‘when two pairs of traits are combined in a hybrid segregation of one pair of traits is independent to the other pair of traits “. 

. The punnett square can be effectively used to understand the 

Non-recognition to Mendal’s work

Mendal work remained unnoticed and unappreciated for several years due to following reasons:-

. communication was not easy in these days and his work could not be easy in these days and his work could not be widely publicised. 

 . His concepts of stable, blending discrete units or factors for various traits did not find acceptance from the contemporaries like charles Darwin etc.

. His approach of using mathematical and statistical analysis to explain biological phenomena was totally new and unacceptable to many of the biologist of that time.

. He could not provide any physical proof for that time.

. He discovery of chromosomes, mitosis and meiosis at the me of Mendel’s work.

. Mendel died in 1884.

After Mendel’s Death :-

. In 1900 three Scientist independentely rediscovered the principles of heredity already worked out by mendel.

. They were De-vries of Holland and Von Tschermak and Carl correns of Germany .

. Also by this advancement in microscopy that were taking place, scientist were able to carefully observe cell divsion.

. this led to the discovery of structure in nucleus that appeared to double and divide just before each cell division.

. By 1902, the chromosomes movement during meiosis had been worked out. 

It was found that, there is striking relationshlip between factors and the chromosomes.

Chromosomal Theory of Inheritance was proposed Independently by Sutton and Boveri

The salient features of chromosomal theory of inheritance are as follow :- 

  1. Like the heredity traits the chromosomes ratein their number , structure and individually throughout the life of an organism and from generation to generation. The two neither get lost nor mixed up.
  2. Both chromosomes as well as genes occur in pairs in the somatic or diploid cells.
  3. A gamete contains only one chromosomes of a type and only one of the two alleles of a trait.
  4. The paired condition of both chromosomes as well as Mendelian factor is restored during fertilization.

Thomas Hunt Morgan Experiment

. Thomas Hunt Morgan was awarded the nobel prize in physiology or medicine in 1933.

. The work for which the prize was completed over a 17- year period at columbia university in 1910 with his discovery of the white- eyed mutation in the fruity fly, Drosophila.

. Morgan carried out several dihybrid crosses as carried out by mendel in peas.

. He carried out several crosses in Drosophila to study the genes that are sex-linked.

. He observed that when the two genes in a dihybrid cross are located on the same chromosomes, the proportion of parental genes combination in the progeny was higher than the non-higher than the non-parental or recombination of genes.

. When the genes are loosely linked they show higher percentage of genes.

Why Morgan Used Only Drosophila Melanogaster For His Experiment?

. It could be grown on simple synthetic medium in laboratory.

. The fly has a short life cycle of about two weeks. 

. A single mating produces hundreds of offspring.

. Female are easily distinguishable from the males by the larger body size and presence of ovipositor (egg laying structure)

. It has many types of heredity variation that can be seen with low power microscopes.

Dihybrid Cross Conducted By Morgan

. He got f1 generation in the form of brown bodied red eyed female and yellow bodied white eyed male.

. He observed that all were brown bodied red eyed as per mendel law.

. In F2 generation obtaining by intercrossing of F1 hybrid . He found 98.7% to be parent 1.3% as recombinants.

Result :-

. In both crosses he found that the ratio obtained is not similar to the Mendel’s ratio of dihybrid cross i.e. 9:3:31 in F2 generation.

. Mendel’s law of independent assortment was not taking place in this case. From this morgan got the idel of linkage.


. He worked on the behaiour of the chromosomes to know the reason behind recombination. He said that these genes were located on X-chromosomes. They were present on same chromosomes so linkage concept was wined by morgan.

. Linkage genes are those genes which are present on same chromosomes. They do not show independent assortment 


. Establishment of sex through differential development in an individual at an early stage of life is called sex determination.

. Different species use very different strategies for this purpose.

. Sex of human being and insects like grass hopper, firefly, drosophila etc is determined genetically.

. Some organisms like turtles rely entirely on environment factors such as temperature for sex determination. 

Chromosomal Basis Of Sex Determination :-

. This was discovered by Henking (1891) .

. He traced a specific nuclear structure all through spermatogenesis in few insects (firefly).

