GENETICS

Genetics is the branch of biology that deals with the study of Heredity and Variations. Heredity is the transfer of parental characters to the off springs. Variation is the differences between the parents and off springs and also between the off springs of a set of parents.

Variations are of two types

1.Somatic variations: These are variations that affect only the somatic cells or body cells. These are not heritable. These are acquired by the effect of environmental factors, use and disuse of organs or by conscious effort.2.Germinal variations: These are variations that affect the reproductive cells. These are heritable. These may be Continuous or Discontinuous. Continuous variations very small indistinct variations. These are also called Fluctuating variations. They develop during gamete formation and will not contribute evolutionary changes. Discontinuous variations develop as sudden changes due to mutations. They are also called Sports or saltations. They are the most important factors that contribute raw materials for evolutionary changes.

Branches of Genetics1.Classical genetics or Transmission genetics: deals with the principles of Mendelian Inheritance2.Molecular genetics: deals with the molecular structure of genes3.Population or biometrical genetics: deals with the behavior and effect of genes in Population

Common Terms in Genetics

1.Phenotype External or visible characters of an organism2.Genotype Genetic makeup of an organism3.Genome Haploid set of chromosome in a diploid cell4.Haploid Organism with half set of chromosomes. Gametes are haploids5.Diploid Organism with full set (2 genomes) of chromosomes6.Genes Physical and chemical basis of heredity. Reside in chromosomes6.Allele Alternate forms of a gene located in the same loci of homologous chromosomes

Also called Allelomorphs7.Locus Position of gene in the chromosome8.Homozygous An organism with identical genes of a character .AA or aa9.Heterozygous An organism with non-identical genes for a character. Aa10.Dominant Character that is expressing.11.Recessive Character that is masked in the presence of a dominant gene.12.Back cross A cross between F1 hybrid with any one of the parents.13.Test cross A cross between F1 hybrid with the recessive parent.

Scientists associated with Genetics

1.Mendel Father of genetics2.Hugo Devries Discovered mutation3.Punnet Introduced Checker board or punnet squre for Mendelian cross4.Carl Correns Rediscovered Mendelism5.Erich Von Tschermak Rediscovered Mendelism6.Correns Discovered Incomplete dominance7.Karl Land Steiner Discovered blood groups8.Von Decastello, Sturli Discovered AB blood group

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9.Bernstein Multiple alleles in blood groups10.Bateson and Punnet Discovered Complementary action of genes in Lathyrus odoratus11.Landsteiner and Weiner Discovered Rh blood group12.Davenport Discovered Polygenic inheritance13.Robert Brown Discovered nucleus14.Hammerling Demonstrated the role of nucleus to control characters.15.Sutton and Bovery proposed the chromosome theory of inheritance16.Waldeyer Coined the term Chromosome17.Balbiani Discovered Giant chromosomes in Drosophila18.Watson and Crick Proposed the double helical model of DNA19.T.H.Morgan Discovered Linkage20.T.H.Morgan Produced artificial mutation in Drosophila for the first time21.Bridges Sex linked inheritance22.Blakeslee Found out trisomic plants for the first time in Datura23.Altenberg Discovered the mutagenic property of U.V rays24.Muller Discovered mutagenic property of X rays.25.Auerbach Discovered mutagenic property of chemicals26.Beadle and Tatum Conducted experiments in Neurospora27.Griffith Discovered bacterial transformation28.Avery, MacLeod, Mc Carthy Proved DNA as the genetic material29.Harshey and Chase Conducted experiments in viral multiplication30.Fraenkel conrat Discovered RNA also act as genetic material31.Friedrich Miescher Isolated nucleic acid from nucleus32.Mary Rosalind Lady behind the DNA.Helped Watson to make the DNA model33.Meselson and Stahl Discovered semi conservative method f DNA replication34.Archibald Garrod Discovered Inborn Errors of Metabolism35.Beadle and Tatum Proposed One Gene One Enzyme hypothesis36.Jacob and Monod Proposed Operon concept37.Holley Proposed the Cloverleaf model of t-RNA38.Philip Sharp, Richard Robert Discovered Split Genes39.Briggs and Thomas Nuclear transplantation experiment40.Francis Galton Founder of Eugenics41.Alec Jeffreys Introduced DNA Finger printing

Mendels’s work on Heredity

Greogor Johan Mendel, an Austrian Monk conducted a number of experiments in garden pea Pisum sativum and formulated the principles and laws of inheritance of characters. Because of his contributions in genetics he was called as the Father of Genetics.

1.Mendel’s pea plantMendel selected pea plant as his experimental material because of the

following reasons

A. Pea plants have large number of observable characters and most of them are true breeds.B.Pea plants are self-pollinating and the keel petal of the flower prevents contamination with foreign pollens.C.they have short life spanD.they produce large number of seeds.E.they are easy to cultivate.

