Heredity and Genetics (BIO 3105)

Correct Answer C. The cell is in a resting state and not actively dividing

Explanation

The G0 phase is a resting or quiescent phase of the cell cycle. Cells that enter the G0 phase are not actively preparing to divide and are often metabolically active but not undergoing cell division. Cells can remain in G0 for an extended period or may re-enter the cell cycle in response to specific signals.

Why other options are wrong

A. The cell is preparing for DNA replication

This occurs during the G1 phase, not G0. During G1, the cell prepares for DNA replication, but in the G0 phase, the cell is not actively preparing to divide.

B. The cell is actively dividing

Active division occurs during the M phase of the cell cycle, not the G0 phase. In G0, the cell is not actively dividing, as it is in a resting state.

D. The cell is undergoing apoptosis

Apoptosis is a programmed cell death process, which is distinct from the G0 phase. Cells in G0 are alive and not undergoing programmed cell death.

Correct Answer C. 3:1

Explanation

When two true-breeding plants with contrasting traits are crossed, the F1 progeny are typically all heterozygous for the trait. When these F1 individuals are crossed with each other, the F2 progeny will exhibit a phenotypic ratio of approximately 3:1, where 75% show the dominant phenotype and 25% show the recessive phenotype. This follows Mendel's Law of Segregation.

Why other options are wrong

A. 2:2

A 2:2 ratio is not typical of Mendelian inheritance, as it would suggest that two phenotypes appear in equal proportions, which is not the expected outcome of a typical monohybrid cross. The correct ratio for a typical dihybrid cross or in some cases incomplete dominance might be 2:2, but it doesn't apply here.

B. 1:3

A 1:3 ratio is not possible in a simple Mendelian cross between true-breeding parents. The correct ratio for such a cross would be 3:1, with three individuals showing the dominant phenotype and one showing the recessive phenotype.

D. 0:4

A 0:4 ratio suggests that all progeny show the same phenotype, which is not typically observed in a cross between true-breeding plants with contrasting traits. Such a result would indicate a genetic anomaly or some other influencing factor, which does not apply in standard Mendelian inheritance.

Correct Answer A. segregation and independent assortment

Explanation

During meiosis, the principles of segregation and independent assortment govern how genes are distributed into gametes. Segregation ensures that each gamete receives one allele from each gene pair, while independent assortment describes how genes on different chromosomes are distributed independently. These principles make it possible to predict the inheritance patterns of traits across generations using tools like Punnett squares.

Why other options are wrong

B. segregation and crossing over

Although segregation occurs during meiosis, pairing it with crossing over does not fully explain the predictability of inheritance patterns. Crossing over introduces genetic variation but does not follow a predictable Mendelian pattern. It disrupts gene linkage but does not substitute for the role of independent assortment.

C. independent assortment and crossing over

This option overlooks the essential role of segregation, which is a foundational concept in Mendelian inheritance. While independent assortment and crossing over contribute to genetic variation, segregation is necessary for ensuring that alleles separate into different gametes, which allows for predictable outcomes.

D. random assortment and segregation

"Random assortment" is not a defined principle in classical genetics. The term is likely a misrepresentation of "independent assortment." While segregation is correct, pairing it with a vague or inaccurate term makes this option incorrect in the context of established genetic principles.

Correct Answer B. Both alleles are expressed in the phenotype.

Explanation

Codominance occurs when both alleles of a gene are equally expressed in the phenotype of a heterozygous individual. In this case, neither allele is dominant or recessive, and both contribute to the observed trait. An example of codominance is seen in human blood types, where both A and B alleles are expressed in individuals with the AB blood type.

Why other options are wrong

A. One allele is dominant over the other allele.

This option describes complete dominance, where one allele is expressed over the other in the phenotype. However, codominance involves both alleles being expressed equally, so this explanation is not correct.

C. Neither allele is expressed in the phenotype.

This option does not apply to codominance. In codominance, both alleles are expressed in the phenotype, so this is not a valid explanation for how codominant traits are expressed.

Correct Answer D. It will be unable to complete either mitosis or meiosis.

Explanation

Spindle fibers are crucial for the proper segregation of chromosomes during both mitosis and meiosis. If a cell cannot produce spindle fibers, it will be unable to properly align and separate its chromosomes, preventing successful cell division. This failure often leads to cell cycle arrest or errors in chromosome distribution.

Why other options are wrong

A. It can successfully complete mitosis.

This is incorrect because spindle fibers are necessary for mitosis. Without them, the cell cannot proceed through the stages of mitosis successfully, as chromosome alignment and separation are disrupted.

