Coral reefs share a symbiotic relationship with a type of algae called zooxanthellae. the hard exoskeleton that makes up a coral reef provides a safe habitat for zooxanthellae. in return, zooxanthellae use sunlight to make food for themselves and for the coral. one of many threats to coral reefs is sedimentation run-off. sedimentation run-off clouds the water, as well as changes its chemistry. if sedimentation run-off continues along coral reef coastlines, how would this most likely lead to the extinction of coral reefs? a. by blocking sunlight from reaching zooxanthellae, sedimentation run-off may deprive coral reefs of the nutrients they need. b. by causing zooxanthellae to rapidly increase in population, sedimentation run-off may greatly reduce the habitat available to coral reefs. c. by causing coral reef waters to become overly warm, sedimentation run-off may cause coral reefs to become overheated. d. by clouding the visibility of coral reefs from humans, sedimentation run-off may result in boats and ships destroying coral reefs.

an organism's phenotype is the  form of a trait  that it  displays. for  flowercolor, a pea  plant  can  display  a phenotype of purple or white flowers. an organism's  genotype  is the alleles of a gene it contains. based on his data, mendel concluded that a phenotype can be  determined  by more than one  genotype.

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.

this age may represent the age of the earth's accretion, of core formation, or of the material from which the earth formed. this dating is based on evidence from radiometric age-dating of meteorite material and is consistent with the radiometric ages of the oldest-known terrestrial and lunar samples.