Mendel’s Experiment

Mendel performed the breeding experiment in three stages

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1.Selection of Parents: Mendel selected 14 pure or true breeds with contrasting characters as parents

2.Hybridisation for F1: Mendel artificially cross-pollinated the F1 plants by dusting the pollens collected from one flower on the stigma of an emasculated flower. Bagging the pollinated flower prevented contamination.

3.Self-breeding F1. Mendel allowed the F1 hybrids to self pollinate and raised F2 and F3

Mendel’s findings

After conducting experiments in garden pea Mendel concluded the results as follows:

1.Hybrids of F1 generation resembled one of their parents. The result of the reciprocal crosses were also same.2.In the F2 generation both the parental characters appeared.

3.Both the parental characters appeared in the ratio 3:1 in the F2.This is called Monohybrid ratio. In the F2 generation of a cross between yellow and green seeds, Mendel obtained 6022 yellow and 2001 green seeds. This was approximately in the ratio 3:1.

4.In the F2 Mendel found that all the recessive plants were true breeding forms.

5.Mendel observed that one of the parental characters that was not appeared in F1 reappeared in F2.

6.He also found that the inheritance was particulate in nature and the characters are transmitted by factors and random mixing of characters during fertilization leads to the appearance of parental characters.

MONOHYBRID CROSS-

Mendel selected true breeding pea plants for the monohybrid cross. He fixed one plant as male and the other as female .The anthers from the female plant were removed (emasculation) and bagged the flowers to prevent contamination with other pollens. Then he dusted the pollens collected from the male plant on the stigma of the female plant. Pollinated flowers were bagged again and raised the F1 generation from the seeds obtained from the cross pollinated flower. He also conducted reciprocal crosses by interchanging the parents. The F1 plants were then allowed to self-pollinate to raise the F2 generation. Similarly F3 and F4 generations were produced.

Results of Monohybrid cross

When true breeding tall and dwarf plants were cross-pollinated only tall plants appeared in the F1 generation. When F1 plants were self-pollinated both tall and dwarf plants appeared in the ratio 3:1.The reciprocal cross also gave the same result. Mendel found that the tall nature of plant appeared in the F1 generation was due to the dominant nature of the factor controlling height of the plant. In the presence of Dominant factor the recessive trait dwarf ness was masked. Thus Mendel concluded that the dominant factor only will appear in some generations and the recessive character will appear only the plant becomes homozygous.

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MONOHYBRID CROSS

Tall X Dwarf Parents

T T X t t Genotype

T T t t P gametes

Tall F1 generation

Tall Tall

T t T t Selfing

T t T t F1 gametes

Male T t

T

Female t

Monohybrid cross Phenotypic ratio: 3 Tall: 1 Dwarf 3:1Genotypic ratio 1 TT : 2 T t : 1 tt 1:2:1

DIHYBRID CROSS

After the monohybrid cross, Mendel conducted dihybrid crosses in pea plant taking two pairs of contrasting characters. He selected two characters namely pod colour and seed shape. Yellow colour of pod is dominant over green colour and round shape of seed is dominant to wrinkled seed. Mendel crossed a pure breed Round Yellow plant with a double recessive Wrinkled Green plant. In the F1 generation he got only Round Yellow plants as in the monohybrid cross. But when he self pollinated the F1 plants, four varieties of plant namely Round Yellow, Round Green, Wrinkled Yellow and Wrinkled Green appeared in the ratio 9:3:3:1.This ratio is called dihybrid ratio. After analyzing the ratio, Mendel found that the factors responsible for colour of pod and shape of seed separated independently and segregated into different gametes of the F1 plants. The random fusion of gametes produced all the four possible varieties in the F2 generation.

T T T t

T t T t

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Mendel’s dihybrid cross

Round Yellow X Wrinkled Green P1Rr Yy rr yy

Rr Yy Round Yellow F1

RY Ry rY ry F1 Gametes

RY Ry rY ry

RY

Ry

rY

ry

9 Round Yellow :3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled green

MEDELIAN PRINCIPLES AND LAWS

After the experiments, Mendel formulated some principles and laws to explain the behavior of factors during inheritance. These are called Mendelian laws and principles. This became the basis of genetics.

Principle of dominance: This principle says that during inheritance some characters will appear while others remain hidden or masked. The character that is appearing is called as dominant character and the one masked as recessive character. Eg. Tall plant is dominant over drawf.

Principle of Combination: This principle says that different characters in an organism are produced by the combination of factors present in the gametes. The random fusion of gametes result in the appearance of new characters. Eg. Round green, Wrinkled Yellow.

