Species of modern stick and leaf insects belonging to the order Phasmatodea are similar to 47-million-year-old fossils of stick and leaf insects. Researchers think that the similar forms of these insects indicate that ancient insects may have engaged in one of the same behaviors as modern insects to escape predators. This behavior, called catalepsy, involves remaining extremely still to deceive predators. This scientific explanation regarding stick and leaf insects indicates that they A
developed a successful survival strategy over the course of 47 million years that decreased their reproductive success.

B
have lived in a stable environment that did not undergo significant change over 47 million years.

C
developed a successful survival strategy 47 million years ago, increasing their reproductive success.

D
have passed mutated genes to each generation, impairing their ability to evolve for 47 million years.

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.

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