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| Name of Board | AHSEC |
| Class: | Assam Board Class 12 |
| Subject: | AHSEC Class 12 Biology |
| Number of Chapter: | 04 |
| Chapter Name | Text, Images, and PDF Format |
| Content Type: | Text, Images and PDF Format |
| Academic Year: | 2024-25 |
| Medium: | English |
| Available Solution Link: | AHSEC Class Biology Notes |
AHSEC Class 12th Biology Chapter: 4 Principles of Inheritance and Variation
(A) FILL IN THE BLANKS (1 MARK EACH)
1. In a Mendelian cross the first hybrid generation is called_______.
Ans: F₁ generation
2. Gregor Mendel, conducted hybridization experiments on_______.
Ans: Pea
3. Heterozygous parent produces ______ kinds of gametes.
Ans: Two
4. Genes which pair for a contrasting traits are known as_______.
Ans: Allele
5. Mendelian factors are presently known as______.
Ans: Genes
6. The physical association of two genes is termed as _______ by Morgan
Ans: Linkage
7. Sickle cell anaemia is caused by______ of Glutamic acid by_____.
Ans: substitution of, valine,
8. Phenylketonuria results in mental_______of brain
Ans: Retardation
9. Turners syndrome results due to loss of an_____ chromosome in human females.
Ans: ‘X’
10. The total number of chromosomes in a normal human cell is_____.
Ans: 46
(B) TRUE OR FALSE (1 MARK EACH)
1. 9:3:3: 1 is the Mendel’s dihybrid ratio.
Ans: True
2. The two alleles of a gene pair are located on homologous sites on the homologous chromosomes.
Ans: True
3. Chromosomal theory of inheritance was proposed by Hugo de Vries.
Ans: False
4. Female heterogamety in terms of sexchromosome is found in crow.
Ans: True
5. In sickle cell anemia substitution of glutamic acid by serine occurs at sixth position of a chain of haemoglobin.
Ans: False
(C) VERY SHORT ANSWER QUESTIONS (1 MARK)
1. Who rediscovered the experimental results of Mendel?
Ans: de Vries, Correns and Van Tschermak.
2. What is pedigree analysis?
Ans: The diagrammatic representation of inheritance of certain genetic character up to two or more successive generations is called pedigree analysis.
3. What is Mendels monohybrid ratio?
Ans: Phenotypic ratio is 3:1 and genotypic ratio is 1:2:1.
4. How many contrasting traits Mendel noted in garden pea?
Ans: 7 pairs of contrast characters.
5. Name the two main types of mutations.
Ans: Gene mutation and chromosomal mutation.
6. Who coined the term linkage?
Ans: T. H. Morgan.
7. Write the genotype of man with blood group AB.
Ans: IAIB.
8. What is codominance?
Ans: Codominance is the phenomenon in which alleles are able to express themselves independently when present together.
9. Which genes show very low recombination.
Ans: Linked genes.
10. Who is called as the father of genetics?
Ans: Gregor Johann Mendel.
11. What are holoandric genes?
Ans: Holoandric genes or Y-linked genes are present in the Y-chromosome which are directly inherited from father to son.
12. Define point mutation.
Ans: When mutation takes place due to single base pair change in DNA sequence is called point mutation.
13. Name the two kinds of linkage.
Ans: Complete linkage and Incomplete linkage.
14. What is frame shift mutation?
Ans: When mutation takes place due to deletion and insertion of base pairs of DNA is called frame shift mutation.
15. Which Mendelian disorder is known as Royal disease.
Ans: Haemophilia.
16. What is a dominant character?
Ans: The allele of a character which expresses its effect in the presence of its alternative allele is called the dominant character.
17. What are mutagens?
Ans: Mutagens are the agents of mutation.
18. What is a phenotype?
Ans: Phenotype is the observable or external characteristics of an organism.
19. What is a test cross?
Ans: Test cross is a special type of back cross which is made between individual with a dominant trait and its recessive parent in order to detect whether the dominant trait is homozygous or heterozygous.
20. How many chromosomes a person with Turner’s syndrome has?
Ans: 45 chromosomes.
21. Define chromosome.
Ans: A thread like structure of nucleic acids and proteins found in the nucleus of most living cell which carries the genetic information.
22.Express the genotype of AB blood group.
Ans. IA IB
23. Name the amino acid which is failed to convert to tyrosine during phenylketonuria.
Ans. Pherylalanine.
