You often hear of questions like “Are you son of Mr.x”, “Do you belong to so and so family” in the society. If the answer is “yes” and the person questions back “How do you guess or how do you know?” The answer simply would be “You resemble Mr.x”. Some children are easily identified as sons or daughters of some known persons due to their resemblance in colour of skin, height, nose, colour of eyes and other distinguishable features with their parents. The same way we can also identify siblings according totheir identical characters. At the same time there are a number of characters by which the parents and offsprings, or siblings differ.
No human is hundred percent similar to his or her parents or his or her siblings. How do these similarities and differences arise? This question is answered by “Genetics”. Genetics also addressed the questions like how an elephant gives rise to an elephant baby, how a human is born to a human, how a pea plant produces only pea plants?
What is genetics?
Genetics is a branch of biology that deals with the inheritance and variations of characters.
Genetics deals with structure and synthesis of genes, inheritance, mutations and variations.
What is inheritance?
Inheritance is the process by which characters pass from parent to offsprings (progeny). It is the basis of heredity.
What is variation?
Variation is the degree by which progeny differ from their parents and among themselves.
Therefore genetics is the study of causes and pattern of inheritance in organisms, variations and their causes.
The idea of inheritance pattern was first given by Johann Gregor Mendel. His work and findings paved the way for the study of inheritance, hence J.G Mendel is considered as the “father of genetics”. Later on many scientists like Watson, Crick, Nirenberg, Khorana, Kornberg, Benzer, Jacob, Monod, Brenner, T.H.Morgan, Hugo de Vries, Sutton, Boveri, Conrat and others made significant contributions in this field.
Some of the important contributions of scientists in genetics are given below:
Without the knowledge of genetics one can understand that characters of an individual are inherited from their parents. This is a common phenomenon observed in plants, animals and microorganisms. It is also very common that individual differences in characters exist in plants or animals. According to Darwin’s theory of natural selection, nature selects those organisms with better characters which is called “Natural Selection”. In fact man started to select those organisms with better characters for his needs which is called “Artificial selection”. For example we breed cows and buffaloes which are high milk yielding and resistant. In plants also we select those varieties which have desirable characters like high yielding, drought resistant, pest and disease resistance etc. , and breed them. Sahiwal cows of Punjab are bred for high milk yield and Ongole bulls in Andhra Pradesh for their agricultural utility as drought breeds. Now let us discuss about the scientific basis of inheritance and variations.
Terminology used to understand Mendel’s experiments and genetics:
Artificial cross pollination: The artificial transfer of pollen grains from the flower of one known plant to stigma of flower of another plant of the same or other species is called artificial crosspollination.
True breeding: Continuous self-pollination in a plant to produce and express stable trait inheritance for several generations.
Mo
ohybrid cross: A cross between two parents differing in one pair of contrasting characters. e.g.: A cross between a tall pea plant (TT) and a dwarf pea plant (tt).
Dihybrid cross: A cross between two plants which differ in two pairs of contrasting characters is called a dihybrid cross.
e.g.: A cross between yellow and round seeded pea plant (YYRR) with a green and wrinkled seeded pea plant (yyrr).
First Filial progeny(generation) (F1): The generation of progeny obtained by crossing two true breeding lines ( two pure parents or homozygous parents) is called a F1 progeny.
Second Filial generation (F2): The generation produced by self-pollinating the F1 progeny.
Factors: Something that was being stably passing down unchanged from parent to offspring though the gametes over successive generations were called as a ‘factor’ by Mendel. It is later replaced by “Gene”.
Gene: Basic unit of inheritance that contain information required to express a particular character (trait) in an organism.
Alleles: Genes which code of a pair of contrasting traits are known as alleles. Alleles are slightly different forms of the same gene. e.g.: Tall (T) and Dwarf (t) alleles are two forms of the gene related to height of the plant.
Homozygous condition: If a allelic pair of genes are identical (similar) this condition is said to be homozygous condition. e.g.: TT (Tall), tt (Dwarf).
Heterozyous condition: If a allelic pair of genes is non-identical or dissimilar this condition is called heterozygous condition. e.g.: Tt (Tall).
