Welcome to the patterns of inheritance
GEM produced by the division of genomic medicine at UC Davis. This video will
walk you through the basic laws of inheritance or how traits are passed on
in a family. A person has two copies of most genes, one copy from their mom and
one copy from their dad. Understanding how changes and genes can affect the
patient is an important part of clinical Diagnostics. Diseases caused by mutations
in a single gene are usually inherited in a simple pattern depending
on a few
factors such as location of the genes, most single gene disorders are rare but
all combined single gene disorders affect millions of people in the United
States; in fact, about 1 in 10 Americans are affected by a rare disease. There are
several basic modes of inheritance that exist for single gene disorders they are
autosomal dominant, autosomal recessive, X-linked and mitochondrial. Let's talk
about these one by one; if the condition associated with the gene is observed
when an individ
ual has only one copy of a mutation in the gene then the allele
is dominant. An autosomal gene refers to a gene on any of the chromosomes 1
through 22 the autosomes ,not on either of the sex chromosomes. Autosomal
dominant conditions are often seen in every generation of a family history as
shown in the pedigree here, conditions that follow an autosomal dominant
pattern of inheritance include Huntington's chorea
Marfan's, achondroplasia and familial hypercholesterolemia. If a person must
inherit
two copies of a particular variant of a gene in order for the
Associated condition to appear, the trait is said to be recessive; as the name
would suggest an autosomal recessive condition is a condition that follows a
recessive pattern of inheritance and is associated with a gene on an autosome.
Autosomal recessive conditions are usually not seen in every generation of
a family history as shown in the pedigree here, conditions that follow an
autosomal recessive pattern of inheritance include Ta
y-Sachs disease,
sickle cell anemia, cystic fibrosis and PKU.
As the name implies,X-linked conditions are conditions
associated with genes on the X chromosome, x-linked conditions
disproportionately affect males this is because males have One X chromosome and
one Y chromosome so males are hemizygous for genes on the X chromosome. A female
often needs to inherit two copies of the mutation in order for the condition to
be expressed however, this is not always the case,
note that fathers cannot pas
s any x-linked traits to their sons. Conditions
that follow an x-linked pattern of inheritance include fragile X,
haemophilia A, fabry disease and Duchenne muscular dystrophy. Another type
of inheritance pattern that you might encounter on the job is mitochondrial
inheritance. Mitochondrial DNA is only passed through the maternal line, a newly
formed embryo receives all of its mitochondria from the mother through the
egg cell, so mothers passe their mitochondrial DNA to all of her children.
Men
never pass any mitochondrial information to any of their children.
Note that mitochondrial conditions can affect both males and females but can
only be passed on by females. Conditions that follow a mitochondrial inheritance
pattern include Leber's hereditary optic neuropathy and Kearns-Sayre syndrome.
It is important to keep in mind however that not all genetic conditions follow
one of these four patterns and for some conditions family members with the same
mutation may not have the same sympt
oms furthermore, sometimes individuals with
different mutations can have the same or similar characteristics, this is in part
because gene expression is influenced by both genes and the environment. To recap,
there are four main patterns of inheritance for single gene conditions,
of course many common medical problems do not have a single genetic cause
rather they come about as a result of the effects of multiple genes in
combination with lifestyle and environmental factors; traits which come
ab
out as a result of the effect of multiple genes or gene environment
interactions are known as complex or multifactorial disorders. Although
complex disorders often run in families, they do not have a clear pattern of
inheritance so it can be difficult to determine a person's risk of inheritance.
Conditions that are complex or multifactorial include heart disease,
diabetes and obesity. Knowing your patient's family history combined
with an understanding of genetic inheritance patterns is an inval
uable
tool for clinical diagnostics. This video covered some of the most common
inheritance patterns for single gene conditions as well as mitochondrial
disorders; however, many health conditions are complex or multifactorial and do not
follow one of these four patterns. If you would like help working through a
patient's family history to determine whether risk of inheritance can be
predicted or to calculate probability of disease inheritance the Division Of
Genomic Medicine is here to help, our
clinic is located in the MIND Institute
on the medical campus in Sacramento. You can refer patients to our genomic
medicine clinic, visit our website or call us with questions or to speak with
a genetic counselor.
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