[D] SHORT ANSWER QUESTIONS (2 MARKS)
1. Differentiate between homozygous and heterozygous.
| Homozygous | Heterozygous |
| It has two same copies of the same allele coding for a particular trait. | It contains two different copies of alleles coding for a particular trait. |
| Contains only one type of allele, either dominant or recessive | Contains different alleles for a trait. Both dominant and recessive |
| Self-fertilization results in the repetition of the same traits over generations | Self-fertilization results in different combination of traits over the next generations |
| Only one type of gamete is produced | Two types of gametes are produced |
| It can be either homozygous dominant or homozygous recessive | Heterozygous alleles can show complete dominance, codominance or incomplete dominance |
| Point | Dominant Gene | Recessive Gene |
| Definition | Gene that expresses its phenotype even in the presence of a recessive allele | Gene whose phenotype is masked when a dominant allele is present |
| Expression | Represented by uppercase letters (e.g., T) | Represented by lowercase letters (e.g., t) |
| Phenotypic effect | Determines the observable trait | Only expressed when the dominant allele is absent |
| Point | Monohybrid Ratio | Dihybrid Ratio |
| Definition | Ratio of phenotypic outcomes in the offspring of a cross involving one trait | Ratio of phenotypic outcomes in the offspring of a cross involving two traits |
| Inheritance pattern | Follows Mendel’s law of segregation | Follows Mendel’s law of independent assortment |
| Example | 3:1 ratio in Mendelian monohybrid crosses | 9:3:3:1 ratio in Mendelian dihybrid crosses |
| Point | Sex Chromosome | Autosome |
| Location | Determine an individual’s sex (X and Y chromosomes in mammals) | Chromosomes other than the sex chromosomes |
| Inheritance | Follows sex-linked inheritance patterns | Follows typical Mendelian inheritance patterns |
| Number in humans | Two types: X and Y chromosomes in males, two X chromosomes in females | 22 pairs in humans, totaling 44 autosomes |
| Point | Genotype | Phenotype |
| Definition | Genetic makeup of an organism | Observable physical or biochemical characteristics of an organism |
| Example | Represented by letters (e.g., TT, Tt, tt) | Examples include height, eye color, and blood type |
| Influence | Determines the potential traits an organism can possess | Represents the actual traits expressed by an organism |
2. Define linkage.
Ans: Linkage is the phenomenon of certain genes staying together during inheritance through generations without any change or separation due to their being present on the same chromosome.
3. Why did Mendel selected pea plant for his experiments?
Ans: Mendel selected pea plant because-
(i) Pure varieties of pea were available.
(ii) Pea plants showed a number of easily detectable Contrasting characters.
4. Define dominance.
Ans: When two alleles of a gene are equally dominant and express themselves in the presence of the other, the phenomenon is called dominance.
5. What is mutation?
Ans: Mutations are new sudden inheritable discontinuous variations which appear in the organisms due to permanent changes in chromosome number, chromosome structure and genes.
6. What is polyploidy?
Ans: Polyploidy is the phenomenon of having more than two sets of chromosomes or genomes. Polyploidy occurs in natue due to the failure of chromosomes to separate at the time of anaphase either due to non-disjunction or due to non formation of spindle.
7. Under which condition, does the law of independent assortment hold good and why?
Ans: Law of independent assortment hold good under the condition that only those alleles can assort independently which are either present on different chromosomes or show regular crossing over. Otherwise all the genes present over one chromosome whould be inherited together which is called linkage.
8. The human male never passes on the gene for haemophilia to his son. Why?
Ans: Haemophilia is X-linked recessive disorder. The father passes only the Y-chromosome to the son and not the haemophilic X-chromosome. Therefore sons of haemophilic father never suffer from haemophilia.
9. Define chromosomal theory of inheritance.
Ans: Chromosomal theory of inheritance states that chromosomes are vehicles of hereditary information, possess mendelian factors or genes, segregate and assort independently during transmision from one generation to the next. It is actually the chromosomes and not genes which segregate and assort independently during meiosis and recombine at the time of fertilization in the zygote.
10. What are the Mendel’s laws of inheritance.
Ans: (i) Law of dominance It states that when two alternative forms of a trait are present in an organism only one factor expresses itsely in F, progeny and is called dominant while the other is recessive.
(ii) Law of segregation: It states that the alleles of a pair segregate from each other during gamete formation such that a gamete receives only one of the two factors.
(iii) Law of independent assortment: The two factors of each character assort independently at the time of gamete formation and get randomly rearranged in the offsprings.
11. What is Filial progeny?
Ans: First filial progeny represents the offsprings of a cross between two genetically. different parents.
Second filial progeny is formed through selfing of individuals of F₁ generation.
