This set of Cytogenetics Multiple Choice Questions & Answers (MCQs) focuses on “Lethal Alleles”.
1. Lethal alleles can only be recessive.
Explanation: Lethal alleles can be both dominant as well as recessive. In case of recessive, it only appears when the individual is homozygous for the gene.
2. In Aurea, a type of snapdragon, only ______________ carrying dominant genes live.
Explanation: In this case the heterozygote produces 2:1 ratio of yellow: green leaves. The homozygous dominant individual germinates but due to lack of chlorophyll fails to grow.
3. The ______________ mouse is lethal.
a) Homozygous white
b) Homozygous black
c) Heterozygous agouti
d) Homozygous yellow
Explanation: The homozygous yellow mouse dies off as it carries the lethal gene in two copies. However in all other options the mouse likes.
4. Cross of two yellow mice will give _____________ normal mouse.
Explanation: Homozygous condition of the yellow allele will lead to lethality thus the living yellow mouse are heterozygous. Their cross will give 1/4th of homozygous yellow allele mice which will die changing the proportion of normal mice to 1.3rd of the living ones.
5. Which of the following is not true for yellow mice?
a) The homozygous yellow mice die
b) A normal ice crossed to yellow mice will always give normal probabilities of F1
c) In yellow mice the yellow coloring allele is under control of Raly promoter
d) The homozygous mouse dies as the yellow allele is produced in excess
Explanation: In homozygous condition the mice die not due to over expression of agouti, or yellow coloring gene but due to absence of expression of Raly gene.
6. The lethal mutation that is seen in the yellow mice is due to _______________ mutation
d) Frame shift
Explanation: The mutation seen in this case is deletion mutation. The raly gene and the agouti genes lie side by side. Deletion of the gene sequence of Rally and the promoter of Agouti gives this colour.
7. Tay Sach’s disease doesn’t agree with which of the following?
a) It causes death in heterozygotes
b) Death occurs within 2-3 years of life
c) HEXA gene is affected
d) Central nervous system is affected
Explanation: Tay Sach’s disease occurs due to the homozygous condition of the lethal allele only. It is a recessive gene and not a dominant gene.
8. A person homozygous for lethal allele______________
a) Always dies
b) Always expresses the phenotype
c) May not express the phenotype
d) They have severe abnormalities
Explanation: A person with a lethal allele doesn’t always die, as in case of temperature sensitive mutant it is seen that the lethality only occurs at higher temperature.
9. Huntington’s disease affects individuals carrying the gene starting from the age of ____________
Explanation: People carrying this dominant allele show the symptoms starting from their 30s. Thus, even with the presence of dominant allele, it is not expressed right from the start.
10. Dominant lethal alleles are quickly removed from the pool while the recessive ones remain.
Explanation: If the lethal allele is dominant it will be expressed even in heterozygotes. This will lead to the death of most individuals carrying the gene and if that is before the mating age the gene will gradually be eradicated from the gene pool.
Sanfoundry Global Education & Learning Series – Cytogenetics.
To practice all areas of Cytogenetics, here is complete set of 1000+ Multiple Choice Questions and Answers.