CBSE Class 12 Biology Principles of Inheritance And Variation Worksheet Set B

Question. Name the respective pattern of inheritance where F1 phenotype
(a) does not resemble either of the two parents and is in between the two.
(b) resembles only one of the two parents.
Answer. (a) Incomplete dominance is the condition in which the F1 phenotype does not resemble both the parents and is in between the two.
(b) Complete dominance is the condition in which the F1 phenotype resembles the dominant parent i.e., one of the two parents.

Question. How would you find the genotype of an organism exhibiting a dominant phenotype trait? 
Answer. By performing a test cross, one can and the genotype of an organism exhibiting a dominant phenotypic trait.

Question. Write the percentage of the pea plants that would be heterozygous tall in F2 generation when tall heterozygous F1 pea plants are selfed. 
Answer. 50% of heterozygous tall pea plant is obtained in F2 generation. For more refer to answer 11.

Question. In a test cross progeny of pea plants, all were bearing violet flowers. Give the genotypes of the parent pea plant. 
Answer. Genotypes of the parent pea plant : Homozygous dominant (violet – VV) and homozygous recessive (white – vv).

Question. Differentiate between dominance and co- dominance. 
Answer. 

Question. Mention the type of allele that expresses itself only in homozygous state in organism.
Answer. Recessive allele, e.g., tt represents dwarf plant.

Question. When at a l l p l ant wa s s e lf-p o l lin at e d , one-fourth of the progeny were dwarf. Give the genotype of the parent and dwarf progenies. 
Answer. Genotype of parent = Tt
Genotype of dwarf progeny = tt

Question. (a) Write the conclusions Mendel arrived at on dominance of traits on the basis of monohybrid crosses that he carried out in pea plants.
(b) Explain why a recessive allele is unable to express itself in a heterozygous state.
Answer. (a) Whenever Mendel carried out a cross between plants for a contrasting trait he found that only one trait out of the two appears in the F1 generation. He concluded that the trait which is expressed in F1 is dominant while the one which remains hidden is recessive. He also said that characters are controlled by discrete unit called factors which occur in pair.
(b) In a diploid organism, there are two copies of each gene, i.e., pair of alleles. These two alleles are not always identical, as in a heterozygote. One of them may be modified due to mutation. The unmodified functional allele that represents the original phenotype behaves as dominant allele and codes for functional protein. The mutated non- functional allele behaves as recessive allele and codes for mutant or non-functional protein. The phenotype of the organism will only be dependent on the functioning of the unmodified allele. Hence, in a heterozygote, the dominant allele will express itself whereas recessive allele will remain hidden.

Question. In pea plants, the colour of the flower is either violet or white whereas human skin colour shows many gradations. Explain giving reasons how it is possible.
Answer. Flower colour in pea plant is a case of Mendelian inheritance where only one of the parental trait appears at F1. The contrasting trait did not show any blending. Human skin colour is an example of polygenic inheritance. The inheritance is controlled by three genes in which the dominant alleles have cumulative effect with each dominant allele expressing a part or unit of the trait.

Question. Human population shown variations in blood groups. Explain the genetic basis for this variation seen in the population. 
Answer. ABO blood groups are controlled by gene I. The gene I has three alleles IA, IB and i. This phenomenon is known as multiple allelism.
The blood groups and their possible genotypes are given below in the table :

Question. Expl ain pleiot ropy w it h t he help of an example. 
Answer. The ability of a gene to have multiple phenotypic effect because it influences a number of characters simultaneously is known as pleiotropy. The gene having a multiple phenotypic effect because of its ability to control expression of two or more characters is called pleiotropic gene. For example, in cotton a gene for the lint also influences the height of plant, size of the boll, number of ovules and viability of seeds.

