Principles of Inheritance and Variation Test - 35

A point mutation, or single base modification, is a type of mutation that causes a single nucleotide base substitution, insertion, or deletion of the genetic material, DNA or RNA.

Sickle-cell anemia is caused by a point mutation in the β-globin chain of hemoglobin, causing the hydrophilic amino acid glutamic acid to be replaced with the hydrophobic amino acid valine at the sixth position. The β-globin gene is found on the short arm of chromosome 11. Hence option D is correct.

Sickle cell trait is a heterozygous condition where the shape of normal haemoglobin becomes sickle cell-shaped.

Sickle-cell anaemia is the most common form of sickle cell disease (SCD). SCD is a serious disorder in which the body makes sickle shaped red blood cells. In sickle cell anaemia, due to mutation the $$\beta$$-chain amino acid sequence is changed causing change in molecular structure.

• Father is the person who determines the sex of the children. 
• There are two types of gametes produced by father - with X chromosome and Y chromosome. 
• Mother contains only X chromosome. 
• If male gemete with X chromosome fuses with X chromosome of mother, it results in a female child. 
• If gamete with Y chromosome fuses with X chromosome of mother, it gives a male child. 
• Thus, there is 50% chance for female child and the same for male child, irrespective of the sex of the five previous children.
• Hence, correct answer is option A.

• The genes that are carried on the Y chromosome are called holandric genes.
• Holandric genes can only be passed by males onto their sons; they code for 'maleness' but sometimes cause rare conditions like hypertrichosis pinnae and color blindness.
  

Human females have XX chromosomes as the sex chromosomes. During the formation of gametes, meiosis takes place. Each gamete thus contains an X chromosome. Human males have XY as the sex chromosomes. Thus, human sperms contain one X or one Y chromosome.

The inactivated X-chromosome in the somatic cell of the female is referred to as Barr body which is present as tightly coiled and visible heterochromatin. Dosage compensation in human is accomplished by the inactivation of one of the X chromosomes in females. It occurs in 200 celled embryo wherein each cell independently select either maternal or paternal X chromosome for inactivation. Once the selection is done, all lineages of that cell make the same selection during cell division and tissue differentiation. This random inactivation of maternal and paternal X chromosome makes a human female mosaic for X gene. 

• In humans, sex is determined by the father. 
• The mother produces only one kind of gamete - having X-chromosome only.
• The father produces two kinds of gametes - where one contains the X chromosome and another one contains the Y chromosome. 
• During fertilization, if the X chromosome of the father fuses with the ovum of the mother, which contains an X chromosome, it becomes a girl child. If the Y chromosome of the father fuses with the ovum containing X in the mother, it results in a boy.

So, the correct answer is (C).

Lyon hypothesis states that gene dosage imbalance between males and females, because of the presence of two chromosomes in female (XX) as opposed to only in one male (XY), is compensated for by random inactivation of one of the X chromosomes in the somatic cells of females. 

Sex chromosomes in plants were first reported in 1917 in the liverwort, Sphaerocarpus. This plant belonging to Bryophyta has a large X chromosome in female gametophyte and small Y chromosome in male gametophyte. Thus the correct answer is option A.