PRINCIPLES OF INHERITANCE & VARIATION

GENERAL TERMINOLOGY:

Ø  GENETICS: - The branch of Biology dealing with Inheritance & variation of characters from parent to offspring.

Ø  INHERITANCE (HERIDITY) is the process by which characters are passed from parent to Offsprings.

Ø  VARIATION is the degree by which Progeny differs from its Parents.

Ø  Through artificial selection & domestication from Wild Cows, Indian Breeds were produced eg. Sahiwal (Punjab).

Ø  Gene is a segment of DNA capable of self expression eg. Gene for Eye Colour. ALLELES are alternate form of genes coding for contrasting Traits eg. Genes for different eye colours like Black, Blue, Grey, Green etc.

Ø  CHARACTER is a sharp Observable feature eg. Height of Plant. TRAITS are different variables of character eg. Tall Plant, Dwarf Plant, Medium Heighted Plant etc..

Ø  PHENOTYPE is the external/observable features & GENOTYPE is the genetic makeup of the individual eg. A phenotypically TALL Plant may be genotypically expressed as TT or Tt.

Ø  The Trait which express phenotypically in the presence of others (Heterozygous condition) is DOMINANT. The trait which remains masked / hidden in the presence of others is RECESSIVE.

Ø  In a cell, When both the alleles for a character are same it is called HOMOZYGOUS eg. TT . When both the alleles for a character in a cell are different then it is HETEROZYGOUS eg. Tt.

Ø  BACK CROSS: - A cross where the Hybrid/ offspring is crossed with either of its parents.

Ø  TEST CROSS: - A Back Cross, where offspring is crossed with Recessive parent. It is to test the zygocity of the offspring. If the cross results in 100% same result then offspring was HOMOZYGOUS & if the result is 50%-50% or 1:1 then the offspring was HETEROZYGOUS for the character.

 

( Test Cross )

Ø  PUNNET SQUARE: - A graphical representation (developed by British Geneticist, R.C Punnet), to calculate the probability of all possible genotypes of Offsprings in a genetic cross.

      MENDEL’S WORK: -

 

(Gregor John Mendel)

ü  Gregor John Mendel (1856-63) conducted Hybridization Experiments on Garden Pea (Pisum sativum).

(Pisum sativum)

ü  His experiments had large sample size which gave greater credibility to his data collection. Also, He confirmed the results of his experiments through experiments on successive generations.

ü  CHARACTERS STUDIED BY MENDEL IN PEA: -  (1) Stem Height -- tall/dwarf (2)Flower Colour –Violet/White      (3) Flower Position – Axial/Terminal (4) Pod Shape – Inflated/Constricted (5) Pod Colour – Yellow/Green (6) Seed Shape – Round/Wrinkled (7) Seed Colour – Yellow/Green.

ü  TRUE/PURE BREEDS:- The Breeds (Plants/animals) which produce same character/trait continuously for many generations, when it is SELFED (crossed with same type of character/trait). Homozygous for that character.eg. TT.

ü  HYBRIDS: - Produced by crossing between two different traits (Heterozygous for that character) eg. Tt

ü  F₁ GENERATION: - First Filial or First Generation produced by Parents.

   MENDEL’S MONO-HYBRID CROSS: -“A cross where inheritance of only one character is observed/studied”.

1.       Mendel obtained PURE/TRUE breeds of contrasting traits of 7-characters in pea plant, by continuous SELFING.

2.       He used PURE BREEDS as Parent (P) Generation with one pair of contrasting character eg. Height of Plant.

3.       On crossing a Tall & a Dwarf plant, The F-1 Generation was Tall (100%).

4.       On Selfing F-1 Generation, The result was roughly 75% Tall & 25% Dwarf (3:1). None of them were of medium height. (Similar results were observed in the study of other Traits). Contrasting Traits do not show BLENDING.

5.       Based on his results of F1 &F2 Generations Mendel Postulated that some “FACTOR” (later known as Gene) was passed down the generations unchanged, from parents to Offsprings.

6.       Mendel Self Pollinated F2 plants & found that Dwarf plants continued to generate dwarf offsprings. He concluded that the genotype for dwarf plant was Homozygous.