Question. In a cross between two tall pea plants some of the offsprings produced were dwarf. Show with the help of Punnett square how this is possible. 
Answer. Tall plants may either have genotype TT or Tt. Two tall pea plants that produce some dwarf plants among their progenies must be heterozygous with the genotype Tt, because TT plants cannot produce dwarf offsprings as they lack the allele for dwarfness (t) and hence cannot transfer it to the progeny. Besides, both of them should have a ‘t’ allele as dwarfness is expressed in homozygous (tt) condition only. It can be expressed using Punnett square as follows:

Question. Inheritance pattern of ABO blood groups in humans shows dominance, codominance and multiple allelism. Explain each concept with the help of blood group genotypes.
Answer. Dominance : The alleles IA and IB both are dominant over allele i as IA and IB form antigen A and antigen B respectively but i does not form any antigen.
Codominance : Both the alleles IA and IB are codominant as both of them are able to express themselves in the presence of each other in blood group AB (IAIB) by forming antigens A and B. Multiple allelism : It is the phenomenon of occurrence of a gene in more than two allelic forms on the same locus. The ABO blood groups in humans are determined by three different allelic forms IA, IB and i.

Question. (a) What is polygenic inheritance? Explain with the help of a suitable example.
(b) How are pleiotropic inheritance different form polygenic pattern of inheritance?
Answer. (b) Polygenic inheritance is a type of inheritance controlled by one or more genes in which the dominant alleles have cumulative effect with each dominant allele expressing a part or unit of the trait, the full trait being shown only when all the dominant alleles are present. Here a cross between two pure breeding parents does not produce dominant trait of one parent but instead an intermediate trait is exhibited. Similarly in F2 generation apart from the two parental types there are several intermediate types which link the two parental traits. E.g., kernel colour in wheat, cob length in maize, skin colour in human beings, etc. Pleiotropy is the ability of a gene to have multiple phenotypic effect because it influences a number of characters simultaneously. Pleiotropy is due to effect of the gene on two or more inter-related metabolic pathways that contribute to formation of different phenotypes. It is not essential that all the traits are equally influenced. Sometimes the effect of a pleiotropic gene is more evident in case of one trait (major effect) and less evident in case of others (secondary effect). E.g., in cotton a gene for the lint also influence the height of the plant, size of the boll, number of ovules and viability of seeds.

Question. (a) How are Mendelian inheritance, polygenic inheritance and pleiotropy different from each other?
(b) Explain polygenic inheritance pattern with the help of a suitable example. 
Answer. (a) Mendelian Inheritance : Mendelian inheritance is a type of inheritance controlled by one or more genes in which only dominant trait was expressed in the F1 generation while at the F2 stage both the traits were expressed The contrasting traits did not show any blending at either F1 or F2 stage on the basis of this Mendel proposed three laws: (a) Law of Dominance, (b) Law of Segregation and (c) Law of Independent Assortment.
Polygenic Inheritance : Polygenic inheritance is a type of inheritance controlled by one or more genes in which the dominant alleles have cumulative effect with each dominant allele expressing a part or unit of the trait, the full trait being shown only when all the dominant alleles are present.
Pleiotropy: The ability of a gene to have multiple phenotypic effect because it influences a number of characters simultaneously is known as pleiotropy. Pleiotropy is due to effect of the gene on two or more inter-related metabolic pathways that contribute to formation of different phenotypes.
(b) Human skin colour is an example of polygenic inheritance. Human skin colour is caused by pigment melanin. The quantity of melanin is due to three pairs of polygenes (A, B and C). If black or very dark (AABBCC) and white or very light (aabbcc) individuals marry, the offsprings or individuals of F1 generation show intermediate colour often called mulatto (AaBbCc). When two such individuals of intermediate colour marry, the skin colour of the children will vary from very dark or black to very light or white. A total of eight allele combinations is possible in the gametes forming 27 distinct genotypes distributed into 7 phenotypes-1 very dark, 6 dark, 15 fairly dark, 20 intermediate, 15 fairly light, 6 light and 1 very light.