Law of segregation: Mendel formulated his first law of inheritance after his monohybrid cross. This law is also called law of purity of gamates. According to the law of segregation, the different factors present in an organism separate and segregate into different gametes at the time of gamete formation. This law can be explained by the monohybrid cross.

Law of Independent Assortment: Mendel formulated his second law after conducting di hybrid cross in pea plants. According to this law, when two or more pairs of contrasting characters are present, the separation or segregation of characters in any one pair is independent to the separation of characters in the other pair. This law can be explained using the dihybrid cross.

VARIATIONS OF MENDELISM

Normally in most of the organisms the hereditary characters are inherited according to the Mendelian pattern. But there are exceptions in which the inheritance shows variations from Mendelism. A few examples of such variations are:

RRYY RRYy RrYy RrYy

RRYy Rryy RrYy Rryy

RrYY rRyY rrYy rrYy

RrYy Rryy rrYy rryy

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INCOMPLETE DOMINANCE

Generally in Mendelian pattern of inheritance, the dominant trait is appear in the F1 generation and the recessive trait does not appear in F1.This is due to the presence of dominant gene that masks the effect of the recessive gene. But in some organisms the character in the F1 generation is a mixture of the effect of both dominant and recessive genes. The dominant gene is expressing partially allowing the recessive gene also to express. This phenomenon is called incomplete or partial dominance.A very good example of incomplete dominance is the flower colour in the common 4–O clock plant Mirabilis jalapa. In this plant the Red flower is dominant over White flower. When a red flowered plant is crossed with white flowered one, all the F1 plants produced Pink flowers. In the F2 generation Red, Pink and White flowered plants will appeared in the ratio 1: 2: 1.That is, the character in the F1 will be a mixture of both dominant and recessive genes. Here the genotypic and phenotypic ratios will be the same.

Red x White Parents

RR rr

R R r r P – gametes

Rr F1 – Pink R r

R

r

1 Red : 2 Pink : 1White Genotypic ratio 1:2:1Phenotypic ratio 1:2:1

Other example 1.Sickle cell anemia Sickle cell trait Hb A / Hb S genotype has partial normal hemoglobin and partial sickle hemoglobin

CO DOMINANCE

In this type of gene expression both the dominant genes of character will express together if they come together and produce a mixture of dominant characters. So neither allele is dominant to the other. The inheritance of AB blood group provides an example for codominance. If a person inherits both I A and I B genes, the blood group will be AB.Both antigen A and B will be present on the surface of RBC.

A – Group X B – Group

I A / I A I B / I B

I A / I B AB Blood group

Eg. 2. Coat colour inheritance in CattleRed coat colour ( I R ) and White ( I W ) have no dominant or recessive relations. In a

cross between Red and White cattles the F1 will be Roan with red and white patches on the body. In the F2 Red, Roan and White will appear in the ratio 1:2:1

RR Rr

Rr rr

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MULTIPLE ALLELISM

Generally a character is controlled by two alleles, either dominant, recessive or in the heterozygous conditions. But some characters are controlled by more than two alleles. This phenomenon is called as multiple allelism and the alleles as multiple alleles. They are different from multiple genes since they are the alternate forms of the same gene located in the same loci of the homologous chromosomes. Even though a number of alleles are present in the group, only two alleles will be present in an organism.

Multiple allelism is exemplified by the blood group inheritance in man. Three alleles are present in man to control blood group inheritance. I A and I B genes are dominant and i gene recessive. The I A and I B genes originated from i gene by two dominant or reverse mutations. Thus all the three alleles control the same character namely blood group inheritance.

Other examples1.Rh inheritance in man Rh factor present on the RBC of man is controlled by 8 alleles. They are treated as multiple alleles by Weiner but Fischer considered them as pseudo genes.

2.Coat colour inheritance in Rabbit: There are four coat colours in rabbit namely Agouti, Chinchilla, Himalayan and Albino. These coat colours are controlled by alleles I C , I ch , I h and I a.

POLYGENIC INHERITANCE OR MULTIPLE GENE INHERITANCE

The quantitative traits of man are controlled by a group of genes called polygenes. They are separate genes forming a group to control a character. Each gene in the group can contributes some amount of character and all the contributions of genes are added together to produce the character. Therefore the effect is called Additive effect or cumulative effect. Davenport in 1913 found that three genes located in the adjacent loci of the cromosome control the skin colour in man. These genes are ABC and abc. Each gene can produce some amount of melanin pigment in the body. A Negro has all dominant genes and hence maximum melanin (around 80 %). But the white has all recessive genes and can produce only 6 % melanin. A cross between Negro and White will produce an intermediate called Mulatto with 38% melanin. The genotype of mulatto is AaBbCc. Crossing of two mulattos will produce individuals with skin colours like Black, Intermediate, Light etc in addition to Negro and White.

