ASSERTION& CORNER

REASONMark the e Tolowing questions, a statement of assertion is followed by a statement ot reason.Mark the correct choice asooth assertion and reason are true and reason is the coect explanation or as5eruon.r Doth assertion and reason are true but reason is not the correct explanation of asserton.(c) f assertion is true but reason is taise.a) t both assertion and reason are false.

1. Assertion: Mendel conductedartificial poilinationexperiments tor his genetic studies using true-breedingpea lines.Reason A true-breeding line shows the stable traitinheritance and expression for several generations.

Reason : When two genes in a dihybrid are on thesame chrormosome, the proportion of parental genecombinations are much higher than the non-parentaltype.

2. Assertion : At Fz stage in monohybrid cross, bothparental traits are expressed in the proportion of

9. Assertion : The maximum frequency of recombination,

thatcan result from crossing over betweenlinked genes,is 50 percent

Reason: Linked genes shown higher frequency ofcrossing over if distance between them is longer.

3:1.

Reason: The contrasting parental traits show blendingat F stage.

3. Assertion: Test cross is the cross between the F, |

progeny and erther of the parent types.

Reason: Back cross is the cross between progenyand the double recessive genotype

10. Assertion: In pigeons, females are heterogametic andmales are homogametic.

Reason: In pigeons, females have ZW sex chromosomesand males have Z sex chromosomes.

4 Assertion: The pink coloured flowers appear in Fz

generation of plant Mirabils jakapa.

Reason This is observed due epistaticsuppression of

white colour alleles in one ot parental tlowers by red

colour alleles.

11. Assertion : Females, homozygous for genes on theX chromosomes do not expres a trait more markedlythan do hemizygous males.

Reason: Dosage compensation mechanism accountsfor etective dosage genes in males and females.

5, Assertion : ABO blood group system provides a good

example of multiple alleles.

Reason : In ABO blood group system, when A and s

alleles are present together, they both expresS their own

Types.

12. Assertion: Variety of fruit colours in Cucurbita pepois result of recessive epistasis.

Reason: In recessive epistatsis, a recessive gene atone locus enhances the expression ot another gene, ata different locus.

13. Asserticn: Sickle-cell anaemia is an autosome-linkedrecessive disorder that can be transmitted if both parentsare heterozygous tor the gene.Reason: In sickle-cellarnaemia, haemoglobin moleculeundergoes polymerisation under low oxygen tensioncausing the change in shape of RBC.

, Assertion : Pairing and separation of pair of

chromosomes would lead to segregation or a pair ot

factors they carried.

Reason Two alleles of a gene pair are located on

Similar sites on non-homologous chromosomes.

7. Assertion : The law of independent assortiment can SErtonPhenypyruvic acid is excreted through urine

be studied by means of dihybrid cross

Reason : The law of independent assortment is

applicable only to linked genes.

in case of phenylketonuria.

Reason: The affected individual lacks enzymephenylalanine hydroxylase.

8, Assertion: When yellowbodied, white eyed Drosophia15. Assertion: Turner's syndrome iscauseddiueto absence

Temales were hybridised with brown-bodied, red eyed

males; andF progenywas intercrossed, , ratro deviated

from 9:3: 3:1.

of any one of the X and Y sex chronosome.Reason: Such individuals show masculine as Wen dfeminine development.

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ASSERTION & REASON CORNER

1. (a): Mendel carried out hybridisation experiments on

garden pea for many years. He performed various types of

cross breeding and then allowed the offspring to self breed.

All selected varieties used by him were pure lines ot true

breeding lines, i.e., they produce offspring resembling the

parents. These lines show the stable trait inheritance and

expression for several generations making them suitable for

genetic studies.

2. (c):In Fz generation both the parental traits of a

character are expressed in the ratio of three dominant to

one recesSIve.

