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Class 12 Biology Chapter 5 Principles of Inheritance and Variation
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Principles of Inheritance and Variation
Chapter – 5
GENETICS AND EVOLUTION
Very Short Answer Type Questions
Q.1. In a Mendelian cross the first hybrid generation is called ____
Ans : F₁ generation.
Q.2. Gregor Mendel, conducted hybridization experiments on ____
Ans : Pea (Pisum sativum)
Q.3. Heterozygous parent produces ____ kinds of gametes.
Ans : Two.
Q.4. Genes which pair for a contrasting traits are known as ____
Ans : Heterozygous.
Q.5. Mendelian factors are presently known as _____
Ans : Gene.
Q.6. The physical association of two genes is termed as_____ by Morgan.
Ans : Homologous.
Q.7. Sickle-cell anaemia is caused by ____ of Glutamic acid by ____
Ans : Removal, valine.
Q.8. Phenylketonuria results in mental ____ of brain.
Ans : Retardation.
Q.9. Turner’s syndrome results due to loss of a _____ chromosome in human females.
Ans : Sexual/sex.
Q.10. The total number of chromosomes in a normal human cell is _____
Ans : 46 (23 pairs)
Q.11. The XO syndrome is called _____
Ans : Turner.
Q.12. _____ coined the term linkage.
Ans : Morgan.
Q.13. The genetic disorder due to additional X chromosome is ____ syndrome.
Ans : Klinefelter’s
(B). True or False (One Mark Each):
Q.1. 9 : 3 : 3 : 1 is the Mendel’s dihybrid ratio.
Ans : True.
Q.2. The two alleles of a gene pair are located on homologous sites on the homologous chromosomes.
Ans : True.
Q.3. Chromosomal theory of inheritance was proposed by Hugo de Vries.
Ans : False.
Q.4. Female heterogamety in terms of sexchromosome is found in crow.
Ans : True.
Q.5. In sickle cell anemia substitution of glutamic acid by serine occurs at sixth position of a chain of haemoglobin.
Ans : False.
Q.6. Grasshopper is an example of XO type of sex determination.
Ans : True.
Q.7. In pea plant the green pod colour is dominant over yellow pod colour.
Ans : False.
(C). Very Short Answer Questions : (One Mark Each)
Q.1. Who rediscovered the experimental results of Mendel?
Ans : Hugode Vrics, Correns and Tschermak.
Q.2. What is pedigree analysis?
Ans : It is considered as the study of transfer and distribution of certain genetic traits through past generations of men and women. By pedigree analysis we can predict the chance of occurrence of genetic diseases in 2. their children even before a woman conceives.
Q.3. What is Mendel’s monohybrid ratio?
Ans : Mendel’s monohybrid ratio is 3:1.
Q.4. How many contrasting traits Mendel noted in garden pea?
Ans : 7 pairs (seven pairs)
Q.5. What is male heterozygosity?
Ans : When the males have different combination of chromosomes like XO and XY, then it is called as heterozygosity.
Q.6. Name the two main types of mutations.
Ans : Two types of mutations are :
(a) Chromosomal mutation.
(b) Gene mutation or point mutation.
Q.7. Who coined the term linkage?
Ans : Morgan first coined the term linkage.
Q.8. Write the genotype of man with blood group AB.
Ans : The genotype of a man of Blood group AB is Iᴬ Iᴮ
Q.9. What is codominance?
Ans : In case of codominance, alleles express themselves independently even when present together.
Q.10. Which genes show very low recombination?
Ans : Closely linked genes.
Q.11. Who is called as the father of genetics?
Ans : Gregor Johann Mendel.
Q.12. What are holandric genes?
Ans : The genes which are located in 4-chromosome and which express the male characters are called holandric genes.
Q.13. Define point mutation.
Ans : The mutation which occurs in gene and which causes physical or chemical change of the gene is called point mutation.
Q.14. Name the two kinds of linkage.
Ans : Linkages are of two types:
(a) Complete linkage.
(b) Incomplete linkage.
Q.15. What is female heterozygosity?
Ans : When the females are with two different sets of chromosomes then they are called as female heterozygosity. Heterozygous female possesses ZW and ZO chromosomes.
Q.16. What is frame shift mutation?
Ans : When the mutation is caused by addition or deletion of the base of DNA is called frame-shift mutation.
Q.17. Which Mendelian disorder is known as Royal Disease?
Ans : Haemophilia.
Q.18. What is a dominant character?
