Question

Refer to the given statements:
(i) Incomplete or mosaic inheritance is an example of the pre-Mendelian concept of blending inheritance.
(ii) A test cross is a special type of back cross.
(iii) Chromosomal aberrations are commonly observed in cancer cells.
(iv) Thalassaemia is a Mendelian disorder.
Which of the above statements are correct?
(a) (i) and (ii) only
(b) (ii), (iii) and (iv)
(c) (ii) and (iv) only
(d) (i) and (iv) only

Answer 1

Hint: Incomplete dominance is seen in the inheritance pattern of flower color in Mirabilis jalapa and Antirrhinum (dog flower). A test cross is used to find out the genotype of a dominant individual. Any change in the chromosomal structure is known as a chromosomal aberration. Thalassemia is a type of blood disorder.

Complete Answer:
Let us understand all the mentioned concepts in detail.
Incomplete or mosaic inheritance: This is the phenomenon in which both the dominant allele of a gene is not able to fully dominate the recessive gene. In such a case the offspring produced has a phenotype that is intermediate of the dominant and recessive phenotype. This is seen in the flower color of Mirabilis jalapa where the dominant red color allele partially dominates the white color allele thus resulting in the phenotype of pink-colored flowers. It is not a pre-Mendelian concept as the inheritance pattern follows the rules proposed by Mendel.
Test cross: Test cross is the cross in which a dominant individual is crossed with its parent that has a recessive phenotype in order to determine whether that individual is a homozygous or heterozygous dominant individual whereas back cross is any cross in which the offspring is crossed with its parent.
Chromosomal aberrations: Sometimes due to error in DNA replication or karyokinesis, a change develops in the chromosome number (gain or loss) or in the structure of the chromosome (deletion, mutation, or inversion). Such a change can sometimes cause cells to divide uncontrollably such as in cancerous cells leading to the formation of tumors.
Thalassemia: Thalassemia is a Mendelian disorder i.e. it is inherited from parents to children through genes. It is associated with improper production of the protein chain of hemoglobin thus causing anemia. Based on the type of protein chain that is affected, thalassemia is divided into alpha and beta-thalassemia.
So, the correct option is ‘(ii) , (iii) and (iv)’.

Note:
- Codominance is a phenomenon in which both the alleles of a gene are dominant on each other as is seen in the AB blood group.
- Changes in the chromosome numbers cause various diseases such as Down syndrome, Klinefelter’s syndrome, and Turner syndrome.
- Sickle cell anemia is another Mendelian blood disorder which causes RBC to become sickle-shaped.