Hemlocks are a common type of tree in the northern forests of the United States. In studying one forest, a forester noticed that mature hemlocks were abundant but there were very few young hemlocks. She wondered why there were so few young hemlocks. By reading the records that other foresters in that area had kept, she learned that the population of deer living in the forest had doubled in the last five years. By reading studies on forests that other scientists had performed, she also learned that deer in similar forests like to eat young hemlocks and the young trees of other species. To test her hypothesis, the forester set up an experiment. She placed fencing around a group of 15 young hemlocks to prevent deer from eating them. She left a group of 15 young trees with no fencing around them. Each day, she examined the trees for evidence that deer had been eating their twigs and branches and counted the number of trees with visible bite marks.

Which example is the control group for this experiment?

the number of young deer in the forest

the young trees without fencing

the number of deer in the forest

the young trees surrounded by fencing

d

explanation:

an individual’s sex (i.e., whether they are a male or female) is determined by the sex chromosomes. most people have two sex chromosomes, one that is inherited from their mother and one that is inherited from their father. typically, females have two x chromosomes (xx) and males have one x chromosome and one y chromosome (xy). conditions caused by changes (“mutations”) in genes located on the x chromosome are considered x-linked.

x-linked recessive inheritance

most x-linked conditions are recessive. this means that in a person with two x chromosomes (most females), both copies of a gene (i.e., one on each x chromosome) must have a change or mutation whereas in a person with one x chromosome (most males), only one copy of a gene must have a mutation. a female with a mutation in one copy of a gene on the x chromosome is said to be a “carrier” for an x-linked condition. a male with a mutation in a gene on the x chromosome is typically affected with the condition. because females have two copies of the x chromosome and males have only one x chromosome, x-linked recessive diseases are more common among males than females. however, x-linked recessive diseases can occur in both males and females.

for x-linked recessive disorders, an unaffected carrier mother who has a mutation in a gene on the x chromosome can transmit either the x chromosome with this mutation or a “normal” x chromosome to her children. if the father is unaffected, none of her daughters will be affected and all of her daughters will be unaffected—since they will inherit at least one normal x chromosome from their father. however, each daughter will have a 50% chance of being an unaffected carrier like her mother and a 50% chance of both x chromosomes being normal.

answer : haversian

[explanation: -(compact bone is organized as parallel columns, known as haversian systems, which run lengthwise down the axis of long bones. these columns are composed of lamellae, concentric rings of bone, surrounding a central channel, or haversian canal, that contains the nerves, blood vessels, and lymphatic system of the bone)]