Question. How do “pleiotropy”, “incomplete dominance”, “co-dominance” and “polygenic inheritance” deviate from the observation made by Mendel? Explain with the help of one example for each. 
Answer. Mendel discovered laws of inheritance. According to law of dominance when two individuals of a species, differing in a pair of contrasting forms of a trait are crossed, the form of the trait that appears in the F1 hybrid is dominant and the alternate form that remains hidden, is called recessive.
Incomplete dominance and co-dominance are exception to this law.
Incomplete dominance is the phenomenon where none of the two contrasting alleles or factors is dominant. The expression of the character in a hybrid or F1 individual is intermediate or a fine mixture of the expression of the two factors. As seen in Mirabilis jalapa where when two types of plants having flower colour in pure state red and white are crossed, the hybrid or F1 generation have pink flowers.
Co-dominance is the phenomenon of expression of both the alleles in a heterozygote, i.e., both alleles are able to express themselves independently when present together. E.g., hair colour in cattle. When red cattle are crossed with white cattle, the hybrid of F1 generation are of roan colour i.e., having a dark coat interspered with white hair.
According to Mendel one gene control the expression of one character only. Pleiotropy is exception to this. The ability of a gene to have multiple phenotypic effect because it influences a number of characters simultaneously is known as pleiotropy. The gene having a multiple phenotypic effect because of its ability to control expression of two or more characters is called pleiotropic gene. For example, in cotton a gene for the lint also influences the height of plant, size of the boll, number of ovules and viability of seeds.
Polygenic Inheritance is a type of inheritance controlled by one or more genes in which the dominant alleles have cumulative effect with each dominant allele expressing a part or unit of the trait, the full trait being shown only when all the dominant alleles are present. The genes involved in quantitative inheritance are called polygenes and inheritance called as polygenic inheritance. E.g., human skin colour. Human skin colour is caused by pigment called melanin. The quantity of melanin is due to three pairs of polygenes (A, B and C). If black or very dark (AABBCC) and white or very light (aabbcc) individuals marry, the offspring show intermediate colour called mulatto (AaBbCc). When two such individuals of intermediate colour marry, the skin colour of the children will vary from very dark or black to very light or white. A total of eight allele combinations is possible in the gametes forming 27 distinct genotypes distributed into 7 phenotypes.

Question. (a) A couple with blood groups ‘A’ and ‘B’ respectively have a child with blood group ‘O’. Work out a cross to show how it is possible and the probable blood groups that can be expected in their other off-springs.
(b) Explain the genetic basis of blood groups in human population. 
Answer. 
(b) ABO blood groups are controlled by the gene I (also called L) located on 9th chromosome that has 3 multiple alleles, out of which any two are found in a person. These groups show Mendelian inheritance (Bernstein, 1924). The IA and IB alleles produce enzyme called glycosyltransferase for the synthesis of sugars. The sugars are attached to lipids and produce glycolipids. These glycolipids then associate with membrane of RBC to form blood group antigens. Allelle i does not produce any enzyme/antigen. Antigenic precursor H is present in RBC membrane. Allele IA produces a-N-acetylgalactosamyl transferase which adds a-N-acetylgalactosamine to sugar part of H to form A antigen. The allele IB produces a-D-galactosyltransferase which adds galactose into H to form B antigen. In case of IAIB heterozygote, both the enzymes are produced. Therefore, both A and B antigens are formed.

Question. (a) Differentiate between dominance and co-dominance.
(b) Explain co-dominance taking an example of human blood groups in the population.
Answer. (a) 

(b) ABO blood groups are controlled by the gene I (also called L) located on 9th chromosome that has 3 multiple alleles, out of which any two are found in a person. These groups show Mendelian inheritance (Bernstein, 1924). The IA and IB alleles produce enzyme called glycosyltransferase for the synthesis of sugars. The sugars are attached to lipids and produce glycolipids. These glycolipids then associate with membrane of RBC to form blood group antigens. Allelle i does not produce any enzyme/antigen. Antigenic precursor H is present in RBC membrane. Allele IA produces a-N-acetylgalactosamyl transferase which adds a-N-acetylgalactosamine to sugar part of H to form A antigen. The allele IB produces a-D-galactosyltransferase which adds galactose into H to form B antigen. In case of IAIB heterozygote, both the enzymes are produced. Therefore, both A and B antigens are formed.

