Free Mendel's Laws of Inheritance MCQs with Answers

1. Mendel carried out his classical experiments on inheritance using

• A. maize
• B. the garden pea
• C. Drosophila
• D. the snapdragon

Explanation: Pisum sativum was an ideal choice because it has clearly contrasting characters, is normally self pollinating so pure lines are easy to keep, can be cross pollinated by hand, and produces a large number of offspring in one season. Drosophila became the standard organism only later, with Morgan's work on linkage and sex linkage, roughly half a century after Mendel.

2. The separation of the two alleles of a gene so that each gamete receives only one of them is Mendel's law of

• A. independent assortment
• B. dominance
• C. segregation
• D. linkage

Explanation: The law of segregation follows from the separation of homologous chromosomes in anaphase I of meiosis, so a heterozygote produces two kinds of gamete in equal numbers. Independent assortment is the separate rule about how two different genes behave relative to each other. Linkage is not a Mendelian law at all but the exception that was discovered later.

3. The phenotypic ratio of the F2 generation of a monohybrid cross is

• A. 1:1
• B. 9:3:3:1
• C. 1:2:1
• D. 3:1

Explanation: Crossing two heterozygotes gives one homozygous dominant, two heterozygotes and one homozygous recessive, and since the first three all look alike the visible ratio is three to one. 1:2:1 is the genotypic ratio of the very same cross, which is why it is the standard trap. 9:3:3:1 belongs to a dihybrid cross and 1:1 to a test cross.

4. The F2 phenotypic ratio of a dihybrid cross between two heterozygotes is

• A. 9:3:3:1
• B. 3:1
• C. 1:1:1:1
• D. 12:3:1

Explanation: Each gene independently gives a three to one ratio, so combining two of them multiplies out to nine both dominant, three of each single dominant and one both recessive. The ratio holds only if the two genes assort independently, which requires them to be on different chromosomes. 1:1:1:1 is what a dihybrid test cross gives, and 12:3:1 arises when one gene masks another.

5. To find out whether a tall pea plant is homozygous or heterozygous, it should be crossed with

• A. another tall plant of unknown genotype
• B. a dwarf plant
• C. a known heterozygous tall plant
• D. itself only

Explanation: A test cross uses the homozygous recessive parent, here the dwarf, because such a parent contributes only recessive alleles and so the offspring show the unknown parent's contribution directly. All tall offspring mean the parent was homozygous, while about half dwarf offspring mean it was heterozygous. Crossing with another tall plant hides the answer, since a dominant allele from either parent masks the recessive.

6. The genotypic ratio of the F2 generation of a monohybrid cross is

• A. 1:2:1
• B. 3:1
• C. 9:3:3:1
• D. 1:1

Explanation: Of four F2 offspring one is homozygous dominant, two are heterozygous and one is homozygous recessive. This becomes the familiar three to one only when the phenotypes are counted, because the homozygous dominant and the heterozygote look the same. The two ratios describe the same cross from two different angles, so read the question carefully before answering.

7. A red flowered four o'clock plant crossed with a white flowered one gives pink flowered offspring. This is an example of

• A. codominance
• B. incomplete dominance
• C. epistasis
• D. multiple alleles

Explanation: In incomplete dominance the heterozygote shows a blend, because one dose of the functional allele makes too little pigment for full colour. It is distinguished from codominance, in which both alleles are fully and separately expressed and the heterozygote shows both, as with the roan coat of cattle. The F2 phenotypic ratio here becomes 1:2:1, matching the genotypic ratio.

8. The AB blood group of humans is an example of

• A. incomplete dominance
• B. sex linkage
• C. codominance
• D. epistasis

Explanation: A person of group AB makes both the A antigen and the B antigen on the red cell surface, so neither allele is diluted or masked and both are expressed fully at the same time. That is codominance, not incomplete dominance, which would give a single intermediate antigen instead of two separate ones. Both alleles are dominant to the recessive O allele.

9. The ABO blood group system illustrates

• A. a single pair of alleles at one locus
• B. two separate genes on two chromosomes
• C. sex linked inheritance
• D. multiple alleles, three of which exist at one locus

Explanation: The gene has three alleles in the population, I A, I B and i, though any one individual carries only two of them, and this is the definition of multiple allelism. The three alleles combine into six genotypes producing four phenotypes. The locus is on an autosome, so the trait appears equally in men and women and has nothing to do with sex linkage.

10. Mendel chose the garden pea for his experiments partly because it

• A. has many easily distinguished contrasting characters
• B. has an unusually long life cycle
• C. cannot self pollinate
• D. blends its characters in every generation

Explanation: Peas offer clear alternatives such as tall versus dwarf, self pollinate to give pure lines and produce many offspring quickly. These practical features let Mendel count ratios, which is where his laws came from.