High School Biology : Understanding Chromosomes

Study concepts, example questions & explanations for High School Biology

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Example Questions

Example Question #1 : Understanding Chromosomes

Traits (alleles) are separated from one another during formation of gametes and are transmitted independently of one another according to __________.

Possible Answers:

the law of the Mendelian genome

the law of independent assortment

the law of mitotic inheritance

the law of segregation

Correct answer:

the law of independent assortment

Explanation:

During metaphase of meiosis, chromosomes form tetrads at the center of the cell. These tetrads are formed from pairs of homologous chromosomes. One chromosomes came from the organism's mother and the other from its father. During alignment, these chromosomes are arranged randomly, such that each gamete will have a combination of maternal and paternal DNA from the organism. This random mixing of DNA during gamete formation is known as the law of independent assortment, and plays a key role in diversifying the genetic background of offspring that form from the gamete.

Example Question #2 : Understanding Chromosomes

In a dihybrid cross (AaBb x AaBb), how many total genotypes are possible in the offspring?

Possible Answers:

Correct answer:

Explanation:

The alleles for gene A assort independently from the alleles of gene B, meaning that the genotype for one does not affect the genotype of the other. Even though there are two genes, we can solve this problem by answering the question separately for the two genes.

There are three possible genotypes with respect to the A gene (AA, Aa, aa) and three possible genotypes with respect to the B gene (BB, Bb, bb). Since genes A and B assort independently, the possible offspring will be the product of the possibilities for each separate gene.

Listed out, these genotypes are: AABB, AABb, AaBB, AAbb, AaBb, aaBB, Aabb, aaBb, aabb.

Example Question #3 : Understanding Chromosomes

If a somatic cell of a diploid organism contains eight chromosomes during interphase, which of the following must be true?

Possible Answers:

A gamete will contain four chromosomes

A somatic cell in metaphase will contain eight chromatids

A gamete will contain eight chromosomes

A germ cell in metaphase I will contain four chromosomes

Correct answer:

A gamete will contain four chromosomes

Explanation:

A somatic cell is a non-sex cell. During interphase (i.e. not during mitosis), a somatic cell of a diploid organism will be in its 2n state with two copies of each chromosome. A diploid somatic cell with eight chromosomes indicates that 2n=8.

When a somatic cell undergoes mitosis it first replicates its chromosomes, so in metaphase it will have sixteen chromatids. At this point, each of the eight chromosomes will be composed of two identical chromatids, for a total of sixteen.

When a germ cell begins meiosis, it also replicates the chromosomes and has sixteen chromatids until meiosis I is complete. During meiosis II, the two daughter cells from meiosis I each contain four chromosomes, each with two chromatids, for a total of eight chromatids. These chromatids are split during meiosis II, giving you four gametes that each have four chromosomes, each made of only one chromatid. This means that the gametes are haploid, since they contain only half of the original genetic material (n=4).

The transition from diploid to haploid occurs after meiosis I, since the first daughter cells only contain one copy of each chromosome after the tetrads are separated.

Example Question #4 : Understanding Chromosomes

What are chromosomes made of?

Possible Answers:

DNA that is coiled into histone proteins make up chromatin, which is organized into chromatids. The chromatids are held together at the centrosome to make a chromosome.

DNA and RNA interact to make a chromatid, which are organized into centrosomes. Two centrosomes are held together at the chromatin to make a chromosome.

DNA that is coiled into histone proteins make up a centrosome, which are organized into chromatids. The chromatids are held together at the centriole to make a chromosome.

DNA coils into circular grains to make up chromatids, which are organized into chromatin. The choromatins are held together at the centrosome to make up a chromosome.

Correct answer:

DNA that is coiled into histone proteins make up chromatin, which is organized into chromatids. The chromatids are held together at the centrosome to make a chromosome.

Explanation:

Strings of DNA coiled around histone proteins make up chromatin. Chromatin condenses into chromatids. Two chromatids line up at the centrosome during cell replication, which results in X-shaped chromosomes.

Example Question #5 : Understanding Chromosomes

A collection of an individual’s genes—inherited alleles—is defined as which of the following?

Possible Answers:

Phenotype 

Locus of alleles 

Genotype

Morphology 

Correct answer:

Genotype

Explanation:

Genotype is defined as the sequence of genetic makeup that determines specific characteristics and traits—phenotypes—of an individual. The genome is defined as the collection of an individual’s genes and consists of DNA. A genotype is expressed when information in DNA makes RNA and protein molecules, which determine the structure and function of cells; however, a genotype can also refer to genes carried by an individual, which includes mutations. Last, phenotype refers to observable physical characteristics within an individual. Some phenotypical traits are determined by the genotype, while others are shaped by environmental factors. 

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