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Mendelian Genetics Questions
Q.
Gregor Mendel's experiments with pea plants established which of the following fundamental principles?
The central dogma of molecular biology
The structure of DNA
The chromosome theory of inheritance
Laws of segregation and independent assortment
Explanation:
Mendel formulated the laws of segregation and independent assortment through systematic crosses of pea plants, founding classical genetics.
Q.
Mendel's Law of Segregation states that:
Two alleles of a gene separate during gamete formation, each gamete receiving one
All offspring resemble the dominant parent
Alleles of different genes assort independently
Alleles blend to produce an intermediate phenotype
Explanation:
During gamete formation, the two alleles of a gene separate so that each gamete receives only one allele. This explains the 3:1 phenotypic ratio in monohybrid crosses.
Q.
Mendel's Law of Independent Assortment applies to:
Only sex-linked genes
Genes located on different chromosomes or far apart on the same chromosome
Only mitochondrial genes
Only genes on the same chromosome
Explanation:
Independent assortment applies to genes on different chromosomes (or far apart on the same chromosome), where alleles of one gene assort independently of alleles of another gene.
Q.
A monohybrid cross between two heterozygotes (Aa × Aa) produces a phenotypic ratio of:
3:1
1:2:1
1:1
9:3:3:1
Explanation:
Aa × Aa yields 1 AA : 2 Aa : 1 aa genotypically, which corresponds to a 3:1 phenotypic ratio (dominant : recessive).
Q.
A monohybrid cross between a heterozygote and a homozygous recessive (Aa × aa) produces a phenotypic ratio of:
1:1
All dominant
3:1
9:3:3:1
Explanation:
Aa × aa yields 1 Aa : 1 aa, which corresponds to a 1:1 phenotypic ratio — the classic testcross ratio.
Q.
A dihybrid cross between two double heterozygotes (AaBb × AaBb) produces a phenotypic ratio of:
1:1:1:1
1:2:1
3:1
9:3:3:1
Explanation:
AaBb × AaBb yields the classic 9:3:3:1 ratio (dominant-dominant : dominant-recessive : recessive-dominant : recessive-recessive) when genes assort independently.
Q.
A dihybrid testcross (AaBb × aabb) produces a phenotypic ratio of:
1:1:1:1
9:3:3:1
1:2:1
3:1
Explanation:
AaBb × aabb yields a 1:1:1:1 ratio (AaBb : Aabb : aaBb : aabb) when genes assort independently. Deviations from this ratio indicate linkage.
Q.
The physical appearance of an organism is called its:
Phenotype
Allele
Genotype
Karyotype
Explanation:
Phenotype refers to observable traits, while genotype refers to the genetic makeup (the specific alleles present).
Q.
An organism with two identical alleles for a gene is called:
Homozygous
Haploid
Hemizygous
Heterozygous
Explanation:
Homozygous organisms carry two copies of the same allele (e.g., AA or aa). Heterozygous organisms carry two different alleles (e.g., Aa).
Q.
An organism with two different alleles for a gene is called:
Diploid
Hemizygous
Homozygous
Heterozygous
Explanation:
Heterozygous organisms have two different alleles (e.g., Aa), often showing the dominant phenotype when dominance is complete.
Q.
A male with an X-linked gene has only one copy of the allele. This condition is called:
Hemizygous
Heterozygous
Haploid
Homozygous
Explanation:
Hemizygous refers to having only one copy of a gene — as males do for X-linked genes. Because males have just one X, they express any allele on it regardless of dominance.
Q.
Which of the following describes complete dominance?
The heterozygote shows the recessive phenotype
The heterozygote shows an intermediate phenotype
The heterozygote shows the same phenotype as the dominant homozygote
Both alleles are expressed equally in the heterozygote
Explanation:
In complete dominance, the heterozygote shows the same phenotype as the dominant homozygote. For example, Aa looks identical to AA.
Q.
In incomplete dominance, the heterozygote:
Shows both parental phenotypes simultaneously
Shows an intermediate phenotype between the two homozygotes
Shows the recessive phenotype only
Shows the same phenotype as the dominant homozygote
Explanation:
Incomplete dominance produces an intermediate phenotype in the heterozygote. A classic example is pink flowers from crossing red and white snapdragons.
