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MCAS Biology practice questions

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Question 1Biology (high school science MCAS)

Which statement best describes the role of the cell membrane in maintaining homeostasis?

  1. It produces ATP through aerobic respiration.
  2. It stores genetic information for protein synthesis.
  3. It regulates which substances enter and leave the cell. Correct answer.
  4. It packages proteins for export from the cell.

Explanation

The cell membrane is selectively permeable, meaning it controls the movement of materials into and out of the cell. This regulation keeps internal conditions such as ion and water balance stable, which is the essence of homeostasis.

Question 2Biology (high school science MCAS)

A student observes a cell that contains a nucleus, mitochondria, and a cell wall but no chloroplasts. Which type of cell is most likely being observed?

  1. A plant cell from a root Correct answer.
  2. A fungal cell
  3. A bacterial cell
  4. An animal cell from muscle tissue

Explanation

Plant root cells have a cell wall and mitochondria but lack chloroplasts because they do not perform photosynthesis. The presence of a nucleus also rules out bacteria, which lack membrane-bound organelles.

Question 3Biology (high school science MCAS)

During which phase of mitosis do sister chromatids separate and move toward opposite poles of the cell?

  1. Anaphase Correct answer.
  2. Telophase
  3. Metaphase
  4. Prophase

Explanation

Anaphase is defined by the splitting of centromeres and the pulling of sister chromatids to opposite poles by spindle fibers. This ensures each daughter cell receives one copy of each chromosome.

Question 4Biology (high school science MCAS)

A mutation changes a DNA codon from GAG to GUG. Using the standard genetic code, what effect does this have on the resulting protein?

  1. The protein is unchanged because the mutation is silent.
  2. The amino acid changes from valine to glutamic acid.
  3. Translation stops early because the new codon is a stop codon.
  4. The amino acid changes from glutamic acid to valine. Correct answer.

Explanation

GAG codes for glutamic acid and GUG codes for valine, so this is a missense mutation that substitutes one amino acid. The change in the first nucleotide position alters the codon's meaning.

Question 5Biology (high school science MCAS)

In a certain species, black fur (B) is dominant to brown fur (b). Two heterozygous black rabbits are crossed. What is the expected phenotypic ratio of the offspring?

  1. All black
  2. 3 black : 1 brown Correct answer.
  3. 1 black : 3 brown
  4. 1 black : 1 brown

Explanation

A cross of Bb × Bb produces genotypes BB, Bb, Bb, and bb in a 1:2:1 ratio. Since B is dominant, three of the four offspring show black fur and one shows brown fur, giving a 3:1 phenotypic ratio.

Question 6Biology (high school science MCAS)

Which structure is primarily responsible for synthesizing proteins in a eukaryotic cell?

  1. Ribosome Correct answer.
  2. Golgi apparatus
  3. Smooth endoplasmic reticulum
  4. Lysosome

Explanation

Ribosomes are the sites of translation, where messenger RNA is decoded into a polypeptide chain. They may be free in the cytoplasm or attached to the rough endoplasmic reticulum.

Question 7Biology (high school science MCAS)

A population of insects develops resistance to an insecticide over many generations. Which mechanism best explains this outcome?

  1. Natural selection acting on existing genetic variation Correct answer.
  2. Inheritance of traits acquired during an individual's lifetime
  3. Gene flow from a neighboring population
  4. Random mutation directed by the presence of the insecticide

Explanation

Some individuals already carry alleles that confer resistance, and these individuals survive and reproduce more successfully when the insecticide is applied. Over generations, the frequency of resistant alleles increases in the population.

Question 8Biology (high school science MCAS)

Which process converts light energy into chemical energy stored in glucose during photosynthesis?

  1. The Calvin cycle Correct answer.
  2. Cellular respiration
  3. The Krebs cycle
  4. Glycolysis

Explanation

The Calvin cycle uses ATP and NADPH produced by the light-dependent reactions to fix carbon dioxide into glucose. This is the light-independent stage of photosynthesis.

Question 9Biology (high school science MCAS)

A plant is placed in a sealed transparent chamber and exposed to light. Measurements show that carbon dioxide levels in the chamber decrease over time. Which conclusion is best supported?

  1. The plant is only performing cellular respiration.
  2. The plant has stopped performing photosynthesis.
  3. The rate of photosynthesis exceeds the rate of cellular respiration. Correct answer.
  4. The rate of cellular respiration exceeds the rate of photosynthesis.