Dominant factor: In a pair of dissimilar factors only one appears in F1 generation which is called dominant factor. e.g.: T for tallness appears in F1 so it is dominant over ‘t’ for dwarfness which does not appear in F1 but reappears in F2.
Recessive factor: In a pair of dissimilar factors the factor that does not appear in F1 but reappears in F2 is called recessive factor. ‘t’ for dwarfness is recessive.
Phenotype: External characters of an organism. e.g.: Tall, Dwarf
Genotype: Genetic composition of a character. e.g.: Tall = (TT) or (Tt)
Dwarf = tt.
The phenotype tall has two genotypes TT and Tt, while the phenotype dwarf has only one genotype tt.
Back cross: A cross between F1 offspring with any of its parents (dominant or recessive) is called a back cross. e.g.: Tt × TT or Tt × tt
Test cross: A cross between F1 with its recessive parent to know the genotype composition is called a test cross. e.g.: Tt × tt.
Punnet Square: A diagram which represents the types of gametes produced by the parents, the formation of zygotes and the progeny is called a Punnet square. It was developed by Reginald C. Punnet. It is commonly called checker board.
Mendel’s Experiments
Gregor Johann Mendel conducted hybridization experiments on garden pea (Pisum sativum) for seven years and proposed laws of inheritance.
He selected plants with contrasting characters regarding
Height - tall /dwarf
Colour of seed - green/ yellow
Shape of seed - round/wrinkled
Colour of flower - violet/white
Position of flower - axial/terminal
Shape of pods - inflated/constricted
Colour of pods - green/yellow
Advantages of selecting garden pea plant by Mendel for his hybridization experiment:
Pea plant is an annual with well-defined characters.
* It can be grown and crossed easily.
* It has bisexual flowers with androecium and gynoecium.
* It is suitable for self-fertilization and cross fertilization.
* It has a short life cycle and produces large number of offsprings.
* It shows number of contrasting characters which can be studied easily.
Mendel’s Monohybrid cross:
Monohybrid cross is across between two true breeding lines which differ in a pair of contrasting characters. Mendel crossed a homozygous tall plant (TT) with a homozygous dwarf plant (tt) and produced heterozygous tall plants (Tt) which are called monohybrids. The details of the monohybrid cross are given hereunder:
The phenotypic ratio of Mendel’s monohybrid cross is 3 : 1 (Tall:Dwarf) while the genotypic ratio is 1 : 2 : 1 (homozygous tall : heterozygous tall : homozygous dwarf). The phenotypic probability of tall plants in F2 is 0.75 and dwarf plants if 0.25. The genotypic probalility of tall plants and dwarf plants is 0.25 homozygous tall, 0.50 heterozygous tall and 0.25 homozygous dwarf plants.
Mendel’s first law – Law of Dominance
From Mendel’s monohybrid cross it is inferred that
* Characters are controlled by factors.
* Factors occur in pairs.
“When a cross is made between two parents differing in a pair of contrasting characters, one of the characters appears in F1 generation which is called dominant character; the other character which does not express in F1 but appears in F2 is called recessive character.”
* The law of dominance explains about the expression of one parental character in F1.
* It also explains 3 : 1 ratio in F2.
Mendel’s Second law - Law of Segregation or Law of Purity of Gametes
* The law of segregation states that “the two alleles of a gene when present together in a heterozygous state do not fuse or blend together in any way but remain distinct and segregate (separate) during meiosis or in the formation of gametes so that each gamete will carry only one of them”.
* Homozygous individual produces only one type of gametes, while heterozygous individual produces two types of gametes each with one type of allele.
Segregation of genes is universal in all organisms reproducing by normal sexual method.
Incomplete dominance
* Sometimes F1 phenotypes do not resemble either of the parental characters but show an intermediate character which is known as incomplete dominance.
e.g.: Flower colour in Snapdragon (Antirrhinum species)
* Both phenotypic ratio and genotypic ratio of incomplete dominance are 1 : 2 : 1 (Red flowers : Pink flowers : White flowers)
Co - dominance:
This is a type of inheritance where the F1 individual resembles both parents.
e.g.: 1. Blood group patterns in human beings (A, AB, B and O)
2. Seed coat pattern and size in lentil (Lens culinaris) plants.
Seed coat pattern in Lentil (Lens culinaris)
Phenotypic and genotypic ratio = 1 : 2 : 1 (Spotted : Spotted and Dotted : Dotted)
In the above example when a pure bred spotted lentil is crossed with pure bred dotted lentil the F1 heterozygotes show both spotted and dotted seed coat pattern.
