Inheritance patterns
Mendel was developed 4 hypotheses:
1. There are alternative forms of genes (discrete portions or sequences of the DNA molecule in
chromosomes), the units that determine heritable traits.We now call alternative forms of genes
alleles.
2. For each inherited trait, an organism has 2 genes (alleles), one from each parent. These genes
may be both the same allele, or they may be different alleles
3. A sperm or an egg carries only one allele for each inherited trait, because allele pairs separate
(segregate) from each other during the formation of the gametes
4. When the 2 genes of a pair are different alleles, one is fully expressed and the other has no
noticeable effect on the organism's appearance
These are called the dominant allele and recessive allele respectively
Our genes, located on our chromosomes in our cells, provide the information for the growth,
development and function of our bodies. When the information in a gene is changed, there is a
different message sent to the cells. A change to the genetic code that causes the gene not to work
properly is called a mutation: the gene is described as faulty.
Inheritance patterns in families of conditions due to faulty(mutation) genes
The inheritance pattern depends on whether the
• Faulty gene is located on one of the chromosomes numbered 1-22 called an autosome or on the
X chromosome which is one of the sex chromosomes.
• Change to the genetic code that makes the gene faulty is ‘recessive’ or ‘dominant’ .
The most common patterns of inheritance:
• Autosomal recessive
• Autosomal dominant
• X-linked recessive
• X-linked dominant
a) Autosomal Dominant Inheritance
Genes are the blueprints for making the substances, called proteins, our bodies need to develop
and work properly. Most genes come in pairs, one of which is inherited from the mother and the
other from the father. A mutation is a change in a gene that prevents it from working properly.
Mutations in genes are inherited from our biological parents in specific ways. One of the basic
patterns of inheritance of mutations is called autosomal dominant inheritance.
Autosomal dominant inheritance means that the gene carrying a mutation is located on one of the
autosomes (chromosome pairs 1 through 22). This means that males and females are equally
likely to inherit the mutation.
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"Dominant" means that having a mutation in just one of the two copies of a particular gene is
all it takes for a person to have a trait, such as an increased risk of developing cancer. When a
parent has a dominant gene mutation, there is a 50 percent chance that any child he/she has will
also inherit the mutation.
The other 50 percent have not inherited the mutation. These four combinations are possible every
time a pregnancy occurs between these two individuals. The gender of the children (whether
they are sons or daughters) does not matter. The chance is 50/50 for each pregnancy.
An important characteristic of dominant gene mutations is that they can have variable
expression. This means that some people have milder or more severe symptoms than others. In
addition, which systems of the body the mutation affects can vary as can the age at which the
disease starts, even in the same family. In some cases, they can have reduced penetrance. This
means that sometimes a person can have a dominant mutation but not show any signs of
disease. The concept of reduced penetrance is particularly important in the case of autosomal
dominant cancer susceptibility genes. If a person has inherited a cancer susceptibility gene, it
does not mean he or she will automatically develop cancer. It simply means that the person has
inherited a mutation in a gene that gives him or her a higher chance to develop cancer than the
general population - someone without the mutation.
Most families know that there is a dominant trait or disorder in their family, because it is passed
from parent to child and can be seen in many generations. When a cancer susceptibility gene
mutation is inherited in an autosomal dominant manner, it means that the mutation can be
inherited from the mother, or the father, who may or may not have ever had any type of cancer.
However, with autosomal dominant inheritance, if a parent does not have the gene mutation
associated with cancer risk in the family, he or she cannot pass it on to his or her children.
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b) Autosomal Recessive Inheritance
Genes are the blueprints for making the substances, called proteins, our bodies need to develop
and work properly. Most genes come in pairs, one of which is inherited from the mother and the
other from the father. A mutation is a change in a gene that prevents it from working properly.
Mutations in genes are inherited from our biological parents in specific ways. One of the basic
patterns of inheritance of our genes is called autosomal recessive inheritance.
Autosomal – this means the disease affects both males and females equally
Recessive – this means both parents must be a carrier for a child to be at risk although the
parents themselves (the carriers) are not affected by the disease
Inheritance – this means the process of genetic transmission of characteristics (in this case the
gene which causes Tay-Sachs) from parents to offspring.
Autosomal recessive inheritance means that the gene carrying the mutation is located on one of
the autosomes (chromosome pairs 1 through 22). This means that males and females are equally
affected. "Recessive" means that both copies of the gene must have a mutation in order for a
person to have the trait. One copy of the mutation is inherited from the mother, and one from the
father. A person who has only one recessive gene mutation is said to be a "carrier" for the trait
or disease, but he or she does not have any health problems from carrying this one mutation.