NEGRO X WHITE

AABBCC aabbcc

Aa Bb Cc Mulatto

Other examples 1.Corolla length in Nicotiana longiflora 2.Kernel colour in wheat 3.Height in man

EPISTASIS AND HYPOSTASIS

This is a gene behavior in which the presence of a gene (not allele) will suppress the expression of another gene for the same character. The gene, which is suppressing is called Epistatic gene and the gene that is undergoing suppression as Hypostatic gene.

Example 1.In Dogs coat colour Black is controlled by the gene B and brown by b. Another allele I inhibits the expression of B and colour becomes white. Here I gene is epistatic and B hypostatic

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Example 2. Coat colour in Mice. In mice the wild coat colour Agouti is controlled by the gene A. Its allele a is responsible for albino. Another gene c controls the spreading of melanin. This gene has a recessive epistatic effect and if if comes with A gene,the colour will be albino instead of agouti.

PLEIOTROPISM

It is a phenomenon in which a gene has multiple phenotypic effects. Such genes are called pleiotropic genes.

Example: Gene responsible for sickle cell anemia. A mutant recessive gene Hbs in the homozygous condition produces sickle cell anemia. In sickle cell anemia the haemoglobin crystallizes in the RBC when the partial pressure of oxygen decreases. This changes the shape of RBC to sickle form, which will block blood vessels causing anemia and haemorrhage. Normal gene for hemoglobin is HbA .The genotype of sickle cell anemia is Hbs / Hb s. The heterozygotes with genotype Hb A / H b s are called Carriers and are showing Sickle Cell Trait. So the same gene produces three phenotypes namely Normal, Sickle cell trait and sickle cell anemia.

Other Examples1.In Drosophila the eye colour gene in different flies produce different eye colours like red, white honey, ebony etc. The same gene will also control sperm storage organs in females.2.In White tiger the gene for fur colour also control the connection between eye and brain.

LETHAL GENES

These are harmful genes, which will destroy the possessor either in the dominant or recessive conditions.

Example 1.A dominant gene Y in the homozygous condition will causes the death of mice. The gene controls yellow coat colour in mice and will be normal in the heterozygous condition and produce yellow coat colour ( Yy ) .In the homozygous recessive condition ( yy ) it produces brown coat. Only in the homozygous condition ( YY ) the gene becomes lethal.

Other examples. 1.Leaf colour in Snapdragon – Antirrhinum majus 2.Thalassemia in man 3.Huntington’s chorea in man

Some times a gene in the homozygous recessive condition will become lethal. Sickle cell anemia in man is caused when the gene for hemoglobin becomes recessive due to mutation. Hbs / Hbs

Other examples 1.Congenital ichthyosis in man – skin disease2.Amaurotic idiocy in man – mental retardation

COMPLEMENTARY GENES

These are genes, which complement other genes to produce a character. The two genes are necessary to produce a particular phenotype. Bateson and Punnet observed this phenomenon in pea plant Lathyrus odoratus. In this plant the genes for purple and white colours of flower are P and p. But the purple or white colour is produced only when another gene C is present. In the dominant form

PpCc Purple PPcc / ppCC White

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PEDIGREE ANALYSIS

Pedigree is the history of inheritance of characters for many generations. Pedigree analysis is the method to trace back the ancestral characters by taking one or more the characters from an individual and the family tree or pedigree tree is constructed. Squares represent males and circles represent females. Shaded squares or circles indicate dominant traits and open squares or circles indicate recessive traits.

Pedigree tree

PROBABLE QUESTIONS DO IT YOURSELF

1. Some plants occur in of the two sizes- tall or dwarf. Tallness is dominant to shortness. Choose suitable letters for the gene.

2. Why are there two genes controlling one character? Do the two genes affect the character in the same way as each other?

3. The gene for red hair is recessive to the gene for black hair. What colour hair will a person have if he inherits a gene for red hair from his mother and a gene for black hair from his father?

4. Read the above question carefully and choose letters for red hair and black hair and write down the gene combinations. Would you expect a red haired couple to breed true? Could a black haired couple a red haired baby?

5. Use the words homozygous, heterozygous, dominant and recessive to describe the following gene combinations- Aa , AA, aa

6. A plant has two varieties, one with red petals and one with white petals. When these two varieties are cross-pollinated, all the offspring have red petals. Which gene is dominant? Choose suitable letters to represent the two genes?

7. Two black guinea pigs are mated together on several occasions and there off springs are invariably black. However when their black offspring are mated with white guinea pigs, half of the matings result in all black litters and the other half produced litters containing equal number of black and white babies. From these results deduce the genotypes of the parents and explain the results of the various matings assuming a single pair of alleles in this case determines that colour.

8. Two black rabbits thought to be homozygous for coat colour were mated and produced a litter, which contained all black babies. The F2 however resulted in some white babies, which meant that one of the grand parents was heterozygous for coat colour. How would you find out which parent was heterozygous?

