Heredity and Genetics (BIO 3105)

Correct Answer A. At the end of telophase I

Explanation

The reduction of chromosome number from diploid to haploid is first achieved at the end of telophase I. During meiosis I, homologous chromosomes are separated into different cells, reducing the chromosome number by half. This results in haploid cells, which is critical for sexual reproduction, ensuring that when two gametes fuse, the resulting zygote has the correct diploid number.

Why other options are wrong

B. During metaphase I

Metaphase I is the stage where homologous chromosomes align at the cell's equator. While this is a crucial step for the separation of chromosomes, the actual reduction in chromosome number does not occur until later, during telophase I.

C. At the end of cytokinesis II

Cytokinesis II follows meiosis II, where the chromatids of each chromosome are separated. The chromosome number has already been halved by the end of telophase I, so cytokinesis II does not contribute to the initial reduction in chromosome number.

D. During anaphase I

Anaphase I is the stage where homologous chromosomes are separated and pulled to opposite poles of the cell. However, the reduction in chromosome number is not fully realized until telophase I, when the cell divides and forms haploid cells.

Correct Answer A. An observable trait

Explanation

A phenotype refers to the observable characteristics or traits of an organism, such as its appearance, behavior, or physiological properties. These traits result from the interaction of an organism's genetic makeup (genotype) and its environment.

Why other options are wrong

B. A derived trait

This is incorrect because a derived trait is one that is different from the ancestral trait and evolved in a specific lineage. It is a specific type of trait, not a general definition of phenotype.

C. An organism's genes

This is incorrect because genes refer to the genetic code that an organism carries in its DNA, which makes up the genotype. The phenotype is the expression of these genes, influenced by both genetics and environmental factors.

D. An evolutionary trait

This is incorrect because an evolutionary trait refers to a characteristic that has evolved in a population over generations. While phenotypic traits can be evolutionary, the term phenotype itself simply refers to the observable traits of an individual organism.

Correct Answer A. ABO blood group

Explanation

The ABO blood group system was one of the first human traits for which inheritance patterns were clearly understood. Discovered by Karl Landsteiner in the early 20th century, it was critical in the development of safe blood transfusions. The inheritance of the ABO blood types follows simple Mendelian genetics, with codominant alleles (A and B) and a recessive allele (O), making it a foundational example in human genetics.

Why other options are wrong

B. Eye color

Although eye color is a commonly discussed inherited trait, it involves multiple genes and is more complex than the ABO blood group system. Its inheritance pattern was not among the earliest to be clearly understood or documented.

C. Hair texture

Hair texture is influenced by multiple genes and environmental factors, making its inheritance more complex. It was not studied or documented early in human genetic research as clearly as ABO blood types were.

D. Sickle cell trait

While the sickle cell trait is significant in medical genetics, especially regarding its relation to malaria resistance, it was identified later than the ABO blood group and was not among the first traits used to understand human inheritance patterns.

Correct Answer B. The maximum number of times a cell can divide before entering senescence

Explanation

The Hayflick Limit refers to the number of times a normal, somatic (non-reproductive) cell will divide before cell division stops and the cell enters a state called senescence. This limit is influenced by the shortening of telomeres, which protect the chromosomes during cell division. Once the telomeres shorten beyond a critical length, the cell can no longer divide and enters senescence.

Why other options are wrong

A. The maximum length of a chromosome

The Hayflick Limit is not about the length of chromosomes but rather the number of times a cell can divide. The length of a chromosome can vary, but this is not related to the Hayflick Limit.

C. The maximum length of a telomere

The Hayflick Limit is related to telomere shortening, not the length of a telomere. As telomeres shorten with each cell division, it contributes to the limit on how many times a cell can divide.

D. The G1/S cell cycle checkpoint

The G1/S checkpoint is a control mechanism in the cell cycle that checks for cell size, DNA integrity, and nutrient availability. It is unrelated to the Hayflick Limit, which specifically deals with the number of divisions before senescence.

E. The maximum number of times a cell can divide before undergoing apoptosis

The Hayflick Limit refers to senescence, not apoptosis. Apoptosis is a programmed cell death process, whereas the Hayflick Limit concerns cellular aging and division cessation.