Ans : The character which can express in F₁ generation is called dominant character.
Q.19. What are mutagens?
Ans : The chemicals or agents which can be used to cause mutation is called mutagen.
Q.20. What is a phenotype?
Ans : The expressive or visible character of an organism is called the phenotypic character.
Q.21. What is a test cross?
Ans : A cross between F₁ hybrid (Tt) and its homozygous recessive parent (tt) is called test cross.
Q.22. How many chromosomes a person with Turner’s syndrome has?
Ans : Such persons are monosomic for sex chromosomes i.e. possess only one X and no Y chromosome (XO). In other words they have chromosome number 2n-1-45.
Q.23. Define chromosome?
Ans : Chromosome can be defined as a highly coiled DNA which is covered by chromatin substance. Chromosomes are the carrier of heredity. The chromosome number is fixed for a specific organism. For example Human being is with 46.
Q.24. Express the genotype of AB blood group.
Ans : The genotype of AB blood group is IᴬIᴮ.
Q.25. Name the amino acid which is failed to convert to tyrosine during phenylketonuria.
Ans : Phenylalanine.
(D). Short Answer Questions (Two Marks Each) :
Q.1. Differentiate between the following. (each pair two marks.
(a) Homozygous and Heterozygous.
Ans :
| Homozygous | Heterozygous | ||
| (i) | Homozygous individuals bear similar alleles (TT, tt) | (i) | Heterozygous individuals bear dissimilar alleles (Tt, Rr) |
| (ii) | Bear either dominant or recessive alleles never both. | (ii) | Always with both dominant and recessive alleles. |
| (iii) | It produces only one type of gametes i.e. T.T, or t, t. | (iii) | It produces both type of gametes i.e. T and t. |
| (iv) | They don’t exhibit extra vigour. | (iv) | They exhibit extra vigour. |
(b) Dominant gene and recessive gene.
Ans :
| Dominance | Recessive | ||
| (i) | It is able to express its effect even in the presence of its contrasting allele. | (i) | It cannot express its effect in presence of its contrasting alleles. |
| (ii) | It does not require the company of another similar allele for producing its effect on the phenotype. | (ii) | It can produce its phenotypic effect only in the company of a similar allele. |
(c). Monohybrid ratio and Dihybrid ratio.
Ans :
| Mono Hybrid | Dihybrid | ||
| (i) | The cross between two indiv iduals taking a single contrasting The or characters at a time. | (i) | The cross between two indivi duals taking two contrasting traits or character at a time. |
| (ii) | It helps to study the inheri tance of a pair of allele. | (ii) | It helps to study the inheritance of two pairs of allele. |
| (iii) | The phenotypic ratio in F₂ generation is 3 : 1. | (iii) | The phenotypic ratio in F₂ generation is 9: 3: 3:1. |
(d) Male heterozygosity and Female heterozygosity.
Ans :
| Male Heterozygosity | Female Heterozygosity | ||
| (i) | Here males are heterozygous. | (i) | Here females are heterozy gous. |
| (ii) | It involves XO and XY Type. | (ii) | It involves ZO and ZW type. |
| (iii) | Absence or presence of Y chromosome helps in determi- nation of sex in males. | (iii) | Absence or presence of W chromosome helps is determin ation of sex is female. |
(e) Sex chromosomes and Autosomes.
Ans :
| Sex Chromosomes | Autosomes | ||
| (i) | The chromosomes which help us to determine the sex i.e. male or female is called sex chromosome. | (i) | The chromosome which are found in all the cells of the organisms is called autosomes. |
| (ii) | They are either X or Y chromosome. | (ii) | There is no such division. |
| (iii) | XX indicates for female and XY indicates male in normal condition. | (iii) | Except X and Y others are autosomes. |
(f) Genotype and Phenotype.
Ans :
| Genotype | Phenotype | ||
| (i) | Genotype is the gene compl ement of an organism i.e. TT or Tt for a tall plant. | (i) | It is the expression of a character i.e. tall or dwarf. |
| (ii) | Genotype remains unchanged. the influence of environment. | (ii) | Phenotype may change under. |
| (iii) | It can be studied directly. | (iii) | It cannot be studied or observed directly. |
| (iv) | organisms with different geno type may have similar pheno- type i.e. Tallness (phenotype) may with TT or Tt. | (iv) | Organisms with different phenotype is always with different genotype. |
Q.2. Define linkage.