9. What are the possible blood groups likely to be inherited by children born to a group A mother and a group B father? Explain your reasoning?

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10. A red cow has a pair of alleles for red hairs. A white bull has a pair of allele for white hairs. If a red cow and white bull are mated, the off springs are Roan. That is they have red and white hairs equally distributed over their body. Is this an example of co-dominance or incomplete dominance?What coat colours would you expect among the offspring of a mating between two Roan cattle?

11. A married couple has four girl children, but no boys. This does not mean the husband produces only X sperms. Explain why not

12. A woman has sickle cell trait. What are the chances of her children inheriting –a. Sickle cell trait b. Sickle disease if she marries a normal man or a man with sickle cell trait or a man with sickle cell disease?

13. Which of the following do you think are – a. mainly inherited characters b. mainly acquired characters or more or less equal mixture?Manual skills, Facial features, Body build, Language, athletism, Ability to talk

14. What new combinations of characters are possible as a result of crossing a tall plant With yellow seeds? (TtYy) with a dwarf plant with green seeds (ttyy)

CHROMOSOMES AND HEREDITY

Chromosomes are structures formed in the cells during cell division. They are visible only in a dividing cell. During inter phase stage chromosomes remain as chromatin reticulum.

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Chromosomes are short and stumpy structures that form the physical basis of heredity. They carry hereditary units called genes.

Chromosomes were first reported by Hofmeister in 1849.The details of the behaviour during cell division were worked out only at the end of 19th century.Experiments conducted by scientists proved that both chromosomes and genes play an important role in heredity and both shows parallelism.Chromosomes and genes exhibit parallelism1.Genes and chromosomes exists in pairs.2.Genes and chromosomes segregate during meiosis.3.Paired condition of both chromosomes and genes will restore during fertilization.4.Genes and chromosomes exhibit independent assortment.

CHROMOSOMES THEORY OF HEREDITY.

alter Sutton and Theodore Boveri in 1902 independently proposed the chromosome theory of heredity. According to this theory hereditary factors are located in the chromosomes.The segregation and independent assortment of genes depends on the segregation and independent assortment of chromosomes.

Structure of chromosome

Chromosomes are rod shaped structures present in the dividing cells.They are stainable and become purple coloured when stained using Acetoorcein ( animal chromosome ) or Acetocarmine ( plant chromosome ).A typical chromosome is called metacentric chromosome.It has two equal chromosome arms connected by a centromere or kinetochore.The centromere is the primary constriction of the chromosome.Besides this some chromosome has a secondary constriction at one end.The part above the secondary constriction is called Satellite.The chromosome with a satellite is called SAT chromosome.The secondary constriction is also called nucleolar organizer because the nucleolus is formed in this region.

The chromosome is covered by a pellicle.Inside the chromosome is a matrix in which the thread like chromonema are present.The chromonema bears equally spaced spherical bodies called chromomeres.They are treated as the genes.

A typical Meta centric chromosome

TYPES OF CHROMOSOMES

Chromosomes are classified in to four types based on the position of centromere. These are1.Metacentric chromosome : The centromere is present exactly in the middle of the

So the chromosome arms are equal in length.2.Sub meta centric The centromere is slightly placed towards one arm. So the

chromosome appears as L shaped during cell division.3.Acro centric The centromere is towards the end of the chromosome. The

chromosome appears as J shaped.4.Telocentric The centromere is at the tip of the chromosome. Usually such

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a chromosome does not exist and is formed when on arm breaks.

Other chromosome types

1.Acentric A chromosome without centromere. This is fomed when one arm

of the chromosome breaks leaving the arm without a centromere.2.Giant chromosomes. These are large chromosomes present in some insects and

amphibian oocytes. They are of two types.

A. Polytene chromosome - These are giant chromosome present

In the salivary gland cells of Drosophila. These are formed by Endo duplication of chromatin. The chromatin duplicate continuously with out cell division. This leads to the formation

of a large chromosome.At some points the chromosome has

swollen parts called chromosome puffs or Balbiani rings. These are

sites of high gene activity.

B. Lamp brush chromosome - These are giant chromosomes present in the oocytes of some amphibians. The chromosomelooks like bottle brush with numerous filaments arranged around a central filament.

3.SAT chromosome Usually one or two chromosome in a cell has a secondaryconstriction for accomodationg the nucleoli.. The secondaryconstriction is called as nucleolar organizer because thenucleolus is supposed to be formed from this region.The part above the secondary constriction is swollen andis called Satellite body and the chromosome as SAT chro-mosome.

