Regents Examination in Living Environment: Questions on Standard 4, Biological Concepts, Key Idea 2—Genetic Continuity

Key Idea 2—Genetic Continuity
Organisms inherit genetic information in a variety of ways that result in continuity of structure and function between parents and offspring.
 

Corn plants grown in the dark will be white and usually much taller than genetically identical corn plants grown in light, which will be green and shorter. The most probable explanation for this is that the
corn plants grown in the dark were all mutants for color and height
expression of a gene may be dependent on the environment
plants grown in the dark will always be genetically albino
phenotype of a plant is independent of its genotype

In order for a substance to act as a carrier of hereditary information, it must be
easily destroyed by enzyme action
exactly the same in all organisms
present only in the nuclei of cells
copied during the process of mitosis

During synapsis in meiosis, portions of one chromosome may be exchanged for corresponding portions of its homologous chromosome. This process is known as
nondisjunction
polyploidy
crossing-over
hybridization

A DNA nucleotide is composed of three parts. These three parts may be
phosphate, adenine, and thymine
phosphate, deoxyribose, and thymine
phosphate, glucose, and cytosine
adenine, thymine, and cytosine

A double-stranded DNA molecule replicates as it unwinds and “unzips” along weak
hydrogen bonds
carbon bonds
phosphate groups
ribose groups

Base your answers to the next 4 questions on your knowledge of biology and the diagrams below.

The diagram on the left represents a portion of a double-stranded DNA molecule. The diagrams at the right represent specific combinations of nitrogenous bases found in compounds transporting specific amino acids.

The amino acid whose genetic code is present in strand I is
lysine
serine
asparagine
phenylalanine

The thymine (T) of strand I is accidentally replaced by adenine (A). This occurrence is called
segregation
disjunction
cytoplasmic inheritance
gene mutation

The number of different amino acids coded by strand I is
1
2
8
12

Which represents the sequence of nitrogenous bases in the molecule of messenger RNA synthesized by strand I?
-T-T-C-G-U-C-
-A-A-C-G-T-C-
-U-U-C-G-A-C-
-A-A-G-C-U-G-
 

Molecules that transport amino acids to ribosomes are known as
protein molecules
RNA molecules
mitochondria
chromosomes
 

A similarity between DNA molecules and RNA molecules is that they
are built from nucleotides
are double-stranded
contain deoxyribose sugar
contain uracil

What is the function of DNA molecules in the synthesis of proteins?
They catalyze the formation of peptide bonds.
They determine the sequence of amino acids in a protein.
They transfer amino acids from the cytoplasm to the nucleus.
They supply energy for protein synthesis.

In pea plants, the trait for smooth seeds is dominant over the trait for wrinkled seeds. When two hybrids are crossed, which results are most probable?
75% smooth and 25% wrinkled seeds
100% smooth seeds
50% smooth and 50% wrinkled seeds
100% wrinkled seeds

A person who is homozygous for blood type A has a genotype that can be represented as
IaIb
IaIa
Iai
ii
 

Animal breeders often cross breed members of the same litter in order to maintain desirable traits. This procedure is known as
hybridization
inbreeding
natural selection
vegetative propagation

Key Idea 2—Genetic Continuity: Answers and Explanations

2 Corn plants grown in the dark will be white. The most probable explanation for this is that expression of the gene for color may depend on the environment. The plants have the genetic information for chlorophyll production. This can be assumed because they are genetically identical to the plants grown in the light. Light is needed to activate the chlorophyll gene.
Wrong Choices Explained:
(1) Mutations are sudden changes in the genetic material. Mutations are inherited. Because the plants grown in the dark were genetically identical to those grown in the light, neither group lacked the genetic information for chlorophyll production.
(3) Albinism is a condition resulting from the absence of a normal gene for color. Both groups of plants had the normal gene for color.
(4) The phenotype is the physical appearance of the organism. The phenotype depends on the genotype.
 

4 The hereditary information is contained in the chromosomes. During mitosis, the chromosomes duplicate. The duplication of chromosomes ensures the equal distribution of identical genetic material to the new cells.
Wrong Choices Explained:
(1) If the hereditary information is destroyed, the cells cannot function. The chromosomes contain the information necessary for carrying out all cellular activities.
(2) No two organisms are exactly alike. No two organisms have the same hereditary material. Identical twins are the only exception to these statements.
(3) Plasmagenes are genes located outside the nucleus. Drug resistance in some bacteria is transmitted through plasmagenes.
 

3 Crossing-over is the exchange of chromosomal material between homologous pairs of chromosomes. This process occurs during synapsis in meiosis.
Wrong Choices Explained:
(1) Nondisjunction is the failure of homologous chromosomes to separate from each other during meiosis. Cells with extra chromosomes and cells with too few chromosomes result from nondisjunction.
(2) Polyploidy is a condition in which the cells have extra sets of chromosomes beyond the normal 2n number.
(4) Hybridization is the crossing of two organisms that are distinctly different from each other. The purpose is to bring together new combinations of genes. Usually the individual with the new gene combinations is more sturdy than either parent. A tangelo is a cross between a tangerine and a grapefruit.
 

2 A DNA nucleotide is composed of a deoxyribose sugar molecule, a phosphoric acid molecule, and a nitrogen base.
Wrong Choices Explained:
(1) Adenine and thymine are bases.
(3) Glucose is not the sugar molecule in DNA.
(4) Adenine, thymine, and cytosine are bases.
 

