Heterozygote advantage confers a fitness advantage on heterozygous individuals. The amount of variance for the phenotype will decrease, though the mean will stay the same. On a graph of frequency vs. phenotype, this is depicted as a narrowing of the curve.

B Prokaryotes (Domains Bacteria and Archaea) have no membrane-bound organelles, so all reactions and processes occur in the cytoplasm. You should have been able to eliminate choices C, D, and E because they did not describe locations.

Monocot vascular tissue is arranged randomly in the stem, while dicot vascular tissue is arranged in a ring. All other answer choices have characteristics reversed.

Speciation requires that two populations be reproductively isolated from each other, allowing natural selection to act independently on each.

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Algae are photosynthetic organisms that thrive in conditions of abundant nutrients such as nitrogen. A eutrophic lake is one that contains an overabundance of nutrients.

Biogeography is the study of the distribution of species over the globe.

The large intestine eliminates digestive wastes rather than metabolic wastes.

Antigens or immunoglobulins produced by B and T cells are specific and provide a "memory" that helps to fight previously encountered pathogens, or infectious organisms. Antigens, produced by pathogens, would not linger in the blood. Macrophages and natural killer cells are part of the innate immune system and would not be specific to a particular virus.

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In ecological succession, a plant community is replaced by a subsequent plant community. This occurs because the plants in the original community alter abiotic factors, such as shade and soil nutrients, making the ecosystem more suitable for the seeds of different plant species.

B The climax community is the final, stable community in succession. The key word here is "stable." That should tip you off that this is the end of the process, or the " climax."

Phagocytes engulf and destroy both foreign cells and damaged or nonfunctional cells in the blood. They do not specifically target an infecting agent.

The extent to which a phenotype is determined by differences in alleles in a population is referred to as heritability. Natural hair color is a highly heritable trait, whereas the number of legs has almost no heritability because there is little genetic variation for this phenotype in the population.

Only a reduction in gene flow is absolutely required for speciation to take place. Populations cannot evolve independently if their gene pools are shared.

D Flowering plants are angiosperms. Gymnosperms are conifers (naked seed plants; C is wrong), bryophytes are mosses (A is wrong), and tracheophytes are non-seed producing plants (ferns; B is wrong). "Endosperm" is not a classification for plants.

A hybrid of two species with differing chromosomal numbers will be incapable of forming its own gametes and thus is considered sterile. In order to form a zygote, homologous chromosomes or each gamete must pair. Because zebra gametes will only possess the haploid number of 23 chromosomes and a horse's gamete a haploid number of 32, the resulting zygote will have 54 chromosomes.

The fact that solute molecules are entering a cell via a membrane protein does not necessarily mean that the cell is expending energy (active transport), that the solute is moving against its concentration gradient (which requires energy), or that the solute concentration is greater inside the cell than outside (implying that the solute is moving against its concentration gradient). If the cell were in a hypotonic solution, solutes would tend to move out of the cell while water moved into it. Facilitated diffusion uses membrane protein to allow polar, hydrophilic molecules or ions into the cell. These solutes cannot pass through the nonpolar, hydrophobic inner layer of the cell membrane.

C The job of the blood is to carry oxygen from the lungs, where it is plentiful, to the tissues, where it is not. Thus hemoglobin should have a high affinity for oxygen in the lungs so it can bind oxygen (choices A, B, and D could be eliminated) and a low oxygen affinity in the tissues (so it can release the oxygen where it is needed). The reverse is true for carbon dioxide. Hemoglobin has a high carbon dioxide affinity in the tissues and a low carbon dioxide affinity in the lungs.

E Recombination occurs when the homologous chromosomes are paired and crossing over can take place. This occurs during prophase I of meiosis. Remember that recombination occurs during prophase, and this would help you eliminate choices A, B, and D.

A Most digestion and absorption occur in the small intestine. A very small amount of digestion (starch only) takes place in the mouth, and a very small amount of digestion takes place in the stomach (acid hydrolysis of food and some protein digestion). As with Questions 7-9, Questions 10-12 require the same type of knowledge-memorization of basic facts.

Deoxygenated blood enters the right atrium of the heart (1) through each vena cava (7). From these, it is pumped to the right ventricle and through the pulmonary arteries, to the lungs.

A silent mutation is not phenotypically visible because the protein product remains unchanged.

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The centromeres joining sister chromatids split in anaphase. The chromosomes are then pulled to opposite poles.

B Choices A, C, D, and E all describe characteristics of the phylum arthropoda. A water vascular system is a characteristic of the phylum echinodermata, the "spiny skinned" animals such as sea stars and sea urchins. Their water vascular system ends in tube feet that play a role in locomotion and feeding. Don't forget your LEAST/EXCEPT/NOT technique of circling the word "EXCEPT" and drawing a vertical line through the answer choices to help you remember to choose the incorrect statement.

When more than one gene affects a trait but only one is expressed, this is called epistasis because one gene "stands over" another.

A These are the characteristics of tundra.

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The ER, plasma membrane, and Golgi body of a eukaryotic cell consist of membrane composed of a phospholipid bilayer. In contrast, the ribosome is not a membrane-bound organelle.

Photosynthesis produces glucose and oxygen from carbon dioxide and water.

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A common mutualism between plants and bacteria occurs in root systems, which host microorganisms that make limiting nutrients, such as nitrogen, available to the plants.

Pith is a source of unspecialized ground tissue that adds structure to a stem at the very center.

C For a population to prevent another population from using a resource, the first population must be a better competitor. Choice C is the only one that deals with competition.

Because its evolutionary history is elsewhere, milfoil has adapted to an environment that lacks the natural predators and pathogens of North America, making it less susceptible to them and therefore more competitive than native plants in North America.

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