Other laws of inheritance
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Other Laws of Inheritance. Mendelian Genetics : characteristics controlled by dominant and recessive paired alleles Many traits follow the patterns outlined by Mendel’s Laws, many do not. Incomplete Dominance. Offspring heterozygous for a trait have an intermediate appearance .

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Other Laws of Inheritance

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Other Laws of Inheritance

  • Mendelian Genetics: characteristics controlled by dominant and recessive paired alleles

    • Many traits follow the patterns outlined by Mendel’s Laws, many do not.


Incomplete Dominance

  • Offspring heterozygous for a trait have an intermediate appearance.

  • Neither allele is completely dominant over the other

  • Ex: Snapdragon flowers

    • Homozygous Red x Homozygous White

    • Results in all PINK flowers


Showing a Cross for Incomplete Dominance

  • Use a capital letter for each allele since each influence trait equally

  • RR (homozygous red) x WW (homozygous white)

  • What is phenotype ratio of offspring?

  • RW x RW (heterozygous pink)

  • What is phenotype ratio of offspring?

    • How is this ratio different from the offspring

      of Mendel’s Monohybrid cross? (Tt xTt)


  • Codominance

    • If individual is heterozygous, both alleles for gene are expressed.

    • Ex: Chickens

      • Black feathers (BB) x White Feathers (WW)

      • Offspring (BW) are checkered and have both black and white feathers


    • Try a codominant cross:

    • Checkered (BW) x checkered (BW)

    • Checkered (BW) x white (WW)

    • Checkered (BW) x black (BB)


    • Ex: Roan Cattle

      • Offspring of pure red and pure white cows

      • Have both red and white hair


    Multiple Alleles

    • A gene that has more than two alleles.

    • Each individual can only have two alleles

    • Ex: Human Blood Types


    Possible Blood Types:

    A, B, AB, O

    • There are 3 alleles for blood type (A, B, O)

    • A and B are codominant alleles (IA, IB)

    • O is a recessive allele (i)


    Six genotypes are possible for blood:

    • IA IA

    • IA IB

    • IB IB

    • IA i

    • IB i

    • i i

    • What blood types would they have?


    • Ex: Type A (homozygous) x Type B

    • What are the blood types of the offspring?


    • Try a Cross:

    • What blood types do the offspring have?

      • IB IB xIA IA

      • IA i x i i

      • IA IB x i i

  • If a Type A mother and a Type B Father produce a type O child, what are the parent’s genotypes?


  • Environmental Influences on Gene Expression

    • Ex: temperature, nutrition, light etc.

  • Ex: Coat color in Himalayan rabbits

    • Black fur present only on areas of body that are colder. (ear, nose, feet and tail)


    • Ice Pack placed on fur = fur turns black

    • If rabbit kept in warm environment, all fur is white.


    Genetic Research and Testing

    • Genetic Research:

      • To study genes for different traits it is best to use an organism that grows and reproduces quickly an produces many offspring.

      • Can do controlled breeding experiments

      • Can do forced “inbreeding”

        • Ex: Drosophila melanogaster (fruit fly)


    Genetic Tests

    • Test Cross:

      • To determine if an individual is homo or heterozygous for a trait.

      • Breed to a homozygous recessive.

      • Ex:

        • Drosophila: red eyes dominant over white.

        • To see if red eyed fly is hetero/homo, breed to white eyed fly


    Sex Determination

    • Humans have 23 pairs of chromosomes

    • 22 pairs are autosomes

    • 1 pair are sex chromosomes

    These pictures are called karyotypes!

    I’m a BOY!

    I’m a GIRL!


    Sex Determination

    • XX = female, XY = male

    • Males can give Y chromosome or X chromosome

    • Females can only contribute X

    • There is always a 50% chance of being boy or girl


    Sex Linked Traits

    • If allele is found on an X or a Y chromosome it is inherited differently in males and females.

    • Ex:

      • Calico Cats

      • Color Blindness

      • Hemophilia

      • Duchenne Muscular Dystrophy


    Calico Cats: (Codominant X Linked Trait)

    • X chromosome has gene for black or orange fur (codominant)

    • Only females can be “Calico” XBlackXOrange

    • Males can only be Black or Orange

      they have only one X chromosome and one Y

    I’m always a girl!