7.       Mendel performed Test Cross between F1 Tall plant and its Dwarf Parent to find that F1 generation was Hybrid Tall (HETEROZYGOUS) as the test cross resulted in 1:1 result.

   Why Mendel Chose Garden Pea (Pisum sativum) for his experiments on genetics?

      Garden Pea has following Qualities- annual plant, easy to grow & low maintenance cost, Closed Bisexual Flower, large number of offsprings (seeds), many Contrasting Traits present.

   Why Mendel’s result were so accurate/ without any exceptions?

      LINKAGE did not interfered in his result as the Seven Characters he chose, luckily, the genes for those characters were present on seven different pairs of chromosomes (Garden pea has only seven pairs of chromosomes in its cells i.e. 2n = 14)

      MENDEL’S LAWS: - (Three laws)

1.       LAW OF DOMINANCE: - “In a Heterozygous Condition, only one Allele express itself Phenotypically (DOMINANT) while the other remains hidden/Masked in its presence (RECESSIVE)”.  Eg. Tt is tall plant. The recessive express only in Homozygous condition eg. tt for dwarf plant.

2.       LAW OF SEGREGATION: -“ The Two alleles of a gene do not remain together forever, but separates during gamete formation”. For any character, a cell will have only two alleles. These separates/segregates at the time of gamete formation (Haploid) and only one allele goes into the gamete.

3.       LAW OF INDEPENDENT ASSORTMENT: -“ When two characters are considered simultaneously (Dihybrid Cross), the inheritance (chance distribution in gametes) of alleles of one character  is independent to that of the other character”.

OR

“When two pairs of Traits are combined in a hybrid, segregation of one pair of characters is independent to the other pair of characters”.

Both characters have paired alleles which will segregate in two gametes only so that the alleles of both will be in same gametes, but pairing is purely on the basis of chance factor.

Ø  While First & The Second Laws could be explained on the basis of Monohybrid/Dihybrid cross, The Third Law of Independent Assortment could ONLY be explained on the basis of a Dihybrid Cross (at least).

      INCOMPLETE DOMINANCE: -

“ When in a Mendelian Cross, Hybrid do not express completely and the Phenotypic ratio comes out as 1:2:1 (and not 3:1)”.

ü Eg. Dog Flower (Snapdragon or Antirrhinum spp.), 4 O’clock (Mirabilis jalapa) etc..

(Antirrhinum spp.)

ü In a Mendelian cross between True Breeding RED Flower (RR) & True Breeding White flowered Plant (rr), the F-1 Hybrid was PINK (Rr) and not red.

ü On Selfing F-1 Hybrid (Pink), The F-2 Generations resulted in 25% Red, 50% pink & 25% White  plants.

ü As gene ‘R’ was not completely dominant during Heterozygous conditions, the Phenotypic Ratio was 1:2:1.

      CO-DOMINANCE: -

 “ In a Heterozygous condition, both the Alleles express simultaneously”.

ü Eg. ABO Blood Group system in Human, Coat colour in Guinea Pig etc..

(ABO System: Codominance & Multiple Allelism)

ü ABO Blood Group system in Human is also an example of MULTIPLE ALLELE (a phenotypic expression controlled by more than two alleles. Another example is eye colour in Drosophila melanogaster / Fruit Fly is regulated by 15 alleles).

ü ABO Blood Group is regulated by a gene ‘I’ with alleles I^A, I^B & i. Here, I^A & I^B produce their Glycoprotein A & B whereas ‘i’ do not produce any Glycoprotein on the cell membrane of R.B.C.

ü Since there are three different allele which can produce Six different combinations (Genotypes) resulting in Four types of Blood Groups.

ü When alleles I^A & I^B  are present together in the RBC both express equally due to Codominance and produce a blood Group called AB. 

   GENE PRODUCT WITH MULTIPLE EFFECT:-

eg. Starch Synthesis in Pea Seed controlled by gene ‘B’. & ‘b’.

Ø Homozygous Dominant (BB) produce Large starch grains, ‘bb’ produce small starch grains & ‘Bb’ seeds are intermediate in size { so for Starch Grain Production as Phenotype, the alleles show Incomplete Dominance}.

