Free Principles Of Inheritance And Variation (Daily Quiz: 8) NEET Biology Questions & Answers with Detailed Explanation & Full Review

Answer: (b)

A cross of $F_1$ hybrid with its recessive homozygous parent is called the test cross. It is done to determine the genotype of a given plant. If the given plant has homozygous dominant traits then on test cross it gives all dominant trait plants but if it is heterozygous dominant then it gives dominant and recessive phenotypes in 1 : 1 ratio.

Answer: (d)

Turner’s syndrome is caused by the absence of X-chromosomes in females. Persons suffering from turner ’s syndrome are monosomic for sex chromosomes i.e., possess only one X and no Y chromosome (XO). In other words they have chromosome number 2n – 1 = 45. They are phenotypic females but are sterile because they have underdeveloped reproductive organs. They are dwarf about 4 feet 10 inches and are flat chested with wide spread nipples of mammary glands which never enlarge like those in normal woman. They develop as normal female in childhood but at adolescence their ovaries remain underdeveloped. They lack female hormone estrogen. About one out of every 5,000 female births results in Turner’s syndrome.

Answer: (b)

Non-disjunction in meiosis results in trisomy, i.e. the presence of an extra chromosome. Such individuals have 47 chromosomes instead of 46. Some examples of such abnormalities include Down's syndrome (21-trisomy), Patau's syndrome (13-trisomy) and Edward's syndrome (18-trisomy).

Answer: (c)

The X body of Henking was observed in half of the sperms during spermatogenesis. During his experiments on insects, Henking found the traces of a nuclear structure all through the process of spermatogenesis which he named it as X body. He also observed that, after spermatogenesis, exact half of the sperms received this X body, while the remaining half did not. Later, scientists found out that this X body of Henking was actually a chromosome and hence, named it X-chromosome.

Answer: (c)

Haemophilia is sex-linked disease which is also known as bleeder's disease as the patient will continue to bleed even from a minor cut since he or she does not possess the natural phenomenon of blood clotting due to absence of antihaemophiliac globulin or factor VIII (haemophilia – A) and plasma thromboplastin factor IX (haemophilia–B, Christmas disease) essential for it. As a result of continuous bleeding, the patient may die of blood loss. It is genetically due to the presence of a recessive sex linked gene h, carried by X-chromosome. A female becomes haemophiliac only when both of her X-chromosomes carry the gene ($X^hX^h$). However, such females generally die before birth because the combination of these two recessive alleles is lethal. A female having only one allele for haemophilia ($XX^h$) appears normal because the allele for normal blood clotting present on the other X-chromosome is dominant. Such females are known as carriers. In case of males, a single gene for the defect is able to express itself as the Y-chromosome is devoid of any corresponding allele ($X^h$Y).