Question. What is the inheritance pattern observed in the size of starch grains and seed shape of Pisum sativum? Workout the monohybrid cross showing the above traits. How does this pattern of inheritance deviate from that of Mendelian law of dominance?
Answer. The starch synthesis in pea plants is controlled by a single gene. It has two alleles B and b. Homozygous for BB produced large starch grains as compared to
that produced by plants which are homozygous for bb. After maturation it was observed that BB seeds were round and bb were wrinkled. When they were crossed the result and progeny were intermediate size Bb seeds showing round seeds.
But in case of seed shape the phenotype is 3 : 1;

Round : Wrinkled.
Deviation from Mendel’s law of dominance : If starch grain size is considered as the phenotype, then the alleles show incomplete dominance. Thus, dominance is not an autonomous feature of a gene,it depends on gene product and production of particular phenotype from this product.

Question. (a) Work out a cross upto F2 generation between two pure breed pea plants, one bearing violet flowers and the other white flowers.
(b) (i) Name this type of cross.
(ii) State the different laws of Mendel that can be derived from such a cross.
Answer. (b) (i) Monohybrid cross (ii) Two laws of inheritance can be derived from such a cross. These are given below: Law of dominance: According to this law, characters are controlled by discrete units called factors, which occur in pairs with one member of the pair dominating over the other dissimilar pair. This law explains expression of only one of the parental character in F1 generation. Law of segregation : Principle of segregation states that, “when a pair of contrasting factor or gene are brought together in a hybrid; these factors do not blend or mix up but simply associate themselves and remain together and, separate at the time of gamete formation”. The above law is also known as “law of purity of gametes” because each gamete is pure in itself.

Question. Name the stage of cell division where segregation of an independent pair of chromosome occurs. 
Answer. During anaphase of meiosis I segregation of an independent pairs of chromosomes occur.

Question. A garden pea plant produced axial white flowers. Another of the same species produced terminal violet flowers. Identify the dominant traits. 
Answer. Axial flower position is dominant over terminal flower position. Violet colour is dominant over white colour.

Question. In a dihybrid cross, when would the proportion of parental gene combinations be much higher than non-parental types, as experimentally shown by Morgan and his group?Answer. When the genes involved are linked, the proportion of parental gene combinations be much higher than non-parental types in a dihybrid cross.

Question. Why did T.H.Morgan s ele c t D ros ophila melanogaster to study sex linked genes for his lab experiments ? 
Answer. T.H. Morgan select Drosophila melanogaster to study sex-linked genes because of following reasons:
(i) They could be grown on simple synthetic medium in the laboratory.
(ii) They complete their life cycle in about two weeks.
(iii) A single mating could produce a large number of progeny flies.
(iv) There was a clear differentiation of sexes - the male and female flies are easily distinguishable.
(v) They have many types of hereditary variations that can be seen with lower microscope.

Question. Write the scientific name of the fruit fly. Why did Morgan prefer to work with fruit-flies for his experiments? State any three reasons.
OR
Linkage and crossing over of genes are alternative of each other. Justify with the help of an example.
Answer. Linkage is the tendency of two different genes on the same chromosome to remain together during the separation of homologous chromosomes at meiosis. Linked genes do not exhibit the dihybrid ratio of 9:3:3:1. It produces offspring with parental characters. Crossing over is the exchange of genes occurring during meiotic prophase I to break old linkage and establish new ones. It produces recombination resulting in new varieties. Thus, they are alternative of one another i.e., if linkage is present in between genes, no crossing over occurs between them and if crossing over occurs between the two genes, they are not linked. Example : In Drosophila a yellow bodied white eyed female was crossed with brown bodied red eyed male, F1 progeny produced and intercrossed. The F2 phenotypic ratio of Drosophila deviate significantly from Mendel’s 9:3:3:1.
This signify that the genes for eye colour and body colour are closely located on the ‘X’ chromosome and are linked. Therefore, inherited together. Recombinants were formed due to crossing over but at low percentage.

Question. How is the phenotypic ratio of F2 generation in a dihybrid cross is different from monohybrid cross?
Answer. In a monohybrid cross, the phenotypic ratio of F2 generation is 3:1 whereas in dihybrid cross, the phenotypic ratio of F2 generation is 9:3:3:1.

