Variations in Populations

Variety H plants were 58% infected when grown alone but 9% in mixed fields. R stayed near 3-4% either way.

• Correct: Variety H was infected far less often among rows of R than when grown alone: Correct: 58% alone versus 9% in mixtures.
• Variety R was infected more often when grown with variety H than when grown alone: R was 4% infected alone and 3% in mixtures, about the same.
• Variety H was infected about as often in mixed fields as in fields of H alone: H infection fell from 58% to 9%, a large drop.
• Variety R lost its resistance when grown in the same field as variety H: R's infection stayed low in mixed fields, so it kept its resistance.

In a uniform field, every neighbor is a host, so the fungus spreads plant to plant. In the mixture, susceptible plants are spaced apart by resistant ones, so many spores are wasted and the outbreak grows more slowly.

• Correct: Spores from infected H plants mostly land on R plants they cannot infect, so spread slows: Correct: resistant neighbors break the chain of spread.
• Variety R plants release a chemical that kills the fungus, which protects the other plants in the field: The stem describes a resistance allele, not a chemical; and R's own infection data do not suggest one.
• Variety H plants grown next to variety R take up R's resistance allele through their roots: Alleles are not passed between neighboring plants through their roots.
• Mixed fields were planted later in the season, after the outbreak had already ended: Nothing in the setup says planting dates differed; the fields were compared in the same outbreak year.

A field of one genotype has no individuals that resist the new strain, so it can spread through the whole field. Mixed fields keep some H plants the new strain cannot infect, and H-alone fields resist it.

• Correct: Fields of R alone, because every plant is a host for the new strain: Correct: a uniform field of a now-susceptible genotype is the most vulnerable.
• Mixed fields, because they contain two varieties the fungus can attack: The new strain cannot infect H, so mixed fields keep some unaffected plants and spaced-out hosts.
• Fields of H alone, because H was the variety most infected the year before: Last year's strain is not this year's; the new strain cannot infect H.
• The three kinds equally, because the new strain spreads by the same kind of spores: The strain can infect only one of the two varieties, so plantings differ in how many hosts they offer.

With few mates available, cats were often related, so kittens more often received the same recessive allele from both parents (inbreeding). Recessive defects then appear, and heterozygosity falls.

• Correct: In a small, isolated population relatives mate, so more kittens get two copies of a harmful recessive allele: Correct: inbreeding exposes harmful recessive alleles.
• Small populations make new harmful mutations much faster, so their kittens carried many more new mutations: Mutation rate per gene does not depend on population size; the issue is how alleles are combined.
• Low heterozygosity means kittens inherited fewer genes, so some body structures could not form: Every kitten has the same set of genes; low heterozygosity means fewer different alleles, not fewer genes.
• The defects were caused by the roads and farms themselves, through stress on the pregnant females: The defects are inherited and fell after new alleles arrived, which stress would not explain.

Surviving a new disease depends on some individuals carrying alleles that resist it. A small population with low heterozygosity has fewer different alleles, so the chance that some cats resist is lower, and one outbreak could wipe it out.

• Correct: Worse in 1990-1994: fewer, more similar cats made resistance alleles less likely: Correct: low variation makes a population vulnerable.
• Better in 1990-1994: with fewer cats, the virus would have had fewer hosts and would have died out: A small population can still pass a virus among its members, and losing cats matters more when there are few.
• The same in both periods, because a new virus is equally deadly to cats of any genotype: Cats differ genetically in how they respond to infection; that variation is what the period comparison is about.
• Better in 1990-1994, because inbred cats have stronger immune systems from mating with relatives: Inbreeding lowers variation, including in immune genes; it does not strengthen immunity.

Heterozygote advantage: selection removes HbS in HbS HbS children and HbA through malaria deaths of HbA HbA children. Carriers, with the highest survival, pass on both alleles, so both persist (balancing selection).

• Correct: Carriers, who hold most HbS alleles, survive better than either homozygote, so both alleles stay: Correct: the heterozygote has the highest fitness where malaria is common.
• New HbS mutations arise each generation fast enough to replace the alleles lost in HbS HbS children: New mutations to HbS are far too rare to keep the allele at such high frequencies.
• Malaria causes the HbA allele to change into the HbS allele in people who are infected: Infections do not change a person's alleles; mutations are random.
• The HbS allele is dominant, so it spreads whether or not it helps the people who carry it: Dominance does not make an allele spread; in fact the disease is recessive.