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Mechanism of Inheritance
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Genetics is the science of the way traits are
passed from parent to offspring
Continuity of the species depends upon the
genetic code being passed from parent to
offspring
Genetic inheritance begins at the time of
conception.
You inherited 23 chromosomes from your
mother and 23 from your father.
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Together they form 22 pairs of autosomal
chromosomes and a pair of sex chromosomes
Homologous chromosomes have the same genes in
the same positions, but may have different alleles(varieties) of those genes
There can be many alleles of a gene within a
population, but an individual within that
population only has two copies, and can be
homozygous (both copies the same) or heterozygous
(the two copies are different) for any given gene
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DNA
DNA or deoxyribonucleic acid,is the
macromolecule that stores the information
necessary to build structural and functional
cellular component.
It also provides the basis for inheritance when
DNA is passed from parent to offspring.
The union of these concepts about DNA
allows us to devise a working definition of a
gene
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Genes
Genes are the blueprint for our bodies.
Almost every cell in the human body contains a
copy of this blueprint, mostly stored inside a special
sac within the cell called the nucleus.
A gene is a segment of DNA that codes for the
synthesis of a protein and acts as a unit of
inheritance that can be transmitted from
generation to generation The gene contain the information used by our cells
to control our growth, development and functioning
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Genes are part of chromosomes. The 23.000 or so genes that make up each person
come in pairs.
A sperm and an egg each contain one copy of everygene needed to make up an individual ( one set of
23 chromosomes each.
When the sperm fertilises the egg, two copies of each
gene are present ( 46 chromosomes) and new lifecan begin.
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The mothers egg contribute an X, while the fathers
sperm provides either an X or a Y.
An XX pairing means the chils is female, while an XY
pairing means the child is male.
Genotype is the actual pair of genes that a person
has for a trait of interest
A Phenotype is the organisms physical appearanceas it relates to a certain trait
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Inheritance Pattern
Autosomal dominant
- Only one mutated copy of the gene is
needed for a person to be affected by an
autosomal dominant disorder
- Each affected person usually has one
affected parent
- There is a 50% chance that a child will
inherit the mutated gene
- Ex : Huntingtons disease
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Autosomal recessive
- Two copies of the gene must be mutated for aperson to be affected by an autosomal recessive
disorder
- An affected person usually has unaffected parents
who each carry a single copy of the mutated gene (
carries)
- Two unaffected people who each carry one copy
of the mutated gene have a 25% chance with eachpregnancy of having a child affected by the
disorder.
- Example: Sickle cell anemia, cystic fibrosis
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X-linked dominant
- X-linked dominant disorders are caused by mutations ingenes on the X chromosome
- Females are more frequently affected than males, and the
chance of passing on an X-linked dominant disorder differs
between men and women.- The sons of a man with an X-linked dominant disorder will
not be affected, and his daughters will all inherit the
condition.
- A woman with an X-linked dominant disorder has a 50%chance of having an affected daughter or son with each
pregnancy
- ex: Aicardi syndrome
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X-linked recessive- X-linked recessive disorders are also caused
by mutations in genes on the X chromosome
- Males are more frequently affected thanfemales, and the chance of passing on the
disorder differs between men and women.
- The sons of a man with an X-linkedrecessive disorder will not be affected, and his
daughters will carry one copy of the mutated
gene
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- With each pregnancy, a woman who carries
an X-linked recessive disorder has a 50%
chance of having sons who are affected and a
50% chance of having daughters who carry
one copy of the mutated gene
ex : hemophilia A
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Y-linked
- Y-linked disorders are caused by mutations
on the Y chromosome.
- Only males can get them, and all of the sons
of an affected father are affected.
- Since the Y chromosome is very small, Y-linked disorders only cause infertility
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Mendels Laws
Principle of segregation : two members of a gene
pair segregate from each other in the formation of
gametes; half the gametes carry one allete, and the
other half carry the other alleleWhat it means : each gene has two copies and a
parent will give only one copy to a child. The other
parent will give another copy, and thus the chils will
receive two copies (alleles) one from each parent.
Each child will literally be half-mom and half-dad
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Principle of independent assortment: Genes
for different traits assort independently of one
another in gamete production
What is means: different genes are inherited
separately. For example, the gene which
codes for eye color is inherited separately
from the gene which codes for nose shape
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Mendelian Concepts
Dominant : only one allele of the gene
necessary to express the trait
Recessive : both alleles of a gene must be
identical to express the trait
Heterozygous : alleles of a particular gene are
non-identical
Homozygous : alleles of a particular gene are
identical
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Applying Mendelian Concepts
A1/A1 A1/A2
A2/A1 A2/A2
Allele 1 Allele 2
Parent 2
Allele 1
Allele 2
Parent 1
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AA AA
AA AA
Homozygous dominat +
Homozygous dominant
A A
A
A
AA AA
Aa Aa
AA
A
a
Homozygous dominat +Heterozygous
Homozygous dominat +
Homozygous resesive
Aa Aa
Aa Aa
aa
A
A
Heterozygous + Heterozygous
Aa
AA Aa
aa
aA
A
a
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Example
Dan and Kim are going to have a baby. Kim
has dimples in her cheeks (a dominant trait),
while Dan does not.
What are the chances Dan and Kims baby
will have dimples?
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Dan has the recessive trait (no dimples)
- he must have two recessive alleles
- dans genotype can be represented as dd Kim has the dominat trait (dimples)
- but kim could be homozygous dominant or
heterozygous dominat- kims genotype can either DD or Dd
- which one is it?
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More information
What if you knew something about Kims
parents?
How could that help?
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Kims Parents
Kims father has dimples in both cheeks, while
her mother does not
Her mother must have the recessive trait and
therefore has to have the genotype dd
kims father has the dominant trait, but we
dont know if he a homozygote or
heterozygote. He could be DD or Dd just
like Kim !
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But we still know what Kims genotype must be.
Why?
Kim must have a recessive allele (d) since that is all
she could have inherited from her mother Since Kim has dimples, we know she inherited
dominant allele (D) from her father
- it doesnt matter if Kims father is DD or Dd;whichever it is, he passed on a D to his daughter
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What are the chances Kim and Dans baby
will have dimples?
D d
d
d
Kim
Dan
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50% chance the baby will have the genotypeDd and have dimples
50% chance the baby have genotype dd
and not have dimples
KimdD
Dan
d
d
Dd dd
Dd dd
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