* This shows that neither spotted nor dotted allele is dominant or recessive to each other
* As the alleles express equally in heterozygote they are termed ‘codominant’.
* Self-pollination of spotted and dotted F1 progeny produces F2 progeny in the ratio 1 : 2 : 1 (spotted : spotted and dotted : dotted).
Pleiotropy
* Pleiotropy is a phenomenon where a single gene may produce more than one effect so that it may be related to more than one character.
e.g.: Starch synthesis in pea seeds is controlled by a gene with B and b alleles.
The genotype BB produces large starch grains and bb homozygotes produce small starch grains.
BB seeds are round whereas bb seeds are wrinkled.
Bb heterozygotes produce round seeds but produce intermediate size starch grains.
So in case of starch grains the alleles show incomplete dominance and in case of shape of the seed the alleles follow law of dominance.
Therefore more than one phenotype may be influenced by the same gene which depends on the gene product that produces a particular phenotype.
Concept of dominance
* Every gene contains information in the form of DNA codes to express a particular trait.
* A diploid organism shows two copies of the same gene called alleles.
* When these two alleles are not identical they are in heterozygous condition.
* One of them may produce an enzyme needed for transformation of a substrate.
* The modified allele may produce
normal or less efficient enzyme,
a non-functional enzyme or no enzyme.
If a normal enzyme is produced the same phenotype or trait is produced.
If a less functional enzyme or non-functional enzyme is produced the phenotype is affected.
The unmodified allele represents the original phenotype dominant allele.
The modified allele represents recessive allele.
Inheritance of two genes (Dihybrid cross)
A cross between two plants which differed in two pairs of contrasting characters is called a dihybrid cross.
e.g.: Cross between a pea plant with yellow coloured and round seeds (YYRR) and another pea plant with green coloured and wrinkled seeds (yyrr).
F1 hybrid of such cross was found to show yellow and round seeds. Thus the yellow and round characters are dominant over green and wrinkled ones.
Law of Independent Assortment
Based on dihybrid crosses Mendel proposed his Law of Independent Assortment as
“When two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters”.
The law of independent assortment can be illustrated as follows:
Probability of F2 generation
Phenotypic ratio of Mendel’s dihybrid cross: 9 : 3 : 3 : 1 (Yellow Round : Yellow Wrinkled : Green Round : Green Wrinkled)
The probability of Yellow round = 9/16
Yellow Wrinkled = 3/16
Green Round = 3/16
Green Wrinkled = 1/16
Now let us work out the genotypic ratio at the F2 stage using the above Punnett square data
By the above table we may derive the genotypic ratio of a dihybrid cross as
1 : 2 : 2 : 4 : 1 : 2 : 1 : 2 : 1
The phenotype Yellow and Round shows 4 types of genotypes YYRR, YYRr, YyRR and YyRr.
The phenotype Yellow Wrinkled shows 2 types of genotypes YYrr and Yyrr.
The phenotype Green Round shows 2 types of genotypes yyRR and yyRr.
The phenotype Green and Wrinkled shows 1 type of genotype yyrr.
Dihybrid test cross
A dihybrid test cross is made between a F1 dihybrid (Yellow and Round YyRr) and its recessive parent (yyrr).
Therefore the test cross ratio of a dihybrid cross would be 1 : 1 : 1 : 1.
Chromosomal Theory of Inheritance
* Sutton and Boveri proposed the “Chromosomal Theory of Inheritance” to explain the Mendel’s laws.
* This theory explains that the behaviour of chromosomes was parallel to the behaviour of genes.
* Both chromosomes and genes occur in pairs.
* The two alleles of a gene are located on the homologous sites of homologous chromosomes.
* Thus the pairing and separation of chromosomes leads to segregation of factors or genes.
* This synthesis of ideas of chromosomal segregation and Mendelian Principles is called as “Chromosomal Theory of Inheritance”.