Question. In a dihybrid cross white eyed, yellow bodied female Drosophila crossed with red eyed, brown bodied male Drosophila produced in F2 generation 1.3 percent recombinants and 98.7 percent progeny with parental type combinations. This observation of Morgan de viated f rom Mendelian F2 phenotypic dihybrid ratio. Explain, giving reasons, Morgan’s observations.
Answer. By conducting the given cross, Morgan conclude that the genes for eye colour and body colour are linked. The linked genes do not show independent assortment but remain together and are inherited, thereby producing only parental type of progeny.

Question. Write the Mendelian F2 phenotypic ratio in a dihybrid cross. State the law that he proposed on the basis of this ratio. How is this law different from the law of segregation?
Answer. Mendelian F2 phenotypic ratio in a dihybrid cross is 9:3:3:1. Law proposed by Mendel on the basis of this ratio is law of independent assortment. It states that in the inheritance of two pairs of contrasting characters, the factors of each pair of characters segregate independently of the factors of the other pair of characters. It is different from law of segregation as law of segregation states that the members of the allelic pair that remained together in the parent, segregate during gamete formation and only one factor enters a gamete.

Question. Mendel published his work on inheritance of characters in 1865, but it remained unrecognised till 1900. Give three reasons for the delay in accepting his work.
Answer. 
90. The following are the three reasons that led to the delay in acceptance of Mendel’s work:
(i) Lack of communication and publicity in those days.
(ii) His concept of factors (genes) as stable and discrete units that controlled expression of traits and, of the pair of alleles that did not blend with each other was not accepted in the light of variations occurring continuously in nature.
(iii) Mendel’s approach to explain biological phenomenon with the help of mathematics was also not accepted.

Question. In pea plant let, symbol Y represent dominant yellow; symbol y, the recessive green; symbol R, the round seed shape and symbol r, the wrinkle seed shape. A typical Mendelian dihybrid cross was carried out in pea plants. Write the genotypes of
(a) Homozygous dominant and recessive parents
(b) Gametes produced by both the parents
Answer. (a) Homozygous dominant = YYRR
Homozygous recessive = yyrr
(b) Gametes produced by both the parents = YR and yr
(c) F1 = YyRr
(d) Gametes produced by F1 offspring
=YR, Yr, yR and yr.

Question. During the studies on genes in Drosophila that were sex-linked, T.H. Morgan found F2 population phenotypic ratios deviated from expected 9 : 3 : 3 : 1. Explain the conclusion he arrived at.
Answer. During his studies on genes in Drosophila, Morgan found that the phenotypic ratio of F2 population deviates from expected 9:3:3:1 ratio. On the basis of his study he conclude the chromosome theory of linkage which states that:
– Linked genes are genes which stay together during transmission from generation to generation and occur on the same chromosome.
– Genes are arranged in a linear fashion on the chromosome.
– Genes tend to maintain original parental combination of alleles with the exception of an occasional crossing over.
– Crossing over occurs due to weakening of linkage between two genes.
– Strength of the linkage between two genes is
inversely proportional to the distance between the two, i.e., two linked genes show higher frequency of crossing over if the distance between them is higher and lower frequency if the distance is small.

Question. Which chromosomes carry the mutant genes causing thalassemia in humans? What are the problems caused by these mutant genes?
Answer. Thalassemia is an autosomal, recessively inherited disorder. The defect can occur due to mutation or deletion of the genes controlling the formation of globin chains (commonly α and β ) of haemoglobin.
α thalassemia is caused by the defective formation of α-globin which is controlled by two genes HBA1 and HBA2 present on chromosome 16. The mutant gene cause anaemia, jaundice, hepatoseplenomegaly and bone changes. All the defective alleles kill the foetus resulting in still birth or death soon after delivery.
β thalassemia is caused due to decreased synthesis
of β globin. The defect is due to alleles of HBB gene present on chromosome 11. It results in severe haemolytic anaemia, hepatosplenomegaly, cardiac enlargement and skeletal deformities.

Question. Why is pedigree analysis done in the study of human genetics? State the conclusions that can be drawn from it. 
Answer. Pedigree analysis is study of pedigree for the transmission of particular trait. It is done to study human genetics because control crosses are not 
possible in human being as the generation time is more in humans. Pedigree analysis is useful in following ways:
(i) It is useful in finding the possibility of absence or presence of that trait in homozygous or heterozygous state in a particular individual and his family members.
(ii) It is useful in detecting genetic defects like haemophilia, colourblindness, alkaptonuria, phenylketonuria, thalassemia, sickle cell anaemia (recessive traits), brachydactyly and syndactyly (dominant traits).
(iii) It helps to detect sex-linked characters and other linkages.
 