Ans : The gene located on the same chromosome may pass together as a single unit form one generation to other due to the phenomenon called likange.
Q.3. Why did Mendel selected pea plant for his experiments?
Ans: Mendel selected the pea plant for experiment for two reasons.
(i) Many varieties were available with observable alternate forms for a trait/character.
(ii) Peas are generally self-pollinated (self fertilised) and can be easily cross- pollinated when self-pollination is prevented.
Q.4. Define codominance.
Ans : When the F₁ generation resembles both the parents, and both the parental character are expressed simultaneously, then the phenomenon is called codominance.
Example : “AB’ type blood group is possible when allele ‘A’ and ‘B’ come together and since both the alleles are expressing their effect in F₁ generation and are therefore, codominance.
Q.5. What is mutation?
Ans : Mutation can be defined as a sudden change in genetic sequence of an individual which may lead to variation of characters.
Q.5. What is Polyploidy?
Ans : The phenomenon of failure of cytokinesis after cell division results in increase of a whole set of chromosome in an organism is called polyploidy.
Q.6. Under which condition, does the law of independent assortment hold good and why?
Ans : In dihybrid cross it appears that some contrasting characters which are recessive remain suppressed in F₁ generation. But during gamete formation, all contrasting characters which remained hidden in F₁ generation come out in different proportion which means the contrasting characters do not blend, rather these segregate and behave independent of each other. This is called independent assortment.
Q.8. The human male never passes on the gene for hemophilia to his son. Why?
Ans : The human male never passes gene to his son for haemophi because it is inherited through an X linked recessive gene. The haemophilia gene (Xʰ) is always associated with X-chromosome. Therefore daughter always receives Xʰ gene from father but not his son who receives Y. chromosome from father.
Q.9. Define chromosomal theory of inheritance.
Ans : The concept of chromosome was not given by Mendel, but he had mentioned as factor. It was found later that the ‘factors’ were the chromosome which were the carrier of heredity. Thus chromosome theory was come to light. Later it was proved that chromosomes were the carrier of heredity characters.
Q.10. What are the Mendel’s Laws of Inheritance?
Ans : Mendel’s laws of inheritance are:
(a) Law of dominance.
(b) Law of segregation.
(c) Law of independent assortment.
Q.11. What is Filial progeny?
Ans : In a cross between two parents, the first hybrid generation is called first filial generation or F₁ generation. Similarly subsequent generation are called F₂F₃ generation etc.
Q.12. Write short notes on : (each carrying two marks)
(i) Sickle cell anaemia.
Ans : It is caused by an autosomal mutant allele Hbs, that in homozygous condition causes the production of an abnormal haemoglobin. Due to presence of abnormal haemoglobin the normal shape of RBCS is changed and they assume elongated, filamentous and sickle celled forms. Such RBCS show a short life span, since they clump together and often cause obstruction in the blood vessels. This causes severe anaemia. This disease sometime even causes death of the patient due to damaged heart, kidney, spleen and brain as a result of bloated blood vessels.
(ii) Pedigree analysis.
Ans : It is considered as the study of transfer and distribution of certain genetic traits through past generations of men and women. By pedigree analysis we can predict the chance of occurance of genetic diseases in their children even before a woman conceives.
(iii) Turner’s Syndrome.
Ans : The persons with 44 autosomes and one X-chromosome (44+X) suffer from Turner’s syndrome. They possess 45 chromosomes instead of normal chromosome number 46. The patient have feminine characters but with rudimentary gonads. They have short stature, shield chest and prominent webbing of the neck.
(iv) Down’s Syndrome.
Ans : The main symptoms are rounded face, broad fore-head, permanently open mouth, protruding tongue, projecting lower lip, short neck, broad palm with characteristic palmar crease and Mongolian type eye-lid fold.
(v) Haemophilia.
Ans : The human male never passes gene to his son for haemophilia because it is inherited through an X linked recessive gene. The haemophilic gene (Xʰ) is always associated with X-chromosome. Therefore daughter always receives Xʰ gene from father but not his son who receives Y- chromosome from father.
Q.13. What is point mutation? Give example.
Ans : The mutation that takes place in gene or DNA chain is called gene mutation or point mutation. Here as a result of gene mutation new alle;es of a gene are formed.
It is found genetically changed plants and animals.
Q.14. Define aneuploidy with example.