Ø After Maturation, BB seeds are ROUND & bb seeds are WRINKLED.  {Shape of Seed → Dominant/ Recessive}.

Ø Hence, Dominance is not an autonomous feature of a gene, it depends on product phenotype. In certain cases a gene may influence more than one Phenotype (PLEOTROPIC GENE).

  MENDEL’S DIHYBRID CROSS: -

“When, in a Genetic Cross, TWO characters are studied/ Observed together”.

1.    Mendel produced pure breeds for Two dominant characters & its Two recessive characters.

2.    On crossing F-1 Generation was “Hybrid Dominant” [ as in Mono-Hybrid Cross].

3.    On Selfing, F-2 Generation results in the Phenotypic Ratio of 9:3:3:1 for RY, Ry, rY & ry alleles.

(Mendel's Di hybrid Cross)

Ø The ratio for Dihybrid cross comes from the expression of Two Monohybrid crosses, Round: Wrinkled (3:1) and Yellow: Green (3:1). Hence, Round Yellow (9); Round Green (3); Wrinkled Yellow (3); Wrinkled: Green (1).

Ø GENOTYPIC RATIO will be 1:2:1:2:4:2:1:2:1.

   POLYGENIC INHERITANCE:

 Traits which are controlled by more than two genes. Eg. Human Skin Colour is controlled by three genes (AABBCC). It is also a case of QUANTATIVE INHERITANCE, where quantity of dominant & recessive genes determine the shade of colour for skin.

   PLEOTROPY: Phenomenon exhibited by single gene showing multiple Phenotypic effect. Eg. Gene for Phenylalanine, due to mutation codes for Phenylketonurea( characterized by mental retardness & loss of body pigmentation). 

( Some pleotropic expressions of Genes)

    CHROMOSOMAL THEORY OF INHERITANCE

( SUTTON & BOVERI, 1902): -

( W. Sutton )           (T. Boveri)

1.       Chromosomes as well as Gene occur in pairs.

2.       The Behavior of Chromosome is parallel to the behavior of Genes.

3.       The two alleles of gene are located on the Homologous Chromosomes.

     THOMAS HUNT MORGAN

( Thomas Hunt Morgan)

(Father of Experimental Genetics on animals)

Ø  Worked on Fruit Fly Drosophila melanogaster.

Ø REASONS FOR CHOOSING Drosophila: - Could be grown on simple synthetic medium in laboratory, Life Cycle ~ One Week, Large number of Progeny, Clear Sexual Dimorphism,  many types of hereditary Variations that can be seen by low power microscope, total chromosomes four pair (2n =8).

 

(Drosophila melanogaster)

    LINKAGE & RECOMBINATION( coined by T.H Morgan): -

      Linkage: - Physical Association of Two Genes on a Chromosome over generations.

      Recombination: - Generation of Non Parental Gene Combination in next generation, mainly due to Meiosis but also due to mutations.

      Possibility of Linkage is Inversely Proportional to the distance between the two genes.

      Possibility of Recombination is Directly Proportional to the distance between the two genes.

      Alfred Sturtevant (student of Morgan) mapped the position of genes on chromosome. Today “Genetic Maps “are extensively used as a starting point in sequencing of while Genome (as in HGP).

  

 SEX DETERMINATION: -

Ø Henking (1891) discovered X – Chromosome.

Ø Chromosomes which determines the sex of an organism are called Sex Chromosomes eg. X- Chromosome. All the other chromosomes present, are called AUTOSOMES.

Ø MALE HETEROGAMY: - (A) XY/XX Type : - (eg. Man, Drosophila) Males have XY pair of sex chromosomes while females have XX pair. (B) XO/XX Type: - (eg. Most Insects) the sperm with X chromosome fuse with egg to form Female while the sperm without X chromosome results in Males.