F. The G2/M cell cycle checkpoint

The G2/M checkpoint is involved in ensuring the cell is ready to undergo mitosis, checking for DNA damage or errors. It is unrelated to the Hayflick Limit.

Correct Answer B. The exchange of genetic material between homologous chromosomes during meiosis

Explanation

Genetic recombination occurs during meiosis when homologous chromosomes exchange genetic material through a process called crossing over. This results in the production of new combinations of alleles in the offspring, contributing to genetic diversity. Recombination is a crucial mechanism in heredity that increases variability within populations.

Why other options are wrong

A. The process by which DNA is replicated.

DNA replication is a process in which a cell copies its DNA before division, ensuring that each daughter cell has a complete set of genetic material. While this is important for cell division, it is not considered genetic recombination, as there is no exchange of genetic material between chromosomes.

C. The mutation of genes due to environmental factors.

Gene mutations caused by environmental factors, such as radiation or chemicals, can change the DNA sequence. However, this is not considered genetic recombination, which specifically involves the exchange of genetic material during meiosis.

D. The formation of new alleles through random mutations.

While mutations can lead to the creation of new alleles, this is not the same as genetic recombination. Recombination involves the mixing of existing genetic material between chromosomes, whereas mutations create new genetic variants.

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 A. the number of divisions a cultured cell can undergo

Explanation

The Hayflick limit refers to the number of times a normal somatic cell population will divide before cell division stops. This phenomenon occurs due to the shortening of telomeres, the protective ends of chromosomes, after each cell division. Once telomeres reach a critically short length, the cell can no longer divide and becomes senescent.

Why other options are wrong

B. the size limit to which a cell can grow

This answer confuses the Hayflick limit with physical cell growth. The Hayflick limit is about the number of cell divisions, not how large a cell can grow. Cell size is regulated by factors like surface-area-to-volume ratio, not telomere length.

C. the most cells an organism can have

This is unrelated to the Hayflick limit. The number of cells in an organism is determined by developmental and biological needs, not by the replicative lifespan of individual cells. The Hayflick limit refers specifically to the lifespan of cultured somatic cells.

D. how rapidly DNA replication can occur

This answer refers to the speed of a molecular process, not the replicative potential of a whole cell. The Hayflick limit has nothing to do with the rate of DNA replication but rather the number of replications (cell divisions) that can occur.

Correct Answer B. Nucleotides

Explanation

Adenine, guanine, thymine, and cytosine are the four nitrogenous bases found in nucleotides, which are the building blocks of DNA. Nucleotides consist of a sugar, a phosphate group, and one of the nitrogenous bases, and they are linked together to form the structure of DNA.

Why other options are wrong

A. Genes

Genes are segments of DNA that code for proteins. While adenine, guanine, thymine, and cytosine are components of DNA, they are not genes themselves; they are the basic units that make up genes.

C. Proteins

Proteins are made from amino acids, not from nucleotides. The nitrogenous bases adenine, guanine, thymine, and cytosine are involved in the structure of DNA, not proteins.

D. Codons

Codons are sequences of three nucleotides that code for specific amino acids in protein synthesis. While adenine, guanine, thymine, and cytosine are part of DNA, they are not codons themselves; they are the individual components that make up codons.

Correct Answer A. Insertion of normal genes to act in place of mutant genes

Explanation

Gene therapy is a medical treatment that involves inserting or altering genes within a person's cells to treat or prevent disease. The goal is typically to replace defective or missing genes with normal ones to correct genetic disorders.

Why other options are wrong

B. Mapping of all human genetic information

This refers to the Human Genome Project, not gene therapy. The Human Genome Project focused on sequencing and mapping human DNA, while gene therapy focuses on treating genetic diseases by replacing or repairing faulty genes.

C. Cloning of genes to produce and purify therapeutically useful proteins

This describes recombinant DNA technology, not gene therapy. While gene therapy may involve genes, recombinant DNA technology focuses on cloning and producing proteins like insulin, but not necessarily inserting them into human cells to treat diseases.

D. Insertion of human genes into other organisms

This describes genetic engineering, where genes from humans (or other organisms) are inserted into different species, not gene therapy. Gene therapy specifically focuses on treating human diseases by directly altering the genetic material of the patient.