Explanation

Photosynthesis consumes carbon dioxide while cellular respiration releases it. A net decrease in carbon dioxide indicates that photosynthesis is taking up more carbon dioxide than respiration produces.

Question 10Biology (high school science MCAS)

Which statement correctly describes the relationship between DNA, mRNA, and proteins?

  1. mRNA is replicated into DNA before protein synthesis begins.
  2. mRNA carries genetic information from DNA to ribosomes for translation. Correct answer.
  3. DNA is translated directly into proteins by ribosomes.
  4. Proteins are transcribed into mRNA in the nucleus.

Explanation

During transcription, DNA is used as a template to make mRNA, which then travels to ribosomes where its sequence is translated into a protein. This flow of information is central to gene expression.

Question 11Biology (high school science MCAS)

A cell is placed in a hypertonic solution. What is the most likely immediate effect on an animal cell?

  1. The cell maintains its size because the solution is isotonic.
  2. Solutes move into the cell to equalize concentration.
  3. Water enters the cell and it swells.
  4. Water leaves the cell and it shrinks. Correct answer.

Explanation

In a hypertonic solution, the solute concentration outside the cell is higher than inside, so water moves out by osmosis. The loss of water causes the animal cell to shrink.

Question 12Biology (high school science MCAS)

Which of the following best explains why enzymes are described as biological catalysts?

  1. They increase the activation energy required for a reaction.
  2. They are consumed during the reaction they catalyze.
  3. They lower activation energy and are not consumed by the reaction. Correct answer.
  4. They change the equilibrium constant of a reaction.

Explanation

Enzymes speed up reactions by lowering the activation energy needed for reactants to reach the transition state. They are not consumed in the process and can catalyze many reactions.

Question 13Biology (high school science MCAS)

A biologist discovers an organism that is multicellular, heterotrophic, and has cells with cell walls made of chitin. To which kingdom does this organism most likely belong?

  1. Plantae
  2. Protista
  3. Fungi Correct answer.
  4. Animalia

Explanation

Fungi are multicellular heterotrophs whose cell walls contain chitin. These features distinguish them from plants, animals, and most protists.

Question 14Biology (high school science MCAS)

In a food web, which trophic level contains the greatest amount of usable energy?

  1. Tertiary consumers
  2. Primary producers Correct answer.
  3. Primary consumers
  4. Secondary consumers

Explanation

Energy is lost as heat at each trophic level, so only about ten percent passes to the next level. Producers therefore hold the largest share of usable energy in a food web.

Question 15Biology (high school science MCAS)

A sudden decrease in the population of a keystone predator in an ecosystem is most likely to cause which outcome?

  1. An immediate increase in the producer population only
  2. No change because other species will fill the same role
  3. A disruption in community structure, possibly reducing biodiversity Correct answer.
  4. A decrease in competition among prey species

Explanation

Keystone predators help regulate prey populations and prevent any one species from dominating. Removing them can cause cascading effects that alter community structure and reduce biodiversity.

Question 16Biology (high school science MCAS)

Which of the following describes a difference between mitosis and meiosis?

  1. Mitosis occurs only in reproductive organs, while meiosis occurs in all body tissues.
  2. Mitosis produces haploid cells, while meiosis produces diploid cells.
  3. Mitosis produces genetically identical cells, while meiosis produces genetically varied gametes. Correct answer.
  4. Mitosis involves two rounds of division, while meiosis involves one.

Explanation

Mitosis produces daughter cells that are genetically identical to the parent cell, while meiosis generates gametes with half the chromosome number and new combinations of alleles. This variation arises from crossing over and independent assortment.

Question 17Biology (high school science MCAS)

A pedigree shows that a trait appears in every generation and affects both males and females equally. Affected individuals always have at least one affected parent. Which inheritance pattern is most consistent with these observations?

  1. Autosomal dominant Correct answer.
  2. Y-linked
  3. Autosomal recessive
  4. X-linked recessive

Explanation

A trait that appears in every generation and affects both sexes equally suggests a dominant allele, since affected individuals inherit the allele from an affected parent. Autosomal dominant traits do not skip generations.

Question 18Biology (high school science MCAS)

Which molecule serves as the primary energy currency of the cell, releasing energy when its phosphate bonds are broken?

  1. Glucose
  2. ATP Correct answer.
  3. NADH
  4. DNA

Explanation

ATP (adenosine triphosphate) stores energy in its phosphate bonds and releases it when converted to ADP and inorganic phosphate. This energy drives many cellular processes.

