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Mendelian patterns of Inheritance

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Title: Mendelian patterns of Inheritance


1
Mendelian patterns of Inheritance
  • Chapter 11

2
Heredity
  • The first scientists to study the laws of
    heredity had some difficult initial problems to
    work with
  • Two parents have to contribute equally to make
    one child
  • Offspring show similar traits to parents OR they
    show traits that havent appeared in a long time
  • Mixed breeds two different species can sometimes
    produce offspring
  • Laws of heredity must explain not only the
    stability but the variation

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Darwin and mendel
  • Darwins Origin of Species was an attempt to
    explain how species evolve from generation to
    generation
  • He had to explain how traits are passed for this
    to make sense
  • Darwin proposed a blending theory, which says
    that parent genes (called particles at the time)
    blend traits to produce an offspring
  • The first to explain heredity using experimental
    data however was an Austrian monk named Gregor
    Mendel
  • Note genes as we know them wont be officially
    explained for another 100 years, but Mendel
    explained how they are passed from parent to
    offspring

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Mendels experiment
  • Mendel was a mathematician, so he attempted to
    explain heredity using statistics and data
  • He used the common garden pea for his experiments
  • The pea is easy to cultivate, has a short
    generation time, and can be self-pollinated or
    cross-pollinated
  • The first experiment involved crossing (mating)
    gametes of a tall pea plant with gametes of a
    short pea plant.
  • The first generation is called the P generation
  • If the blending theory is correct, all offspring
    should be medium length
  • The first offspring generation is called the F1
    generation

10
Mendels Experiment
  • Mendel crossed plants thousands of times to
    ensure the accuracy of his data
  • When the F1 generation grew, his results were
    contrary to his hypothesis 100 of the plants
    were tall plants. Only one parental trait was
    passed on.
  • Did the short parental genes disappear?
  • Mendel then crossed members of the F1 generation
    with each other to produce the F2 generation.
  • The results 787 tall plants and 277 short plants
  • The ratio was pretty close to 31 (74 26)
  • The short trait had disappeared for a
    generation, but reappeared later

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Mendels Experiment
  • Perhaps, however, this was just a trait with the
    height of plants
  • Mendel repeated the same experiments over the
    next few months with the following traits pea
    pod shape, seed shape, pod color, flower color,
    seed color, flower position.
  • Each time, the same results 100 of one trait in
    the F1 generation, a 31 ratio is the F2.
  • Note this ratio represents the simplest type of
    gene. Only a small percentage of all genes in all
    organisms are actually this simple and easy to
    calculate.
  • In other wordsMendel got really lucky.

14
Mendels Explanation
  • After analyzing all possible mathematical
    explanations for his results, Mendel wrote his
    first of two laws The Law of Segregation
  • Each organism has two factors for each trait
  • During meiosis (formation of gametes) the factors
    separate into different cells
  • (From diploid to haploid)
  • Each gamete contains only one factor for each
    trait
  • When gametes fertilize, each new organism
    contains one factor from each parent for each
    trait

15
Modern Genetics
  • We now know that these factors are the strands
    of DNA that contain our genes
  • Each gene has a minimum of two possible alleles
  • An allele is an alternate form of the same gene
  • Gene plant size. Alleles tall and short
  • One of these genes is a dominant allele and the
    other is recessive
  • Dominant alleles means the trait they code for
    will always appear in an organism
  • Recessive alleles can be masked (covered, but not
    absent) by the dominant allele

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Modern Genetics
  • Because you receive a set of genes from each
    parent, eukaryotic organisms all have two alleles
    for each gene (one from mom, one from dad)
  • The combination of alleles an organism has is
    called their genotype
  • Each allele in a genotype is given a
    single-letter label
  • Capital letters are dominant, lower-case are
    recessive
  • The actual trait that appears in an organism is
    called a phenotype
  • Ttall plants, tshort plants
  • TT or Tt genotypes tall phenotypes
  • tt is only genotype that codes for a short
    phenotype
  • Genotypes with the same allele are called
    homozygous. Different alleles are called
    heterozygous

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Punnett Squares
  • When you understand the vocabulary and the
    process behind genetics, youre not only able to
    calculate what genes an organism likely has, but
    what genes its offspring are likely to have
  • In other words, you can calculate the likelihood
    of what your kids will look like.
  • We use a tool called Punnett Squares to calculate
    these odds.
  • Punnett Squares are especially helpful for humans
    since we cant make lots of humans just to see
    what traits come up

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Punnett Squares
  • Monohybrid Crosses
  • Use a monohybrid cross if you want to calculate
    the odds of only one gene
  • On the top, put the two alleles of the female
    parent
  • On the left side, put the two alleles of the male
  • Then fill in the boxes according to the allele
    from each parent

T
t
TT Tt
Tt tt
T
t
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Punnett Squares
  • Monohybrid Crosses
  • Each of the squares represent the potential
    sequence of genes in the offspring of the parents
  • Once the squares are filled in, you can start
    calculating ratios
  • Genotypic Ratios
  • 121
  • TT Tt tt
  • Phenotypic Ratio
  • 31
  • Tall Short

T
t
TT Tt
Tt tt
T
t
21
Punnett Squares
  • Dihybrid Crosses
  • What if you want to calculate the possibility of
    two genes at once?
  • Dihybrid crosses are larger, but the overall
    concept is the same.
  • For this example, we will use Mendels
    experiment The shape of peas AND the color of
    peas
  • Round (R) is dominant over Wrinkled (r)
  • Yellow (Y) is dominant over Green (y)

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Punnett Squares
  • First, find the four possible allele combinations
    for each parent.
  • Assign one combo for each row and column.
  • Fill in the squares as with the monohybrid cross