Ans : In aneuploids, the chromosome complement either has one or more chromosomes or one or more chromosomes less than the normal complement. The sematic chromosome number of an aneuploid differs from normal in the sense that it is not an exact multiple of basic haploid number. The number may be 2n+1, 2n+2, 2n-1, 2n-2 and so on.
Aneuploidy can be either due to less of one or more chromosomes (hypoploidy) or due to addition of one or more chromosomes to complete chromosomal complement (hyperploidy). Monosomy (2n-1), Nullisomy (2n-2), are the example of polyploidy while the trisomy (2n+1) and tetrasomy (2n+2) are the hyperploidy.
(E). Short Answer Question (3 Marks) :
Q.1. Elucidate Mendel’s law of segregation with the help of monohybrid cross.
Ans : Mendel selected garden pea for his experiment because it shows many contrasting characters. He selected seven pairs of contrasting characters in Pisum sativum.
Mendel first crossed pea plants differing in a single pair of contrasting character, such as tallness and dwarfness, smooth seeded and wrinkled seeded, yellow seeded and green seeded etc. He called the initial cross between two varieties as parental or P₁ generation. The offspring of the F₁ generation were found to bear, the character of one of the parents only and not of both in F₁ generation. The contrasting characters never mix up F₁ generation is the offspring of P₁ generation. When Mendel observed that in F₂ generation the contrasting characters were not evident he allowed the plants of F₁ generation to self-pollinate.
He collected all the seeds and sown them to produce F₂ generation. He noted with great interest that in F₂ generation the contrasting character came out in the proportion of 3:1. That was out of every 4 plants 3 plants showed the characters of one of the parent of P₁ generation grid and the other one showed the character of the other parent. He noted with interest that out of the 3 plants which showed dominant character did not breed true. Out of the 3 plants only one breed pure and the other-2 segregated in the ratio of 3 :1 again. But out of the 4 plants in F₂ generation the one which showed only recessive character did not segregate but breed true to its type.
Mendel called this as Monohybrid Ratio and the cross between the plants in which one pair of contrasting characters were taken was called Monohybrid cross.
In monohybrid cross in F₁ generation the offsprings are tall. Tallness is dominant and dwarfness is recessive. The dominant factor masked the recessive character. In F₂ generation the recessive character segregate out as pure dwarf. In a allelic pair always there will remain one dominant and one recessive trait. In F₁ generation, although the plants look tall, it is not pure tall. It has in it the recessive character (dwarfness) also. So it is only phenotypically tall but genotypically it carries the traits of both tallness and dwarfness (Tt).
Due to dominant factor, all plants of F₁ of their dominant parent. The recessive characters will however remain present in the somatic cells of the F₁ individuals throughout their whole life. But during the formation of gametes by reduction division each contrasting character segregates out and if such plant of F₁ generation are allowed to be self-pollinated then the resultant individuals will show character of either parent.
That is both the parental characters of P₁ generation will segregate out again in the F₂ generation. This is to say that if the characters of the parents remains suppressed in F₁ ; generation, it comes out in the F₂ generation. It means that the contrasting characters does not blend, contaminate or affect each other while they are together in the hybrid in the F₁ generation and segregare out in subsequent generation.
TT, Tt, Tt = 3 (Tall)
tt = 1 (dwarf)
∴ The ratio of Tall (TT/Tt) And dwarf (tt) is 3: 1.
Q.2. What does Mendel’s law of independent assortment state?
Ans : In F₁ generation only, the dominant character is visible. Recessive characters remain masked. But in F₂ generation, all the contrasting characters appear in different proportions. This shows that characters never blend. They remain pure and reappear at certain stage. This phenomenon is governed by the Mendel’s law of independent assortment. This also indicates that the gametes remain pure and reappear with distinct character.
Q.3. What is codominance?
Ans : Long years of study has shown that man inherits the blood group. There are three alleles which determine the blood group. A man may possess any one or two alleles which are located in one of the 22 autosomes. No person can possess antigen-A or antigen-B without hav their corresponding genes in autosomes. Similarly, if a person in his blood possesses antigen-A and antigen-B then he must carry the alleles corresponding to both the antigens. Here both the alleles-A and B are dominant and neither of them is suppressed by the other. Because of this unusual method of inheritance it has become customary to use the letter T(Isohemagglutinin) as symbol of the genes and with a second letter as an exponent to indicate which variation of the alleles is represented. For example Iᴬ represents the gene which produces antigen-A and Iᴮ is gene which produces antigen-B.