Question 19Biology (high school science MCAS)

A scientist treats a culture of cells with a chemical that blocks the formation of the spindle apparatus. During which stage of the cell cycle would these cells most likely arrest?

  1. Metaphase Correct answer.
  2. S phase
  3. Cytokinesis
  4. G1 phase

Explanation

The spindle apparatus is required to align and separate chromosomes during mitosis. Without it, cells cannot properly align chromosomes at the metaphase plate, so division arrests around metaphase.

Question 20Biology (high school science MCAS)

Which statement best explains why a population of organisms with a high degree of genetic variation is more likely to survive an environmental change?

  1. Variation guarantees that all individuals will adapt during their lifetime.
  2. Some individuals may possess traits that allow them to survive and reproduce under the new conditions. Correct answer.
  3. Genetic variation prevents mutations from occurring.
  4. Variation increases the mutation rate in response to stress.

Explanation

Genetic variation means that some individuals may already carry alleles suited to the new environment. These individuals survive and pass those alleles to offspring, allowing the population to persist.

Question 21Biology (high school science MCAS)

Which of the following correctly pairs a macromolecule with its monomer?

  1. Proteins - nucleotides
  2. Lipids - monosaccharides
  3. Carbohydrates - amino acids
  4. Nucleic acids - nucleotides Correct answer.

Explanation

Nucleic acids such as DNA and RNA are polymers made of nucleotide monomers. Each nucleotide consists of a sugar, a phosphate group, and a nitrogenous base.

Question 22Biology (high school science MCAS)

In a certain ecosystem, a disease drastically reduces the population of nitrogen-fixing bacteria in the soil. Which effect is most likely to occur in nearby plants?

  1. Plants will absorb more nitrogen directly from the atmosphere.
  2. Plants will increase photosynthesis to compensate for the loss.
  3. Plants may show reduced growth due to limited available nitrogen. Correct answer.
  4. Plants will switch to performing nitrogen fixation themselves.

Explanation

Nitrogen-fixing bacteria convert atmospheric nitrogen into forms plants can absorb. Without them, plants may lack usable nitrogen, which is needed for proteins and nucleic acids, leading to reduced growth.

Question 23Biology (high school science MCAS)

Which statement best describes the function of the mitochondria in a cell?

  1. They convert light energy into chemical energy.
  2. They generate ATP through cellular respiration. Correct answer.
  3. They contain enzymes that digest worn-out organelles.
  4. They synthesize lipids and detoxify drugs.

Explanation

Mitochondria carry out aerobic cellular respiration, using oxygen to convert glucose into ATP. This ATP supplies energy for the cell's activities.

Question 24Biology (high school science MCAS)

Two populations of the same bird species live on opposite sides of a wide river and do not interbreed. Over many generations, their songs and mating behaviors diverge. Which type of speciation is most likely occurring?

  1. Polyploid speciation
  2. Allopatric speciation Correct answer.
  3. Sympatric speciation
  4. Artificial selection

Explanation

Allopatric speciation occurs when a geographic barrier, such as a river, separates populations and prevents gene flow. Over time, genetic and behavioral differences accumulate, leading to reproductive isolation.

Question 25Biology (high school science MCAS)

A student measures the rate of an enzyme-catalyzed reaction at several temperatures. The rate increases up to about 40°C and then drops sharply at higher temperatures. Which explanation best accounts for the drop?

  1. The enzyme becomes more specific at higher temperatures.
  2. The reaction becomes endergonic at higher temperatures.
  3. The enzyme is denatured, losing its functional shape. Correct answer.
  4. The substrate concentration decreases at high temperatures.

Explanation

High temperatures break the bonds that maintain an enzyme's three-dimensional shape, denaturing it and destroying the active site. Once the active site is distorted, the enzyme can no longer bind substrate efficiently.

Question 26Biology (high school science MCAS)

Which property of water allows insects such as water striders to move across the surface of a pond without sinking?

  1. The polarity of water molecules dissolving gases
  2. The high specific heat of water
  3. Surface tension caused by hydrogen bonding between water molecules Correct answer.
  4. The density of water decreasing as it freezes

Explanation

Water molecules form hydrogen bonds with one another, pulling the surface molecules inward and creating a cohesive film called surface tension. This film can support the weight of small organisms. The other properties listed are real features of water but do not explain why a lightweight object rests on the surface.