Question. Identify ‘a’, ‘b’, ‘c’, ‘d’, ‘e’ and ‘f ’ in the table given below.

Answer. (a) (i) Partially opened mouth with furrowed tongue.
(ii) Palm is broad with palm crease
(b) Both
(c) Klinefelter’s
(d) Male
(e) (i) Sterile female with poorly developed ovaries and under developed breasts.
(ii) Webbed neck and broad chest
(f ) Female

Question. Why is haemophilia rare in human females?
Mention a clinical symptom for the disease.
Answer. The patient having haemophilia will continue to bleed even from a minor cut since he/she does not possess the natural phenomenon of blood clotting due to the absence of antihaemophilic globulin or factor responsible for clotting.

Question. Name a blood related autosomal Mendelian disorder. Why is it called Mendelian disorder? How is the disorder tansmitted from parents to offsprings? 
Answer. Sickle-cell anaemia is a blood related autosomal Mendelian disorder. It is called Mendelian disorder because it is transmitted to the offspring as per Mendelian principles. The gene for sickle-celled erythrocytes is represented by Hbs while that of normal erythrocytes is written as HbA. The homozygotes for the two types are Hbs Hbs and HbA HbA. The heterozygotes are written as HbA HbS. When two sickle cell heterozygotes marry they produce three types of children–homozygous normal, heterozygous carrier and homozygous sickle celled in the ratio of 1 : 2 :1. However, homozygous sickle-celled individuals (Hbs Hbs) die in childhood (before reproductive age) due to acute anaemia. Therefore, a ratio of one normal to two carriers is obtained.

Question. State the three principles of Mendel’s law of inheritance.
Answer. 
The three principles of Mendel’s law of inheritance are :
Law of dominance : It explains that when two individuals of a species, differing in a pair of contrasting forms of a trait are crossed, the form of the trait that appears in the F1 hybrid is dominant and the alternate
form that remains hidden, is called recessive.
Law of segregation : It states that the members of the allelic pair that remained together in the parent, segregate during gamete formation and only one factor enters a gamete.
Law of independent assortment : It states that in the inheritance of two pairs of contrasting characters, the factorsofeachpairofcharacterssegregateindependently of the factors of the other pair of characters.

Question. Dif ferent i ate b et we en m a le and fem a le heterogamety.
Answer. The type of sex determination mechanism shown in female XX with male XY is called male heterogamety because male produces two different types of gametes. Example - Drosophila
The type of sex determination mechanism shown in female ZW with male ZZ is female heterogamety because female produces two different types of gametes. Example - Birds

Question. Explain mechanism of sex-determination in birds. 
Answer. Birds have ZW - ZZ type of sex determination mechanism. In this type the male has two homomorphic sex chromosomes (ZZ) and is homogametic, and the female has two heteromorphic sex chromosomes (ZW) and is heterogametic. There are, thus, two types of eggs: with Z and with W, and only one type of sperms, i.e., each with Z. Fertilisation of an egg with Z chromosome by a sperm with Z chromosome gives a zygote with ZZ chromosomes (male). Fertilisation of an egg with W chromosome by a sperm with Z chromosome yields a zygote with ZW chromosomes (female).

Question. A male honeybee has 16 chromosomes whereas its female has 32 chromosomes. Give one reason.
Answer. In honeybees an unfertilised egg develops into a male and a fertilised egg develops into a female. Therefore, the female is diploid (2n), and the male is haploid (n).

Question. How many chromos omes do drones of honeybee possess? Name the type of cell division involved in the production of sperms by them.
Answer. Drones of honeybees are haploid and possess 16 chromosomes. Mitosis is involved in the production of sperms.

Question. Identify and write the correct statement :
(a) Drosophila male has one X and one Y chromosome.
(b) Drosophila male has two X chromosomes.
Answer. Drosophila male has one X and one Y chromosome.