These alleles are found either in homozygous condition or heterozygous condition. Iᴬ in homozygous condition (Iᴬ Iᴬ) produces antigen-A, Iᴮ in homozygous condition (Iᴮ Iᴮ) produces antigen-B. But both alleles in heterozygous condition (Iᴬ Iᴮ) produces antigen-A and antigen-B (A and B being both dominant) I° is recessive to Iᴬ and Iᴮ. Therefore Iᴬ I° produces antigen-A and Iᴮ I° produces antigen-B. But I° in homozygous condition (I°I°) produces neither antigen-A nor antigen-B.
| Sl. No. | CONTENTS |
| Chapter 1 | Reproduction in Organisms |
| Chapter 2 | Sexual Reproduction in Flowering Plants |
| Chapter 3 | Human Reproduction |
| Chapter 4 | Reproductive Health |
| Chapter 5 | Principles of Inheritance and Variation |
| Chapter 6 | Molecular Basis of Inheritance |
| Chapter 7 | Evolution |
| Chapter 8 | Human Health and Disease |
| Chapter 9 | Strategies for Enhancement in Food Production |
| Chapter 10 | Microbes in Human Welfare |
| Chapter 11 | Biotechnology: Principles And Processes |
| Chapter 12 | Biotechnology and its Applications |
| Chapter 13 | Organisms and Populations |
| Chapter 14 | Ecosystem |
| Chapter 15 | Biodiversity and Conservation |
| Chapter 16 | Bioresources of Assam |
| Chapter 17 | Environmental Issues |
Q.4. Different between incomplete dominance and codominance.
Ans : Difference between incomplete dominance & codominance:
| Incomplete Dominance | Codominance | ||
| (i) | One of the two alleles shows more conspicuous effect. | (i) | Both the alleles shows equal effectiveness. |
| (ii) | Hybrid is the intermediate expression of two alleles. Iᴬ and I°, Hbˢ and Hbᴬ. | (ii) | Both the alleles shows their effect independently eg. |
| (iii) | Expression of hybrid is mixture of two alleles. | (iii) | No mixing takes place due to effect of two alleles. |
Q.5. Give the purpose and results of a test cross.
Ans : Test cross is a cross between phenotypically dominant F₁ hybrid with recessive parent to test the former whether it is homozygous or heterozygous. For example in F₁ generation there may be two types of tall plants such as TT and Tt. Both are tall. The former is homozygous and the later is heterozygous. To know which one is homozygous a test cross is conducted between all the tall plants and the recessive parent (tt). In cross between tall plants and the dwarf parent it will be observed that in certain cases the dominant amd the recessive character will appear in 50: 50 ratio. The F₁ heterozygous tall (Tt) plants can be identified on the basis of the result of the test cross result that produce hybrid tall plants and dwarf plants in the ratio of 50: 50.
Q.6. What are the various causes of human genetic disorders?
Ans : (i) Mendelian Disorder : Mendelian disorder occur due to alteration or mutation of single gene. Such disorder is associated with defect of enzyme action as because each gene control one enzyme. Such disease is inherited in Mendelian fashion.
Phenylketonuria, alkaptonuria are such diseases.
(ii) Chromosomal Disorder : Various genetic diseases are known which are caused by abnormality of the autosomal chromoscme (not connected with determination of sex). Autosomal abnormality may be due to polyploidy (more than normal diploid set of chromosome) or may be due to aneuploidy (one chromosome may be more or less than the normal). Aneuploidy has several forms. Sometime 21st pair or 18th pair or 13th pair of chromosomes fail to separate during meiosis. Fertilization between one normal gamete and one abnormal gamete with non- disjunctioned chromosome gives rise to individual having one extra chromosome such as 3 sets of 21st or 18th or 13th Chromosome.
This phenomenon is called trisomy. Similarly one chromosome of 21st pair or 18th pair may deleted. In such condition the diploid cell will contain one chromosome less. This phenomenon is called monosomy. In trisomic condition the body cells contain 47 chromosomes (one chromosome more) but in monosomic condition the body cells contain 45 chromosomes (one chromosome less).
Q.7. Explain the concept of dominance.
Ans : Of the two contrasting hereditary unit characters, the one that expresses itself in the F₁ generation is called dominant and the other which remains latent is called recessive. For example, if tall and dwarf plants are crossed then ‘fallness’ being the dominant character over the “dwartness’ all plants in the F₁ generation will be tall. But dwarfness which remains latent in F₁ generation will express itself in F₁ generation. Similarly, round seeded plant is dominant over wrinkled seeded plants, yellow seeded plants is dominant over the green suede plants.