T.H. Morgan provided experimental verification to this theory while working on fruit flies (Drosophila melanogaster).
Linkage and Recombination
When two genes in a dihybrid cross were situated on the same chromosome the proportion of parental gene combinations was higher than non-parental type.
This is due to physical association or linkage of the two genes on a chromosome.
T.H. Morgan defined Linkage as the physical association of two genes on the same chromosome.
The non-parental gene combinations that would result in a dihybrid cross are called Recombinations.
When the genes are tightly linked on the chromosome the frequency of recombinations is very low.
e.g.: White eyed and yellow body genes of Drosophila were tightly linked hence show only 1.3% recombination.
When the genes are loosely linked the frequency of recombinations is high.
e.g.: White eye and miniature wing genes of Drosophila were loosely linked hence show 37.2% of recombination.
Alfred Sturtevant used the frequency of recombination between genes as a measure of distance between genes and prepared gene maps.
Genetic maps are used in sequencing of genomes (Human Genome Sequencing).
Mutations
* The alteration of genes which results in changes in the genotype or phenotype of an organism is called a ‘Mutation’.
* Mutation leads to variation in DNA.
* Mutation was first observed by Hugo de Vries in Oenothera lamarckiana (Evening primrose).
* Mutations may occur as deletions or insertions in a segment of DNA.
* Chromosomal abnormalities or aberrations are also mutations that are commonly observed in cancer cells.
* The mutations that arise due to change in a single base pair of DNA are called ‘Point Mutations’. e.g.: Sickle cell anemia
* Mutations that occur due to deletions and insertions of base pairs of DNA are called frame-shift mutations.
* The chemical and physical factors that induce mutations are called ‘Mutagens’.
e.g.: UV radiation.
Mutations cause great variability in a population that may produce desirable traits. Such crop plants are improved by selection and hybridisation.
Bits
1. In lentil plant when spotted variety crossed with dotted variety all F1 plants are spotted and dotted. If F1 plants are self pollinated the F2 progeny are spotted, spotted-dotted, dotted
varieties. Choose the correct statement.
1) This experiment proves the law of dominance |
2) All F1 plants are spotted varieties due to co-dominance
3) Spotted, spotted-dotted, dotted varieties are 2 : 1 : 2 ratio
4) Law of segregation does not apply in this experiment
2. Which among the following statement is incorrect
1) Reginald C. Punnett formulated Punnett square
2) Crossing of two plants with one pair of desirable contrasting characters is dihybrid cross
3) Crossing of F1 hybrid with any one of the parent is back cross
4) Crossing of F1 hybrid with its homozygous recessive parent is test cross
3. Identify mismatch
1) Chromosome theory - Sutton and Boveri
2) Linkage - Correns
3) Chromosome maps - Sturtevant
4) Mutations - Hugo de Vries
4. Checker board was designed by
1) Reginald C. Punnet 2) T.H. Morgan 3) Sturtevant 4) Hugo de Vries
5. What is the genotypic ratio of dihybrid test cross
1) 1 : 2 : 1 2) 3 : 1 3) 9 : 3 : 3 : 1 4) 1 : 1 : 1 : 1
6. Chromosomal theory of inheritance was experimentally verified by T.H. Morgan in this organism.
1) Lens Culinaris 2) Pisum sativum
3) Drosophila melanogaster 4) Musca domestica
7. The method of gene mapping of the chromosomes was developed by
1) Marshall Nirenberg
2) Franklin Stahl 3) Alfred Hershey
4) Alfred Sturtevant
8. Sickle cell anaemia is caused due to
1) Deletions 2) Insertions 3) Single base alterations 4) Polyploidy
9. The Phenomenon of Linkage was discovered by
1) T.H. Morgan 2) Hugo de Vries 3) G.J. Mendel 4) Bateson
10. Match Column I with Column II related to the F2 progeny of di hybrid cross in Pea plant
Column - I Column - II
A) Number of genotypes I) 9:3:3:1
B) Number of phenotypes II) 1:2:2:4:1:2:1:2:1
C) Phenotypic ratio III) Four
D) Genotypic ratio IV) Nine
The correct match is:
A B C D
1) III I IV II
2) IV III II I
3) III II I IV
4) IV III I II
Answers: 1-4; 2-2; 3-2; 4-1; 5-4; 6-3; 7-4; 8-3; 9-1; 10-4.