Question 27Biology (high school science MCAS)

A single-celled organism is observed under a microscope and its genetic material is not enclosed by a membrane. Its cell also lacks mitochondria and other membrane-bound organelles. To which group does this organism most likely belong?

  1. Fungi
  2. Bacteria Correct answer.
  3. Animalia
  4. Protista

Explanation

Bacteria are prokaryotes, meaning their DNA is not enclosed in a nucleus and they lack membrane-bound organelles such as mitochondria. The other listed groups are eukaryotes whose cells contain a nucleus and internal membrane-bound structures.

Question 28Biology (high school science MCAS)

During which phase of the cell cycle is DNA replicated so that each chromosome consists of two sister chromatids?

  1. Prophase of mitosis
  2. S phase of interphase Correct answer.
  3. G1 phase of interphase
  4. Anaphase of mitosis

Explanation

DNA synthesis occurs during the S phase of interphase, producing two identical sister chromatids joined at the centromere. Prophase and anaphase are stages of mitosis in which the already-replicated chromosomes are separated, and G1 is a growth phase before replication.

Question 29Biology (high school science MCAS)

A mutation changes a single DNA base so that one codon in an mRNA molecule is altered, but the amino acid sequence of the resulting protein is unchanged. What type of mutation is this?

  1. Silent mutation Correct answer.
  2. Frameshift mutation
  3. Missense mutation
  4. Nonsense mutation

Explanation

A silent mutation changes a codon but not the amino acid it specifies, because the genetic code is redundant and multiple codons encode the same amino acid. Missense mutations change one amino acid, nonsense mutations create a stop codon, and frameshifts shift the entire reading frame.

Question 30Biology (high school science MCAS)

In a cross between two heterozygous pea plants (Tt × Tt), where T is the dominant tall allele and t is the recessive short allele, what fraction of the offspring is expected to show the recessive short phenotype?

  1. 1/4 Correct answer.
  2. 3/4
  3. 1/2
  4. 0

Explanation

A Tt × Tt cross produces offspring in a 1 TT : 2 Tt : 1 tt ratio. Only the tt genotype expresses the recessive short phenotype, which is one of four possible outcomes, or 1/4.

Question 31Biology (high school science MCAS)

Which level of ecological organization includes all the living organisms in a given area together with the nonliving factors such as soil, water, and sunlight?

  1. Biome
  2. Population
  3. Community
  4. Ecosystem Correct answer.

Explanation

An ecosystem consists of all the interacting living organisms in an area plus the abiotic, or nonliving, components of that environment. A population is one species, a community includes only the living organisms, and a biome is a large geographic region with characteristic climates and communities.

Question 32Biology (high school science MCAS)

Which process allows a white blood cell to engulf and internalize a bacterium during an immune response?

  1. Exocytosis
  2. Facilitated diffusion
  3. Osmosis
  4. Phagocytosis Correct answer.

Explanation

Phagocytosis is a form of endocytosis in which the cell membrane surrounds and engulfs large particles such as bacteria, bringing them inside the cell. Osmosis and facilitated diffusion move small molecules across membranes without engulfing particles, and exocytosis releases material from the cell.

Question 33Biology (high school science MCAS)

A scientist measures the rate of photosynthesis in a leaf exposed to increasing light intensity while carbon dioxide and temperature are held constant. The rate rises and then levels off. Which factor most likely limits the rate at that plateau?

  1. Light intensity
  2. Chlorophyll color
  3. Carbon dioxide concentration Correct answer.
  4. Oxygen production

Explanation

Once light is no longer limiting, another input must limit the reaction; with light abundant and temperature constant, carbon dioxide concentration becomes the limiting factor. Increasing light further cannot raise the rate because the Calvin cycle is limited by available CO2.

Question 34Biology (high school science MCAS)

A student examines a slide of onion root tip cells and identifies cells with chromosomes lined up along the cell's equator. Which stage of mitosis is shown?

  1. Telophase
  2. Metaphase Correct answer.
  3. Prophase
  4. Anaphase

Explanation

During metaphase, replicated chromosomes align at the metaphase plate, the midpoint of the cell. Prophase involves condensing chromosomes, anaphase involves chromatid separation, and telophase involves the formation of two new nuclei.

Question 35Biology (high school science MCAS)

A cell is placed in a hypotonic solution. Assuming the cell membrane is permeable to water but not to the solute, what will happen to a plant cell?

  1. It will take in water and become turgid. Correct answer.
  2. It will lose water and plasmolyze.
  3. It will show no net movement of water.
  4. It will take in water until it bursts.