Most people do not know they carry a recessive gene mutation for a disease until they have a
child with the disease. Once parents have had a child with a recessive disease, there is a one out
of four, or 25 percent chance, with each subsequent pregnancy, for another child to be born with
the same disorder. This means that there is a three out of four, or 75 percent chance, for another
child to not have the disease.
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The birth of a child with a recessive condition is often a total surprise to a family, since in most
cases, there is no previous family history of the condition. Many autosomal recessive conditions
occur this way. It is estimated that all people carry some recessive genes that cause genetic
diseases or conditions. It is only when a person has a child with a partner that carries the same
recessive gene mutation, that there is a chance of having a child with a recessive disorder.
Mutations in certain genes have occurred over time in different parts of the world.
a) X-linked Recessive: Red-Green Color Blindness, Hemophilia A
Genes are inherited from our biological parents in specific ways. One of the basic patterns of
inheritance of our genes is called X-linked recessive inheritance.
In genetics, the term "recessive gene" refers to an allele that causes a phenotype (visible or
detectable characteristic) that is only seen in a homozygous genotype (an organism that has two
copies of the same allele) and never in a heterozygous genotype. Every person has two copies
of every gene on autosomal chromosomes, one from mother and one from father. If a genetic
trait is recessive, a person needs to inherit two copies of the gene for the trait to be expressed.
Thus, both parents have to be carriers of a recessive trait in order for a child to express that trait.
X-linked inheritance means that the gene causing the trait or the disorder is located on the X
chromosome. Remember, females have two X chromosomes, while males have one X and one
Y. Genes on the X chromosome can be recessive or dominant, and their expression in females
and males is not the same because the genes on the Y chromosome do not exactly pair up with
the genes on the X chromosome.
X-linked recessive genes are expressed in females only if there are two copies of the gene (one
on each X chromosome). However, for males, there only needs to be one copy of an X-linked
recessive gene in order for the trait or disorder to be expressed. For example, a woman can
carry a recessive gene on one of the X chromosomes unknowingly, and pass it on to a son,
who will express the trait:
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There is a 50 percent chance that daughters carry the gene and can pass it to the next generation.
There is a 50 percent chance that a daughter will not carry the gene and, therefore, cannot pass
it on. There is a 50 percent chance that sons do not have the gene and will be healthy. However,
there is a 50 percent chance that a son will have inherited the gene and will express the trait or
disorder.
Examples of X-linked recessive conditions include red-green color blindness and hemophilia A:

Red-green color blindness. Red-green color blindness simply means that a person cannot
distinguish shades of red and green (usually blue-green). Their visual acuity (ability to see) is
normal. There are no serious complications; however, affected individuals may not be considered
for certain occupations involving transportation or the Armed Forces where color recognition is
required. Males are affected 16 times more often than females, because the gene is located on the
X chromosome.
Hemophilia A. Hemophilia A is a disorder where the blood cannot clot properly due to a
deficiency of a clotting factor called Factor VIII. This results in abnormally heavy bleeding that
will not stop, even from a small cut. People with hemophilia A bruise easily and can have
internal bleeding into their joints and muscles. The occurrence of hemophilia A (Factor VIII
deficiency) and hemophilia B (Factor IX deficiency) combined is one in 10,000 live male births,
with hemophilia A accounting for 80 percent of all cases. Treatment is available by infusion of
Factor VIII (blood transfusion). Female carriers of the gene may show some mild signs of Factor
VIII deficiency such as bruising easily or taking longer than usual to stop bleeding when cut.
However, not all female carriers present these symptoms. One third of all cases are thought to be
new mutations in the family (not inherited from the mother). Another examples such as muscular
dystrophy and fragile X syndrome.
b) X-linked dominant inheritance

Sometimes referred to as X-linked dominance, is a mode of genetic inheritance by which a
dominant gene is carried on the X chromosome. As an inheritance pattern, it is less common
than the X-linked recessive type. In medicine, X-linked dominant inheritance indicates that a
gene responsible for a genetic disorder is located on the X chromosome, and only one copy of
the allele is sufficient to cause the disorder when inherited from a parent who has the disorder.

X-linked dominant traits do not necessarily affect males more than females (unlike X-linked
recessive traits). The exact pattern of inheritance varies, depending on whether the father or the
mother has the trait of interest. All daughters of an affected father will also be affected but none
of his sons will be affected (unless the mother is also affected). In addition, the mother of an
affected son is also affected (but not necessarily the other way round).
Males are normally hemizygous for the X chromosome, having only one copy. As a result, Xlinked dominant disorders usually show higher expressivity in males than females.
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a) When the mother is an affected X-linked dominant faulty gene carrier, in every
pregnancy, there is
 1 chance in 2, or 50% chance, that both her sons and daughters will inherit the faulty
gene copy from her and be affected by the condition.