Ø FEMALE HETEROGAMY: - ZW/ZZ type: -(eg. Birds) dissimilar sex chromosomes present in females while similar set present in males (To demarcate the nature of Heterogamy, terms Z & W are used)

Ø SEX DETERMINATION IN HUMAN: - 2n = 46 (23 pairs) = 22 pairs (Autosomes) + 1 Pair Sex Chromosome. Males are Heterozygous (XY) while Females are Homozygous (XX). Males produce two type of sperms while female produce one type of egg. Sperm carrying X chromosome fusing with egg results in a female & sperm carrying Y chromosome results in a Male. (50% probability of each type of sex). Hence, it is clear that Females have no say in sex determination & Male’s sperm decide the sex of the baby.

Ø In HUMAN, Sperm decides the sex of the offspring (Males are responsible). Opposite is true in Birds where egg is responsible (Females) for the sex of the offsprings.

Ø SEX DETERMINATION IN HONEY BEE: [ Haplo-Diploidy Type]

è A fertilized egg (2n) develops into a Female (Queen/ Workers) While an Unfertilized egg (n) form Drones (Males).

è 2n = 32 chromosomes; n = 16 chromosomes.

è Males/ Drones produce sperm by Mitosis (as the main body is Haploid) while female/ Queen produce eggs by normal meiosis (as the main body is diploid) Workers are sterile with rudimentary/ underdeveloped gonads..

è Interestingly, due to male Haploidy, Drones do not have father and will not have sons though they have Grandfather and will have Grandsons.

(Haplo-Diploidy in Honey Bee)

 MUTATION: - It is a SUDDEN INHERITABLE change in the Genotype.

Ø    Substances causing mutation are called MUTAGENS. Eg. Physical (UV rays, X-ray), Chemical (Nitric acid).

Ø  Single base or few bases in a gene involved in mutation is called POINT (Gene) Mutation.

Ø  Change involving more than one gene in a chromosome or many chromosomes as called Chromosomal Mutation.

Ø  FORMS OF MUTATIONS: - Deletion, Addition/Duplication, Inversion & Translocation

Ø  Deletion & Addition results in “FRAME SHIFT MUTATION”.

  

     GENETIC DISORDERS [PEDIGREE ANALYSIS]; -

It is the analysis/study of Inheritance of a Genetic Trait through the Family History using Family Tree method.

ü  Pedigree helps to study/ trace the extent of a specific trait, abnormality or disease in the family history.

ü It tells whether the trait is Autosomal/ Sex Chromosomal, X/Y linked, Dominant/ Recessive.

ü Standard symbols are used to construct a pedigree chart. (NCERT Text Book, Fig. 5.13) eg. Male is ¨, Female is ¡, Unspecified Sex is u , mating is ¨--¡, Consanguineous mating is ¨=¡, Carrier female is ¤etc..

   MENDELIAN DISORDERS: Alteration/ Mutation in Single Gene. Eg. Cystic Fibrosis, Thalassemia, Colour Blindness.

  HAEMOPHILIA: - (SEX LINKED RECESSIVE DISEASE)

1.        X- Linked Recessive Trait/Disease. Single Protein (a part of cascade involved in blood clotting) in affected due to which a simple cut results in non-stop bleeding.

2.       Heterozygous female (carrier) pass the disease to her son, Possibilities of a Female becoming Hemophilic is extremely rare as only homozygous condition of affected gene express as disease (her mother should be at least a carrier & father definite Hemophilic).

3.       Pedigree of Queen Victoria has a number of Hemophilic Descendents, as she was a Carrier.

 SICKLE CELL ANAEMIA:- (AUTOSOMAL RECESSIVE DISEASE)

1.       Both parents (Heterozygous) are carrier for gene ( controlled by a single pair of allele Hb^A & Hb^S)

2.       Only Homozygous condition (Hb^SHb^S) shows phenotype of disease. Heterozygous (Hb^AHb^S) are apparently normal but have 50% probability to transfer diseased gene to their offsprings.

3.       The defect is caused due to substitution of GLUTAMIC ACID (Glu - coded by GAG) to VALINE (Val- coded by GUG) by Point Mutation (Substitution)  at 6^th Position of Second Beta-Globin Chain.

4.       Mutant Hemoglobin undergoes polymerization under low oxygen tension causing the change in the shape of RBC from Biconcave to Sickle Shaped.