. Henking also observed that only 50% of the sperms received this structure. This structure was termed X-body by him but he could not explain its significance.

. Further investigation by other scientists led to the conclusion that x body was actually a  chromosomes it was given name X-chromosomes.

. Stevens (1902) discovered Y-chromosomes X$Y chromosomes named as sex chromosomes by wilson and stevens.

. Chromosomes basis of sex determination is of following types. 

Male Heterogamety

. In this type male individuals produces two different of gametes. Thus the sperm determines the sex of offspring. 

. It involves two types of sex determining mechanism XO type.

i) XY Type

. In a large number of other insects like drosophila and mammals including human being, the male contanin two type of sex chromosomes (X and Y).

. While females possess two similar type of sex chromosomes (XX).

. Both male and female have some number of chromosomes.

. In males, Y chromosomes is often shorter than the x chromosomes.

ii) XO Type

. It is observed in large number of insects. Example :- Grasshopper. 

. Number of chromosomes are different in male and female individual. 

. It is clear that all eggs (ova) bear an additional X- chromosomes besides the autosomes while only 50% of the sperm bear X- chromosomes.

. In grasshopper, eggs fertilized by (A+O) type sperm become males.

. Therefore sperm determine the sex of offspring.

Female Heterogamety :-

. In this type female individuals produces two different types of gametes. Thus the egg determines the sex of offspring.

. It involves two types of sex determining mechanism ZW and ZO type.

i) ZW type

. In birds both the sexes possess two sex chromosomes. Unike human being the female contain heteromorphic sex choromosomes. 

. While males possess have homorphic sex chromosomes.

ii) ZO type

. It is observed in butterflies, sex determination is exactly opposite the condition found in grasshopper. 

. In this type, the females have only one sex chromosome and hence represented as ZO. 

. Females are heterogametic. On fertillzation by a Z-carrying sperm the Z-carrying ovum would develop into a male (ZZ) and on fertilization, the Z-lacking ovum would develop into a female.

. This type of sex determination is seen in some and butterflies.


. Human being have 22 pairs of autosomes and one pair of sex chromosomes. 

. All the ova (haploid) fomed are similar in their chromosomes type (22+X) Therefore females are homogametic. 

. Male individuals produces two types of sperm during the process of spermatogenesis.

. 50% of the totale sperm produces two types of sperm during the process of spermatogenesis.

. There is an equal probability of fertilization of ovem (22+X) with the sperm carrying either X and Y chromosomes.

. If ovum fertilization (22+X) type sperm ,the zygote develops into a female( 44+XX).

. If the ovum fertilizes with ( 22+Y) type sperm, the zygote develops into a male (44+XY).

. Thus, the genetic make up or the sperm determines the sex of child.

. It is also clear that in each case of pregnancy there is always 50% of probatility of either a male or female child.

Sex Determination In Honey Bea

. Honeybees show haplodiploid method of sex determination.

. Haplodiploidy is a sex determination mechanism in which males develop from unfertilized eggs and are haplod. And females develop from fertilized eggs and are diploid.

. This mode of sex determination was first discovered by Johann, a Catholic priest, in 1845.

. In honeybees, sex is normally determined by the fertilization or non-fertilization of the eggs, rather then the presence or absence of sex chromosomes.

. In honeybees, the male progeny normally develops from unfertilized eggs, which are haploid and have just one set of chromosomes

. The fertilized honeybee eggs, which are diploid and have two sets of chromosomes, differentiate into queens and worker bees.

Topics To Be Covered :-

  1. Mutation
  2. Genetic mutation
  3. Chromosomal mutation 
  4. Genomatic mutation 
  5. Non disjunction
  6. Aneuploidy 
  7. Euploidy


    1. i) Smoking, Radiations, chemicals 
    2. ii) DNA replication error 

      . Mutation, an alteration in the genetic material ( the genome) of a cell of a living organism.

      . Mutation is sudden, discontinuous variation in genotype and phenotype of an organism due to change in chromosomes and genes.

      . In addition to Recombination, mutation is another phenomenon that leads variation in DNA.

    3. . Factors which are responsible for Mutation :- 

    1. i) Smoking, Radiations, chemicals 
    2. ii) DNA replication error

    . The organisms which undergoes mutation is called mutant. The agent of mutation is called mutagen.