It was found later that dominance is absent in many cases In such cases, the hybrid does not resemble either of the presents exactly. Sometimes, they resemble in between the two. For example, when while snapdragon plant is crossed with red snapdragon plant, the hybrid is pink flowered. So, the dominance is incomplete . In such case the term incomplete dominance is applied.
(F). Long Answer Question (5 Marks) :
Q.1. Explain the inheritance of haemophilia in man.
Ans : Haemophilia : This is a disease occur due to sex-linked inheritance. It is called bleeder’s disease. It is caused by a mutant recessive gene (h) present on X-chromosome. In such disorder the blood lacks thromboplastin which helps in blood clotting. So in the absence of blood clotting substance a minor cut or injury may cause prolonged bleeding leading to death. A female may become haemophilic only when both the X-chromosomes carry the recessive gene. When the mutant gene is present in one of the X-chromosome the female becomes the carrier without being haemophilic herself. In male again if the X-chromosome carries the mutant gene then he becomes haemophilic because the corresponding Y-chromosome does not carry a corresponding dominant gene. Therefore, haemophilic disease is always caused by X-linked gene. It may also be called sex-linked inheritance.
Q.2. Describe the various kinds bene mutation.
Ans : Gene Mutations : Gene or point mutations are stable changes in genes i.e. DNA chain. Although each gene is a potential site for a mutation, yet some genes mutate more frequently then other genes. Such mutable genes are widely found in plants and animals. In bacteria spontaneously arising from gene mutations occur with a frequency of about one in 10⁶ gene duplications. Many times a change in a gene or nucleotide pair doesn’t produce detectable mutation. Thus the point or gene mutation means the process by which new alleles of a gene are produced. The smallest part of gene that can mutate is called muton. The smallest muton in a gene is a single base pair of DNA.
Sickle cell anaemia arises due to a change of amino acid glutamine to valine in the sixth position of B. chain of haemoglobin.
Q.3. How is sex determined in human beings?
Ans : The same XX-XY method of sex determination is found in human. In human the diploid cells contain 46 chromosomes (23 pairs). Of these, 44 chromosomes are autosomes and the other 2 chromosomes are sex- chromosomes. The pair of sex-chromosomes may be constituted either.
by a pair of XX chromosomes or XY chromosomes. Male diploid cells contain 44+XY chromosomes and female diploid cells contain 44+XX chromosomes. So the sperms contain 22+X (50%) or 22+Y (50%) sets of chromosomes. But the eggs always contain 22+X (100%) sets of chromosomes. Therefore, when sperm with 22+X unites with egg having 22+X the resultant zygote is produced with 44+XX chromosomes. Thus the offspring becomes a female baby. On the contrary if sperm carrying 22+Y chromosomes unites with the egg having 22+X chromosomes the zygote that is produced developes into a male baby (44+XY). Therefore the Y-chromosome takes leading part in determination of sex. In the absence of Y chromosome the baby is female and in its presence the baby is a male.
Q.4. How would you distinguish between Klinefelter’s syndrome and Turner’s syndrome?
Ans :Turner’s Syndrome : A female with one chromosome shows Turner’s syndrome. Such males are sterile, of short stature, webbed necked and with short hair and have low intelligence.
Klinefelter Syndrome : This syndrome develops when abnormal egg have XXY chromosomes. In such case an abnormal male child is born. Such child will have small testicles, longer arms, low breast, high pitched voice. They are also mentally retarded.
Q.5. Write an account of Mendel’s experiments.
Ans : Gregor Johann Mendel was born in 1822 in a village of Heinzendort (new czechoslovakia) and in 1857 he began his famous experiment on peas in monastery garden. He considered pea (Pisum sativum) for his experiment because.
(a) It was easy to cultivate.
(b) Many contrasting characters.
(c) Prefers cross as well as self pollination.
(d) Result can be obtained within short period only.
(e) Flowers were bisexual.
Mendel considered the following character only :
(1) Seeds Smooth Wrinkled
(2) Colour of seed Yellow Green
(3) Height of plant Tall Dwarf –
(4) Portion of flower Axial Terminal
(5) Seed coat White Grey
(6) Endosperm/cotyledon Yellow Green
Mendel kept notice of each character separately and carefully in each and every generations during the course of his experiment.
