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Genes and Genetic Defects. In which we examine normal genetic transmission and genetic defects. Part 1: Genes. Genes and DNA. A gene is the unit of heredity. A gene contains hereditary information encoded in the form of DNA, which is located at a specific position on a chromosome.

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Genes and Genetic Defects

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Genes and genetic defects l.jpg

Genes and Genetic Defects

In which we examine normal genetic transmission and genetic defects.


Part 1 genes l.jpg

Part 1: Genes


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Genes and DNA

  • A gene is the unit of heredity.

  • A gene contains hereditary information encoded in the form of DNA, which is located at a specific position on a chromosome.

  • Genes determine many aspects of anatomy and physiology by controlling the production of proteins.


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Genotype & Phenotype

  • Allele: Any of the alternate forms of a given gene. A gene is a code that produces a protein. The alternate forms of a gene produce slightly different proteins.

    The alleles for blood type are A, B, and O.

  • Genotype: The pair of alleles for a given gene.

    The six possible genotypes for blood type are:

    ABBB

    AABO

    AOAO


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Genotype & Phenotype

  • Phenotype: The physical trait produced by a genotype.

    The genotypes for blood type and their phenotypes are:

    genotypephenotype

    ABAB

    AA A

    AO A

    BB B

    BO B

    OO O


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Dominant & Recessive Alleles

  • Dominant: produces the same phenotype whether it is paired with same or different allele

  • Recessive: produces the same phenotype only if it is paired with the same allele

  • Codominant: when neither of two alleles dominate the other

    For blood type:A is dominant over O

    B is dominant over O

    therefore, O is recessive

    A and B are codominant


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Why do some alleles dominate over others?

  • A gene contains code for producing a protein.

  • The different alleles for a gene produce slightly different versions or different amounts of that protein.

  • Note: some alleles are code that fail to produce the protein. (e.g., hemophilia)

  • The protein produced by one allele may be stronger than the protein produced by other alleles. The allele producing the stronger protein is dominant.


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Example of proteins

  • The exact color of the human eye is determined by the amount of a single pigment called melanin that is present in the iris of the eye.

  • Melanin is a dark brown pigment that is deposited on the front surface of the iris.

  • If a lot of melanin is present, the eye will appear brown or even black.

  • If very little melanin is present the iris appears blue.

  • Intermediate amounts of melanin produces gray, green, hazel or varying shades of brown.


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Some Examples of Dominance


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More Examples


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Heterozygous & Homozygous

  • Rh factor: a protein found on the surface of red blood cells, which produces an antigenic reaction

  • There are 2 alleles for the Rh factor: + (produces the protein) and - (does not produce protein)

  • + is dominant over -

    Genotype:Phenotype:

    + +Homozygous DominantRh+

    - -Homozygous RecessiveRh-

    + -HeterozygousRh+


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Part 2: Genetic Defects


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What is a Genetic Disorder?

Genetic disorder: a disorder that is caused by a a faulty allele that programs the body to be built in a maladaptive way.

  • Hemophilia: normal allele produces proteins that cause the blood to clot; faulty allele does not produce the blood-clotting proteins.

  • Sickle Cell Anemia: the faulty allele causes a mutation of a blood protein (beta globin), which in turn causes red blood cells to be sickle-shaped, stiff, and sticky, which cause severe organ damage.


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Types of Genetic Disorders

Genetic disorders carried on the autosomes

Dominant genetic disorder: A genetic disorder in which the faulty allele is dominant. A person with just one faulty allele will have the disorder.

  • Recessive genetic disorder: A genetic disorder in which the faulty allele is recessive. Only people with two faulty alleles will have the disorder.

    Genetic disorders carried on the X chromosome

  • Sex-linked genetic disorder: The faulty allele is recessive. A female must have two faulty alleles in order to have the disorder; a male will have the disorder with only one faulty allele.


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Dominant Genetic Disorders

  • Caused by a gene on one of the autosomes, the faulty allele is dominant.

  • Anyone who has the faulty allele will get the disorder.

