Directions : In the following questions, a statement of assertion is followed by a statement of reason. Mark the correct choice as :
(a) If both assertion and reason are true and reason is the correct explanation of assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of assertion.
(c) If assertion is true but reason is false.
(d) If assertion is false but reason is true.
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Assertion : Mendel conducted artificial pollination experiments for his genetic studies using true-breeding pea lines.
Reason: A true-breeding line shows the stable trait inheritance and expression for several generations.
(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 of 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.
Assertion : At \(F_2\) stage in monohybrid cross, both parental traits are expressed in the proportion of \(3: 1\).
Reason : The contrasting parental traits show blending at \(F_2\) stage.
(c) : In \(F_2\) generation both the parental traits of a character are expressed in the ratio of three dominant to one recessive. Hence, there is no blending of traits at \(F_2\).
Assertion : Test cross is the cross between the \(F_1\) progeny and either of the parent types.
Reason : Test cross is used to predict the genotype of an individual of unknown genotype.
(d) : A cross of F, hybrid with either of the two parents is known as back cross. Crossing of \(F\), individual having dominant phenotype with its homozygous recessive parent is called test cross. It is used to predict the genotype of an individual of unknown genotype.
Assertion : The pink coloured flowers appear in \(F_1\) generation of plant Mirabilis jalapa.
Reason: This is observed due epistatic suppression of white colour alleles in one of parental flowers by red colour alleles.
(c) : The pink colour flower appear in \(F1\), generation due to incomplete dominance of red (dominant) over white (recessive).
Assertion : ABO blood group system provides a good example of multiple alleles.
Reason: In ABO blood group system, when \(I^A\) and \(B\) alleles are present together, they both express their own types.
(b)
Assertion : Pairing and separation of pair of chromosomes would lead to segregation of a pair of factors they carried.
Reason : Two alleles of a gene pair are located on similar sites of non-homologous chromosomes.
(c) : According to the chromosomal theory of inheritance, the chromosomes possess Mendelian factors or genes. The chromosomes segregate and assort independently during meiosis and recombine in zygote at the time of fertilisation. Chromosomes, like genes occur in pairs. The two alleles of a gene pair are located on homologous sites of homologous chromosomes.
Assertion : The law of independent assortment can be studied by means of dihybrid cross.
Reason : The law of independent assortment is applicable only to linked genes.
(c) : The law of independent assortment states that two factors of each character assort or separate independent of the factors of other characters at the time of gamete formation and get randomly rearranged in the offspring producing both parental and new combinations of traits. The principle of 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.
Assertion : When yellow bodied, white eyed Drosophila females were hybridised with brown-bodied, red eyed males; and \(F_1\) progeny was intercrossed, \(F_2\) ratio deviated from \(9: 3: 3: 1\)
Reason : When two genes in a dihybrid are on the same chromosome, the proportion of parental gene combinations are much higher than the non-parental type.
(a) : In Drosophila, the genes for body and eye colour are located on X chromosome. When two genes in a dihybrid cross are situated on the same chromosome, the proportion of parental gene combination are higher than non-parental type. This occurs due to physical association or linkage of the two genes while non-parental gene combinations due to recombination between two genes. Thus, linkage and recombination deviates the ratio from Mendelian ratio of a dihybrid cross, \((9: 3: 3: 1)\).
Assertion : The maximum frequency of recombination, that can result from crossing over between linked genes, is 50 percent.
Reason : Linked genes show higher frequency of crossing over if distance between them is longer.
(b)
Assertion : In pigeons, females are heterogametic and males are homogametic.
Reason : In pigeons, females have ZW sex chromosomes and males have ZZ sex chromosomes.
(a) : In pigeons, ZW-ZZ type of sex determination mechanism is found. 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.
Assertion: In honeybees, drones have 16 chromosomes while queen has 32 chromosomes.
Reason: Male bees develop from unfertilised eggs and female bees from fertilised eggs.
(a)
Assertion: Human skin colour is controlled by 3 pairs of genes.
Reason : Skin colour in humans show pleiotropic inheritance.
(c) : Human skin colour is a polygenic trait.
Assertion: Sickle-cell anaemia is an autosome-linked recessive disorder that can be transmitted if both parents are heterozygous for the gene.
Reason : In sickle-cell anaemia, haemoglobin molecule undergoes polymerisation under low oxygen tension causing the change in shape of RBC.
(b)
Assertion : Phenylpyruvic acid is excreted through urine in case of phenylketonuria.
Reason : The affected individual lacks enzyme phenylalanine hydroxylase.
(a) : Phenylketonuria is an inherited error of metabolism caused by a deficiency in the enzyme phenylalanine hydroxylase. It is an autosomal recessive trait. It is a hereditary human condition resulting from the inability to convert phenylalanine into tyrosine. This leads to over production of phenylalanine and its conversion to phenylpyruvic acid and other derivatives. These are then excreted in urine due to poor absorption by kidneys.
Assertion : Turner’s syndrome is caused due to absence of any one of the \(X\) and \(Y\) sex chromosome.
Reason : Turner’s syndrome is an example of aneuploidy.
(d) : Turner’s syndrome is one of the example of aneuploidy that occurs due to absence of \(X\) chromosome. Individuals having a single \(X\) chromosome \(2 A+X 0(45)\) have female sexual differentiation but ovaries are rudimentary. Other associated phenotypes of this condition are short stature, webbed-neck, broad chest, lack of secondary sexual characteristics and sterility. Thus, any imbalance in the copies of the sex chromosomes may disrupt the genetic information necessary for normal sexual development.
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