QCM : Genetics Fundamentals and Inheritance — 18 questions

Questions et réponses du QCM

1. What do the terms homozygous and heterozygous describe for a single gene?

Having one gene on each chromosome or one gene only
Having two identical alleles or two different alleles
Carrying one allele from the mother and one from the father
Showing a dominant trait or showing a recessive trait

Having two identical alleles or two different alleles

Explication

Homozygous means two identical alleles, while heterozygous means two different alleles for the same gene. The other options describe related inheritance ideas but not these terms directly.

2. When does a recessive trait appear in an individual?

Whenever at least one dominant allele is present
Only in the F1 generation after a true-breeding cross
Only when the individual carries two dominant alleles
Only when the individual is homozygous for the recessive allele

Only when the individual is homozygous for the recessive allele

Explication

A recessive trait is observed only in the homozygous recessive condition. A dominant allele would mask it in a heterozygote.

3. What is a single-factor cross designed to test?

How two traits are inherited together in one family
How chromosomes separate during meiosis I
How one gene’s alleles are inherited across generations
How gametes combine at random in fertilization

How one gene’s alleles are inherited across generations

Explication

A single-factor cross focuses on inheritance of a single gene’s alleles across generations. The other choices describe different genetic ideas, but not the purpose of this cross.

4. What is the F2 generation in a single-factor cross?

The offspring produced directly from two true-breeding parents
The first generation of pure-breeding parents
The offspring produced by crossing F1 individuals with each other
The gametes formed by the original parental generation

The offspring produced by crossing F1 individuals with each other

Explication

F2 is the generation that results when F1 individuals are crossed with one another. The direct offspring of the true-breeding parents are the F1 generation.

5. What does a gene locus refer to?

The observable effect of a gene on the body
The process by which alleles separate into gametes
The chromosome site where a particular gene is found
The ratio of dominant to recessive offspring

The chromosome site where a particular gene is found

Explication

A gene locus is the specific position of a gene on a chromosome. The other options describe phenotype, segregation, or ratios instead.

6. What does the genotype notation Tt indicate in the height example?

A homozygous dominant genotype with two tall alleles
A homozygous recessive genotype with two dwarf alleles
A heterozygous genotype with one tall allele and one dwarf allele
A gene located on two different chromosomes

A heterozygous genotype with one tall allele and one dwarf allele

Explication

Tt represents a heterozygous individual carrying one dominant tall allele and one recessive dwarf allele. TT would be homozygous dominant, and tt would be homozygous recessive.

7. What is the main use of a Punnett square?

To identify the physical appearance of a trait without knowing the genes
To predict genotypes and proportions of offspring from known parental genotypes
To determine whether a gene is dominant or recessive from one parent only
To show how chromosomes pair during meiosis

To predict genotypes and proportions of offspring from known parental genotypes

Explication

A Punnett square is a grid used to predict offspring genotypes and their proportions from parental genotypes. It works after the parental genotypes and dominant allele are known.

8. What genotype ratio is expected from crossing two heterozygous tall plants, Tt × Tt?

1 TT : 1 tt
3 TT : 1 tt
1 TT : 2 Tt : 1 tt
All Tt

1 TT : 2 Tt : 1 tt

Explication

A Tt × Tt cross produces four combinations: TT, Tt, tT, and tt, which gives a 1:2:1 genotype ratio. The phenotype ratio is different because T is dominant.

9. How is probability defined in genetics and chance problems?

The number of times an event has already happened
The difference between expected and observed results
The total number of offspring produced in a cross
The fraction of all possible outcomes that correspond to a particular event

The fraction of all possible outcomes that correspond to a particular event

Explication

Probability is calculated as the number of ways the event can occur divided by the total possible outcomes. It expresses chance, not the observed count itself.

