BIOL180 Practice Exam 4 Genetics
and Mendelian Inheritance Questions
And Correct Answers (Verified
Answers) Plus Rationales 2026 Q&A |
Instant Download Pdf
1. Which term describes the observable characteristics of an organism
resulting from its genotype and environmental influences?
A. Allele
B. Locus
C. Phenotype
D. Genome
Answer: C. Phenotype
Rationale: The phenotype is the observable expression of an
organism's traits, including physical, physiological, and sometimes
behavioral characteristics. Although genotype strongly influences
phenotype, environmental factors can also affect how traits are
expressed. An allele is an alternative form of a gene, a locus is the
location of a gene on a chromosome, and the genome is the
organism's complete genetic material.
2. Which statement best defines an allele?
A. An alternative form of a particular gene
B. The physical appearance of an organism
C. A complete set of chromosomes
D. A pair of homologous chromosomes
,Answer: A. An alternative form of a particular gene
Rationale: Alleles are alternative versions of the same gene that
occupy the same locus on homologous chromosomes. For example, a
gene affecting flower color may have a purple allele and a white allele.
Different alleles can produce different versions of a characteristic.
Phenotype refers to observable traits, whereas the genome refers to all
genetic material.
3. An organism with two identical alleles for a particular gene is
described as:
A. Heterozygous
B. Homozygous
C. Hemizygous
D. Polygenic
Answer: B. Homozygous
Rationale: An organism is homozygous for a gene when it possesses
two identical alleles, such as AA or aa. A heterozygous organism
possesses two different alleles, such as Aa. The terms homozygous and
heterozygous describe the genetic composition at a particular locus
rather than the organism's overall genetic makeup.
4. Which genotype represents a heterozygous individual?
A. AA
B. aa
C. BB
D. Bb
Answer: D. Bb
Rationale: A heterozygous genotype contains two different alleles of
the same gene. Bb therefore represents a heterozygous individual. AA,
,aa, and BB each contain two copies of the same allele and are
consequently homozygous. Heterozygosity is particularly important
when studying dominant and recessive inheritance patterns.
5. Mendel's law of segregation states that:
A. Dominant alleles always occur more frequently in populations.
B. Genes on different chromosomes always assort together.
C. The two alleles of a gene separate during gamete formation.
D. Recessive alleles disappear when dominant alleles are present.
Answer: C. The two alleles of a gene separate during gamete
formation.
Rationale: Mendel's law of segregation states that the two alleles
carried by an individual separate during gamete formation so that
each gamete receives only one allele. During fertilization, gametes
combine and restore the pair. This principle explains why
heterozygous organisms produce gametes carrying either allele.
6. Which type of genetic cross is used to determine whether an
organism showing a dominant phenotype is homozygous or
heterozygous?
A. Dihybrid cross
B. Testcross
C. Reciprocal cross
D. Monohybrid self-cross
Answer: B. Testcross
Rationale: A testcross involves crossing an individual with an
unknown genotype but dominant phenotype with an individual that is
homozygous recessive. If all offspring show the dominant phenotype,
the unknown parent may be homozygous dominant. If recessive
, offspring appear, the unknown parent must have contributed a
recessive allele and is therefore heterozygous.
7. In a monohybrid cross between two heterozygous individuals with
complete dominance, what phenotypic ratio is expected?
A. 1:1
B. 1:2:1
C. 3:1:1
D. 3:1
Answer: D. 3:1
Rationale: A cross between Aa × Aa produces genotypes AA, Aa, Aa,
and aa. Three of these genotypes express the dominant phenotype,
while one expresses the recessive phenotype. Therefore, the expected
phenotypic ratio is 3 dominant to 1 recessive. The corresponding
genotypic ratio is 1:2:1.
8. What is the expected genotypic ratio from a cross between two
heterozygous individuals for a single gene showing complete
dominance?
A. 1 AA : 2 Aa : 1 aa
B. 3 AA : 1 aa
C. 2 AA : 2 aa
D. 1 AA : 1 Aa : 2 aa
Answer: A. 1 AA : 2 Aa : 1 aa
Rationale: In an Aa × Aa cross, each parent contributes either A or a
with equal probability. The four equally likely combinations are AA,
Aa, Aa, and aa. Combining the two heterozygous outcomes gives a
genotypic ratio of 1 AA : 2 Aa : 1 aa. Complete dominance changes the
phenotype ratio but not the underlying genotype probabilities.
and Mendelian Inheritance Questions
And Correct Answers (Verified
Answers) Plus Rationales 2026 Q&A |
Instant Download Pdf
1. Which term describes the observable characteristics of an organism
resulting from its genotype and environmental influences?
