Explain why hydrophobic molecules can easily cross the plasma membrane, while hydrophilic molecules cannot.

Membranes are semipermeable lipid bilayers. Permeability refers to the ease with which molecules cross biological membranes. Because of the chemical and structural nature of the phospholipid bilayer (hydrophobic core), only lipid-soluble molecules are able to freely pass through the lipid bilayer because it attracts these non polar molecules.

Water soluble or hydrophilic molecules cannot pass through these lipid membranes because hydrophobic bilayer repels hydrophilic molecules. Ions and large polar molecules cannot pass through the lipid bilayer.  

But more specifically, whether a molecule can pass through the membrane depends on its size and its electrical nature.

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.