These questions ask you to separate selective breeding from natural selection and to read a simple pedigree without assuming that every genotype is visible. In both parts, trace what is actually being selected or shown before drawing an inheritance conclusion.
Use one method for breeding and one for pedigrees
For selective breeding, identify who chooses the parents. People select individuals showing a desired inherited characteristic, breed them, then repeat the choice among their offspring for many generations. This raises the frequency of helpful alleles already present. Because similar individuals contribute repeatedly, the resulting population usually contains less genetic variation. Natural selection also changes allele frequencies, but survival and reproduction in the environment determine which individuals leave more offspring; there is no human goal.
For a pedigree, decode the symbols before reasoning about alleles. A square represents a male and a circle a female. Shading means the individual shows the characteristic being followed; it does not by itself reveal a hidden allele. A horizontal line joins parents, and lines descending from that union lead to their children. Rows therefore help you follow generations.
| Pedigree evidence | Safe inference |
|---|---|
| An unshaded symbol | The phenotype is not shown. The individual may still carry a recessive allele, so a pair of dominant alleles is not certain. |
| A shaded symbol for a recessive characteristic | The individual has two recessive alleles, one inherited from each parent. |
| Two unaffected parents with an affected offspring | The characteristic follows recessive inheritance, and each parent supplied a recessive allele. |
| A parent able to supply a recessive allele | That parent cannot have two dominant alleles, even if the recessive phenotype is hidden. |
Mistakes that look plausible
| Tempting reading | How to reject it |
|---|---|
| Calling environmental survival selective breeding | Look for a person choosing parents. Without that deliberate choice, the process is natural selection. |
| Reversing the square and circle meanings | Shape records sex consistently throughout the diagram; generation is shown by position, not shape. |
| Saying a new breed means variation within it increased | Compare individuals inside the breed. Repeatedly choosing similar parents narrows the alleles represented there. |
| Reading shading as proof of one hidden allele | Shading records an expressed characteristic. Whether an unshaded individual carries a hidden allele requires inheritance reasoning and is not a general meaning of shading. |
| Reading a horizontal couple line as a sibling link | Follow the vertical descent: the joined pair are parents, and the lower symbols are their children. |
| Saying breeding invents an allele | Breeders change which existing alleles are passed on most often. New alleles arise through mutation, not through choosing parents. |
Try it
A farmer wants a wheat variety with a much higher grain yield. How long does selective breeding for this take?
- One cross, since the offspring inherit the wanted characteristic directly
- Many generations of choosing and breeding the best plants
- One growing season, provided enough plants are crossed
- It cannot be done at all, because yield is not inherited
Selective breeding and natural selection both change how common an allele is in a population. What is the difference between them?
- Only selective breeding changes how common an allele is
- Who or what decides which individuals become parents
- Natural selection is the faster of the two because it is natural
- Only natural selection can act on characteristics that are inherited
In a rabbit pedigree following a recessive coat characteristic, one offspring is shaded and neither parent is. What can be stated about the alleles carried?
- The shaded offspring has two recessive alleles, and neither parent has two dominant ones
- The shaded offspring has one allele of each kind, like both of its parents
- One of the parents may carry two dominant alleles, because only one parent needs to supply the recessive allele
- Both parents must show the characteristic themselves for it to appear