Mechanisme of Inheritance

8/22/2019 Mechanisme of Inheritance

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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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Mechanisme of Inheritance

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