12. Write short notes on
(i) Sickle cell anaemia- This is an autosome linked recessive trait that can be transmitted from parents to the offspring when both the partners are carrier for the gene. The disease is controlled by a single pair of allele, HbA and Hbs. The defect is caused by the substitution of glutamic acid by valine at the sixth position of the beta globin chain of the haemoglobin molecule. The mutant haemoglobin molecule undergoes polymerisation under low oxygen tension causing the change in the shape of the RBC from biconcave to elongated sickle like structure.
(ii) Pedigree analysis: The study of inheritance of genetic traits in several generations of a human family in the form of a family tree diagram is called pedigree analysis. In pedigree inheritance of genes follow Mendelian principles and there are linkages and crossing overs. The advantages of pedigree are it helps in genetic counselling to avoid disorders and also shows the origin of a trait.
(iii) Turner’s syndrome: It is caused due to the absence of one of the X chromosomes, i.e. 45 with XO. They are sterile female with rudimentary ovaries, have shield shaped thorax, webbed neck, poor development of breasts, short stature, small uterus and puffy fingers.
(iv) Down’s Syndrome: the cause of this genetic disorder is the presence of an additional copy of the chromosome number 21 (21 trisomy). This disorder was first described by Langdon Down (1866). The affected individual is short statured with small round head, furrowed tongue and partially open mouth. Palm is broad with characteristic palm crease. Physical, psycho mental development is retarded. rand
(v) Haemophilia: It is a sex linked dessive disease which shows its transmission from unaffected carrier female to male progeny In this disease a single protein that is a part of the cascade of proteins involved in blood clotting is affected. Due to this, in an affected individual a simple cut will result in nonostop bleeding. The possibility of a female becoming a haemophilic is extremely rare because mother of such a female has to be at least carrier and the father shold be haemophilic.
13. What is point mutation? Give example. Ans. The gene mutation that involve a change in only a single nucleotide or nitrogen base of the cistron is called point mutation. Eg. Frame shift mutation due to insertion of one nucleotide. ATT CGC unsertiono of G→ TGA AGT TCG CTGA
14. Define aneuploidy with example.
Ans. Aneuploidy is a condition of having fewer or extra chromosomes than the normal genome number of the species. Aneuploidy is of two types, hypoploidy or loss of chromosomes and hyperploidy or addition of chromosomes.
E.g. Hyperploidy Trisomic (2n+1), Downs syndrome.
Hypoploidy monosomic (2n-1), Turner’s syndrome.
[E] SHORT ANSWER QUESTION (3 MARKS)
1. Elucidate Mendel’s law of segregation with the help of monohybrid creess.
Ans: When a cross is performed between two individuals taking a single contrast character at a time is called monohybrid cross. The character height has two alleles ‘T’ and ‘t’. ‘T’ exhibits tallness whereas ‘t’ exhibits dwarfness when a pure tall (TT) pea plant is crossed with a pure dwary (tt) plant, in the F_{1} generation, It (tall) plants are obtained. In the F_{2} generation (tall) TT, tt, (dwarf) and Tt plants are obtained after selfing F_{1} plant. Thus the contrasting trait did not show any blending at either F_{1} or F_{2} stage. Thus it follows the law of segregation which states that allele pairs segregate during gamete formation such that gametes receives only one of the two factors and do not show any blending.
2. What does Mendel’s law of independent assortment state?
Ans: Mendel’s law of independent assortment states that Cactors or alleles of different characters assort or separate ndependently of one another at the time of gametogenesis and get randomly rearranged in the offspring at the time of fertilisation. Law of independent assortment is applicable to only those factors or genes which are either located distantly on the same chromosome or occur on different chromosomes. All the genes or factors present on a chromosome are inherited together except when crossing over takes place.
3. What is codominance?
Ans: Chromosomal theory of inheritance states that chromosomes are vehicles of hereditary information, possess mendelian factors or genes, segregate and assort independently during transmision from one generation to the next. It is actually the chromosomes and not genes which segregate and assort independently during meiosis and recombine at the time of fertilization in the zygote.
4. How does it differ from incomplete dominance.
Ans :
| Dominance Type | Characteristics |
| Complete Dominance | – The effect of both alleles is equally conspicuous |
| – Both alleles produce their effects independently | |
| – Phenotypic ratio differs from genotypic ratio | |
| Incomplete Dominance | – Effect of one allele is more conspicuous |
| – Effect in hybrid is intermediate of the two alleles | |
| – Phenotypic and genotypic ratios are the same |
5. Give the purpose and results of a test cross.
Ans: Test cross is done between individual with a dominant trait and its recessive parent in order to know whether the individual is homogygous of heterozygous for the trait. The result is, if the individual was homozygous dominant offspring will be 100% dominant. The progeny will consist of 50% dominant and 50% recessive for trait if the individual is heterozygous, i.e. ratio is 1:1. In case of dihybrid, the test cross will give a ratio of propeny.
