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Pedigrees
4/13/2010
Pedigrees
• The risks of passing on a genetic disorder to offspring can be
assessed by genetic counseling, prenatal testing, and by
analyzing a pedigree.
• A pedigree is a family history diagram that shows how a trait is
inherited over several generations.
• A pedigree can be mapped out to determine if individuals are
carriers or if their children might inherit the disorder.
• Carriers are individuals who are heterozygous for an inherited
disorder but do not show symptoms. Carriers can then pass
the allele for the disorder on to their children.
Pedigrees
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In a pedigree, females are indicated by
circles, males are indicated by squares.
Shaded figures represent individuals
with the trait, a carrier could be 1/2
shaded (but not always!).
Generations are numbered with roman
numerals (I, II, III, IV) from top to
bottom.
People within generations are
numbered (1, 2, 3) from left to right.
People that are married (or just having
children together) are connected by a
horizontal (left to right) line. Offspring of
individuals are connected to their
parents by a vertical (up and down) line.
Pedigrees
• By analyzing a pedigree you can tell if a
trait is dominant or recessive and if it is
autosomal or sex-linked.
One parent has the disease, and none of
the three children inherited it. We can
tell that this is a recessive trait because
not many people in the family have it. If
it were a dominant trait, many more
would have inherited it.
Both males and females show the trait, so
we know this is not sex-linked but is an
autosomal trait.
Pedigrees
• We can also analyze a pedigree to
figure out people’s genotypes. If we
know this is an autosomal recessive
trait, then anyone shaded in must
have the genotype (nn). Anyone
not shaded must have either (NN or
Nn).
• What is the genotype of person
I1?
nn
• Person IV2?
Nn or NN
• Who in this pedigree must be
heterozygous?
II1, II2, II3, III1 and III2
Pedigrees
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Only women are
carriers, and only
men show the trait.
Therefore, this must
be a sex-linked trait.
• We can also tell this is a recessive trait,
because not many people have it. In order
for a trait to have carriers, it must be
recessive. If a trait is dominant, people
either have it or they don’t.
• Since this is a sex-linked recessive trait,
what is the genotype of Alice?
XHXh
• What is the genotype of Fred?
XhY
Karyotyping and genetic
disorders
4/14/2010
Karyotypes
• Genetic disorders may be detected by using prenatal testing
and pedigrees. They can also be detected using karyotypes.
• A karyotype is a photograph of an individual’s chromosomes in
a dividing cell during mitosis. The chromosomes are arranged
by size and numbered.
Karyotypes
• A karyotype can show you two things:
1. Chromosome abnormalities: missing chromosomes, extra
chromosomes, or if chromosomes are malformed
2. The sex of the person
karyotypes
Normal Karyotype
Down’s Syndrome
Genetic disorders
Down’s Syndrome:
o A chromosomal disorder caused by an extra chromosome 21. For this
reason it is also known as Trisomy 21 (which means 3 chromosome
21’s).
o Caused by nondisjunction, which means that during meiosis a gamete is
produced with an extra copy of chromosome 21. This is not an inherited
trait, it happens in the egg or the sperm before fertilization.
o Symptoms: learning disabilities, developmental disabilities, and impaired
physical growth.
o Occurrence: about 1 of every 9,000 births.
Genetic disorders
Genetic disorders
Cystic Fibrosis:
o Symptoms: causes thick mucus to coat the lungs leading to severe
breathing problems. It also causes the pancreas to not secrete enzymes
as efficiently as it should, causing poor growth, diarrhea, and vitamin
deficiency.
o Inheritance: autosomal recessive disease caused by a mutation in a
gene.
o Occurrence: 1 in 3,900 children are born with this disease, and there is no
cure.
Genetic disorders
Genetic disorders
Huntington’s Disease:
o Symptoms: a genetic neurological disorder characterized after onset by
uncoordinated, jerky body movements and a decline in some mental
abilities. People with Huntington’s Disease have too many CAG’s in a
gene on their DNA and so form a mutant protein from too many
glutamines.
o Occurrence: Up to 7 people in 100,000 have this disorder.
o Inheritance: This is an autosomal dominant trait, so an affected individual
needs just one copy of the gene to show the disease.
Genetic disorders
Genetic disorders
Sickle Cell Anemia:
oA blood disorder in which the red blood cells are not flexible and round but
are rigid and sickle-shaped (like a crescent moon). This restricts the
blood cells’ movement throughout the blood stream and decreases the
amount of oxygen the cells can carry through the body.
oInheritance: a recessive trait.
oSymptoms: misshapen blood cells cause the blood to not carry enough
oxygen throughout the body. Individuals most often feel fine, but their
lives are interrupted by periodic painful attacks. The only treatment is pain
medication during these attacks.
oOccurrence: 1 out of every 10 African-Americans has this trait.
Genetic disorders