This set of Cytogenetics Multiple Choice Questions & Answers (MCQs) focuses on “Dihybrid Cross”.
1. The expected phenotypic ratio obtained by crossing the F1 generation in dihybrid cross would be?
a) 3:1
b) 1:2:1
c) 9:3:3:1
d) 1:2:1:2:4:2:1:2:1
View Answer
Explanation: The phenotypic ratio obtained by the dihybrid cross is 9:3:3:1 while that by a monohybrid cross is 3:1. The genotypic ratio of a monohybrid cross is 1:2:1 while the genotypic ratio of dihybrid cross would be 1:2:1:2:4:2:1:2:1.
2. Which of the following will be pure breeding for only one character?
a) Hh Rr
b) HH Rr
c) HH RR
d) hh rr
View Answer
Explanation: In this case of HH Rr it will be true breeding for H locus but as it is heterozygous for R locus, R character will not breed true. On the other hand HH RR and hh rr will breed true for both the H and R characters.
3. If you cross homozygous recessive short plant with heterozygous purple flower to heterozygous tall plant with heterozygous purple flower, what will be the expected phenotypic ratio?
a) 9:3:3:1
b) 4:4:2:2
c) 3:1:3:1
d) 1:2:1:2:4:2:1:2:1
View Answer
Explanation: In this cross the ratio would be as follows 3(tall purple): 1 (tall white): 3 (short purple):1 (short white). This is a test cross for height gene but not for colour gene.
4. Considering a dihybrid cross, what is the probability of the progeny being heterozygous at both the alleles?
a) 1/16
b) 4/16
c) 7/16
d) 9/16
View Answer
Explanation: If you make a punett square the diagonal will have the progeny probability that are heterozygous for both alleles.
5. If you have an organism with the genotype Pp HH rr Ww this can be used as a parent in test cross for________
a) H locus
b) R locus
c) W locus
d) P locus
View Answer
Explanation: The organism has a homozygous recessive phenotype for the R locus, thus it can be used to test cross other organisms for R locus.
6. If you include more number of genes in the cross, a branched diagram will be easier to find progeny phenotype.
a) True
b) False
View Answer
Explanation: In a branched diagram the next phenotype could be processed by adding one column or branch, but in case of Punett square considering greater than 2 alleles would include a huge table which is quite hazardous to handle.
7. In an organism you observe that phenotypes of the progeny are as – 315 H/H R/R, 108 H/-r/r, 101 h/h R/-, 32 h/h r/r. This shows_____
a) Linkage
b) Lethal alleles
c) Epistasis
d) Normal ratio
View Answer
Explanation: If the ratio of the progeny is calculated it is very close to 9:3:3:1 which is the ratio of normal digybrid cross.
8. By crossing homozygous short purple flowering parent with a heterozygous tall purple flowering parent you get 41 tall progeny. What will be the expected number of short progenies?
a) 12
b) 42
c) 60
d) 80
View Answer
Explanation: This is a case of a test cross for the Height allele. The ratio should be 1:1 for short: tall. If there are 41 tall progeny there must be a similar number say 42 of short ones.
9. What will be the dyhybrid ratio for a cross between HH TT with hh tt?
a) 1:2:1
b) 9:3:3:1
c) All same
d) 3:1
View Answer
Explanation: Both the phenotypic and genotypic ratio will be the same i.e. all will be heterozygous for H loci and T loci. In this case, it will express the dominant trait in both cases.
10. In an experiment with drosophila eye colour purple/red and wing shape normal/vestigial you observe that offspring mainly have either purple vestigial trait or red normal trait. What is the expected parental trait?
a) Purple long and Red long
b) Red Vestigial and purple long
c) Red vestigial and purple vestigial
d) Purple vestigial and red vestigial
View Answer
Explanation: In this case, drosophila shows the linkage between these two gens, so the parents must having the trait that is mostly present in the offspring.
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