Compare sexual and asexual reproduction and explain why offspring vary from their parents and one another

Learning goal: Compare asexual reproduction, in which one parent transfers its genetic information to offspring, with sexual reproduction, in which reproductive cells, usually from two parents, join so that both parents contribute DNA; explain why offspring of sexual reproduction vary; and explain how environmental factors shape the expression of inherited traits.

Two ways to transfer genetic information. Reproduction is the transfer of genetic information (DNA) from one generation to the next, and the ability to reproduce is what defines living things. In asexual reproduction, a single parent produces offspring, and the lone parent contributes all of the offspring's DNA. A strawberry plant sends out runners that root and grow into new plants; a potato sprouts new plants from the eyes of its tuber; a hydra grows a bud that pinches off as a new animal; some sea stars regrow a whole body from a large fragment. In each case the offspring's inherited information matches the parent's, apart from rare mutations. The offspring are made by the same kind of cell division that grows and repairs the parent's body (mitosis, introduced in grade 6 as the division that makes new cells for growth and repair), which copies a cell's complete genetic information into each new cell; that is why the body cells of an organism carry essentially the same genetic information (rare mutations aside), and why an asexual offspring carries its parent's. Bacteria also reproduce, in ways with their own features, but they are not the subject here.

In sexual reproduction, usually two parents each contribute DNA. Each parent forms special reproductive cells, an egg from the female and sperm from the male, that carry one copy of each paired gene rather than the parent's two. In animals these are made by a different kind of cell division (meiosis) whose end product is a cell with half the parent's nuclear genetic information; comparing the two end products, a complete copy from mitosis against a half set from meiosis, is the key difference between the two modes at the level of cells. (Flowering plants also make egg and sperm cells with a half set, the sperm cells carried inside pollen grains, by a longer route that is left for later courses.) When an egg and a sperm join, the offspring receives two copies of each paired gene, one from each parent, and its combination is usually new; the reproductive cells, with their half set, are the main exception to the pattern that an organism's cells carry essentially the same genetic information.

Whichever mode an organism uses, the line of ancestors described two lessons ago continues only when genetic information is actually passed on: every organism alive today, sexual or asexual, comes from a line in which that transfer succeeded in every generation.

Why sexual offspring vary. Consider one gene for which a parent is Pp. When that parent forms a reproductive cell, the cell receives either P or p, with equal chance. Now consider a parent that carries two different alleles for each of ten genes. Use the simple model in which each gene is passed on independently of the others (Mendel's law of independent assortment, which holds in simple cases; some genes tend to be passed on together, a topic for high school). In that model the choice is made separately for each gene, so the number of different combinations one parent can pass on is 2 multiplied by itself ten times, 1,024. The other parent contributes its own independently chosen combination, so the number of possible pairings of reproductive cells from just ten genes is 1,024 × 1,024, more than a million (some pairings give the offspring the same pair of alleles at every gene, so the number of genetically distinct offspring is smaller, but still enormous). Real organisms have thousands of genes. This is why brothers and sisters usually differ from each other and from both parents while still resembling them: they draw from the same two sources of genetic information but receive different samples. Identical twins, who develop from one fertilized egg, are a useful check: they share the same inherited information and are far more alike than other siblings.

Why asexual offspring do not vary in the same way. With one parent and no mixing, offspring receive the parent's complete set of information, so a patch of strawberry plants grown from runners is genetically the same plant repeated. Variation among such offspring comes only from rare mutation and from environmental differences, discussed below. This is not a defect. If a parent is well suited to a stable environment, copying it exactly is a fast way to produce many suited offspring without needing a mate.

Trade-offs, not a winner. Asexual reproduction can be rapid, needs only one individual and preserves a successful combination of traits. Its offspring are alike, so a change in conditions that harms one tends to harm all. Sexual reproduction is often slower, usually needs two parents (a few plants, pea plants among them, can fertilize their own flowers, and their reproductive cells still mix the alleles) and breaks up combinations, but it produces varied offspring, and variation is what gives a population some members that may survive and reproduce when the environment changes. Many organisms use both: a strawberry plant produces runners and also flowers and seeds. Which strategy is favored depends on the organism and its conditions, so neither is simply better.

Genes and environment together. Expression of all traits is determined by genes and environmental factors, to varying degrees. Some traits depend almost entirely on inherited information; pea flower color is a case. Many depend heavily on conditions during growth. A plant's height depends on its alleles and on its light, water and soil. The flowers of a bigleaf hydrangea can be blue in acidic soil, where the roots can take up aluminum, and pink in more alkaline soil, with no change in the plant's genes. Siamese cats carry an allele that produces dark pigment only where the skin is cooler, which is why their ears, paws and tail are dark while the warmer body stays pale. Human skin darkens with sun exposure and muscles grow with use. In every case the inherited information sets the range of what is possible, and the environment affects where in that range the organism ends up.