 1 chance in 2, or 50% that her children (both sons and daughters) will inherit the working
copy of the gene from her (‘d’) and not be affected by the condition. This pattern of
inheritance is superficially similar to that of autosomal dominant inheritance
b) When the father is affected by a condition due to an X-linked dominant faulty gene
the unaffected mother will only give working copies of the gene to her children but the
father will pass his X chromosome to his daughters.
This means that in every pregnancy:
None of his sons can inherit the faulty gene since the son only inherits from his father the Y
chromosome that does not have the faulty gene copy. They will inherit the working copy from
their mother. None of his sons will have the condition.
All of his daughters will inherit from their mother the working gene copy and the faulty gene
copy from him. All of his daughters will have the condition
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Mitochondrial Inheritance: Leber's Optic Atrophy
The normal 46 chromosomes in our body are contained in the center of our cells, which is called
the nucleus. Mitochondria are structures in the cytoplasm of the cell, located outside of the
nucleus in the cytoplasm, that make energy for the cell. Mitochondria also contain their own
genes that are separate from the ones in the nucleus.
Unlike nuclear genes, which are inherited from both parents, mitochondrial genes are inherited
only from the mother. If there is a mutation in a mitochondrial gene, it is passed from a mother to
all of her children; sons will not pass it on, but daughters will pass it on to all of their children,
and so on. The first human disease that was associated with a mutation in mitochondrial DNA is
called Leber's Hereditary Optic Neuropathy, or LHON.
What is Leber's hereditary optic neuropathy (LHON)?
LHON causes a painless loss of central vision due to the death of optic nerve cells. It leads to
blindness in young adult life, typically between 12 and 30 years of age, and both eyes are
affected at the same time. Males are approximately four times more likely to be affected than
females. Males will not pass the gene to any of their children, but females with the mutation will
pass it to all of their children, regardless of whether they are sons or daughters. LHON is also a
multifactorial disease; both alcohol and tobacco use are important environmental factors
associated with increased risk of blindness in carriers of the mutation.
Some of the estimated 20,000 genes in the human genome are located in small compartments in
the cell called the mitochondria, rather than on chromosomes in the cell’s nucleus. Some cells
contain many hundreds of mitochondria
• The genes found within the mitochondria contain the information that codes for the production
of many of the important enzymes that drive the biochemical reactions to produce the body’s
source of energy: a chemical called ATP (adenosine
triphosphate). The cells in the body, especially in organs such as the brain, heart, muscle,
kidneys and liver, cannot function normally unless they are receiving a constant supply of energy
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• Faulty mitochondrial genes can result in absence of these enzymes, or enzymes that are
impaired and do not work properly. This leads to a reduction in the supply of ATP, and may
result in problems with the body’s functions
• The pattern of inheritance of conditions due to faulty mitochondrial genes is often called
maternal inheritance. This is because a child inherits the great majority of their mitochondria
from their mother through the egg
• Usually a mother will have a mixture of mitochondria containing the working gene copy and
others containing the faulty gene.
For a condition to develop, the number of mitochondria with the faulty gene must be above a
critical level
Another place in the cell where DNA is found is in very small compartments called
mitochondria (plural) or mitochondrion (singular), that are found randomly scattered in the
cytoplasm of the cell outside the nucleus. The cells in the body, especially in organs such as the
brain, heart, muscle, kidneys and liver, cannot function normally unless they are receiving a
constant supply of energy. The cell’s energy source is a chemical called ATP (adenosine
triphosphate) that is used by the body to drive the various reactions essential for the body to
function, grow and develop. A number of biochemical reactions that occur in an ordered
sequence within the mitochondria are responsible for this process of ATP production. These
reactions are under the control of special proteins called enzymes. The genes found within the
mitochondria contain the information that codes for the production of some of these important
enzymes.
Inheritance of faulty mitochondrial genes (maternal inheritance). The number of mitochondria in
every cell of a person’s body varies from a few, to hundreds.
• All of these mitochondria, and therefore the DNA within the mitochondria, descend from the
small number of mitochondria present in the original egg cell at the time of that person’s
conception
• The sperm contributes very few mitochondria to the baby An individual’s mitochondria are
generally only inherited from his or her mother. A change (mutation) in one of the mitochondrial
genes that makes it faulty, can therefore be passed by the mother to a child in her egg cells.
• This pattern of inheritance is therefore often referred to as maternal inheritance.
The egg cell contains many mitochondria, each having on average one to several copies of the
mitochondrial genes. If a particular gene in every mitochondrion in an egg cell is faulty and is
therefore sending the incorrect instructions, the disruption to energy production would be so
severe that the early embryo would probably not survive. The fact that a person survives to birth
and is affected with a mitochondrial condition means that they must have inherited two types of
mitochondria from his or her mother: some containing the working copy of the gene, and some
containing the faulty gene.
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