Genetics Overview of MLDNext Topic
Almost all the cells in the body have 23 pairs of chromosomes. In each pair, one chromosome comes from the mother and one
from the father. These chromosomes are very long strands of DNA all wrapped together. Along the DNA strand are certain sections
called genes, which control all the body’s processes and give people their individual traits. Eye color, skin color, and gender
are just a few of the traits encoded in a person’s genes. Unfortunately, disorders such as MLD are also encoded by genes passed
down from generation to generation.
Every gene is composed of two alleles, one on the chromosome from the mother and one on the chromosome from the father. It
is the alleles that determine which traits are recognized, or expressed. Alleles are classified as either dominant or
recessive. A dominant allele will always be expressed no matter what the other allele is. For example, brown is the dominant
allele for eye color. (See diagram below.) If either one of your chromosomes has a brown eye color allele, then you will have
brown eyes. A recessive allele, however, is expressed only if both alleles are the same. Blue is a recessive allele for eye
color, which means every person with blue eyes has two blue alleles for eye color, one from his or her mother and one from the
father. So how does a child get blue eyes if both parents have brown eyes? That question introduces the final genetic puzzle
piece—the carrier.
Eye color genetics.
A carrier has one dominant allele and one recessive allele. This person carries the recessive allele, which can be passed
along to children but it will only be expressed if paired with another recessive allele. Often people will not even know that
they are carrying certain recessive alleles because those alleles may never be expressed. Many generations can pass before a
recessive allele finds a match from the other parent and gets expressed. Some recessive alleles are common, such as blue eye
color; others are extremely rare, such as MLD.
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