Molecular basis of Inheritance
1. Which among the following statement is incorrect
1) DNA and RNA are two types of nucleic acids
2) DNA acts as genetic material and RNA acts as genetic material in some viruses
3) Nucleotide has three components nitrogen base, sugar and phosphate
4) Purines are single ringed and pyramidines are double ringed compounds
2. Identify the wrong statement
1) Thymine present in DNA and Uracil present in RNA
2) Nitrogen base is linked to pentose sugar by glycosidic bond
3) Phosphate group is linked to 5’ -OH of a nucleoside by phosphodiester bond
4) Meischer identified the acidic nature of DNA
3. Find out mismatch
1) DNA - negative charge 2) Amino acids - positive charge
3) Histone - negative charge 4) Zwitter ion - dual charge
4. Identify wrong match
1) Mutation - mistake in replication
2) Leading strand - continuous strand of DNA
3) Lagging strand - discontinuous strand of DNA
4) Discontinuous synthesized DNA - Watson strand
5. DNA finger printing was discovered by
1) Wilmut 2) A. Jeffreys 3) Eithoven 4) Kary Mullis
6. DNA is wrapped around the histone octamer to form
1) Nuclein 2) Nucleosome 3) Nucleolous 4) Nucleus
7. RNA mutates faster than DNA because
1) RNA is unstable 2) RNA is more stable
3) RNA is less reactive 4) RNA is non reactive
8. Semi conservative replication of DNA was first observed in
1) Drosophila melanogaster 2) Diplococcus pneumoniae
3) Eicheritia coli 4) Neurospora crassa
9. Molecular gum or glue is
1) DNA polymerase 2) DNase
3) DNA dependant DNA polymerase
4) DNA ligase
10. Identify correct statement from the following
1) AUG is termination codon 2) UAA,UAG are starting codons
3) Genetic code is ambiguous 4) Genetic code is commaless
11. The number of nucleotides present in the ds DNA of Escheritia coli.
1) 3.3 × 109 bp 2) 6.6 × 109 bp 3) 4.6 × 109 bp 4) 5.6 × 109 bp
12. The nitrogen bases common to DNA and RNA are
1) A, G and U 2) A, G and T 3) A, T and U 4) G, A and C
13. Each histone Core is wrapped by how many turns of DNA
1) 2 2) 3 3) 4 4) 5
14. If a length of DNA has 45,000 base pairs, how many complete turns will the DNA molecule take ?
1) 4,500 2) 45,000 3) 450 4) 45
15. In their Semiconservative experiment Meselson and Stahl used
1) C14 2) P32 3) N15 4) S35
16. This RNA brings the amino acids and reads the genetic code
1) m-RNA 2) r-RNA 3) t-RNA 4) Hn-RNA
17. Reverse transcription is depicted as
1) DNA → RNA → Protein 2) RNA → RNA → Protein
3) RNA → DNA → Protein 4) RNA → Protein → RNA A + G
18. In ds DNA the A + G/T + C = 1 existence was first reported by
1) Wilkins and Franklin 2) Watson and Crick
3) Meischer and Franklin 4) Chargaff
19. Which of the following organisms does not follow the central dogma of molecular biology ?
1) Chlamydomonas 2) Yeast 3) HIV 4) Bacteria
20. A ds DNA molecule consists of 200 nucleotides with 20% adenine. What is the number of hydrogen bonds in that molecule?
1) 260 2) 360 3) 180 4) 240
21. Match Column I with Column II
Column - I Column – II
A) Guanine I) Purines pairs with Purines
B) Adenosine II) 2 C-N ringed N2 base
C) Non - Complementary
base pair III) Nucleoside
D) Deoxy Cytidylic
acid IV) Nucleotide
The correct match is:
A B C D
1) II III IV I
2) II III I IV
3) II IV III I
4) II III IV I
Answers: 1-4; 2-3; 3-3; 4-4; 5-2; 6-2; 7-1; 8-3; 9-4; 10-4; 11-3; 12-4; 13-1; 14-1; 15-3; 16-3; 17-3; 18-4; 19-3; 20-1; 21-2.