    Genotype:Phenotype:

    D DHomozygous Dominanthas disorder

    N NHomozygous Recessivenormal

    N DHeterozygoushas disorder

    N = normal allele, D = faulty allele


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Examples of Dominant Disorders

  • Huntington’s Chorea: faulty allele produces an abnormal protein that in middle age begins to destroy brain cells that control movement. (1 in 10,000 births)

  • Adult Polycystic Kidney Disease: faulty allele results in growth of fluid-filled cysts in kidneys, which replace healthy tissue and eventually cause kidney failure and death.

  • Neurofibromatosis: faulty allele results in growth of benign tumors on nerve cells throughout body. (1 in 2500 births)


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Example 1: Parents NN & NN


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Example 2: Parents NN & ND


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Example 3: Parents ND & ND


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Recessive Genetic Disorders

  • Caused by a gene on one of the autosomes, the faulty allele is recessive.

  • Anyone who has the faulty allele will get the disorder.

  • Person with one faulty allele will not have disorder, but will be a carrier.

    Genotype:Phenotype:

    N NHomozygous Dominantnormal

    D DHomozygous Recessivehas disorder

    N DHeterozygouscarrier

    N = normal allele, D = faulty allele


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Examples of Recessive Disorders

  • Sickle Cell Anemia: found in populations descended from Africa. Incidence among African-Americans, 1 in 375 births.

  • Tay-Sachs Disease: faulty allele does not produce protein needed to break down gangliosides in nerve cells, which accumulate and destroy the nerve. Incidence among Ashkenazi Jews, 1 in 27 are carriers. General population, 1 in 250 are carriers.


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Another Example

  • Phenylketonuria: faulty allele that produces mutated enzyme, which in turn fails to metabolize phenylalanine, which accumulates in the brain, causing retardation and epilepsy.

  • PKU test: blood test given to infants shortly after birth to determine if there is abnormally high level of phenylalanine in the blood.

  • Incidence: U.S. Caucasians, 1 in 8,000. U.S. Blacks: 1 in 50,000.Irish, 1 in 4500. Japanese, 1 in 143,000. Countries with low immigration from Celtic countries have low rates in Phenylketonuria.


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Example 1: Parents NN & NN


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Example 2: Parents NN & ND


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Example 3: Parents ND & ND


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Example 4: NN & DD


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Sex-Linked Genetic Disorders

  • Caused by a gene on the X chromosome, the faulty allele is recessive.

  • Males who have the faulty allele in their X chromosome will have the disorder

  • Females who have the faulty allele on one X chromosome will be carriers; those who have the faulty allele on both X chromosomes will have the disorder.


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How do we know the faulty allele is on the X chromosome?

Traits on X chromosome

Traits on Y chromosome


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Besides male sex determining gene, only other trait found on Y chromosome

Come to class to see slide!


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Sex Differences in Sex-Linked Genetic Disorders

  • Y chromosome only has genes for male sex differentiation (and hairy ears).

  • Therefore, for all traits on X chromosome, males have only one gene. The single allele of a gene determine phenotype for males:

    Genotype:Phenotype:

    XNYnormal

    XDYdisorder


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Sex Difference, continued

  • Females have two X chromosomes. Thus, for all traits on X, females have two genes.

  • Disorders on X are recessive. Therefore, to have disorder, females must have two faulty alleles. A female with one faulty and one normal allele is a carrier.

    Genotype:Phenotype:

    XN XN normal

    XN XD carrier

    XD XD disorder


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Examples of Sex-Linked Disorders

  • Hemophilia: faulty allele is mutation of blood-clotting gene. Persons with disorder have blood that clots very slow or does not clot at all. Incidence: 1 in 4000 males, 1 in 16,000,000 females.

  • Duchenne Muscular Dystrophy: faulty allele fails to produce muscle protein, dystrophin, the lack of which causes muscle cells to die. 70% of cases are inherited, 30% are spontaneous mutations. Incidence: 1 in 3500 males, 1 in 12,500,000 females (theoretical).


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Example 1: XNXN & XNY


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Example 2: XNXD & XNY


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Example 3: XNXN & XDY


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Example 4: XNXD & XDY


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Chromosomal Abnormalities

Genetic Defects

Number of units

too many or too few chromosomes

correct number of genes

Normalcy of units

chromosomes are normal

faulty allele is abnormal

Number of symptoms directly caused

many symptoms

one direct symptom (may be many indirect)

Chromosomal Abnormalities vs. Genetic Defects


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