Principles of Inheritance and Variations

Important Words

Genetics: The branch of biology that deals with the study of genes are called genetics. Gregor Johan Mendel is often regarded as the "Father of Genetics.
Gene: The paired factor that controls a particular trait of a character is called a gene. Or As per molecular concept the segment of DNA that codes for a character is called a gene.
Allele or Allelomorph: The alternative form of the gene for a particular trait is called an allele.
Dominant: The trait which appears both in homozygous as well as heterozygous conditions is called dominant. The dominant alleles are denoted by capital letters.
Recessive: The traits which appear only in homozygous conditions are called recessive. Recessive alleles are represented in the small letter of the respective dominant.
Homozygous: When both the alleles for particular traits are the same.
Heterozygous: When both the alleles for a particular trait are different, one dominant and the other recessive.
Parental generation: The generation of plant from where wedding programs begins. It is represented by P.
F1 or First filial generation: The generation of the plant obtained after crossing the parental generation is called the F1 generation.
Cross: When pollination is man-made it is called the cross.
Self cross: When the F1 generation plant is crossed with the F1 generation itself it is called a self cross.
Back cross: When the F1 generation plant is crossed with the parental generation first type of process is called back cross.
Test Cross: When the F1 generation plant is crossed with a recessive parent such type of back cross is called test cross.

A Pea plant with a round seed was crossed with a wrinkle where the round is dominant over a wrinkle. Find out the phenotypic and genotypic ratio when the F1 generation was test crossed.

Phenotypic ratio= 1:1 Genotypic ratio= 1:1

Emasculation: The process of removal of the male part from a bisexual flower in order to prevent self-pollination is called emasculation.
Bagging: After the cross, the process of covering the flower with butter or parchment paper is called bagging.
Tagging: The process of attachment of a tag where date and time, name of the plant, name of the emasculator is mentioned.
F2 or Second filial generation: The generation of the plant obtained by crossing the F1 generation is called the F2 generation.
Monohybrid cross: When the cross is made between one pair of contrasting traits, it is called a monohybrid cross.

When a tall plant is crossed with a dwarf where tall is dominant over a dwarf.

Find out the phenotypic and genotypic ratio in F2 generation when F1 generation is self crossed?

Phenotypic ratio= 3:1 Genotypic ratio= 1:2:1

Dihybrid cross: When the cross is made between two pairs of contrasting traits is called a dihybrid cross.

When a pea plant with round and yellow seed is crossed with wrinkle and green. Find out the phenotypic and genotypic ratio when F1 generation was self crossed.

Phenotype: The external character of an organism is called the phenotype.
Genotype: The genetic constitution of a phenotype is called genotype.

Mendel's Law of Inheritance

After doing a monohybrid cross Mendel proposed three laws of Inheritance.

Law of Unit Factor: This law states that characters of a trait are controlled by factors that occur in pairs.
Law of Dominance/ Recessive: This law states that the trait which occurs both in homozygous, as well as heterozygous conditions, is called dominant and the trait which appears only in the homozygous condition is called recessive.
Law of Segregation/Separation: This law states that during the formation of gamete separation of factors takes place independently in each gamete.

After doing dihybrid cross Mendel give Law of independent assortment

Law of independent assortment: In dihybrid cross during the formation of gamete segregation of unit factor of opposite character takes place independently in each gamete.

Or This law states that when two pairs of traits are combined in a hybrid segregation of one pair of character is independent of the other pair of character at the time of gamete formation.

Characters studied by Mendel in Pea:-

Characters	Dominant trait	Recessive trait
Height	Tall	Dwarf
Seed shape	Round	Wrinkle
Seed color	Yellow	Green
Pod shape	Full	Constricted
Pod color	Green	Yellow
Flower color	Violet	White
Flower position	Axial	Terminal

Incomplete dominance:

The phenomena in which the dominant allele is not completely dominant over recessive due to which an intermediate character is obtained in heterozygous condition, this process is called incomplete dominance.

For Example When a 4 O'clock plant having red flower was crossed with white where red is dominant over white all the plants in the F1 generation were pink flowers when the F1 generation plant was self crossed red, pink and, white were obtained in a ratio of 1:2:1, the gene that controls redness is not totally dominant over gene controlling whiteness due to which and intermingle character is obtained in heterozygous conditions.