Q.
In codominance, the heterozygote:
Shows an intermediate phenotype
Shows only the dominant allele
Shows no phenotype
Expresses both alleles fully and simultaneously
Explanation:
In codominance, both alleles are fully expressed. The AB blood type is a classic example — both A and B antigens appear on red blood cells.
Q.
A cross between two pink-flowered snapdragons (incomplete dominance) produces a phenotypic ratio of:
3:1
1:2:1
1:1
9:3:3:1
Explanation:
Pink (Rr) × Pink (Rr) yields 1 Red (RR) : 2 Pink (Rr) : 1 White (rr), i.e., a 1:2:1 ratio — diagnostic of incomplete dominance.
Q.
Human ABO blood groups are an example of:
Sex-linked inheritance
Multiple alleles and codominance
Incomplete dominance only
Polygenic inheritance only
Explanation:
The ABO blood group system is governed by a single gene with three alleles (IA, IB, i). IA and IB are codominant, while both are dominant over i.
Q.
A blood type AB individual has children with a blood type O partner. What is the possible blood type of their children?
All four types possible
Type O only
Type A or Type B only
Type AB only
Explanation:
AB (IAIB) × O (ii) yields IAi (type A) and IBi (type B) offspring. Children can be type A or type B — not AB or O.
Q.
A testcross is used to determine:
Whether the gene is X-linked
The gender of the offspring
The genotype of an individual showing the dominant phenotype
Whether mutation has occurred
Explanation:
A testcross (crossing to a homozygous recessive individual) reveals the genotype of an individual showing the dominant phenotype — because the recessive parent cannot mask alleles passed from the unknown parent.
Q.
In a testcross where the unknown parent is homozygous dominant, all offspring will:
Show the recessive phenotype
Show a 1:1 ratio of dominant to recessive
Show an intermediate phenotype
Show the dominant phenotype
Explanation:
AA × aa yields all Aa (heterozygous) offspring, all expressing the dominant phenotype. A 1:1 ratio would indicate the unknown was heterozygous.
Q.
The genotype of a person with type O blood is:
IBIB
IAIA
IAIB
ii
Explanation:
Type O requires two i alleles (ii) since i is recessive to both IA and IB. The absence of A or B antigens on red blood cells defines type O.
Q.
In a monohybrid cross Aa × Aa, what fraction of offspring will show the recessive phenotype?
3/4
1/4
1/2
All offspring
Explanation:
The cross produces 1/4 AA, 1/2 Aa, and 1/4 aa. Only the 1/4 aa genotype shows the recessive phenotype.
Q.
What is the probability that a child of two heterozygous parents (Aa × Aa) will be homozygous dominant?
1/2
3/4
1/4
Explanation:
Aa × Aa produces 1/4 AA, 1/2 Aa, and 1/4 aa. The probability of AA (homozygous dominant) is 1/4.
Q.
A plant with genotype AaBb is self-crossed. What fraction of offspring will be double homozygous recessive (aabb)?
1/16
1/2
1/4
9/16
Explanation:
For each gene, Aa × Aa gives 1/4 aa. Since genes assort independently, P(aabb) = 1/4 × 1/4 = 1/16.
Q.
Why did Mendel choose pea plants for his genetic experiments?
They had no genetic variation
They had complex traits influenced by many genes
They had very long generation times
They had clear contrasting traits, short generation time, and easy controlled breeding
Explanation:
Pea plants offered clear-cut contrasting traits, short generation time, self-fertilization ability, and easy cross-breeding — ideal properties for quantitative analysis of inheritance.
Q.
A pure-breeding red-flowered plant is crossed with a pure-breeding white-flowered plant (complete dominance). F1 offspring are:
All pink (intermediate phenotype)
All red (dominant phenotype)
All white (recessive phenotype)
Half red, half white
Explanation:
In complete dominance, crossing homozygous dominant (RR) with homozygous recessive (rr) yields all heterozygous (Rr) F1 offspring — all showing the dominant phenotype.