The Genetics of Missing Teeth
A
pproximately one in five peo-
ple are born without the
ability to develop a full set of teeth.
One form of this condition, known
as hypodontia,is caused by an
autosomal-dominant mutation.
Therefore, each child of an
affected parent has a 50 percent
chance of inheriting the condition.
The genetic basis for autosomal-
dominant hypodontia was
discovered by researchers at
Baylor College of Medicineand
The University of Texas at
Houston.
Identifying the Gene
The researchers studied a Hou-
ston family in which 21 members
had hypodontia. They compared
the DNA of those 21 individuals to
that of 22 of their relatives who did
not have hypodontia. This com-parison revealed a difference
between the affected individu-
als and their relatives in a small
region of chromosome 14. In-
cluded in that region is a gene
called Pax-9,which is required
for tooth formation in mice.
When the researchers looked
specifically at the Pax-9 gene
sequence in the Houston fam-
ily, they found that all 21 family
members who had hypodontia
had a mutation in the Pax-9 gene.
Their unaffected relatives did not
have the mutation nor did 150 un-
affected individuals outside the
family.
Pinpointing the Mutation
A base-by-base analysis of the
mutated Pax-9 gene in this family
showed that the gene contained
an inserted cytosine nucleotide.This insertion leads to a premature
termination of translation and a
smaller-than-normal protein. This
is not the only mutation, however,
that can result in hypodontia.
When the same research group
studied another family affected by
the condition, they found a differ-
ent type of point mutation—
a substitution—in the Pax-9 gene.
This substitution leads to a non-
functional protein.Because the genetic message is read as a series of triplet nucleotides,
insertions and deletions of one or two nucleotides can upset the
triplet groupings. Imagine deleting the letter C from the sentence
“THE CAT ATE.” Keeping the triplet groupings, the message would
read “THE ATA TE,” which is meaningless. A mutation that causes
a gene to be read in the wrong three-nucleotide sequence is called a
frameshift mutation.220 CHAPTER 10How Proteins Are Made
Describethe effect a repressor has on the lac
operon when lactose is present. 6A 6BExplainthe role of transcription factors and
enhancers in eukaryotic gene expression. 4B 6BDifferentiatebetween exons and introns.Critical ThinkingEvaluating Significance
Which type of mutation would have a greater
effect on the sequence of amino acids in a pro-
tein, a base-pair substitution or a frameshift
mutation? Explain your answer. 4B 6A 6CA mutation in which one
nucleotide in a gene is replaced with a different
nucleotide is called 6C
Aa deletion. Ca substitution.
Ban insertion. D a frameshift mutation.TAKS Test PrepTAKS Test PrepSection 2 Review
4A 6A