In the cross AaBb × Aabb with complete dominance, what fraction of offspring shows both dominant phenotypes?
Correct answer A. 3/8
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MDCAT Biology MCQs
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In the four o’clock plant, red (RR) crossed with white (rr) gives pink F₁. This is an example of ____.
Correct answer D. incomplete dominance
When pink-flowered four o’clock plants (Rr) are crossed among themselves, the F₂ phenotypic ratio is ____.
Correct answer A. 1 red : 2 pink : 1 white
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A person with blood group AB has both A and B antigens on the red cells. This illustrates ____.
Correct answer C. codominance
How do codominance and incomplete dominance differ in the heterozygote?
Correct answer B. In codominance both parental traits appear fully; in incomplete dominance an intermediate appears
The ABO blood group system is controlled by ____.
Correct answer C. three alleles of one gene, Iᴬ, Iᴮ and i
How many genotypes and phenotypes are possible in the ABO blood group system?
Correct answer C. 6 genotypes and 4 phenotypes
A man of genotype Iᴬi marries a woman of genotype Iᴮi. What is the probability that their child has blood group O?
Correct answer A. 1/4
Both parents have blood group O. Which blood group can their children have?
Correct answer D. O only
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A child has blood group AB. Which pair of parental blood groups is NOT possible?
Correct answer D. O and AB
Erythroblastosis foetalis is most likely when ____.
Correct answer B. an Rh-negative mother carries an Rh-positive foetus after an earlier Rh-positive pregnancy
Genes that lie close together on the same chromosome and tend to be inherited together are said to be ____.
Correct answer C. linked
Gene linkage was first worked out in detail by T. H. Morgan using ____.
Correct answer D. Drosophila melanogaster
Crossing over takes place between ____.
Correct answer D. non-sister chromatids of homologous chromosomes in prophase I
The visible points where non-sister chromatids remain joined after crossing over are called ____.
Correct answer C. chiasmata
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A dihybrid test cross gives 1000 offspring, of which 160 are recombinant types. What is the distance between the two genes?
Correct answer D. 16 map units
Genes A and B are linked 12 map units apart. What proportion of gametes from an AB/ab individual will be Ab?
Correct answer C. 6%
If two genes show complete linkage, a test cross of AB/ab × ab/ab gives ____.
Correct answer B. only two parental types in a 1 : 1 ratio
The maximum recombination frequency observed between two genes is ____.
Correct answer D. 50%
The frequency of crossing over between two linked genes increases with ____.
Correct answer D. the distance between them on the chromosome