Genetic basis of incomplete dominance: Gene Express themselves by producing proteins, in incomplete dominance the dominant allele does not suppress the recessive allele due to which an intermediate type of protein is produced which is responsible for intermingle character.

Codominance and Multiple Allele Effect:

Co-dominance

When the two alleles neither show dominant or recessive relationship nor show intermediate condition but both of them express themselves simultaneously in the heterozygous condition, this phenomenon is known as co-dominance.

Multiple allele

Most of the gene occurs in two alternative forms called alleles that controls the same character but certain gene may exist in more than two alternative forms, this form of allele is called multiple allele.

Various Types Of Inheritance

Pleiotropy

Chromosomal theory of Inheritance

This theory was proposed by Sutton and Boveri independently in 1902 which was supported by T.H Morgan, C.B Bridge and Stuartvent.

This theory believes that chromosomes are the unit of heredity that contains gene or Mendelian factors which segregate during gamete formation and transmits characters from one generation to another.

The main postulates of this theory are-

Bridge between one generation to another is sperm and ovum.
Both sperm and ovum contributes equally in the heredity of offspring.
Every chromosomes have a definite role in the development of offspring and loss of a part or entire chromosome causes disorders.
Both chromosomes as well as gene occurs in pair.
Diploid condition of both chromosomes as well as Mendelian factor occurs during zygote formation.
Homologous chromosomes synapses during meiosis and then separated or segregate independently in different cell(gamete) which stabilizes the principle of independent assortment.

Parallelism (similarity) between Genes (Mendelian factors) and chromosomes

Both chromosome as well as Mendelian factors are transmitted from generation to generation without any change.
In diploid cells, chromosomes occurs in homologous pairs. Genes also occurs in allelic pairs.
During gametes formation meiosis takes place and chromosome numbers is reduced to half.
The diploid condition is restored during zygote formation.
Both chromosomes as well as Mendelian factors follow law of segregation.
Both chromosomes and Mendelian factors shows independent assortment.

Linkage

The phenomenon by which genes remain linked together in a chromosome is called linkage.
The genes which are linked together are called linked genes.
Linkage was first told by Sutton and Boveri but it was explained in detail by Morgan, who also proposed chromosomal theory of linakge.

The main points of chromosomal theory of linakge are

The linked genes are present on the same chromosome.
All the linked genes remains linearly arranged.
The strength of linkage is inversely proportional to the distance between them.
There is tendency to maintain parental combination except during crossing over.

Linkage is of two types

(1 ) Complete linkage The type of linkage where genes are completely linked to one another and there is no crossing over, is known as complete linkage.

Eg. It was seen in male Drosophila. Morgan when crossed purple eyed normal winged homozygous dominant male (PPNN) with white eye vestigial wing recessive female (ppnn) , all the F1 generation had purple eye normal wing. When F1 generation male was crossed with recessive female, F2 generation had only parental type characters.

From the above experiment they concluded that genes in chromosomes of male Drosophila are completely linked to one another and there is no crossing over, hence complete linkage come into existence.

(2) Incomplete linkage It is the phenomenon of an occasional crossing over between two homologous chromosomes so that one or more alleles present in a linkage group are replaced by other alleles. It produces both parental (more than 50%) and recombinant individuals (less than 50%).

Eg.

Crossing over/ Recombination

Factors affecting crossing over

Crossing over is directly proportional to distance between Genes.
The frequency of crossing over decreases with age.
Crossing over increases with physical radiation like x-ray, u.v rays.

Mechanism of Sex determination

Mutation

Genetic Disorder

Red green Colourblindness (Daltonism)

It is a x linked recessive disorder in which human being cannot differentiate between red and green colour.

Colourblindness occurs due to recessive gene "c" present on x chromosome. Males are always colourblind (XcY) since Y chromosome cannot supress the gene for colourblindness.

Females may be carrier (XcX) where symptoms of disease do not appear but can transmit the disease.

Females are colourblind only when both the x chromosome contains gene for colourblindness (XcXc).

The defect occurs in about 8% of the males and 0.4% of the females.

Marriage between normal visioned male and carrier woman produces normal girls, carrier girls, colour blind boys and normal boys in the ratio 1:1:1:1.

Haemophilia (Bleeder's disease)

It is a sex linked recessive disorder discovered by John Otto (1803). In this disease there is severe bleeding even when there is a small cut, so it is also known as Bleeder's disease.

