All Free Biology MCQs with Answers

701. An individual carrying two different alleles of a gene, such as Tt, is described as

• A. homozygous dominant
• B. homozygous recessive
• C. heterozygous
• D. hemizygous

Explanation: Heterozygous means two unlike alleles at a locus, and such individuals are often called carriers when the recessive allele is hidden. Hemizygous is a different state, having only one copy of a gene, as with X linked genes in males.

702. A genetic cross in which the inheritance of a single pair of contrasting characters is followed is called a

• A. dihybrid cross
• B. test cross
• C. back cross
• D. monohybrid cross

Explanation: Mono means one character, such as height, and the classic F2 ratio of 3 to 1 comes from it. A dihybrid cross follows two characters at once and gives the 9:3:3:1 ratio.

703. In pea plants, tall (T) is dominant over dwarf (t). What proportion of the offspring of the cross Tt x Tt will be tall?

• A. 3/4
• B. 1/4
• C. 1/2
• D. All of them

Explanation: The cross gives TT, Tt, Tt and tt, so three of four genotypes contain a dominant allele and appear tall. Only the tt quarter is dwarf, which is the origin of the famous 3:1 phenotypic ratio.

704. Mendel's law of independent assortment states that

• A. dominant alleles always mask recessive ones
• B. the alleles of one pair separate into gametes independently of the alleles of other pairs
• C. genes on the same chromosome always recombine
• D. opposite traits blend together in the F1 generation

Explanation: During meiosis each pair of homologues lines up independently, so the allele a gamete gets for one character does not influence the other characters. The law needs the genes to sit on different chromosomes, as linkage questions make clear.

705. How many genetically different types of gamete are produced by an individual with the genotype AaBb, assuming independent assortment?

• A. 2
• B. 3
• C. 4
• D. 8

Explanation: Each gene pair contributes two choices, so 2 times 2 gives four gamete types: AB, Ab, aB and ab. The rule is two to the power of the number of heterozygous pairs, which scales to dihybrids and beyond.

706. When a red shorthorn cow is crossed with a white bull, the offspring carry a mixture of red and white hairs called roan. This illustrates

• A. complete dominance
• B. linkage
• C. crossing over
• D. codominance

Explanation: Both alleles express themselves side by side in the heterozygote, producing distinct red and white hairs rather than a blended colour. In incomplete dominance, by contrast, the phenotype is an intermediate such as pink.

707. A recessive allele expresses itself in the phenotype

• A. only when present in the homozygous condition
• B. in both the heterozygous and homozygous conditions
• C. only in males
• D. only in the F1 generation

Explanation: In a heterozygote the dominant allele masks the recessive one, so the recessive trait appears only when both alleles are recessive. This hiding is why traits can skip a generation and reappear in grandchildren.

708. However many alleles of a gene exist in a population, a diploid human individual can carry at most

• A. one
• B. two
• C. three
• D. four

Explanation: Each person inherits one allele from each parent, so only two can be present, one on each homologue. The ABO blood group has three alleles in the population, but any one person holds only two of them.

709. All the genes located on one chromosome together form a

• A. genome
• B. gene pool
• C. linkage group
• D. gene family

Explanation: A linkage group is everything that travels together on one chromosome during meiosis unless crossing over intervenes. Humans have 23 linkage groups, matching the haploid chromosome number.

710. Crossing over involves the exchange of corresponding segments between

• A. two sister chromatids of the same chromosome
• B. any two chromatids chosen at random
• C. the two strands of one DNA molecule
• D. non sister chromatids of homologous chromosomes

Explanation: Exchange between non sister chromatids of homologues shuffles maternal and paternal alleles, creating recombinant gametes. Exchange between sister chromatids would swap identical material and produce nothing new.