Title: patterns of inheritance
Post by: 170584 on May 12, 2012
I need help on session 2 basic genetic concepts pedigree exercise power point, on determing the patterns of inheritance? Can anyone help?
Title: Re: patterns of inheritance
Post by: shohrehjoon on May 12, 2012
Untitled 2
Pedigree Analysis
Basic principles
If more than one individual in a family is afflicted with a disease, it is a
clue that the disease may be inherited. A doctor needs to look at the family
history to determine whether the disease is indeed inherited and, if it is, to
establish the mode of inheritance. This information can then be used to predict
recurrence risk in future generations.
A basic method for determining the pattern of inheritance of any trait (which
may be a physical attribute like eye color or a serious disease like Marfan
syndrome) is to look at its occurrence in several individuals within a family,
spanning as many generations as possible. For a disease trait, a doctor has to
examine existing family members to determine who is affected and who is not. The
same information may be difficult to obtain about more distant relatives, and is
often incomplete.
Once family history is determined, the doctor will draw up the information in
the form of a special chart or family tree that uses a particular set of
standardized symbols. This is referred to as a pedigree. In a pedigree, males
are represented by squares and
females by circles .
An individual who exhibits the trait in question, for example, someone who
suffers from Marfan syndrome, is represented by a filled symbol or .
A horizontal line between two symbols represents a mating .
The offspring are connected to each other by a horizontal line above the symbols
and to the parents by vertical lines. Roman numerals (I, II, III, etc.)
symbolize generations. Arabic numerals (1,2,3, etc.) symbolize birth order
within each generation. In this way, any individual within the pedigree can be
identified by the combination of two numbers (i.e., individual II3).
Dominant and recessive traits
Using genetic principles, the information presented in a pedigree can be
analyzed to determine whether a given physical trait is inherited or not and
what the pattern of inheritance is. In simple terms, traits can be either
dominant or recessive. A dominant trait is passed on to a son or daughter from
only one parent. Characteristics of a dominant pedigree are: 1) Every affected
individual has at least one affected parent; 2) Affected individuals who mate
with unaffected individuals have a 50% chance of transmitting the trait to each
child; and 3) Two affected individuals may have unaffected children.
Recessive traits are passed on to children from both parents, although the
parents may seem perfectly "normal." Characteristics of recessive pedigrees are:
1) An individual who is affected may have parents who are not affected; 2) All
the children of two affected individuals are affected; and 3) In pedigrees
involving rare traits, the unaffected parents of an affected individual may be
related to each other.
The reason for the two distinct patterns of inheritance has to do with the genes
that predispose an individual to a given disease. Genes exist in different forms
known as alleles, usually distinguished one from the other by the traits they
specify. Individuals carrying identical alleles of a given gene are said to be
homozygous for the gene in question. Similarly, when two different alleles are
present in a gene pair, the individual is said to be heterozygous. Dominant
traits are expressed in the heterozygous condition (in other words, you only
need to inherit one disease-causing allele from one parent to have the disease).
Recessive traits are only expressed in the homozygous condition (in other words,
you need to inherit the same disease-causing allele from both parents to have
the disease).
Penetrance and expressivity
Penetrance is the probability that a disease will appear in an individual when a
disease-allele is present. For example, if all the individuals who have the
disease-causing allele for a dominant disorder have the disease, the allele is
said to have 100% penetrance. If only a quarter of individuals carrying the
disease-causing allele show symptoms of the disease, the penetrance is 25%.
Expressivity, on the other hand, refers to the range of symptoms that are
possible for a given disease. For example, an inherited disease like Marfan
syndrome can have either severe or mild symptoms, making it difficult to
diagnose.
Non-inherited traits
Not all diseases that occur in families are inherited. Other factors that can
cause diseases to cluster within a family are viral infections or exposure to
disease-causing agents (for example, asbestos). The first clue that a disease is
not inherited is that it does not show a pattern of inheritance that is
consistent with genetic principles (in other words, it does not look anything
like a dominant or recessive pedigree).
Title: Re: patterns of inheritance
Post by: boot1 on May 12, 2012
Untitled 2
Single Gene Inheritance |
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Genetic conditions caused by a mutation in a
single gene follow predictable patterns of inheritance within families.
Single gene inheritance is also referred to as Mendelian inheritance as
they follow transmission patterns he observed in his research on peas.
There are four types of Mendelian inheritance patterns:
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Autosomal dominant
Autosomal recessive |
X-linked recessive
X-linked dominant |
Autosomal: the
gene responsible for the phenotype is located on one of the 22 pairs of
autosomes (non-sex determining chromosomes). X-linked: the
gene that encodes for the trait is located on the X chromosome.
Dominant: conditions
that are manifest in heterozygotes (individuals with just one copy of
the mutant allele).
Recessive: conditions
are only manifest in individuals who have two copies of the mutant
allele (are homozygous).
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Autosomal Dominant
Dominant conditions are expressed in individuals who have
just one copy of the mutant allele. The pedigree on the right
illustrates the transmission of an autosomal dominant trait. Affected
males and females have an equal probability of passing on the trait to
offspring. Affected individual's have one normal copy of the gene and
one mutant copy of the gene, thus each offspring has a 50% chance on
inheriting the mutant allele. As shown in this pedigree, approximately
half of the children of affected parents inherit the condition and half
do not. |
Autosomal Dominant
Conditions:
• Huntington Disease
• acondroplasia (short-limbed dwarfism)
• polycystic kidney disease
|
Autosomal Recessive
Recessive conditions are clinically manifest only when an
individual has two copies of the mutant allele. When just one copy of
the mutant allele is present, an individual is a carrier of the
mutation, but does not develop the condition. Females and males are
affected equally by traits transmitted by autosomal recessive
inheritance. When two carriers mate, each child has a 25% chance of
being homozygous wild-type (unaffected); a 25% chance of being
homozygous mutant (affected); or a 50% chance of being heterozygous
(unaffected carrier). |
Affected individuals are
indicated by solid black symbols and unaffected carriers are indicated
by the half black symbols.
Autosomal recessive
diseases:
• Cystic fibrosis
• Tay-Sachs
• hemochromatosis
• phenylketonuria (PKU)
|
X-linked Recessive
X-linked recessive traits are not clinically manifest when
there is a normal copy of the gene. All X-linked recessive traits are
fully evident in males because they only have one copy of the X
chromosome, thus do not have a normal copy of the gene to compensate for
the mutant copy. For that same reason, women are rarely affected by
X-linked recessive diseases, however they are affected when they have
two copies of the mutant allele. Because the gene is on the X chromosome
there is no father to son transmission, but there is father to daughter
and mother to daughter and son transmission. If a man is affected with
an X-linked recessive condition, all his daughter will inherit one copy
of the mutant allele from him. |
X-linked Recessive Disorders:
• Duchenne muscular dystrophy
• hemophilia A
• X-linked severe combined immune disorder (SCID)
• some forms of congenital deafness
|
X-linked Dominant
Because the gene is located on the X
chromosome, there is no transmission from father to son, but there can
be transmission from father to daughter (all daughters of an affected
male will be affected since the father has only one X chromosome to
transmit). Children of an affected woman have a 50% chance of inheriting
the X chromosome with the mutant allele. X-linked dominant disorders are
clinically manifest when only one copy of the mutant allele is present. |
X-linked Dominant
Disorders
• some forms of retinitis pigmentosa
• Chondrodysplasia Punctata
• hypophosphatemic rickets
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