Mendel first crossed pea plants differing in a single pair of contrasting characters, such as tallness and dwarfness, smooth seeded and wrinkled seeded, yellow seeded and green seeded etc. He called the initial cross between two varieties as parental or P₁ generation. The offspring of the P₁ generation were found to bear the character of one of the parents only and not of both. For example, a cross between tall and dwarf plants, always produce tall plant in F₁ generation. The contrasting characters never mix up. F₁ generation is the offspring of P₁ generation. When Mendel observed that in F₂ generation the contrasting characters were not evident he allowed the plants of F₁ generation to self-pollinate.
He collected all the seeds and sown them to produce F₂ generation. He noted with great interest that in F₂ generation the contrasting character came out in the proportion of 3: 1. That was out of every 4 plants 3 plants showed the character of one of the parent of P, generation and the other one showed the character of the other parent. He noted with interest that out of the 3 plants, which showed dominant character did not breed true. Out of the 3 plants only one breed pure and the other-2 segregated in the ratio of 3 :1 again. But out of the 4 plants in F₂ generation the one which showed only recessive character did not segregate but breed true to its type.
The above facts can be explained taking into consideration of one pair of contrasting characters. For example, in P₁ generation the contrasting characters are tallness and dwarfness. They were pure tall and dwarf respectively as they breed true to their types generation after generation having in them the tallness of both the parents and dwarfness of both the parents. Their charactery therefore can be represented as TT and tt respectively. The tall (TT) and dwarf (tt) will produce gametes T and t respectively. These two gametes after fertilization will produce offspring in the F₁ generation which will exhibit tallness. The dwarfness will remain latent in them as it will contain both characters T and t in its gametes.
If these plants are allowed to self-pollinate then in F, generation tallness and dwarfness will be segregated in the ratio of 3: 1. The plants carrying the characters- TT, Tt and Tt will all be tall act being dominant characters). The other plant receiving tt character will be pure dwarf. Out of the TT, Tt and tt plants the TT will breed pure tall plants and the Tt and Tt will breed again in the ratio of 3:1. Mendel called this as Monohybrid Ratio and the cross between the plants in which one pair of contrasting character were taken was called Monohybrid cross. TT and Tt show different genetic characters (genotype) but same visible character (phenotype).
Q.6. Give an account on chromosomal disorders.
Ans : Various genetic diseases are known which are caused by abnormality of the autosomal chromosomes (not connected with determination of sex). Autosomal abnormality may be due to polyploidy (more than normal diploid set of chromosomes) or may be due to aneuploidy (one chromosomes may be more or less than the normal). Aneuploidy has several forms. Sometime 21st pair of 18th pair or 13th pair of chromosomes fail to separate during meiosis. Fertilization between one normal gamete and one abnormal gamete with nondisjunction chromosome gives rise to individual having one extra chromosome such as 3 sets of 21st or 18th or 13th chromosome. This phenomenon is called trisomy.
Similarly one chromosome of 21st pair or. 18th pair may be deleted. In such condition the diploid cell will contain one chromosome less. This phenomenon is called monosomy. In trisomic condition the body cells contain 47 chromosomes (one chromosome more) but in monosomic condition the body cells contain 45 chromosomes (one chromosome less). Abnormality of sex chromosome also causes genetic diseases of many types. Three or more sets of sex-chromosome results form non-disjunction of sex chromosome during gamete formation. The normal pair of sex- chromosome in human is either XX or XY. But due to abnormality the sex-chromosomes set may be XXY or XYY or XXXY etc.
Genetic disorder may also be caused by change in the sequence of gene in chromosome. Change in the sequence of gene may be caused by mutagenic agents or may be due to loss of genes by accidental breakage of chromosomal part or it may be due to addition of genes by attachment of broken chromosomal part with another chromosomes.
Some inherited diseases have been briefly mentioned below :
(i) Down’s Syndrome : This syndrome is also called mongolism. In the middle aged female the 21st pair of chromosome may not be able to separate during oogenesis. Thus a fertilized agg may contain 3 sets of 21st chromosome. Thereby total chromosomes in the diploid nucleus becomes 47 instead of normal 46. A child having 3 sets of chromosome number 21 (trisomy-21) may suffer from physical and mental defects. Mental retardation is severe in such case. They may have below average height, widely separated eyes, flattened nose, short but broad feet etc.
(ii) Monosomy-21 : If the fertilization egg is devoid of complete set of 21st chromosome the body cell will contain 45 chromosomes. Loss of one chromosome will produce a monosomic child with large ears and closely placed eyes.