B. It can successfully complete meiosis.

This is incorrect because meiosis also requires spindle fibers to ensure the proper division of chromosomes. Without spindle fibers, meiosis cannot be completed correctly, resulting in errors during gamete formation.

C. It will undergo apoptosis.

While apoptosis could be triggered due to errors in cell division, it is not a guaranteed consequence. The inability to produce spindle fibers may first result in a halted cell cycle, with apoptosis potentially occurring later as a response to the damage. However, it doesn't always lead directly to apoptosis.

Correct Answer A. Determined only by genes

Explanation

Hereditarianism is the belief that human traits, including intelligence, behavior, and physical characteristics, are solely determined by genetic inheritance, with minimal or no influence from environmental factors. This perspective views genetics as the primary determinant of an individual's traits.

Why other options are wrong

B. Partly influenced by the environment

This is incorrect because hereditarianism specifically emphasizes the role of genetics over environmental influences. The idea that traits are partly influenced by the environment would be more aligned with modern views that emphasize gene-environment interactions.

C. Influenced equally by genes and the environment

This contradicts hereditarianism, which posits that traits are determined predominantly by genetics, not by an equal balance of genetic and environmental factors.

D. Traceable to our earliest ancestors

While human traits may be influenced by ancestry, hereditarianism focuses specifically on the belief that genetics, rather than ancestral or environmental factors, are the primary determinants of traits.

Correct Answer A. The phenotype refers to traits in an organism that can be observed

Explanation

The phenotype is the observable physical or behavioral traits of an organism, which are determined by its genetic makeup (genotype) and environmental factors. These traits can include characteristics like eye color, height, and hair texture. The phenotype represents what is visible or measurable in an organism, distinguishing it from the genotype, which refers to the underlying genetic code.

Why other options are wrong

B. The phenotype is the set of genes in an organism.

This is incorrect because the phenotype refers to the observable traits, not the set of genes. The set of genes in an organism is called the genotype. The genotype influences the phenotype but is not the same as it.

C. A gene consisting of a single allele is said to be polymorphic.

Polymorphism refers to the occurrence of two or more different alleles at a particular locus within a population, not a gene consisting of a single allele. A gene with only one allele would not be considered polymorphic.

D. Genotype is a synonym of locus.

This is incorrect. The genotype refers to the genetic makeup of an organism, while the locus refers to the specific location of a gene on a chromosome. They are not synonymous; the locus is a location, while the genotype refers to the actual genetic information present at that location.

Correct Answer A. Alleles

Explanation

Alleles are different forms of a gene that can exist at a specific locus on a chromosome. Each individual has two alleles for each gene, one inherited from each parent. The variations in these alleles can result in different traits being expressed.

Why other options are wrong

B. Chromosomes

Chromosomes are structures made of DNA and protein that carry genes. A chromosome can contain many genes, but it is not used to describe the different forms of a gene at a specific locus.

C. Genotypes

Genotypes refer to the genetic makeup of an organism in terms of the alleles it possesses. While alleles contribute to a genotype, the term genotype itself does not describe the variations of a gene at a locus.

D. Phenotypes

Phenotypes refer to the observable traits or characteristics of an organism, which result from the interaction of the genotype and the environment. It is not used to describe variations of a gene

Correct Answer D. The nuclear envelope disappears and spindle fibers form.

Explanation

Prophase II is a stage in meiosis II where cells prepare for the second round of division. During this phase, the nuclear envelope breaks down, and spindle fibers begin to form, just as in prophase I or mitotic prophase. This prepares the cell to properly align and separate sister chromatids during metaphase II and anaphase II. No new crossing-over events occur at this stage.

Why other options are wrong

A. Homologous chromosomes align at the center of the cell

This event takes place during metaphase I of meiosis, not prophase II. During metaphase I, homologous chromosomes pair and align at the metaphase plate before being separated. In prophase II, the cells are already haploid, and homologous chromosomes have been separated, making this statement incorrect for that stage.

B. Sister chromatids align and crossing-over occurs

Crossing-over is a hallmark event of prophase I, not prophase II. During prophase II, the chromosomes are already recombined and condensed from the previous division, and no new crossing-over happens. Alignment of sister chromatids occurs later in metaphase II, not prophase II.

C. Sister chromatids separate and move towards opposite ends of the pole

This occurs during anaphase II, not prophase II. Prophase II is preparatory, where the spindle forms and the nuclear envelope dissolves, setting the stage for chromatid separation later. Thus, this option confuses the sequence of events in meiosis II.