3. (d): A cross of F, hybrid with either ofthe two

IS known as back cross. Crossing of F, individual having

dominant phenotype with its homozygouS recessive parent

is called test cross.

rents

4. (c): In Mirabilis jalapa (four o'clock) there are two

types of flower colour in pure state: red and white. VWhen

the two types of plants are crossed, the hybrid or plants of

Fi generation have pink tlowers. If the latter are selfed, the

plants of F, generation are of three types-red, pink and white

flowered in the ratio of 1:2:1.The pinkcolour apparentlyappears due to incomplete dominance of red (dominant) over

white (recessive). Thus, pink is differentiated from red andwhite.

5. (b)

6. (c): According to the chromosomal theory of inheritance,

the chromosomes possess Mendelian tactors or genes. The

chromosomes segregate and assort independently duringmeiosis and recombine in zygote at the time of fertilisation.Chromosomes, like genes occur in pairs. The two alleles of agene pair are located on homologous sites on homologouschromosomes.

7. (c): The law of independent assortment states that twofactors of each character assort or separate independent of thefactors of other characters at the time of gamete formationand get randomly rearranged in the offspring producing botnparental and new combinations of traits. The principle of tawof independent assortment is applicable to only those tactorsor genes which are either located distantly on the sameChromosome or occur on different chromosomes. Actually,

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wutG Objective NCERT at your Fingertips134

a chromosome bears hundreds of genes. All the genes ofTactors present on a chromosome are inherited togetherexcept when crossing over takes place.

others. Therefore, dosage compensation regulates the enzymelevel of males and females.

12. (d): Fruit colour of summer squash (Cucurbita pepo) is

governed bya gene which produces yellow colour in dominantstate (Y) and green colour in recessive state (yy). There is

8. (a): In Drosophila, the genes for body and eye colourare located on X chromosome. When two genes in a dihybridCross are situated on the same chromosome, the proportion a non-pigment forming epistatic gene which in dominantof parental gene combination are higher than non-parentaltype. This occurs due to physical association or linkage ofthe two genes while non-parental gene combinations dueto recombination between two genes. Thus, linkage andrecombination deviates the ratio from Mendelian ratio of a

dihybrid croSs, (9 :3:3:1).

state (W- produces white colour (W- Y-, W- yy) whilein recessive state it allows the yellow (wwY-) and greenwwyy) expression. This shows dominant epistasis where adominant epistatic allele suppresses the expression of a nonallelic gene whether the latter is dominant or recessive.

13. (b)9. (b)

14. (a): Phenylketonuria is an inherited error of metabolism10. (a): In pigeons, ZW-7Z type of sex determination caused by a deficiency in the enzyme phenylalanine hydroxylasemechanism is found. In this type, the male has two homomorphic It is an autosomal recessive trait. It is a hereditary humansex chromosomes (ZZ) and is homogametic, and the female has condition resulting from the inability to convert phenylalaninetwo heteromorphic sex chromosomes (W) and is heterogametic. into tyrosine. This leads to over production of phenylalanineThere are, thus, two types of eggs: with Z and with W, and and its conversion to phenylpyruvic acid and other derivatives.only one type of sperms, i.e., each with Z.These are then excreted in urine due to poor absorption by11. fa):Females have two X chromosomes and males kidneys.

have only one X chromosome. Thus, a dosage compensation 15. (d): Turner's syndrome occurs due absence of Xmechanism must be present through which the effective chromosome. Individuals having a single X chromosomedosage of genes of the two sexes is made equal. This is 2A+X0 (45) have female sexual differentiation but ovariesachieved by inactivation of one X chromosome in Temales are rudimentary. Other associated phenotypes of this conditionwhich gets condensed and become heteropyknotic. Since are short stature, webbed-neck, broad chest, lack of secondaryonly one X chromosome is required for normalmetabolismin sexual characteristics and sterility. Thus, any imbalance incells of femaies, the other X chromosome has the capability the copies of the sex chromosomes may disrupt the geneticof being heterochromatic in some cells and euchromatic in information necessary for normal sexual development.

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