Farmers have selected for increased oil content from corn. For a long time, the plants responded with steadily increasing oil co
ntent, but recently the response slowed and then stopped. When this line of corn is crossed to another line and the hybrids are selected, the oil content resumed its climb. What is a plausible reason for the lack of response? A. The environmental conditions improved.
B. The genetic variation for the trait was exhausted.
C. There was too much genetic variation for the trait.
D. The environmental conditions deteriorated.
E. Artificial selection is different than natural selection.
B. The genetic variation for the trait was exhausted.
Explanation:
Oil content is a quantitative trait, ie., a phenotypic feature that depends on the accumulative effects of many genes and the interaction of a genotype with its environment. During crop improvement, the plants (i.e., lines) containing desired alleles involved in the variation of this trait will be selected, thereby increasing oil content but simultaneously decreasing the genetic variation of the population. In consequence, after successive rounds of artificial selection, lines will not have sufficient genetic diversity to be again selected for this trait.
During prophase I, the homologous chromosomes condense and become visible as the x shape we know, pair up to form a tetrad, and exchange genetic material by crossing over.
The volume of blood is reduced as the vessels are more narrow due to the contraction, so less blood reaches the respiring tissues, causing the force of blood flow to increase as there is less space for normal blood flow to occur. This means that blood pressure increases.
