Biology: TestAS practice testlets

Gregor Mendel's experiments with pea plants in the 1860s laid the foundations of genetics. He studied traits with two clearly different forms, such as round or wrinkled seeds and yellow or green seeds. He concluded that each individual carries two copies of a hereditary factor, now called a gene, for each trait, and that these copies, the alleles, separate when gametes form, so that each gamete receives only one (law of segregation). When one allele masks the other in a heterozygous individual, it is dominant; the masked allele is recessive.

Mendel also crossed plants that differed in two traits. When he crossed plants that were heterozygous for both seed shape and seed colour (RrYy × RrYy), the offspring showed the four possible combinations of traits in a ratio of about 9 : 3 : 3 : 1. This showed that the alleles of different genes are distributed to gametes independently of each other (law of independent assortment). Today we know that this holds for genes on different chromosomes; genes lying close together on the same chromosome tend to be inherited together and are said to be linked.

A test cross shows whether an individual with the dominant phenotype is homozygous or heterozygous: it is crossed with a homozygous recessive individual. If any offspring show the recessive trait, the tested individual must be heterozygous.

Not all traits follow simple dominance. In incomplete dominance, the heterozygote has an intermediate phenotype: crossing red and white snapdragons gives pink flowers. In codominance, both alleles are expressed fully in the heterozygote. The human ABO blood group system shows both codominance and multiple alleles. The alleles IA and IB are codominant, and both are dominant over the allele i. People with the genotypes IAIA or IAi have blood group A, IBIB or IBi group B, IAIB group AB, and ii group O.

Many human traits, such as height or skin colour, are not controlled by a single gene but by many genes, each with a small effect, together with environmental influences. Such polygenic traits vary continuously rather than falling into clear classes, and they do not show simple Mendelian ratios.

Punnett squares help to predict the outcomes of crosses. The possible gametes of one parent are written along the top, those of the other parent down the side, and each cell of the grid gives one equally likely genotype of the offspring.

Almost all ecosystems are powered by sunlight. Producers, mainly green plants and algae, capture light energy in photosynthesis and store it in organic matter. Herbivores, the primary consumers, eat plants; carnivores that eat herbivores are secondary consumers; and carnivores that eat these are tertiary consumers. Each of these feeding steps is a trophic level. Decomposers such as bacteria and fungi break down dead matter from all levels and return nutrients to the soil.

Energy flows through an ecosystem in one direction and is not recycled. At each trophic level, most of the energy taken in is used for movement, maintaining body temperature and other life processes and is lost as heat, or it leaves the level in faeces and uneaten parts. On average, only about 10% of the energy at one level is stored in the biomass of the next level. This is why energy pyramids narrow sharply towards the top, why food chains rarely have more than four or five links, and why a given area can feed more people on a plant-based diet than on a meat-based one.

Nutrients, in contrast, cycle. Carbon, nitrogen and phosphorus pass from the environment into organisms and back again. Some pollutants that are not broken down, such as mercury or certain pesticides, accumulate as well: because each predator eats many prey, their concentration rises from one level to the next, a process called biomagnification. Top predators such as eagles or tuna can therefore contain high concentrations even when levels in water are low.

Populations grow when births exceed deaths. With unlimited resources, a population grows exponentially, adding more individuals the larger it becomes. In reality, food, space and other resources are limited. The population then follows logistic growth: growth is fast at first, slows as the population approaches the carrying capacity K, the largest population the environment can support in the long run, and stops at K. In the logistic model, the population grows fastest, in absolute numbers per year, when it is at half the carrying capacity. Fisheries managers use this idea when setting catch limits.

Populations also interact. In predator-prey cycles, such as those of lynx and snowshoe hares in Canada, the prey population rises, then the predators increase, the prey declines, and finally the predators decline too, with the predator peaks lagging behind those of the prey.