Different Types Of Mutation

    1. Genetic mutations :- changes in gene structure.

      Chromosomes mutations :- changes in chromosomes number or structure.

Gene Mutation

    1. . It is alteration of DNA due to change in nucleotide sequence. Gene mutation may occur due to change in a single base pair of DNA known as point mutation. 

      Example :- Sickle cell anemia 

      . Change in more than one nucleotide base pair is called gross mutation. 

      Gene mutation occur by following method :- 

      1. Substitution :- Incorrect bases matched with each other. 
      2. Insertion :- Addition of one or more bases in nucleotide chain it can lead to change in whole sequence of nucleotide.
      3. Deletion :- removal of bases pair from the sequence.

Chromosome Mutation

    1. Alteration in the sequence and number of genes on chromosomes can cause such mutations. 

      . Duplication :-

      A part chromosomes is duplicated such that there are more genes on it.

      . Deletion :-

      A part of chromosomes is deleted.

      . Inversio :-

      A part of chromosomes is broken and added inversely.

      . Translocation :

      A part of non homologous chromosomes is attached to the chromosomes and vice versa. 


    1. . It is change in chromosome number that bring about visible effects on a the phenotype. 

      It is of two types :- 

      Aneuploidy :-

      In aneuploidy any change in number of chromosomes is an organis would be different that the multiple of basic set ol chromosomes.

      • It commonly arise due to non-disjuction absence of separation of two homologous chromosomes during of two homoiogous chromosomes during cell division ) of the two chromosomes of homologous pair.
      • So that one gamete comes to have an extra chromosomes (n+1)while the other becomes deficient in one chromosomes (n-1).
      • Fusion of these gametes (n-1 or n+1) with normal gametes (n) give rise to different types of aneuploids.
      • Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell divsion.

Gametic fusion

Types of aneuploid



2n-1 monsomic 


(n-1) (n-1)

2n-2 (nullisome condition)


n (n+1)

2n-1 (trissomic condition)


(n+1) (n+1)

2n+2 (tetrasomic condition)

Example of aneuploidy:-
  • Trisomy – Down’s syndrome, klinefelter’s syndrome.
  • Monosomy-tumer syndrome.

(b) Euploidy-

. In Euploidy any change in the numbers of chromosomes is the multiple of the multiple of the number of chromosomes is basic set 

. These may be haploidy or polypiody.

  (I) Haploidy-

. One set of chromosome.

. there is only one allele of each gene present in each cell.

(ii) Polyploidy-

. More than two set of chromosomes.

. Failure of cytokinesis after telophase stage of cell division results in an increase is a whole set of chromosomes in an organism and this phenomenon is called as polyploidy.

. It is often seen in plants.

. In case of animais, polyploidy is rare in animal.

Genetic Disorder

A disease or problem present from birth due to genetic abnormalities

The genetic disorders can be divided as :

The disorders are transmitted in next generation accoration to the principle of inheritance and can be studied by pedigree analysis. These can be dominant or recessive

Pedigree Analysis

 A technique to study inheritance of traits over several generations in a family.

. It is useful to determine the possibility of an offspring with a genetic disorder.

. To determine weather the trait is dominant or recessive.

. Slmilary the trait may also be linked to the chromosomes as in case of haemophillia.

Mendelian Disorders

Mutation in single gene.

They are transferred to offsprings as per Mendelin principle of inheritance.

Inheritance pattern similar to the principles of mendelian inheritance.

Example :- Haemophilia, sickle cell anaemia, phenylketonuria, colour blindness

Chromosomal Disorder

 Abnormal arrangement of one or more chromosomes.

Example :- Down’s syndrome , Turner’s syndrome, klinefelter’s syndrome

Autosomal Dominant Trait :-

In autosomal dominant trait, the character condition can be seen in parent and then there is 50% of chances of transfer of their defect from parent to offspring may be girl or boy.