One gene, many traits; one trait, many genes. Many genes influence more than one trait. In some white cats, the allele responsible for the white coat is also associated with blue eyes and, often, deafness, so one inherited difference shows up in coat, eyes and hearing. And many traits are influenced by more than one gene: height, skin color and the shape of a face are each shaped by many genes with small effects, which is why they show a smooth range across a population rather than two or three sharp categories. A trait is rarely a one-gene, one-outcome matter.

Misconceptions. "Asexual offspring are exact copies in every way" ignores mutation and the environment; two strawberry runners grown in sun and shade look different despite identical genes. "Sexual reproduction is more advanced" is a judgment the evidence does not support; each mode is favored under some conditions. "If a trait is inherited, the environment cannot change it" is refuted by height, hydrangea color and sun-darkened skin. "Offspring get half their traits from each parent" is wrong as stated; offspring get half their genetic information from each parent, and traits result from the combination and from conditions.

Investigation you can do. Take four cuttings from one healthy houseplant, such as a coleus, so that all four share the same genetic information, and root them in matching containers of water or moist potting mix. Place two in a bright window and two in a dim corner, keeping water and temperature the same, and wash your hands after handling the plants. After three weeks, measure stem length between leaves, leaf size and leaf color for each cutting, and record daily light conditions. Because the plants are genetically identical, a consistent difference between the bright pair and the dim pair cannot be inherited and, if everything but the light was kept the same, is due to the light, which supports the claim that environmental factors shape the expression of inherited traits; a typical result is longer, paler stems in dim light. Then plant twelve seeds from one packet of the same plant species side by side in identical conditions and, after three weeks, measure the same features. Differences among those seedlings under the same conditions are consistent with inherited variation from sexual reproduction. The investigation cannot show which genes are involved, cannot separate small environmental differences between individual pots from genetic ones without more replicates, and cannot show what mutation does.

Worked count of combinations: A parent carries two different alleles for each of 4 genes. Each reproductive cell gets one allele per gene, chosen independently, so the number of different combinations is 2 × 2 × 2 × 2 = 16. If the other parent also has 4 such genes, the pairings of reproductive cells number 16 × 16 = 256; several pairings can give an offspring the same alleles, so the count of distinct offspring is smaller (for 4 genes, 3 × 3 × 3 × 3 = 81 if both parents carry the same two alleles at each gene), but still large. For 10 genes per parent the numbers are 1,024 and 1,048,576 pairings. This is a simplified count that treats every gene as independent, which is not always true, but it shows why two parents can produce enormous variety.

Worked comparison of a strawberry patch (a hypothetical case): A gardener plants one strawberry plant. By the end of summer, runners have produced 12 new plants, and the original plant has also set seeds after its flowers were pollinated by a neighbor's plant. The 12 runner plants share the parent's complete genetic information (asexual: lone parent contributes the DNA). Seedlings grown from the seeds carry half their information from the gardener's plant and half from the neighbor's (sexual: both parents contribute DNA), so they vary in berry size and flavor while the runner plants do not, apart from environmental effects such as one growing in a shadier spot.

Worked genes-and-environment case: Two bigleaf hydrangea plants grown from cuttings of the same parent are planted in different beds; one produces blue flowers and the other pink. The plants have identical genetic information, so the difference must be environmental; for this plant the known cause is soil chemistry: acidic soil lets the roots take up aluminum, which turns the flowers blue. Moving the pink plant to the acidic bed would be expected to shift its later flowers toward blue, provided that bed has aluminum available, as most garden soils do. The inherited information made either color possible; the soil decided which appeared.

Worked reading of the cutting investigation (hypothetical figures): Four cuttings of one plant are grown for three weeks. Bright pair: stem segments between leaves 3 cm and 3.5 cm, leaves deep green. Dim pair: 7 cm and 8 cm, leaves pale. Since all four share the same genes, the longer paler growth is explained by low light, an environmental factor. The difference of 0.5 cm within the bright pair and 1 cm within the dim pair shows that even identical plants in nominally identical conditions differ a little, so small variations should not be over-interpreted.

Supported representation: Make a two-column comparison of asexual and sexual reproduction with rows for number of parents, source of the offspring's DNA, how much offspring vary and why, an example organism, and one advantage and one cost of each. Below it, list three traits and rate each on a line from "mostly inherited" to "strongly shaped by environment," with one sentence of justification per trait.

Independent performance: Carry out the cutting-and-seedling investigation, or analyze a set of measurements supplied by your teacher, and write a conclusion. Success criteria: (a) the design states which plants share the same genetic information and which do not, and why that matters; (b) measurements are recorded for every plant with the growing conditions; (c) the conclusion attributes the bright-versus-dim difference to environmental factors and explains why identical genetic information, with the other conditions held the same, makes that attribution valid; (d) it explains why variation among the seedlings is consistent with inherited differences from sexual reproduction, using the words alleles, both parents and DNA correctly; (e) it names one thing the investigation cannot show and one improvement, such as more replicates, that would strengthen the claim.