6. What are the various causes of human genetic disorders?
Ans: The various causes of human genetic disorders are-
(i) Mendelian disorders are caused due to alteration or mutation in single gene.
(ii) Chromosomal disorders are caused due to excess, absence or abnormal arrangement of one or more chromosomes. Failure of segregation of chromatids during cell division results in the gain or loss of a chromosome called aneuploidy. Failure of cytokinesis after telophase stage results in an increase in a whole set of chromosomes called polyploidy.
7. Explain the concept of dominance.
Ans. Genes contain information about the expression of particular characters. Diploid organisms have two copies of each gene. They are called alleles. The two alleles may be similar or dissimilar. The dissimilarities appear due to mutational changes in the nucleotides of the gene. The changes alter the information contained in the allele that results in altered expression. It may produce a defective protein/ enzyme, no protein or altered protein/ enzyme. Accordingly the allele fails to express the original trait. As it generally forms a defective tent product, the biochemical formed by the original allele that gives a defective product is called recessive factor or allele. However in case the mutated allele gives rise to on altered but functional product it behaves as codominant.
[F] LONG ANSWER QUESTIONS (5 MARKS)
1. Explain the inheritance of haemophilia in man.
Ans: Haemophilia is a sex linked recessive disorder which is transmitted from unaffected carrier female to some of the male progency. The heterozygous female (carrier) for haemophilia may transmit the disease to sons. The possibility of a female becoming a haemophilic is extremely rare because mother of such a female has to be a least carrier and the father should be haemophelic. The various possibilities of inheritance of haemophilia are
(i) Marriage betwwen a carrier woman (XX^h) and a normal man (XY)
(ii) Marriage between haemophiliac man (X^h Y) and normal woman
(iii) Marriage between haemophiliac man (X^hY) and carrier woman (XX^h)
2. How is sex determined in human beings.
Ans: Out of 23 pairs of chromosomes present, 22 pairs are exactly same in both males and females; called autosomes. A pair of X Chromosomes are present in the female, whereas the presence of an X and Y chromosome are determinant of the male characteristics. During spermatogenesis males produce two types of sperms 50% carrying X chromosome and 50% carrying Y chromosome. However females produce only one type of ovum carrying X-chromosome, If the sperm carrying X-chromosome fertilises the ovum, the zygote will develop into a female (XX) and if the sperm carrying Y-Chromosome fertilises the ovum, the zygote develops into a male (XY).
3. How would you distinguish between Klinefelter’s syndrome and Turner’s syndrome
Ans:
| Characteristic | Klinefelter’s Syndrome | Turner’s Syndrome |
| Chromosomal Abnormality | Extra X chromosome (XXY karyotype) | Missing or incomplete X chromosome |
| Gender | Male | Female (but can have ambiguous genitalia) |
| Physical Features | – Tall stature | – Short stature |
| – Reduced muscle mass | – Webbed neck | |
| – Gynecomastia (enlarged breasts) in some cases | – Broad chest with widely spaced nipples | |
| – Sparse facial and body hair | – Low hairline at the back of the neck | |
| Reproductive Organs | Usually have small testes and may be infertile | Underdeveloped ovaries, leading to infertility |
| Hormonal Effects | Elevated levels of estrogen, decreased levels of testosterone | Hormonal imbalances, including decreased estrogen and progesterone |
| Intellectual Development | Generally normal intelligence | Normal intelligence unless there are associated complications |
| Associated Conditions | Increased risk of learning disabilities, developmental delays, and behavioral issues | Heart defects, kidney abnormalities, infertility, and other health concerns |
5. What is Pedigree analysis? Suggest how such an analysis can be useful.
Ans: The diagrammatic representation of inheritance of certain genetic character upto two or more successive generations is called pedigree analysis.
Advantages
(i) It is used to know the possibility of expressive or recessive allele which may cause genetic disorder.
(ii) It shows the origin of a trait and flow of that trait in a family.
(iii) It helps in genetic counselling to avoid disorders in children.
(iv) It is extensively used in medical research.
6. Mention any two autosomal genetic disorders with their symptoms.
Ans: Two autosomal genetic disorders are –
(i) Sickle cell anaemia: It is transmitted from parents to the offspring when both the partners are carrier for the gene (heterozygous). The disease is controlled by single pair of allele, HbA and Hbs. Heterozygous (HbA HbS) individuals appear apparently unaffected but they are carrier of the disease. The defect is caused by the substitution of glutamic acid by Valine at the sixth position of the beta globin chain of the haemoglobin molecule.
