Questions: Inbreeding Depression and Genetic Rescue Mechanisms

Question 1

A critically endangered wolf population of 25 individuals shows reduced litter sizes, increased pup mortality, and higher parasite loads. Conservationists propose introducing 3 unrelated wolves from a distant population. What is the most likely immediate effect?

• A. The deleterious recessive alleles will be permanently removed from the population by intensified selection
• B. The immigrants' alleles will restore heterozygosity at many loci, masking deleterious recessives and improving fitness
• C. The immigrants' better-adapted alleles will replace the local population's inferior alleles over several generations
• D. Purging will intensify as inbreeding temporarily increases due to the small immigrant group size

Answer: (B)

Genetic rescue works by restoring heterozygosity — masking deleterious recessives that were being expressed because inbred individuals were homozygous for them. This effect is immediate (within one generation) and does not require selection to remove alleles. Option A describes purging, which is a separate, slower, and unreliable process. Option C mischaracterizes the mechanism — immigrant alleles don't 'replace' local alleles as superior; they restore complementarity that hides recessive damage. Option D is wrong — immigration reduces inbreeding, not increases it.

Question 2

A population of desert lizards has undergone severe inbreeding. Conservationists consider rescuing it with immigrants from a cold-montane population. The key genetic risk of this approach is:

• A. The immigrant individuals will outcompete local lizards for food resources before their offspring are produced
• B. Introducing alleles adapted to very different conditions may disrupt locally adapted gene combinations
• C. Immigrants may carry novel pathogens to which the inbred population has no immunity
• D. Gene flow from immigrants will halt purging, preventing removal of deleterious alleles

Answer: (B)

Outbreeding depression is the risk that hybrid offspring are poorly adapted to either parental environment because locally adapted gene combinations are disrupted. Desert adaptations (heat tolerance, water conservation) may be incompatible with cold-montane adaptations, producing hybrid offspring less fit than either parent population. This is why conservation biologists must evaluate genetic distance, severity of inbreeding depression, and degree of local adaptation before recommending rescue. Option A is an ecological competition concern, not a genetic mechanism risk. Option D is wrong — gene flow does not halt purging.

Question 3

Purging is a reliable mechanism for eliminating inbreeding depression in small populations because natural selection becomes more efficient at removing deleterious alleles when they are expressed in homozygous form.

• T. True
• F. False

Answer: False

Purging is explicitly described as unreliable. While increased homozygosity does allow selection to act more efficiently against strongly deleterious recessives, purging fails against the many mildly deleterious alleles that collectively drag down fitness. In very small populations, drift removes alleles randomly before purging can operate, and the extinction vortex may accelerate faster than purging can rescue the population. Purging can work under moderate inbreeding but is not a reliable conservation strategy for populations already in severe decline.

Question 4

Inbreeding depression is caused primarily by the accumulation of new harmful mutations in inbred populations, rather than by the unmasking of deleterious alleles already present in the population's standing genetic variation.

• T. True
• F. False

Answer: False

Inbreeding depression is caused by increased homozygosity *revealing* deleterious recessive alleles already present in the population — not by generating new mutations. In a large outbreeding population, most deleterious recessives are masked in heterozygotes and hidden from selection. Inbreeding increases the probability that an individual inherits the same allele from both parents, homozygosing these already-existing alleles and exposing their effects. The mechanism is exposure of existing variation, not accumulation of new mutations.

Question 5

Why does genetic rescue work within a single generation, whereas purging takes multiple generations — and what does this difference reveal about the mechanism of inbreeding depression?

Model answer: Genetic rescue is immediate because it directly restores heterozygosity: immigrant alleles pair with local alleles at deleterious loci, creating heterozygotes where the harmful recessive is masked. The fitness benefit appears in F1 offspring — heterozygous at thousands of loci that were previously homozygous. Purging, by contrast, works through selection: deleterious alleles must be expressed in homozygous form, reduce fitness, fail to reproduce, and thereby decrease in frequency — a multigenerational process. The difference reveals that inbreeding depression is about *masking*, not allele removal. The deleterious alleles are still present after genetic rescue; they are just hidden again in heterozygotes.

This has practical importance: genetic rescue is not a permanent fix. If the rescued population again becomes isolated and inbred, deleterious alleles can be unmasked again in future generations. Ongoing gene flow — not a one-time rescue event — is what maintains heterozygosity and suppresses inbreeding depression long-term.