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Mendelian Genetics Questions
Q.
Gregor Mendel's experiments with pea plants established which of the following fundamental principles?
The chromosome theory of inheritance
The central dogma of molecular biology
Laws of segregation and independent assortment
The structure of DNA
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:
All offspring resemble the dominant parent
Alleles blend to produce an intermediate phenotype
Two alleles of a gene separate during gamete formation, each gamete receiving one
Alleles of different genes assort independently
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 genes on the same chromosome
Genes located on different chromosomes or far apart on the same chromosome
Only mitochondrial genes
Only sex-linked genes
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
9:3:3:1
1:2:1
1:1
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:
9:3:3:1
All dominant
1:1
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:
3:1
1:1:1:1
1:2:1
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
3:1
1:2:1
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:
Allele
Phenotype
Karyotype
Genotype
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:
Hemizygous
Haploid
Heterozygous
Homozygous
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:
Homozygous
Heterozygous
Diploid
Hemizygous
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:
Heterozygous
Haploid
Homozygous
Hemizygous
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 an intermediate phenotype
The heterozygote shows the recessive 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 the recessive phenotype only
Shows an intermediate phenotype between the two homozygotes
Shows the same phenotype as the dominant homozygote
Shows both parental phenotypes simultaneously
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:
Expresses both alleles fully and simultaneously
Shows an intermediate phenotype
Shows only the dominant allele
Shows no phenotype
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:
1:1
9:3:3:1
1:2:1
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:
Incomplete dominance only
Sex-linked inheritance
Multiple alleles and codominance
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:
The genotype of an individual showing the dominant phenotype
The gender of the offspring
Whether the gene is X-linked
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 dominant phenotype
Show an intermediate phenotype
Show a 1:1 ratio of dominant to recessive
Show the recessive 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:
IAIB
ii
IAIA
IBIB
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
1/2
1/4
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?
1/4
1/2
3/4
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
1/4
9/16
1/2
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 clear contrasting traits, short generation time, and easy controlled breeding
They had complex traits influenced by many genes
They had very long generation times
They had no genetic variation
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 red (dominant phenotype)
All pink (intermediate phenotype)
Half red, half white
All white (recessive phenotype)
Explanation:
In complete dominance, crossing homozygous dominant (RR) with homozygous recessive (rr) yields all heterozygous (Rr) F1 offspring — all showing the dominant phenotype.