Genetics quiz: 25 questions with answers

Questions and answers

By CanQuiz

An AI wrote the questions in this quiz from its creator’s material. It may contain mistakes.

1. 1.In genetics, what is an allele?

   • The fixed position of a gene on a chromosome
   • One of the alternative forms of a gene
   • A pair of matching chromosomes in a cell
   • The visible trait an organism shows
   Show answer

   Answer: One of the alternative forms of a gene

   Explanation: An allele is a version of a gene; a diploid organism carries two alleles for each gene on its non-sex chromosomes, one inherited from each parent. The gene's position on the chromosome is called its locus.

2. 2.Which of the following best describes an organism's genotype?

   • The characteristics you can observe in it
   • The number of chromosomes in each of its cells
   • The conditions in which its genes are expressed
   • The combination of alleles it carries for a trait
   Show answer

   Answer: The combination of alleles it carries for a trait

   Explanation: The genotype is the genetic make-up for a trait, written as letters such as Tt. What we can observe, such as height or colour, is the phenotype.

3. 3.The specific position that a gene occupies on a chromosome is called its locus.

   • True
   • False
   Show answer

   Answer: True

   Explanation: Each gene sits at a particular place on a particular chromosome, and that place is its locus. Homologous chromosomes carry the same genes at matching loci, though possibly different alleles.

4. 4.An individual with the genotype Aa for a gene is described as:

   • Heterozygous
   • Homozygous dominant
   • Homozygous recessive
   • Hemizygous
   Show answer

   Answer: Heterozygous

   Explanation: Heterozygous means the two alleles of the gene are different. AA would be homozygous dominant and aa homozygous recessive.

5. 5.Under complete dominance, an individual shows the recessive phenotype only if it is homozygous for the recessive allele.

   • True
   • False
   Show answer

   Answer: True

   Explanation: With complete dominance, a single dominant allele is enough to produce the dominant phenotype. The recessive trait therefore appears only when both alleles are recessive.

6. 6.In pea plants, the tall allele (T) is completely dominant over the short allele (t). What fraction of the offspring of a Tt x Tt cross is expected to be short?

   • 1/2
   • 3/4
   • 1/4
   • 0
   Show answer

   Answer: 1/4

   Explanation: Each parent passes on t with a probability of 1/2, so the chance of a tt offspring is 1/2 x 1/2 = 1/4. Only tt plants are short.

7. 7.What genotypic ratio is expected among the offspring of a Tt x Tt cross?

   • 1 TT : 1 tt
   • 3 TT : 1 tt
   • 2 TT : 1 Tt : 1 tt
   • 1 TT : 2 Tt : 1 tt
   Show answer

   Answer: 1 TT : 2 Tt : 1 tt

   Explanation: A Punnett square for this cross gives one TT box, two Tt boxes and one tt box. Under complete dominance, this 1:2:1 genotypic ratio becomes a 3:1 ratio of phenotypes.

8. 8.What does Mendel's law of segregation state?

   • The offspring show a blend of their parents' traits
   • The two alleles of a gene separate when gametes form, so each gamete carries only one of them
   • Genes found on different chromosomes are inherited together
   • Dominant alleles are more frequent in a population than recessive ones
   Show answer

   Answer: The two alleles of a gene separate when gametes form, so each gamete carries only one of them

   Explanation: During meiosis, the homologous chromosomes carrying the two alleles are separated into different gametes. Fertilisation then restores the pair, with one allele from each parent.

9. 9.A test cross is used to find out the genotype of an individual with the dominant phenotype. Which individual is it crossed with?

   • A homozygous dominant individual
   • A heterozygous individual
   • A homozygous recessive individual
   • An individual with the same phenotype
   Show answer

   Answer: A homozygous recessive individual

   Explanation: A homozygous recessive partner can only pass on recessive alleles, so the offspring's phenotypes reveal which alleles the tested individual carries.

10. 10.In a test cross, a plant with the dominant phenotype produces about half dominant and half recessive offspring. What is the plant's genotype?

    • Heterozygous
    • Homozygous dominant
    • Homozygous recessive
    • It cannot be worked out from this result
    Show answer

    Answer: Heterozygous

    Explanation: Recessive offspring must have received a recessive allele from the tested plant, so it cannot be homozygous dominant. A 1:1 ratio is the expected result when the tested parent is heterozygous (Aa x aa).

11. 11.Two organisms with exactly the same genotype can show different phenotypes if they grow in different environments.

    • True
    • False
    Show answer

    Answer: True

    Explanation: Phenotype results from the interaction of genes and environment. For example, genetically identical plants grown in poor soil or low light may end up much smaller than those grown in good conditions.

12. 12.Mendel's law of independent assortment applies to genes that are:

    • Very close together on the same chromosome
    • Found only on the Y chromosome
    • Present in more than two alleles in a population
    • Located on different chromosomes
    Show answer

    Answer: Located on different chromosomes

    Explanation: Genes on different chromosomes are sorted into gametes independently because each pair of homologous chromosomes lines up at random during meiosis. Genes close together on one chromosome tend to be inherited together.

13. 13.How many genetically different types of gamete can an AaBb individual produce if the two genes are on different chromosomes?

    • 2
    • 4
    • 8
    • 16
    Show answer

    Answer: 4

    Explanation: Each gamete receives one allele of each gene, giving the combinations AB, Ab, aB and ab. Because the genes assort independently, all four types are equally likely.