 PHENYLKETONURIA: - (AUTOSOMAL RECESSIVE DISEASE)

1.       Inborn Error of Metabolism due to Inheritance of an Autosomal Recessive Trait.

2.       Patient lacks an Enzyme that converts Phenylalanine (Phe) in to Tyrosine (Tyr). Accumulation of amino acid Phenylalanine results in conversion to Phenyl Pyruvic Acid & other derivatives.

3.       Accumulation of these in brain causes Mental Retardation. They are also excreted through urine (Black Urine Disease) due to their poor absorption by Kidney.

      THALASSEMIA: [Autosome linked Recessive Blood Disease]

      Gene defect may be due to Mutation or Deletion that results in the reduced rate of synthesis of One of the Globin chains for Hemoglobin. [Abnormal Hemoglobin resulting in Anaemia.]

      In a Thalassemia, production of á globin chain is affected. It is controlled by two closely linked genes HBA1 and HBA2 on chromosome 16 of each parent (Total Four Genes) due to MUTATION or DELETION. The more genes affected, the less alpha globin molecules produced.

      In â (Beta) Thalassemia, production of â globin chain is affected. It is controlled by a single gene HBB in chromosome No. 11 of each parent (Total Two Genes) due to MUTATION. . The more genes affected the less BETA globin molecules produced.

        Thalassemia (QUANTITATIVE Inheritance) differs from Sickle Celled Anaemia (QUALITATIVE Inheritance) as it synthesize few globin molecules (due to deletion) while Sickle Cell Anaemia produce a Irregular structure & function of Hemoglobin Molecule(due to substitution).

      COLOUR BLINDNESS:  [ X-Linked Recessive Disorder]

(Colour Blindness Test - Find the identity in the circle?)

è Due to defect in either Red or Green Cones (as a result of Point/Gene mutation in X-Chromosome). Hence, a person is not able to differentiate between red-green colour. (This defect occurs in ~ 8% males and 0.4% Females )

è This is because males being Heterozygous for sex chromosomes, with defected gene on X-Chromosome Males are always Patients, while female being Homozygous may be carrier/ patient.

 

CHROMOSOMAL DISORDERS: -(MORE THAN ONE GENE OR CHROMOSOME INVOLVED)

Ø  ANEUPLOIDY (2N + 1, 2N -1, 2N+2 etc..) Addition or Deletion of one or more chromosomes in the normal set (may be due to Non Disjunction of Chromosomes during anaphase in cell division).

(Polyploidy resulting in Gigantic Effect)

Ø  EUPLOIDY (N, 3N, 4N etc..) Change in the entire set of chromosome due to Failure of cytokinesis after Telophase of cell cycle. Eg. Plants (Bread wheat Triticum aestivum is a Hexaploid – 6N)

(Triticales)

Ø  ANEUPLOIDY is common in Human.eg. Trisomy (2n+1=47) & Monosomy (2n-1=45)

    Down’s Syndrome : -

1.       Described firstly by Langdon Down (1866).

2.       Trisomy of Autosome (chromosome no. 21) resulting in 2n+1=47 chromosomes.

3.       Affected person is Short Statured, Small round Head, Furrowed Tongue, Partially Opened Mouth, Broad palm with characteristic palm Crease. Physical, Psychomotor & Mental development is retarded.

(Karyotype of Down's Syndrome)

(Features of Down Syndrome)

   Klinefelter’s Syndrome: -

1.       Trisomy of Sex Chromosome (X-Chromosome; XXY) in males as 2n+1=44+XXY=47 chromosomes.

2.       Overall Masculine development, but Feminine development (development of Breasts i.e. Gynacomastia) is expressed. Individuals are Sterile. Low pitch Voice, Sparse body hair, Long Hands.

3.       Rarely, Extreme Klinefelter’s (XXXY) are seen associated with brutal crimes.

   Turner’s Syndrome: -

1.       Monosomy of Sex Chromosome (X-Chromosome) resulting in 2n-1= 44 + XO = 45 chromosomes.

2.       Such Females are sterile (rudimentary ovaries), lack of other secondary sexual characters.

For figures and images : Courtesy Google Images