10. Why do observed offspring ratios usually become closer to expected ratios when the sample size is larger?

Because deviations from expected ratios become fewer as the number of trials increases
Because probability stops applying once many offspring are counted
Because every large sample guarantees an exact ratio
Because larger samples change the dominant allele into a recessive one

Because deviations from expected ratios become fewer as the number of trials increases

Explication

With more trials, random fluctuations matter less, so observed frequencies tend to match expected probabilities more closely. Exact ratios are not guaranteed, especially in small samples.

11. What does Mendel’s law of segregation state about the two copies of a gene during transmission from parent to offspring?

They blend together before fertilization
They separate into different gametes
They remain paired in every gamete
They are copied so each gamete gets both

They separate into different gametes

Explication

The law of segregation says the two alleles for a gene separate during gamete formation, so each gamete receives only one copy. This is the basis for the patterns seen in monohybrid crosses.

12. At which stage do homologous chromosomes separate as part of the segregation of alleles?

Telophase of meiosis II
Prophase of meiosis I
Anaphase of meiosis II
Anaphase of meiosis I

Anaphase of meiosis I

Explication

Homologous chromosomes separate into different cells during anaphase of meiosis I. Sister chromatids separate later, during anaphase of meiosis II.

13. In a cross between two true-breeding parents that differ in one characteristic, what is the F1 generation?

The offspring produced by crossing F1 individuals
The offspring produced by crossing the two parents
The parental generation itself
Only the recessive offspring from the cross

The offspring produced by crossing the two parents

Explication

The F1 generation is the first offspring produced from crossing the two true-breeding parents. Crossing F1 individuals with each other produces the F2 generation.

14. What offspring genotype ratio is expected from a Tt × Tt cross?

1 TT : 2 Tt : 1 tt
1 TT : 1 tt
All Tt
3 TT : 1 tt

1 TT : 2 Tt : 1 tt

Explication

A heterozygous cross produces four genotype combinations in a 1:2:1 ratio: TT, Tt, Tt, and tt. This ratio is the standard result shown by the Punnett square for Tt × Tt.

15. What is the purpose of a test cross?

To compare two recessive phenotypes directly
To determine whether a dominant-phenotype individual is homozygous or heterozygous
To increase the number of offspring in a cross
To find the exact nucleotide sequence of a gene

To determine whether a dominant-phenotype individual is homozygous or heterozygous

Explication

A test cross is used to reveal the genotype of a dominant-phenotype individual by crossing it with a recessive homozygote. The offspring phenotypes then indicate whether the unknown parent was TT or Tt.

16. In a test cross, what offspring pattern indicates that a dominant-phenotype parent is heterozygous rather than homozygous dominant?

About half of the offspring show the recessive phenotype
All of the offspring show the dominant phenotype
Only the offspring with two dominant alleles are affected
About three-quarters of the offspring show the dominant phenotype

About half of the offspring show the recessive phenotype

Explication

A heterozygous dominant parent crossed with a recessive homozygote can produce recessive offspring, giving about a 1:1 dominant-to-recessive ratio. If the dominant parent were homozygous dominant, all offspring would show the dominant phenotype.

17. In the dog trait EIC, what inheritance pattern is described?

X-linked dominant
Autosomal dominant
Mitochondrial inheritance
Autosomal recessive

Autosomal recessive

Explication

EIC is described as an autosomal recessive trait in dogs. That means affected dogs must carry two recessive alleles.

18. For the short-legs trait described in animals, which statement is correct?

It is inherited only from the mother
It appears only when two recessive alleles are present
It is caused by an autosomal recessive allele
It is caused by an autosomal dominant allele

It is caused by an autosomal dominant allele

Explication

The short-legs example is described as an autosomal dominant trait, so one dominant allele is sufficient for the phenotype. This contrasts with recessive traits such as EIC.

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Alleles — definition?

Alternative forms of a gene.

Dominant vs recessive — difference?

Dominant shows in heterozygotes; recessive only in homozygotes.

Homozygous vs heterozygous — meaning?

Homozygous has identical alleles; heterozygous has different alleles.

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