5
328-333
Family pedigrees
Understanding how genetic materials can be passed down from
our ancestors
Outcomes:
 Identify and explain the key symbols in family pedigrees;
 Identify and explain the structure of family pedigrees; and
 Ability to construct and analyse family pedigrees and explain the patterns of
inheritance;
 Ability to appreciate the importance of family pedigrees and their contributions in
allowing scientists to further understanding genetic inheritance.
Content:
Introduction
 A family pedigree is a chart that illustrates the pattern of inheritance of a trait in a
family, often showing the passing-down of a characteristic through generations.
 The pattern may indicate if:
o The trait concerned is dominant or recessive; or
o The controlling gene is located on an autosome or on a sex chromosome.
 It is important that the teacher instructs students that a single pedigree may not
always provide conclusive evidence.
Key symbols and pedigree structure
Autosomal dominant pattern
(1) Features of an autosomal dominant pattern:
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Both males and females can be affected
All affected individuals have at least one affected parent
Transmission can be from fathers to daughters and sons, or from others to daughters
and sons
Once the trait disappears from a branch of the pedigree, it does not reappear
In a large sample, approximately equal numbers of each sex will be affected
(2) Example of an autosomal dominant pattern:
Hypercholesterolaemia (abnormally high levels of cholesterol in blood) controlled by the
LDLR gene. High blood cholesterol level is dominant (B) and normal level is recessive (b),
thus both dominant homozygote (BB) and heterozygote (Bb) will express abnormally high
cholesterol levels.
(3) Other examples of autosomal dominant traits:
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Huntington disease, a degenerative brain disorder; controlled by the HD gene on the
short arm of the number-4 chromosome (4p16)
achondroplasia, a form of dwarfism; controlled by the ACH gene on the short arm of
the number-4 chromosome (4p16)
familial form of Alzheimer disease; controlled by the AD1 gene on the long arm of
the number-21 chromosome (21q21)
defective enamel of the teeth; controlled by the DGI1 gene on the number-4
chromosome (4q13)
neurofibromatosis, the ‘Elephant man’ disease; controlled by the NF1 gene on the
number-17 chromosome (17q11)
familial breast cancer controlled by the BRCA1 gene on the number-17 chromosome
(17q21)
lip pits and cleft palate; controlled by the LPS gene on the number-1 chromosome
(1q32).
Autosomal recessive pattern
(1) Features of an autosomal recessive pattern:
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Both males and females can be affected
Two unaffected parents can have an affected child
All the children of two persons with the condition must also show the condition
The trait may disappear from a branch of the pedigree, but reappear in later
generations
Over a large number of pedigrees, there are approximately equal numbers of
affected females and males.
(2) Example of an autosomal recessive pattern:
Oculo-cutaneous albinism (condition in which pigmentation is absent from skin, eyes and
hair). Albinism is an autosomal recessive trait (a) and normal pigmentation is dominant (A).
(3) Other examples of autosomal recessive traits:
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cystic fibrosis, controlled by the CFTR gene on the number-7 chromosome (7q31)
thalassaemia, a blood disorder; controlled by the HBB gene on the number-11
chromosome (11p15)
Tay-Sachs disease, a degenerative disease of the central nervous system that causes
death in infancy and is common in Jewish people of middle-European ancestry;
controlled by the HEXA gene on the number-15 chromosome (15q23)
Wilson disease, a disease in which copper metabolism is abnormal; controlled by the
WND gene on the number-13 chromosome (13q14)
insulin-dependent diabetes mellitus-1; controlled by the IDDM1 gene on the
number-6 chromosome (6p21)
phenylketonuria; controlled by the PKU1 gene on the number-12 chromosome
(12q24)
red hair colour (see figure 9.55), controlled by the HCL2 gene; located on the
number-4 chromosome (4q28)
a form of osteogenesis imperfecta (‘brittle bones’), a disorder affecting the collagen
protein of bones; controlled by the COL1A1 gene on the number-17 chromosome
(17q21)
galactosaemia, a disease in which a person cannot metabolise the major sugar
(galactose) found in milk; controlled by the GALT gene on the number-9
chromosome (9p13).
X-linked dominant pattern
(1) Features of X-linked dominant pattern:
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a male with the trait passes it on to all his daughters and none of his sons
a female with the trait may pass it on to both her daughters and her sons
every affected person has at least one parent with the trait
if the trait disappears from a branch of the pedigree, it does not reappear
over a large number of pedigrees, there are more affected females than males.
(2) Example of X-linked dominant pattern:
A form of vitamin D-resistant rickets (controlled by HYP gene). The trait is expressed even if
a person has just one copy of the allele responsible.
(3) Other examples of X-linked dominant traits:
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incontinentia pigmenti, a rare disorder that results in the death of affected males
before birth; controlled by the IP2 gene on the X chromosome (Xq28)
Xg(a) blood group; controlled by the XG gene on the X chromosome (Xp22).
X-linked recessive pattern
(1) Features of X -linked recessive pattern:
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All the sons of a female with the trait are affected
All the daughters of a male with the trait will be carriers of the trait and will not
show the trait; the trait can appear in their sons
None of the sons of a male with the trait and an unaffected female will show the
trait, unless the mother is a carrier
All children of two individuals with the trait will also show the trait
In a large sample, more males than females show the trait.
(2) Example of X -linked recessive pattern:
Appearance of favism. Favism is a disorder in which the red blood cells are rapidly destroyed
if the person with this condition comes into contact with certain agents (e.g. broad beans
and mothballs). Favism (f) results from a missing enzyme and is controlled by the G6PD gene
on the long arm of the X chromosome.
(3) Other examples of X -linked recessive traits:
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Ichthyosis, an inherited skin disorder; controlled by the STS gene on the X
chromosome (Xp22)
One form of red–green colour-blindness; controlled by the CBD gene on the X
chromosome (Xq28)
One form of severe combined immunodeficiency disease; controlled by the SCIDX
gene on the X chromosome (Xq13)
Haemophilia, an inherited blood-clotting disorder; controlled by the F8C gene on the
X chromosome (Xq28)
One form of mental retardation, known as fragile X syndrome; controlled by the
FMR1 gene on the X chromosome (Xq27)
Duchenne muscular dystrophy; controlled by the DMD gene on the X chromosome
(Xp21).
Y-linked pattern
(1) Features of Y-linked patterns
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Only the male offspring (sons) will inherit the trait
All Y-linked traits are expressed
Activity:
 Part 10 of worksheet:
http://www.svusd.k12.ca.us/schools/lhhs/mccormacks/apbiowebpage/Assignments
/genetics%20wkst.pdf
 Answers are at the end of the document.