SEX DETERMINATION

In higher organism males and females are found and the maleness and femaleness are determined by various factors. The process by which the sex of an individual is established is called sex determination. Chromosomes , hormones , environment etc. are various factors that determines the sex of the organisms. Various mechanisms of sex determination are found among organisms and theories have been put forwarded to explain the mechanism of sex determination. Some of the important theories are :

1.Chromosome theory of sex determination : Chromosomes play an important role in sex -mination. Egs. Man , Drosophila etc.

2.Gene balance theory : Put forwarded by Bridges.This theory says that the balance between the

genes present in the sex chromosomes and autosomes determine the sex.3.Environmental theory : The environment of the developing organism influence the expression of

sexual characters.

1.XX Femele XY Male mechanism or Homogametic female Heterogametic male method.

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Y-LINKED INHERITANCE

The somatic genes present in Y chromosome are called Y linked genes or holandric genes. They express only in males and no counter part allele is present in the X chromosome.Eg.Hypertrychosis – hair in the ear pinna

SEX LIMITED CHARACTERS

These are characters express only in one sex , either in male or female. It is not expressed in the other sex even though the gene is present.

Example 1.Feather pattern in poultry. In poultry two feather types are seen. Males have cock feathering and females have hen feathering .The cock feathering is limited to males only. Females will not develop cock feathering even though the gene is present. The hen feathering is a dominant character controlled by the gene h+ .Females show hen feathering both in the h+h+ and H+h conditions. Cock feathering appears in males only when both the genes becomes recessive hh .Cock feathering will not develop in females if hh genes are present. The hormone from the ovary of female inhibits the expression of the genes in females.

Genotype PhenotypeMale Female

h+h+ Hen feathered Hen featheredh+h Hen feathered Hen featheredhh Cock feathered Hen feathered

Example 2. Premature baldness in human malesExample 3. Milk production in cattles. Gene express only in females.

SEX INFLUENCED CHARACTERS

These are characters behave as dominant in one sex and recessive in the opposite sex. So the phenotype is different even though the genotype is same. This is mainly due to the influence of sex hormones.

Example 1.Horn development in sheep : In sheep, horned character is dominant in male and recessive in female. In the heterozygous condition horn appears in males and not in females.

Genotype PhenotypeMale Female

h+h+ Horned Hornedh+h Horned Hornlesshh Hornless Hornless

Example 2.Baldness in man.- This character is more frequent in males.Example 3.Hare lip - More frequent in malesExample 4.Spina bifida – forked spinal cord – more frequent in females.

MUTATION

Mutations are sudden or spontaneous heritable changes in the genotype of an organism. The mutation was observed first time by a Duch botanist Hugo de Vris in the evening prim rose Oenothera lamarkiana. This plant has narrow leaves and yellow flowers. Hugo de Vris got a

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new variety of oenothera with broad leaves and large flowers. He named the plant as Oenothera gigas. Experiments conducted in O.gigas revealed that it was formed as result of a sudden change in the genes of Oenothera lamarkiana. This lead to the formulation of the Mutation theory. According to the mutation theory , mutations are sudden changes in the genotype and are responsible for evolution. Thus evolution is a sudden change without intermediate stages.

TYPES OF MUTATIONS

Mutations are classified in to various types based on the site of occurrence and size.

Somatic mutation - Mutation in the somatic or body cells – non-heritable – no role in evolutionGerminal mutation – Mutation in the germ cells – heritable – form the raw materials of evolution

Germinal mutation may be Gametic (in gametes) or Zygotic (in zygote)

Natural mutation - Mutations that develop naturally.Induced mutations – Artificially produced mutation by mutagens.

Gene mutation - Mutation in the DNA or geneChromosome mutation - Mutation in the chromosomes – It may be structural changes

(Aberrations) or numerical changes (ploidy)

MECHANISM OF MUTATION

Mutations are of two types based on the effect on the genotype. These are gene mutations and chromosome mutations.

MUTATIONS

GENE MUTATION CHROMOSOME MUTATION

1.Deletion 1.Aberrations2.Addition or Insertion a.Deletion3.Substitution b. Addition

c. Inversiona. Transitions d. Translocationb. Transversions

2.Ploidya. Aneuploidy

1.Monosomy2.Nullisomy3.Trisomy

b.Euploidy1.Haploidy2.Diploidy3.Polyploidy

GENE MUTATIONS

These are changes in the structure or chemical nature of genes. Genes are made up of DNA and any change in the DNA will affect the hereditary characters of the organisms. Gene mutations will reflect as a defective phenotype. Since gene mutations affect only smaller

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regions of DNA, they are called as Point mutations. The following mechanisms are responsible for gene mutations.

1.Deletion.These are removal of one or more nitrogen bases or nucleotides from the DNA. Certain

Ionizing radiations, alkylting agents cause deletion of nitrogen bases.

2.Addition When one or more nitrogen bases or nucleotides are added to the DNA, the mutation is

called addition. Some mutagens like Acridine dyes (Acridine orange) causes additions.