Symptoms: 1) Under oxygen stress condition carythrocytes become sickle shaped which clogs the blood capillaries due to which oxygen supply is affected.
2) Anaemia
3) Jaundice
4) Headache and damage to brain.
(ii) Phenylketonuria: The affeced individual lacks an enzyme that converts the amino acid phenylalanine into tyrosine. As a result phenylalanine is accumulated and converted into phenyl pyruvic acid and other derivatives.
Symptoms:
1) Mental retardation
2) Skin and hair show hypo pigmentation
3) They are excreted through urine because of its poor absorption by kidney.
7. Give an account on chromosomal disorders.
Ans: Chromosomal disorders are caused due to absence or excess or abnormal arangement of one or more chromosomes. Failure or segregation of chromatids during cell division results in the gain or loss of a chromosome called aneuploidy. For eg. Down’s syndrome results in the gain of extra copy of chromosome 21. Turner’s syndrome results due to loss of an X chromosome in human females.
Failure of cytokinesis after telephase stage of cell division results in an increase in a whole set of chromosomes in an organism and the phenomenon is called polyploidy. It is often seen in plants. Sometimes either an additional copy of a chromosomes may be included in an individual or an individual may lack one of any one pair of chromosomes. These situations are called trisomy or monosomy respectively. Few chromosomal disorders are-
1) Down’s syndrome: It is caused due to the presence of an additional copy of the chromosome number 21 (Trisomy 21).
2) Klinefelters syndrome: Caused due to the presence of an additional copy of X chromosome (XXY).
3) Turner’s syndrome: Caused due to absence of one of the X chromosome (XO).
8. Write an account of Mendel’s experiments.
Ans: Mendel conducted hybridisation experiment on garden peas (1856-63) and proposed the laws of inheritance in living organisms. During Mendel’s investigations it was for the first time that statistical analysis and mathematical logic were applied to problems in biology. Mendel conducted such cross pollination experiments using several true breeding pea lines. Mendel conducted two experiments, Monohybrid & dihybrid cross, In monohybrid cross only one pair of contrast characters (tall & dwarf pea) were used and he observed that F₁ generation were all tall. Then he self pollinated tall F, plants and observed that some of the F₂ plants were dwarf some homozygous tall & some heterozygous tall in the ratio of 3:1.
In dihybrid cross Mendel choosed Round yellow and wrinkled green seeds. In the F₁ generation all the plants were round yellow indicating it as the dominant character. After self pollinating F₁ plants royndly yellow, prorisytall gren plants as well as round green and wrinkled yellow plants were also produced in the phenotypic ratio of 9:3:3:1.
Based on his experiments Mendel proposed the laws of inheritance. He proposed that during inheritance of characters from parents to offspring there is no blending rather each character separate independently at the time of gamete formation and get randomly rearranged.
9. Describe the various kinds of gene mutation.
Ans: Gene mutation is the change in expression of a gene which is caused by change in number sequence and types of nucleotides. Gene mutation may occur due to change in a single nucleotide. It is called point mutation. Mutation due to change in more than one nucleotide pair is called gross mutation. Most of the gene mutations develop due to errors in DNA replication and transcription. It is called copy error mutation. All genes undergo mutatious. A mutation from wild gene type to a new type is called forward mutation. Reversal of mutated gene to wild type is called reverse or back mutation. Gene mutation affecting somatic cells is called somatic mutation, Mutations occurring in germ cells which is transferable to the progeny is called germinal mutation. A mutation which does not cause any change in the product is called silent mutation. Mutation involving change in one codon and one amino acid in one protein is called mis sense mutation. A mutation which brings about early stoppage of polypeptide formation is called non sense mutation. Pleiotopic mutation is a gene mutation which influences a number of characters.
ONE WORD TECHNICAL TERMS (SPECIAL 1 MARK QUESTION)
1. Genes which code for a pair of contrasting traits.
Ans: Alleles,
2. Alteration of DNA and subsequent changes in genotype & phenotype.
Ans: Mutation.
3. Traisomy of 21 chromosome.
Ans: Down’s syndrome.
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Last Words on AHSEC Class 12 Biology Chapter: 4 Principles of Inheritance and Variation
The Chapter 4 of AHSEC Class 12 Biology deals with Principles of Inheritance and Variation for a brief understanding. You can Download this HS 2nd Year Biology Notes in PDF 2025.