A. Allele
B. Locus
C. Phenotype
D. Genome
Answer: C. Phenotype
Rationale: The phenotype is the observable expression of an
organism's traits, including physical, physiological, and sometimes
behavioral characteristics. Although genotype strongly influences
phenotype, environmental factors can also affect how traits are
expressed. An allele is an alternative form of a gene, a locus is the
location of a gene on a chromosome, and the genome is the
organism's complete genetic material.
2. Which statement best defines an allele?
A. An alternative form of a particular gene
B. The physical appearance of an organism
C. A complete set of chromosomes
D. A pair of homologous chromosomes
,Answer: A. An alternative form of a particular gene
Rationale: Alleles are alternative versions of the same gene that
occupy the same locus on homologous chromosomes. For example, a
gene affecting flower color may have a purple allele and a white allele.
Different alleles can produce different versions of a characteristic.
Phenotype refers to observable traits, whereas the genome refers to all
genetic material.
3. An organism with two identical alleles for a particular gene is
described as:
A. Heterozygous
B. Homozygous
C. Hemizygous
D. Polygenic
Answer: B. Homozygous
Rationale: An organism is homozygous for a gene when it possesses
two identical alleles, such as AA or aa. A heterozygous organism
possesses two different alleles, such as Aa. The terms homozygous and
heterozygous describe the genetic composition at a particular locus
rather than the organism's overall genetic makeup.
4. Which genotype represents a heterozygous individual?
A. AA
B. aa
C. BB
D. Bb
Answer: D. Bb
Rationale: A heterozygous genotype contains two different alleles of
the same gene. Bb therefore represents a heterozygous individual. AA,
,aa, and BB each contain two copies of the same allele and are
consequently homozygous. Heterozygosity is particularly important
when studying dominant and recessive inheritance patterns.
5. Mendel's law of segregation states that:
A. Dominant alleles always occur more frequently in populations.
B. Genes on different chromosomes always assort together.
C. The two alleles of a gene separate during gamete formation.
D. Recessive alleles disappear when dominant alleles are present.
Answer: C. The two alleles of a gene separate during gamete
formation.
Rationale: Mendel's law of segregation states that the two alleles
carried by an individual separate during gamete formation so that
each gamete receives only one allele. During fertilization, gametes
combine and restore the pair. This principle explains why
heterozygous organisms produce gametes carrying either allele.
6. Which type of genetic cross is used to determine whether an
organism showing a dominant phenotype is homozygous or
heterozygous?
A. Dihybrid cross
B. Testcross
C. Reciprocal cross
D. Monohybrid self-cross
Answer: B. Testcross
Rationale: A testcross involves crossing an individual with an
unknown genotype but dominant phenotype with an individual that is
homozygous recessive. If all offspring show the dominant phenotype,
the unknown parent may be homozygous dominant. If recessive
, offspring appear, the unknown parent must have contributed a
recessive allele and is therefore heterozygous.
7. In a monohybrid cross between two heterozygous individuals with
complete dominance, what phenotypic ratio is expected?
A. 1:1
B. 1:2:1
C. 3:1:1
D. 3:1
Answer: D. 3:1
Rationale: A cross between Aa × Aa produces genotypes AA, Aa, Aa,
and aa. Three of these genotypes express the dominant phenotype,
while one expresses the recessive phenotype. Therefore, the expected
phenotypic ratio is 3 dominant to 1 recessive. The corresponding
genotypic ratio is 1:2:1.
8. What is the expected genotypic ratio from a cross between two
heterozygous individuals for a single gene showing complete
dominance?
A. 1 AA : 2 Aa : 1 aa
B. 3 AA : 1 aa
C. 2 AA : 2 aa
D. 1 AA : 1 Aa : 2 aa
Answer: A. 1 AA : 2 Aa : 1 aa
Rationale: In an Aa × Aa cross, each parent contributes either A or a
with equal probability. The four equally likely combinations are AA,
Aa, Aa, and aa. Combining the two heterozygous outcomes gives a
genotypic ratio of 1 AA : 2 Aa : 1 aa. Complete dominance changes the
phenotype ratio but not the underlying genotype probabilities.