(iii) 18-Trisomy : A Child having 3 sets of 18th chromosome possesses laterally flas head and the helix of the car scarcely develops. The hands are short and show little development of the second phalanx.
(iv) 13-Trisomy : A child having 3 sets of 13th chromosome shows severe body and organ malformation. Such children are also mentally retarded. Head is small and eyes are often very small or absent.
Genetic Disorders:
(i) Mendelian Disorders:
(a) These are mainly due to alternation or mutation in single gene.
(b) The disorder may be dominant or recessive.
(c) These may be metabolic abnormalities.
(i) autosomal (eg. sickle-cell anaemia, phenylketonuria)
(ii) sex-linked (eg. haemophilia, colour blindness)
(ii) Chromosomal Disorder :
(a) These may be due to excess of certain chromosome or abnormal arrangement or structural defect of certain chromosome.
(b) Some of the more serious disorders-Down’s syndrome, Klinefelter’s syndrome, Turner’s syndrome.
Q.7. Give an note on phenylketonuria.
Ans : It is a Mendelian disorder. It causes metabolic disorder due to non production of specific enzyme at particular time of the relation. This happen when the gene controlling certain enzyme become in effective due to mutation. As enzyme is available for conversion of certain substrate it accumulates and cause metabolic malfunctioning.
Phenotypic expression in individual depends upon the end product of a particular series of reaction where a large number of enzymes take part and for which a large number of genes are involved. In the following scheme, it may be noted that in the normal phenylalanine metabolism if block appears at step 1, phenylalanine will not be converted to tyrosine and as a result of which the reaction will not proceed further. Phenylalanine will accumulate in excess in blood resulting into a disease called phenylketonuria (mental retardation, pale skin, epileptic tendency). Similarly blockage at step 4 causes excessive accumulation of homogentisic acid that passes through urine. Oxidation of homogentisic acid by air turns urine black. The disease is called alkaptonuria.
Phenylalanine → tyrosine → p-hydroxyphenylpyruvate → homogentisic acid → male-ylacetoacetic acid → fumarylacetoacetate acid → fumaric acid→ acetoacetic acid → citric acid cycle.
Q.8. What is pedigree analysis? Suggest how much an analysis can be useful?
Ans : Pedigree analysis is an analysis of the distribution and movement of traits in a series of generation of a family.
It is useful. Because:
(i) It is an important method to study human genetics.
(ii) Genetic disorders occurring in a family can be identified and the movement of the trait in the future generation can be predicted.
Q.9. Mention any two autosomal genetic disorder with their symptoms.
Ans : Two autosomal genetic disorders are:
(i) Down’s syndrome : The main symptoms are rounded face, broad fore-head, permanently open mouth, protruding tongue, projecting lower lip, short neck, broad palm with characteristic palmar crease and Mongolian type eye-lid fold.
(ii) Phenylketonuria : It is an inborn error of metabolism. The effected individual lack an enzyme called phenylalanine hydroxylase that converts the amino acid phenylalanine into tyrosine. As a result of this phenylalanine is accumulated and converted into phenylpyruvic acid and other derivatives. Accumulation of this in brain causes mental retardation. There are also excreted through urine due to their absorption be kidney.
Q.10. Explain the law of Independent assortment using Punnett square.
Ans : The law of Independent assortment states that the genes of different characters created in different pairs of chromosomes are in dependent of one another in their segregation during gamete formation (meiosis), The principle of Independent assortment can also be defined as ‘ If we consider the inheritance of two or more genes at a time, their distribution in the gametes and progeny of subsequent generations is independent of each other.
In the dihybrid cross that each pair of characters (alletes) behave independently of other pair of characters. One character has no influence on another character and assort independent of other.The dihybrid ratio and independent assortment of alleles can be represented schematically and the ratio is 9:3:3:1
(i) Yellow round – 1,2,3,4,5,7,9,10,13 = 9
(ii) Yellow Wrinkle – 6,8, 14 = 3
(iii) Green round – 11, 12, 15 = 3
(iv) Green wrinkle – 16
One Word Technical Terms (Special 1 Mark Question)
Q.1. Genes which code for a pair of contrasting traits.
Ans : Allele.
Q.2. Alteration of DNA and subsequent changes in genotype phenotype.
Ans : Mutation.
Q.3. Trisomy of 21 chromosome.
Ans : Down’s syndrome/Mongolism.
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