Explanation

In a hypotonic solution, water enters the cell by osmosis because the solute concentration inside is higher. A plant cell resists bursting because its rigid cell wall creates turgor pressure that stops further water uptake, leaving the cell firm and turgid.

Question 36Biology (high school science MCAS)

A segment of one DNA strand has the sequence 5'-ATG CCA-3'. What is the sequence of the complementary DNA strand, written in the 3'-to-5' direction?

  1. 3'-ATG CCA-5'
  2. 3'-TAC GGT-5' Correct answer.
  3. 3'-UAC GGU-5'
  4. 3'-TGG AAT-5'

Explanation

DNA bases pair by complementary rules: A pairs with T and C pairs with G. The complement of ATG CCA is therefore TAC GGT, and it runs antiparallel to the original strand.

Question 37Biology (high school science MCAS)

In a certain species, a gene for coat color has two alleles that show incomplete dominance. A homozygous black individual is crossed with a homozygous white individual, and all offspring are gray. What phenotypic ratio is expected when two gray individuals are crossed?

  1. 1 black : 1 white
  2. All gray
  3. 3 black : 1 white
  4. 1 black : 2 gray : 1 white Correct answer.

Explanation

With incomplete dominance, heterozygotes show an intermediate phenotype. Crossing two heterozygotes produces a 1:2:1 ratio of homozygous dominant, heterozygous, and homozygous recessive offspring, which here corresponds to black, gray, and white.

Question 38Biology (high school science MCAS)

Which statement best explains how a population of bacteria can become resistant to an antibiotic over many generations?

  1. The antibiotic causes bacteria to develop new resistance genes in response to exposure.
  2. Individual bacteria gradually strengthen their cell walls during their lifetime.
  3. Bacteria pass on acquired traits from exposure directly to their offspring.
  4. Bacteria with existing resistance alleles survive and reproduce more than susceptible bacteria. Correct answer.

Explanation

Genetic variation exists in the population before the antibiotic is applied. When the antibiotic kills susceptible individuals, those already carrying resistance alleles survive and reproduce, increasing the frequency of those alleles in later generations.

Question 39Biology (high school science MCAS)

A student builds a model of a food chain: grass → grasshopper → frog → snake → hawk. If the grass contains 10,000 units of energy, approximately how much energy is available to the hawk?

  1. 100 units
  2. 10 units Correct answer.
  3. 1,000 units
  4. 1 unit

Explanation

On average, only about 10 percent of the energy at one trophic level is transferred to the next, with the rest lost as heat and used in metabolism. From grass to hawk is four transfers, so 10,000 × 0.1⁴ equals about 10 units.

Question 40Biology (high school science MCAS)

A segment of messenger RNA has the sequence 5'-AUG GCA UAA-3'. Using the genetic code, what is the result of translating this sequence?

  1. No protein, because no start codon is present
  2. A protein with one amino acid
  3. A protein with three amino acids
  4. A protein with two amino acids Correct answer.

Explanation

AUG is the start codon and codes for methionine, GCA codes for alanine, and UAA is a stop codon that terminates translation without adding an amino acid. The resulting polypeptide therefore contains two amino acids.

Question 41Biology (high school science MCAS)

Two species of finches live on the same island. One has a thick beak suited to cracking large seeds, and the other has a thin beak suited to eating small seeds. Which term describes this pattern?

  1. Competitive exclusion
  2. Primary succession
  3. Mutualism
  4. Resource partitioning Correct answer.

Explanation

Resource partitioning occurs when species divide a shared resource by using different parts of it, reducing competition. Here the two finches specialize on different seed sizes, allowing them to coexist.

Question 42Biology (high school science MCAS)

A couple has a child with a recessive genetic disorder. Both parents are unaffected. What is the probability that their next child will be affected?

  1. 100 percent
  2. 50 percent
  3. 0 percent
  4. 25 percent Correct answer.

Explanation

Because the disorder is recessive and both parents are unaffected, both must be heterozygous carriers. A cross of two carriers gives a 25 percent chance that a child inherits two recessive alleles and is affected.

Question 43Biology (high school science MCAS)

Which feature distinguishes the cell wall of a plant cell from the cell membrane that lies just inside it?

  1. The cell wall contains phospholipids arranged in a bilayer.
  2. The cell wall contains proteins that pump ions across the membrane.
  3. The cell wall is a rigid structure made mainly of cellulose that provides support. Correct answer.
  4. The cell wall is selectively permeable and controls which substances enter the cell.