Sex Linked :-

. Dominant X Linked

. Recessive X linked

. Y linked 

Topics To Be Covered This Video :-

Mendelian disorders

  1. i) Colourblindness 
  2. ii) Haemophillia 

iii) sickle cell anemia 

  1. iv) Phenyketonuria
  2. v) Thalassemia

Chromosomal disorder

    1. Down syndrome 
    2. Klinefelter’s syndrome

    iii. Turners syndrome

Mendelian disorder

    1. . Mutation in single gene. 

      . They are transferred to offfsorings as per mandolin principle of inheritance. 

      1. Color Blindness 
      2. Haemophillia

      iii. Sickle cell anemia 

      1. Phenyketonuria
      2. Thalassmia

Mendelian disorder

    1. . Mutation in single gene. 

      . They are transferred to offfsorings as per mandolin principle of inheritance. 

      1. Color Blindness 
      2. Haemophillia

      iii. Sickle cell anemia 

      1. Phenyketonuria
      2. Thalassmia

 Color Blindness

. Color blindness is a recessive sex linked trait in which eye fails to distinguish red and green colours.

. The normal gene its recessive allele carried by X- chromosomes.

. In females colours blindness appears only when both the sex chromosomes carry the recessive gene. (XX)

. The females have normal vision but function as carrier if a single recessive genes for colour blindness is present (XX).

. human males the defect appear in the presence of a single recessive (XY).


. It X- linked recessive trait.

. Due to absence of defection form of blood clotting factor (protein) exposed blood of affected individuals fails to coaguate.

. The possibility of a female becoming a heamophillic is extremely rare because month of such a female has to be atleast carriers and the father should be haemophillic.

. Haemophillic female dies before birth. 

. The family of queen Victoria shows a number of haemophillc descendent as she was a carrier of the disease.

. The person suffering from this disase cannot synthesize a normal blood protein called antihaemophillic globulin (AHG) required for normal blood clotting.

. Therefore ever a very small cut may lead to continuous bleeding for a long time.

. This gene is located on X- chromosomes and is recessive.


. It is an autosomal recessive disarse therefore it can be transmitted from parents to the offspring when both male and female individuals are carrier (heterozygous) for the gene.

. The disease is controlled by a single pair of allele , Hb and Hb .

. Thus three genotypes are possible in

. Heterozygous individual appear unaffected but they are carrier of the disease as there is 50% probability of transmission of mutant gene progeny , thus exhibit sickle cell trait 

.The disease is caused by mutation of the gene controlling 

. It cause change in shape of RBC from biconcave shape to elongated sickle cell structure.

. It is autosomal recessive trait.

. The affected individual lacks a liver enzyme called phenylalanine into tyosine.

. As a result of this phenylpyruvic acid and other derivatives.

. Accmulation of these in brain results in mental retardation. 


. It is a autosomal recessive trait.

. It is responsible for lysis od cell and in anaemia.

  1. Chromosomal disorder

. Down’s syndrome

. Klinefelter’s syndrome

. Tuner’s syndrome

  1. Question time

    1. The Infected Individuals are sterile.
    2. Lacks Female secondary sexual characteristic. 
    3. Poor development of breast,

      Chromosomal disorder

      . Disorders can also be created by imbalance in chromosome number and chromosomal rearrangement these are called as chromosomal disorders.

      Down’s syndrome

      . It was first described in 1866 by langdon down. 

      . This disorder develops due to trisomy of chromosome number 21.

      Features :-

      Individual is short 

      Small round head 

      Furrowed tongue 

      Partially open mouth 

      Broad palm 

      Big and wrinkled tongue

      Physical , mental development is retarted 

      Klinefelter’s Syndrome

      . It is called due to presence of an additional copy of X- chromosomes on sex chromosomes (XY) in males and resulting into 44 + XXY type chromosomes. 

      . Such a persons are sterile males with overall.

      . Have masculine development.

      . Some female characteristics like feminine pitched voice , development of breast.

      Turner’s Syndrome 

      . The disorders is due to monosomy (2n-1).

      . It is caused in females due to absence of One X- Chromosomes.

      . The females with this syndrome has XO Sex chromosomes.

      Symptoms :-

    Important questions :

    Q1. A child suffering from thalassemia is born to a normal couple. But the mother is being blamed by the family for delivering a sick baby. 

    1. What is thassmia ?
    2. How would you counsel the family not to blame the mother for delivering child suffering from disease ? Explain.