14. 14.In a dihybrid cross AaBb x AaBb, with unlinked genes and complete dominance, what phenotypic ratio is expected in the offspring?

    • 3:1
    • 9:3:3:1
    • 1:2:1
    • 1:1:1:1
    Show answer

    Answer: 9:3:3:1

    Explanation: Each gene on its own gives a 3:1 ratio, and multiplying the two independent ratios gives 9:3:3:1. The 9 show both dominant traits and the 1 shows both recessive traits.

15. 15.In the cross AaBb x AaBb, with unlinked genes, what fraction of the offspring is expected to have the genotype aabb?

    • 1/4
    • 3/16
    • 9/16
    • 1/16
    Show answer

    Answer: 1/16

    Explanation: The probability of aa is 1/4 and the probability of bb is also 1/4. Since the genes assort independently, the probability of both is 1/4 x 1/4 = 1/16.

16. 16.Two genes that lie very close together on the same chromosome follow the law of independent assortment just like genes on different chromosomes.

    • True
    • False
    Show answer

    Answer: False

    Explanation: Genes that are close together on one chromosome are linked and tend to be passed on together. They are only separated when crossing over occurs between them, which is rare when they are very close.

17. 17.In snapdragons, crossing a true-breeding red-flowered plant with a true-breeding white-flowered plant gives only pink-flowered offspring. If two of these pink plants are crossed, what fraction of their offspring is expected to be pink?

    • 1/4
    • 3/4
    • 1
    • 1/2
    Show answer

    Answer: 1/2

    Explanation: This is incomplete dominance: each pink plant carries one red allele and one white allele, and neither allele masks the other. Crossing two pink plants gives 1 red : 2 pink : 1 white, so half the offspring are pink.

18. 18.Which statement describes codominance?

    • The heterozygote shows a phenotype intermediate between the two homozygotes
    • One allele completely hides the effect of the other
    • Both alleles are fully expressed in the heterozygote
    • The allele is expressed in males only
    Show answer

    Answer: Both alleles are fully expressed in the heterozygote

    Explanation: In codominance, the heterozygote shows the effects of both alleles at once. Blood group AB is a classic example: red blood cells carry both the A and the B antigens.

19. 19.In the ABO blood group system, which pair of inherited alleles gives a person blood group O?

    • An A allele from one parent and an O allele from the other
    • A B allele from one parent and an O allele from the other
    • An O allele from each parent
    • An A allele from one parent and a B allele from the other
    Show answer

    Answer: An O allele from each parent

    Explanation: The O allele is recessive to both A and B, so group O appears only when a person inherits an O allele from each parent. An A or B allele paired with O gives group A or B, and A with B gives group AB.

20. 20.One parent has blood group A with genotype AO and the other has blood group B with genotype BO. Which blood groups are possible among their children?

    • Only A and B
    • Only AB
    • A, B, AB and O
    • A, B and AB, but not O
    Show answer

    Answer: A, B, AB and O

    Explanation: The possible children are AB, AO, BO and OO, each with a probability of 1/4. These correspond to groups AB, A, B and O.

21. 21.In humans, which parent's gamete determines the biological sex of a child?

    • The mother's, because an egg can carry either an X or a Y chromosome
    • The father's, because a sperm cell can carry either an X or a Y chromosome
    • Each parent contributes equally, because both pass on an X chromosome
    • Neither parent's, because sex depends on the temperature during development
    Show answer

    Answer: The father's, because a sperm cell can carry either an X or a Y chromosome

    Explanation: Every egg carries an X chromosome, while half of the sperm carry an X and half carry a Y. An XX zygote develops as female and an XY zygote as male.

22. 22.Red-green colour blindness is caused by a recessive allele on the X chromosome. Why is it much more common in males than in females?

    • Males have only one X chromosome, so a single copy of the allele is enough to cause the condition
    • The allele is carried on the Y chromosome
    • Males inherit two X chromosomes from their mother
    • The allele is dominant in males and recessive in females
    Show answer

    Answer: Males have only one X chromosome, so a single copy of the allele is enough to cause the condition

    Explanation: A male has no second X chromosome that could carry a normal allele to mask the recessive one. A female needs two copies of the allele, one on each X, to be colour-blind.

23. 23.A woman who carries the allele for red-green colour blindness has children with a man whose colour vision is normal. What is the probability that a son of theirs is colour-blind?

    • 1/4
    • 1/2
    • 0
    • 1
    Show answer

    Answer: 1/2

    Explanation: A son receives his Y chromosome from his father and his only X chromosome from his mother. The carrier mother passes on the X with the recessive allele half of the time, so each son has a 1/2 chance of being colour-blind.

24. 24.A colour-blind father passes the allele for red-green colour blindness to all of his sons.

    • True
    • False
    Show answer

    Answer: False

    Explanation: A father gives his Y chromosome to his sons and his X chromosome to his daughters. His sons therefore cannot inherit an X-linked allele from him, while all his daughters receive it.

25. 25.Two parents who do not have a genetic condition have a daughter who has it. Which pattern of inheritance best explains this?

    • Autosomal dominant
    • X-linked recessive
    • Autosomal recessive
    • Y-linked
    Show answer

    Answer: Autosomal recessive

    Explanation: Unaffected parents can each be heterozygous carriers of a recessive allele and pass it to a child. An X-linked recessive condition in a daughter would need an affected father, and a dominant condition would normally show in at least one parent.