Explanation

The plant cell wall is a rigid outer layer composed mainly of cellulose that gives the cell shape and structural support. The plasma membrane inside it is the selectively permeable phospholipid bilayer that regulates transport.

Question 44Biology (high school science MCAS)

A scientist exposes a plant to carbon dioxide labeled with a radioactive isotope and tracks where the label first appears during photosynthesis. In which stage is the labeled carbon first incorporated into an organic molecule?

  1. The light-independent reactions, where carbon is fixed Correct answer.
  2. The light-dependent reactions, where water is split
  3. The Krebs cycle in the mitochondria
  4. The electron transport chain in the thylakoid membrane

Explanation

Carbon dioxide is incorporated into organic molecules during the light-independent reactions, also called the Calvin cycle, which take place in the stroma. The light-dependent reactions split water and produce ATP and NADPH but do not fix carbon.

Question 45Biology (high school science MCAS)

A human body cell has 46 chromosomes. How many chromosomes does each cell produced by meiosis contain, and why?

  1. 23, because sister chromatids separate during mitosis
  2. 46, because meiosis preserves the chromosome number
  3. 23, because homologous chromosomes separate during meiosis I Correct answer.
  4. 92, because chromosomes replicate before division

Explanation

Meiosis reduces the chromosome number by half so that gametes carry one set of chromosomes. Homologous chromosomes are separated during meiosis I, leaving 23 chromosomes in each resulting cell.

Question 46Biology (high school science MCAS)

The graph of a population's size over time shows a rapid increase followed by a plateau near a maximum value that the environment can support. What does this plateau represent?

  1. The population's mutation rate
  2. The rate of immigration into the population
  3. The carrying capacity of the environment Correct answer.
  4. The exponential growth rate of the population

Explanation

Carrying capacity is the maximum population size that an environment can sustain given available resources. Logistic growth levels off at this value as limiting factors such as food and space slow population growth.

Question 47Biology (high school science MCAS)

In a laboratory experiment, an enzyme is added to a substrate solution and the reaction rate is measured. When the substrate concentration is very high, adding more substrate does not increase the rate. Which conclusion best explains this observation?

  1. The enzyme has been consumed by the reaction and is no longer present.
  2. The substrate has denatured the enzyme at high concentration.
  3. All enzyme active sites are occupied, so the enzyme is saturated. Correct answer.
  4. The pH of the solution has dropped below the enzyme's optimum.

Explanation

At high substrate concentrations, every active site of the enzyme is occupied, so the reaction rate reaches a maximum called saturation. Adding more substrate cannot increase the rate until more enzyme is available.

Question 48Biology (high school science MCAS)

A mutation deletes one nucleotide near the beginning of a gene's coding region. Which outcome is most likely?

  1. The chromosome is duplicated, doubling the gene copy number
  2. A single amino acid substitution with no other changes
  3. The entire reading frame shifts, altering many downstream amino acids Correct answer.
  4. The gene is transcribed but the mRNA is not translated at all

Explanation

Because the genetic code is read in non-overlapping triplets, deleting one nucleotide shifts the reading frame from that point onward. This frameshift changes every subsequent codon and usually produces a very different or nonfunctional protein.

Question 49Biology (high school science MCAS)

In a population of organisms, a trait is controlled by a gene with two alleles. One allele is lethal when homozygous, and heterozygotes show a distinct phenotype. If two heterozygotes are crossed, what phenotypic ratio is expected among the surviving offspring?

  1. 2:1 Correct answer.
  2. 3:1
  3. 1:1
  4. 1:2:1

Explanation

The cross produces a 1:2:1 genotypic ratio, but the homozygous recessive class does not survive. Removing that class leaves two heterozygotes for every one homozygous dominant, a 2:1 ratio among survivors.

Question 50Biology (high school science MCAS)

An ecologist removes all the sea stars from a rocky intertidal area. Within a year, mussels spread across the rocks and crowd out several other species. Which concept does this result best illustrate?

  1. Invasive species always increase local biodiversity.
  2. Primary succession begins when a disturbance removes a dominant species.
  3. Energy transfer between trophic levels is about 10 percent efficient.
  4. Keystone species influence community structure far beyond their abundance. Correct answer.

Explanation

Sea stars function as a keystone species because their predation on mussels keeps the mussel population in check and frees space for other species. Removing them allows one competitor to dominate and reduces community diversity.