Acridine dye gets incorporated between the nucleotides and makes a gap between them.

in to the gap additional base or nucleotide will be inserted.

3.Substitution :When a nitrogen base, either purine or pyrimidine is replaced by another ` base , then it is called substitutions. These are of two types: Transition and

Transversion

Transition When a purine is replaced by another purine or a pyrimidine is replaced byanother pyrimidine, the substitution is called transition. Transitions are mainly due to the following mechanisms.

A. Tautomerization - The rare forms of nitrogen bases are called Tautomers.The normal Nitrogen bases undergo tautomeric shifts – shifting of protons from one position to another – and becomes the tautomers. Such changes will alter the normal bases and the abnormal base will mis- pair with an unusual base leading to substitution.

B. Base analogoue - These are chemicals, which have structure identical to the bases. For example the 5-bromo uracil – 5BU – is a base analogue of thymine In the presence 5BU the adenine pairs with 5 BU instead of pairing with thymine. This changes the base sequences and causes substitution.

C. Deamination - The removal of amino group - NH2 group - from the nitrogen base is called deamination. Nitrous acid – HNO2 – remove amino group from bases and causes substitution.

Transversion When a purine is replaced by a pyrimidine or vice versa, then it is called tranversion.

GENE MUTATION AND FRAME SHIFTS

The most important effect of gene mutation is the Frame shifting. In the DNA molecule the nucleotides are arranged in a linear fashion. A group of three nitrogen bases form a genetic code that is transcribed to the m-RNA during protein synthesis. The code will rearrange after the base change and new codes are formed. These new codes will represent new amino acids and the proteins thus formed will be an abnormal one. This change in the reading frame of DNA will reflect as a defective phenotype. This is called frame shifts and the mutations as frame shift mutations.

Mis sense mutations : Mutations resulting in a different amino acid in the protein

Non sense mutations : Mutations resulting in termination codon . Protein synthesis stops in the termination codon

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Silent mutations : These are mutations that do not alter the amino acid sequence.

CHROMOSOME MUTATIONS

These are mutations affecting the structure or number of chromosomes. Structural changes are called aberrations and numerical changes are called Heteroploidy.

CHROMOSOME ABERRATIONS.

These are deletions , additions and translocations.

Deletion: Loss of a segment from the chromosome is called deletion. Some mutagens causes breaks in the chromosomes. This will result in the loss of many genes so that the characters controlled by the deleted part of the chromosome will be absent in the organism. Loss of the segment from the tip of the chromosome is called Terminal deletion and between the centromere and tip is called intercalary or interstitial deletion.

A B C D A D

Segment with genes B and C deleted

Clastogenic agents : These are chromosome breaking agents cause deletions. Egs. X ray Gamma rays.Cri–du–chat syndrome : It is a defect caused due to chromosome deletion in the short arm of the 5 th chromosome .Children with this defect shows mental retardation and physical defects. The cry of such children is cat like.

Addition or Duplication : When a new segment is added to the chromosome , the mutation is called addition. This causes repetition of genes in the chromosome and hence it is called duplication.

A B C D A B C B C D

Tandem duplication : The repeated genes may in the same order.Reverse tandem duplication : The repeated genes may be in the reverse order.

Duplication may increase the dose of genes which may beneficial or harmful.Example : In Drosophila the duplication in the 16 – A locus causes bar eye - narrow eye with reduced facets.

Inversion : If a chromosome segment is brocken and rejoined in the reverse order, the mutation is called Inversion.Paracentric inversion – Centromere is not included in the inverted segment.Pericentric - Centromere is included.

Inversition causes position effect, suppress crossing over etc.

Position effect : The inversition affect the mutual relationship between the genes. This is called position effect.G-banding : It is a method used to study the inversions in man. The chromosome is karyotyped and stained using Giemsa stain. The different regions of the chromosome stain differently.

Translocation : This the breakage of chromosome segment followed by its transfer to a non homologous chromosome.

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Simple translocation : Translocation of only one segment.Reciprocal translocation : Mutual transfer of chromosome segments between the non homologous chromosomes.Translocation may affect the phenotype because of the change in gene relations. It may lead to sterility in some plants. Example – Translocation in plants like Rhoeo and Oenothera.

Philadelphia chromosome :Recoprocal translocation in man produces a smaller 22nd

chromosome called Philadelphia chromosome .It is due to the translocation between chromosome 9 and 22.It is seen in patients with Chronic myeloid leukemia ( CML )

All chromosome aberrations play an important role in evolution because gene rearrangements and variations are produced by aberrations.

HETEROPLOIDY

These are numerical changes in chromosome make up. The normal number of chromosome is diploidy and any variation from the normal number leads to mutations.