1 The two strands of DNA are held together by hydrogen bonds. The hydrogen bonds form weak links between the base pairs of each strand.
Wrong Choices Explained:
(2) The sugars and bases of the nucleotides of DNA are organic compounds. Each individual compound is made up of carbon bonds.
(3) The nucleotides in each strand are joined together by phosphate groups.
(4) There are no ribose groups in DNA.
 

1 Lysine is the amino acid whose genetic code is present on DNA strand I. Strand I of the DNA molecule contains two triplet codons (a triplet codon is a three-base sequence): AAG-CTG. Each triplet codon represents a specific amino acid. The amino acids are carried by tRNA molecules. The tRNA codon matches the DNA codon (except that U replaces T). There are two possible tRNA codons that could match the DNA strand I sequence; they are AAG and CUG. Of these, only the AAG tRNA appears in the diagram. The AAG tRNA carries the amino acid known as lysine.

Wrong Choices Explained:
(2) The DNA triplet code for serine is TTC. This codon does not appear on strand I of the DNA molecule in the diagram.
(3) The DNA triplet code for asparagine is GAC. This codon does not appear on strand I of the DNA molecule in the diagram.
(4) The DNA triplet code for phenylalenine is TTT. This codon does not appear on strand I of the DNA molecule in the diagram.
 

4 A gene controls the production of a protein. The substitution of one base for another changes the triplet code. One amino acid will be substituted for another. The result is a mutation. The replacement of glutamic acid by valine in a hemoglobin molecule causes sickle cell anemia.
Wrong Choices Explained:
(1) Segregation is the separation of alleles from each other during the formation of gametes.
(2) Disjunction is the separation of homologous chromosomes during the process of meiosis.
(3) Cytoplasmic inheritance is the inheritance of genes located in the cytoplasm, not in the nucleus. The cytoplasmic genes are called plasmagenes.
 

2 Two different amino acids are coded by strand I. Strand I has two triplet codes, six bases.
Wrong Choices Explained:
(1) Only three bases would have to be shown in the diagram to code for 1 amino acid.
(3) 24 bases are needed for 8 amino acids.
(4) 48 bases are needed for 12 amino acids.
 

3 UUCGAC is the correct sequence. Base pairing is an important concept in DNA duplication and RNA synthesis. Adenine pairs with thymine; cytosine pairs with guanine. There is no thymine in RNA. Uracil takes its place.
Wrong Choices Explained:
(1) TTCGUC is not correct. Because thymine is present in the base sequences, the molecule cannot be RNA.
(2) AACGTC is not correct. The base sequences are not complementary to either strand I or strand II.
(4) AAGCUG is not correct. The base sequences are complementary to strand II in the diagram not strand I.
 

2 Amino acids are transported to the ribosomes by RNA molecules known as transfer RNA, tRNA.
Wrong Choices Explained:
(1) Protein molecules are synthesized in the ribosomes. The code for the synthesis is contained in mRNA.
(3) Cellular respiration takes place in the mitochondria.
(4) Chromosomes are structures found in the nucleus. They are composed of DNA and protein. The genes are located on the chromosomes.
 

1 Both DNA and RNA are built from nucleotides.
Wrong Choices Explained:
(2) Only DNA is double-stranded. RNA is single-stranded.
(3) Deoxyribose is the sugar in the DNA nucleotides. Ribose is the sugar in the RNA nucleotides.
(4) The base thymine is replaced by uracil in RNA nucleotides.
 

2 The sequence of amino acids in a protein is determined by the triplet codes in DNA. The codes are carried to the ribosomes when mRNA is synthesized. A DNA strand is the template in mRNA synthesis.
Wrong Choices Explained:
(1) Enzymes catalyze the formation of peptide bonds. A peptide bond is a C–N bond formed by the dehydration synthesis of amino acids.
(3) Amino acids are transferred from the cytoplasm to the ribosomes, not to the nucleus. Transfer RNA is the carrier molecule.
(4) ATP molecules supply the energy for protein synthesis.
 

1 A hybrid is an individual that has two different alleles for a particular trait. The hybrid may be represented by the symbols Ss. The hybrid is smooth; the smooth trait is dominant over the wrinkled trait. When two hybrids are crossed, 75% of the offspring will have the smooth trait and 25% of the offspring will have the wrinkled trait.

Wrong Choices Explained:
(2), (3), (4) These choices are incorrect based on the information provided by the Punnett square.
 

2 The term homozygous means pure for the trait. A homozygous individual has two identical alleles for a gene. There are three alleles for blood type: Ia, Ib, and i. The allele Ia produces a protein for blood type A; the allele Ib produces a protein for blood type B; the allele i does not produce either protein. The type of blood is determined by the combination of alleles. The genotype refers to the allelic combination. Because the person in the question is homozygous for type A blood, his genotype is IaIa.
Wrong Choices Explained:
(1) The Ia and Ib alleles are both dominant over the i allele. When both Ia and Ib alleles occur in the same person, the person has type AB blood.
(3) The type of blood represented by the genotype Iai is type A.
(4) The genotype of a person with type O blood is ii.
 

2 The mating of members of the same litter to maintain desirable traits is known as inbreeding. Because mating pairs come from the same litter, they are genetically similar to each other. Inbreeding is used to maintain pure breeds.
Wrong Choices Explained:
(1) Hybridization, or outbreeding, is the mating of organisms with contrasting traits. It is the opposite of inbreeding.
(3) Factors in the environment select the organisms that are best adapted to survive in the environment. This principle is known as natural selection. It is an essential feature in the theory of evolution.
(4) Vegetative propagation is asexual reproduction in plants.