    Colorblindness: (Recessive X linked trait)

    • Certain colors cannot be distinguished from others (usually red or green)

    • Most often seen males

    • Females are more likely to be “carriers”

    • Father cannot pass gene to son, only mother


    Hemophilia: (Recessive X-linked trait)

    • Blood has trouble clotting

  • XHXhCarrier female

  • XHYNormal Male

  • XhYAffected Male

  • XhXhAffected Female

  • XHXHNormal Female


    • Examples of Crosses for Hemophilia


    • Queen Victoria of England passed this gene onto some of the royal family offspring

    She must have been a “carrier”

    Queen Victoria


    Chromosomal Disorders

    • Due to the presence of absence of an entire chromosome or part of a chromosome.

    • Can be determined by doing a Karyotype from blood or amniotic fluid


    • Amniocentesis:

      • Prenatal test

      • Grow cells from amniotic fluid around fetus.

      • Helps to rule out major chromosomal abnormalities


    Nondisjuction

    • Nondisjunction:

      • homologous chromosomes fail to separate properly during meiosis.

      • Results in egg or sperm with one or more extra chromosomes.


    Nondisjunction

    Normal Meiosis


    What is Wrong?


    Nondisjunction of Sex Chromosomes

    • Klinefelter’s Syndrome (XXY)

      • 2 X’s from mother, Y from father

      • Often tall, long arms and legs

      • May have some gynomastia and small testicles


    • Turner’s Syndrome (XO)

      • Offspring has only one X (always female)

      • Usually sterile, short, thick neck


    Nondisjunction of Autosomes

    • Trisomy 21 (Down’s Syndrome) (47, +21)

      • Most commonly seen chromosomal disorder resulting in live birth

      • Extra Chromosome 21


    • Other trisomys can happen as well

    • Babies may make it to term but usually don’t live very long (nonviable)

      • Ex: Trisomy 18, Trisomy 13


    Gene Linkage

    • When alleles for different traits are located on the same chromosome

    • They tend get inherited together.

      • They are “linked”

      • They do no “assort independently”


    • Ex: Red hair and freckles


    Crossing Over and Linked Genes

    Crossing Over: homologous chromosomes

    line up during synapsis and exchange

    material.

    • Increases variation in offspring.


    Sometimes linked

    genes will separate

    when crossing over

    occurs.

    The closer genes are

    on the chromosome,

    the less chance they

    will be separated during

    crossing over


    Autosomal Genetic Disorders

    • Caused by defective alleles on autosomes

    • Can be recessive or dominant


    Sickle Cell Anemia: (Autosomal recessive)

    • Red blood cells have a sickle shape

    • Very fragile and break easily

    • Clump up on blood vessels (causing pain)


    • Reduces oxygen carrying capacity of red blood cells

    • Caused by a single flaw in gene

    • One nitrogenous base in DNA sequence is different

    • Cause wrong amino acid in protein chain for hemoglobin, changing it’s shape

    • Hemoglobin doesn’t transport oxygen as well


    • Mostly found in people of African American descent

      • (1 in 500 births heterozygous)

    • If disorder has negative effects why so common in population?

      • Malaria is a deadly disease common in Africa

      • Heterozygous individuals for sickle cell have malarial resistance, more likely to survive than people who aren’t carriers.

    I spread malaria!


    Phenylketonuria (PKU):(Autosomal recessive)

    • Enzyme that breaks down amino acid phenylalanine doesn’t function

    • Chemical builds up in system and eventually forms substances that can damage the brain and cause mental retardation

    • Can test for it at birth and treat with a low phenylalanine diet


    Tay-Sachs Disease: (Autosomal Recessive)

    • Incurable inherited disorder that damages the brain

    • Enzyme doesn’t function that helps breakdown lipid in brain.

    • Brain tissue deteriorates

    • Most often found in eastern European Jews

    • Death occurs several years after birth

    • http://www.youtube.com/watch?v=SeoPF74QSms&safe=active


    Gene-Chromosome Theory

    • Explains the hereditary patterns that Mendel and others observed.

    • The genetic characteristics of an organism are determined by the genes for different traits present on their chromosomes.


    Mutations

    • Sometimes a genetic disorder is caused by a mutation of a gene.

    • The mutation can only be passed to the next generation if it happens in a sex cell.

    • Mutagenic Agents:

      • Exposure to x-rays

      • Radiation

      • Certain chemical toxins


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