Ploidy can be classified into Aneuploidy and Euploidy.

Aneuploidy : Numerical changes involving addition or deletion of one or more individual chromosomes is called aneuploidy. Full chromosome set is not involved. It may be1.Monosomy Deletion of only one chromosome from the diploid set. 2n-1 2.Nullisomy Deletion of two chromosomes from the diploid set. 2n-23.Trisomy Addition of one chromosome to the diploid set. 2n+14.Tetrasomy Addition of two chromosomes to the diploid set. 2n+2

Aneuploidy is the result of genetic non-disjunction.It is the abnormal separation of chromosomes at the time of cell division. Due to the abnormal spindles , some chromosomes may lag during the movement .This may lead to changes in the chromosome number in the new cells.

Examples 1.In man Turner’s syndrome is due to monosomy in the X chromosome2.Down’s syndrome Or Trisomy 21 is due to the addition of one chromosome in the 21st pair.3.In Datura aneuploidy produces changes in fruit shape

EUPLOIDY

This is the numerical change due to addition or deletion of entire set of chromosomes ( n )Euploidy is classified in to haploidy, diploidy, and polyploidy.

Haploidy This is the conditition in which the organism has only one set of chromosomes.The haploid condition of gametes is normal but in a diploid organism the haploidy is abnormal and produce mutation. Haploidy is produced naturally in some plants. They are usually weak and sterile. Haploidy can be induced in plants artificially by anther culture. Haploids produced from pollen grains are called Androgenic haploids. Datura plant can be produced by anther culture method. Honey bee male is a natural haploid. It is produced from unfertilized egg by parthenogenesis. Its chromosome number is 16 while that of female is 32.

DiploidyThis is the normal condition found in all most all organisms. In this condition two sets of chromosomes (paternal and maternal sets ) are present. This represents the 2n number. Each set is called a genome ( n ).

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Polyploidy Presence of more than two sets of chromosomes in the cell is called polyploidy. This may be triploidy ( 3n ),tetraploidy ( 4n ), pentaploidy (5n ), hexaploidy ( 6n )etc.Polyploidy is rare in animals but is a common phenomenon among plants. Polyploidy arises due to abnormalities in the cell division. Sometimes the cell division stops after the chromosome doubling. This converts a normal cell in to a polyploid cell. Polyploids may be autopolyploid or allopolyploid .In Autopolyploid the same genome is multiplied .For example potato is a autotetraploid. ( 4n =48).When different genomes in a hybrid are multiplied then it is Allopolyploid.For example the common wheat is allohexaploid .A new genus Tricale is produced by crossing Wheat with Rye.The hybrid is then subjected to induced polyploidy to produce an allopolyploid.

Wheat Rye

Hybrid ( sterile )

Induced polyploidy

Tricale ( allopolyploid ) Tricale is the first man made cereal.

Induced polyploidy Induced polyploidy is the artificially produced polyploidy. Polyploidy can be induced in many plants for agricultural purposes. A number of vegetables like tomato, potato, wheat, banana etc are polyploids .Banana is a sterile tatraploid. Colchicine is used induce polyploidy. It is a Mitotic Poison that can destroy the spindle fibres. If colchcine is applied to the growing seed or shoot , the mitotic division stops at the metaphase stage and the cells become polyploids. Colchicine is an alkaloid obtained from a plant Colchicum autumnale of Liliaceae.

MUTAGENS

Agents either physical or chemical that can produce a mutation is called Mutagen or Mutagenic agent. Mutagens are classified in to physical and chemical mutagens.

Physical Mutagens High energy radiations like electromagnetc or particulate radiations are capable of producing mutations. Most of the high energy radiations are ionizing radiations. They produce ion pairs in the genes and produce mutations. Ultra violet is a non ionizing radiation that can cause mutations.

Radiation Physical property Effect

X-ray Electromagnetic IonizingGamma rays Electromagnetic IonizingAlpha rays Particulate IonizingBeta rays Particulate IonizingNeutrons Particulate IonizingUltraviolet Electromagnetic Excitation

Chemical Mutagens Several chemicals act as mutagens. The first chemical mutagen identified is mustard gas. Many of the chemical mutagens can alter the chemical nature of genes and produce mutations.

Chemical mutagen Type Effect

Nitrous acid Deaminating agent Deamination of baseEthyl Methane

Sulphonate ( EMS ) Alkylating Add alkyl groups to basesEthyle Ethane

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Sulphonate (EES ) Alkylating Add alkyl groups to bases

5-Bromo uracil Base analogue Substitution

Acridine orange Fluorescent dye Additions

Site directed mutagenesis This is the modern technique to induce mutation in plant breeding., Molecular method is used to induce mutation only at the desired sites of the DNA.

Sharbati Sonora A variety of Wheat produced by induced mutation.

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