Speciation is the event that occurs when two populations of a particular species can no longer interbreed. Speciation is not defined by physical barriers or by the time that two populations are separate from one another. In fact, two populations of the same species can be apart any distance or time, and if they can still interbreed they are considered the same species even if they look completely different.

Note that carrying capacity refers to the maximum number of individuals in a population that the natural resources of the surrounding environment can support. It is, essentially, the maximum healthy population size, and is not a measure of distance as implied in the answer choice.

In this example, some individuals with a certain genotype do not express the expected phenotype (symptoms of the disease) at all. The only term that properly describes this effect is reduced penetrance. Penetrance refers to the percent of individuals with a specific genotype who express the associated phenotype. In most common examples given in genetics courses, autosomal dominant diseases are 100% penetrant, meaning that all individuals with one disease allele will show symptoms to some extent. Here, however, the disease appears to show 80% penetrance.

One term often confused with penetrance is expressivity. This refers to the extent that the phenotype is expressed, and is only applicable when penetrance is 100%. If all of the individuals showed symptoms of the disease, but some showed slightly different defects than others, the disease would have variable expressivity. The other answer choices refer to unrelated genetics concepts.

This problem asks you to use concepts of inheritance and Mendelian genetics. The best approach to this problem is to rule out possiblities rather than to find the actual mode of inheritance, as the latter can be a much more difficult and time-consuming process. First off, we know that Y-linked inheritance could not explain this pattern because we see that in generation 1 (G1), the male is affected. If he is affected, all of his sons (who inherit his Y chromosome) would also be affected. There is one son in G2 who is not. Similarly, dominant X-linked inheritance could not explain this pattern; recall that the daughters inherit two copies of the X chromosome, and one is always inactivated. Were the trait X-linked dominant, then the girls of generation 3 (G3) would be affected, having received a copy of the affected gene from their father. Revisiting all other options, we see that any of the remaining inheritance patterns could possibly explain what we see.

Individual 22 is male, and the trait it X-linked recessive. We know he will inherit the Y-chromosome from the unknown father, and a singular X-chromosome from the affected mother. Because the mother is affected, we know she must have two affected X-chromosomes. No matter which chromosome is passed to individual 22, he will inherit the trait.

Since the disease is found on the X-chromosome, we need to find the scenario in which both sons and daughters have an equal 50% probability of getting the disease. Regardless of gender, mothers will always donate one X-chromosome to the offspring. If the mother is heterozygous for the disease, she has a 50% chance of giving an offspring the diseased allele. As a result, a heterozygous mother will have children that display the disease in the observed ratio.

Parents: XX^R x XY

Offspring: XX, XX^R, XY, X^RY

Note that this ratio of expression is only possible when the allele for the disorder is dominant; otherwise the heterozygous female would be a carrier, and not express the disorder.

Duchenne Musclar Dystrophy is a well-studied genetic disorder, generally resulting from a frameshift mutation on the X chromosome. Its pattern of inheritance is considered X-linked recessive.

In order for a girl to have this disorder, she must possess two copies of the recessive allele (one on each X chromosome). In contrast, a boy would only need one copy of the affected allele, but it must come from the mother (he will only have one X chromosome).

Let us use as a healthy chromosome, and to represent the recessive allele. Since we know that the girl has the disorder, her genotype must be . She received one affected allele from the mother, and one from the father. From this information, we know that the father must have the disease. We also know that the mother either has the disease OR is a carrier.

Father:

Mother:

The only certain conclusion that we can make is that the father has the disease.

All of the other listed answer choices are possible, but cannot be concluded with certainty unless additional information was provided.

When assessing patterns of inheritance, a genetic disorder that preferentially affects males over females will most commonly be X-linked recessive.

Females have two copies of the X chromosome (one from each parent), while males have one X chromosome (from the mother) and one Y chromosome (from the father). In a recessive allele is present on the X chromosome, a female can carry the allele without expressing it. Males, however, lack the shielding provided by a second X chromosome, and will express all recessive alleles present on their single X chromosome.

Autosomal disorders originate from non-sex chromosomes (autosomes), and do not generate any sex-linked patterns. The question also notes that females do rarely get the disease, indicating that the Y chromosome is not involved (a female will not receive the Y chromosome).

Codominance is evidenced when the phenotypes of both parents show up in the offspring. A dog that has fur that consists of colors of both parents will be an example of codominance. Only one trait can be expressed at a time, since they are both dominant phenotypes. This results in regions of one dominant allele and regions of the other, showing a spotted or mottled pattern.

Incomplete dominance occurs when neither trait is truly dominant over the other. This means that both traits can be expressed in the same regions, resulting a blending of two phenotypes. If a white and black dog produce a gray offspring, this is an example of incomplete dominance.

The answer that suggests a red offspring from a black parent and tan parent could result from one of two scenarios. The first possibility is that there are three alleles for color, with red recessive to both black and tan. Both parents carry the red allele, but do not display it, and then pass it to the offspring. Something similar happens with the O blood type. The other possibility is that red color is a new mutation.

This example represents the incomplete dominance crossing a black chicken with a white chicken resulted in the offspring taking on a third phenotype that is an intermediate, blended phenotype of both parents' phenotypes.

This is an example of codominance -- crossing a black chicken with a white chicken results in the offspring taking on a third phenotype where both of the parents' phenotypes appear together. This is different than incomplete dominance, in which the offspring take on a third phenotype that is a "blend" of the parents' phenotypes (gray chickens as opposed to spotted chickens).