The defect has been inherited by many royal families of Europe through queen Victoria, therefore it is also known as Royal disease.

It occurs due to a defective gene "h" present on the x chromosome.

Males are always haemophilic (XhY) since Y chromosome cannot supress the gene for haemophilia.
Females may be carrier (XhX) where symptoms of disease do not appear but can transmit the disease to next generation.
Females are diseased (XhXh) when defective gene present on both the x chromosomes; but dies at foetal stage.

Hemophilia is of two types

Haemophilia a - due to absence of platelet factors VIII ( antihaemophilic globulin)
Haemophilia b (Christmas disease) - due to absence of factor IX ( plasma thromboplastin)

Marriage between a carrier woman and a normal man will produce normal girl, carrier girl, normal boy and haemophilic boy in the ratio of 1:1:1:1.

Sickle cell Anemia

It is an autosomal recessive disorder which occurs due to point mutation (chromosome number 11). In this disorder RBC become sickle shaped than normal biconcave.

It occurs due point mutation when codon GAG is replaced by GUG at the sixth position of Beta globin chain that caused replacement of glutamic acid by valine and individual suffers from sickle cell Anemia.

Normal haemoglobin chain is denoted by HB(A)HB(A), carrier as HB(A)HB(S) (symptoms of disease can be seen during oxygen deficiency) and diseased as HB(S)HB(S) (dies at foetal stage or within two year after birth).

Note- This disorder can be beneficial in malaria prone area as the malarial parasite are unable to feed and multiply inside HB(S) carrying erythrocytes.

Thalassemia /Cooley's anemia

It is an autosomal genetic disorder in which there is change in the structure of either alpha, beta or delta chain of haemoglobin.

Haemoglobin is a quaternary protein that contains three types of polypeptide chain - alpha, beta and delta. If change in the structure of these polypeptide chains takes place human being suffers from thalassemia.

It shows quantitative inheritance.

In this disorder oxygen carrying capacity of haemoglobin decreases, rate of cellular respiration decreases, less energy is released a person is very lean and weak, get fatigue very quickly and sometimes may proof fatal.

Thalassemia is of three types

Alpha thalassemia
Beta thalassemia
Delta thalassemia

Alpha thalassemia

It occurs due to defective alpha polypeptide chain of haemoglobin. The alpha chain is coded by two genes - HBA1 and HBA2 present on chromosome number 16 that have 4 alleles.
When there is mutation in one of the four allele then no significant change appears in alpha chain , and individual are called silent carrier.
When mutation occurs in two allele than individual suffers from alpha thalassemia minor.
When mutation occurs in three allele than individual suffers from thalassemia Bart's where oxygen carrying capacity of haemoglobin decreases.
When all the four alleles gets mutated both the alpha chain were not formed, severe haemolytic anemia takes place (which is fatal), such type of thalassemia is called alpha thalassemia major or hydrops foetalis.

Beta thalassemia

In this type of thalassemia there is defective beta chain of haemoglobin.
The beta chain of haemoglobin is coded by a gene present on chromosome number 11 called HBB that have two alleles.
When mutation occurs in one of the allele than individual suffers from thalassemia minor and when mutation occurs in both the allele than individual suffers from beta thalassemia major or Cooley's anemia.

Delta thalassemia

The delta chain of haemoglobin is coded by a gene present on chromosome number 11 called HBD that have two allele.

When mutation occurs in one of the allele than individual suffers from delta thalassemia minor and when mutation occurs in both the allele than individual suffers from delta thalassemia major.

Phenylketonuria

It is a pleiotropic inborn error of metabolism caused by autosomal recessive allele present on chromosome number 12.
In this disease there is failure of conversation of amino acid phenylalanine to tyrosine due to lack of formation of phenylalanine hydroxylase. A number of phenylalanine derivatives are also formed viz. Phenyl pyruvate, phenyl acetate, phenyl lactate, etc.
Accumulation of these in cerebrospinal fluid leads to mental retardation.
There is increased concentration of amino acid phenylalanine in blood and urine.

Cystic Fibrosis

It occurs due to mutation in the CFTR (cystic Fibrosis transmembrane protein) gene present on chromosome number 7. The defective gene encodes defective protein that cause formation of excess mucus in the trachea and in the small intestine which provides favourable conditions for growth of bacteria. The defective protein cause release of excess of sodium chloride and such type of patients sweat is much more salteir than normal people.