Principles of Inheritance and Variation Test - 48

Radioactive materials give out ionizing radiation that cause DNA mutation by double-stranded breaks in DNA. These breaks can be repaired by the cell. Mutations occur if the repair mechanisms are defective and the wrong piece of DNA are joined together. This may lead to the deletion of important genes or change in the location of a gene within the DNA strand. Such mutations lead to cancer. Thus the correct answer is option B.

A mutation is a change that occurs in our DNA sequence, either due to mistakes when the DNA is copied or as the result of environmental factors such as UV light and cigarette smoke. Mutations are inherited and affects all generations. It is not responsible for plant growth. Thus, the correct answer is option A.

A gene mutation can be defined as a permanent alteration in the DNA sequence that makes up a gene, such that the sequence differs from what is found in most people. Mutations range in size. They can affect anywhere from a single DNA building block (base pair) to a large segment of a chromosome that includes multiple genes. Gene mutation does not occur due to reproduction, linkage or changes in the size of the DNA. Thus, option B is correct and other options are wrong.

A mutation is a sudden inheritable change as it occurs in our DNA sequence, either due to mistakes when the DNA is copied or as the result of environmental factors such as UV light and cigarette smoke. Mutations contribute to genetic variation within species. Thus, option A is correct.

Mutations were discovered by Hugo De Vries in Oenothera lamarckiana. This plant is commonly known as Evening Primrose. He formulated observations by experimenting on this plant. He took a normal plant and self pollinated it to raise seeds. He found that when these seeds grown into plants, all the plants were normal and they were showing only minor variations. A few were different from the parents in many characters. In further generations, these different plant gave rise to few more different plants than their parents. De Vries named these minor variations as 'mutations'. He found that mutations were sudden and they were inherited from the previous generations.

The monohybrid cross-refers to inheritance of singe contrasting trait by two parents at a time. For example, a cross of tall (T) and dwarf (t) pea plants is a monohybrid cross as it exhibits the inheritance of plant height (TT x tt). A monohybrid cross between two pure breeding varieties obtains all dominant $$F_1$$ generation which in turn produces $$F_2$$ generation upon selfing and gives the monohybrid ratio of 3 (dominant):1 (recessive). The dihybrid cross-refers to the inheritance of two pairs of contrasting characters by two parents at the same time. For example, a cross of two pea plants, one with yellow and round seeds (YYRR) and other with green and wrinkled seeds (yyrr). Selfing of $$F_1$$ hybrids give dihybrid ratio of 9:3:3:1 in $$F_2$$ generation. Likewise, the inheritance of three traits by two parents at a time refers to the trihybrid cross. Example, inheritance of plant height, seed shape, and seed color (TTYYRR x ttyyrr). The phenotypic ratio for a trihybrid cross is 27 : 9 : 9 : 9 : 3 : 3 : 3 : 1. Thus, the correct answer is option A.

Pleiotropy is the condition in which a single gene influences more than one trait. Polyploidy is a condition in which individuals have more than two complete sets of chromosomes. Apomixis is a reproductive process in plants that superficially resembles normal sexual reproduction but there is no fusion of gametes.

Polytene chromosomes in salivary glands of Drosophila are formed as a result of the replication of the chromosomes and the synapsis of homologues without cell division. The enlarged part of polytene chromosomes, presence between two bands in staining process, are termed as balbiani rings. Thus, the correct answer is option B.

The ratio of the phenotypes is 3:1, typical for a monohybrid cross. Therefore, (b) is the correct answer.

Sickle cell disease is an inherited blood disorder that affects red blood cells. People with the sickle cell disease have red blood cells that contain mostly haemoglobin S, an abnormal type of haemoglobin. Sometimes, these red blood cells become sickle-shaped (crescent shaped) and have difficulty passing through small blood vessels.