Round R Wrinkled r Yellow Y Green y
RY
Ry
rY
ry




RY
Ry
rY
ry
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Punnett Squares
  • When you calculate the different squares, you end
    up with a 9331 ratio

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Mendels experiment
  • Mendel noticed that in his dihybrid crosses all
    possible combinations occurred.
  • This must mean that genes are not connected to
    each other.
  • This led to Mendels second law, the Law of
    Independent Assortment
  • Each gene separates independently from itself
  • Every theoretical combination of genes is
    possible within an individual organism

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Non-Mendelian Traits
  • Theres lots of other sets of rules that genes
    follow
  • Incomplete dominance
  • In incomplete dominance, the heterozygote
    phenotype actually is a blending of the two
    alleles
  • Snapdragons show incomplete dominance
  • Rred flowers
  • Rwhite flowers
  • RR red flowers
  • RR white flowers
  • RR pink flowers
  • Hair styles (straight, wavy, curly) is an
    incomplete dominance trait in humans

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Non-Mendelian Traits
  • Codominance
  • Codominant traits are traits that show both
    alleles equally in the genotype
  • Dairy cow fur has two alleles and is codominant
  • B Black fur
  • W White fur
  • BB All black
  • WW All white
  • BW Black and White spotted
  • A human example of codominance is sickle blood
    cells
  • The two traits are round blood cells and
    sickle-shaped blood cells

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Non-Mendelian Traits
  • Multiple Allele Traits
  • Multiple allele traits are traits that have three
    or more alleles
  • The alleles all have an order of dominance
  • Labrador fur shows multiple alleles
  • YYellow
  • Y1Black
  • Y2Chocolate
  • YY2 Yellow
  • Y1Y2 Black
  • For humans, a multiple allele trait is blood type

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Multiple Alleles in Humans
  • Your immune system must recognize the difference
    between foreign substances and your own blood
  • To do this, your blood has specific proteins
    called antigens on its plasma membrane.
  • Antigens are glycolipids
  • Your immune system recognizes these proteins and
    knows that the blood cell belongs to you and
    isnt an intruder

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Multiple Alleles in Humans
  • The different antigens are labeled A and B
  • Alleles IAA-Type Blood IBB-Type Blood
    iNeither type
  • There are 4 possible phenotypes of blood, arising
    from 6 possible genotypes

Genotype Antigens Present Phenotype (Blood Type)
IAIA (AA) IAi (AO) A-Type only Type A
IBIB (BB) IBi (BO) B-Type only Type B
IAIB (AB) Both A and B Types Type AB
ii (O) Neither A or B Types Type O
38
Multiple Alleles in Humans
  • It is important for you to know what your blood
    type is BEFORE you get a blood transfusion
  • If you get blood with a different protein than
    what your immune system is used to, it will
    attack the blood
  • This results in blood clots and, usually, is
    deadly
  • (Because O type blood has NO proteins on it,
    your cells wont recognize the WRONG proteins.)

If you have this blood type You can receive these blood types
Type A Type A or Type O
Type B Type B or Type O
Type AB Type A or Type B or Type O
Type O Type O only
39
Non-Mendelian Traits
  • Polygenic Traits
  • Polygenic traits follow all normal Mendelian
    rules, but are combinations of multiple different
    genes
  • Seed color in wheat three different genes
  • Human height and skin color an unknown number of
    genes, but we think its between 5 and 12.
  • Polygenic traits tend to result in what appears
    to be multiple or infinite different phenotypes

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Non-Mendelian Traits
  • Epistasis
  • Epistasis is when one gene masks the effect of
    another gene.
  • Albinism in humans is an example of epistasis
  • Humans have multiple genes for what their skin
    color will be.
  • The different tones of color are controlled by
    how much melanin is produced by skin cells
  • The more melanin, the darker the skin
  • They have another gene elsewhere that controls
    whether or not melanin will be produced
  • If no melanin is produced, then the organism will
    be an albino and their skin color genes no longer
    matter

43
Autosomal recessive disorders
  • Three diseases that are recessive alleles that
    can be passed from parents to offspring
  • 1. Tay-Sachs disease
  • Neurological impairment at 7-8 months old
  • Blindness, seizures, paralysis possible
  • 2. Cystic Fibrosis
  • Mucus forms in the bronchial tubes and prevents
    lungs from working properly
  • CF children develop more slowly and only live to
    20-30 years
  • Phenylketonuria
  • Cannot digest the amino acid phenylalanine
  • Results in severe mental retardation

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Autosomal Dominant disorders
  • Neurofibromatosis
  • Tumors covering the nerve endings may cause
    deformations in bone and tissue structure
  • Huntingtons Disease
  • Brain cells begin to deteriorate at age 40-50
  • Victims lose motor and cognitive function as well
  • You can be tested to see if you have the gene for
    Huntingtons, but there is no cure
  • Question If these diseases are dominant, how
    come hardly anyone ever gets them?

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Chromosomal patterns of inheritance
  • Chapter 12

46
Chromosomes
  • Humans have a total of 46 chromosomes (23 from
    mom, 23 from dad
  • Homologous chromosomes 1-22 are all called
    autosomes. These chromosomes contain the same
    genes no matter which parent they came from
  • Not necessarily the same alleles.
  • There is one set of homologous chromosomes that
    may be different. These are the sex chromosomes
  • Two X chromosomes (Female)
  • One X chromosome, one Y chromosome (Male)
  • These chromosomes also contain genes. Any trait
    controlled by one of these chromosomes is called
    a sex-linked trait.

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Sex-Linked Traits in Humans
  • Females have two X chromosomes. Therefore,
    mothers can only donate an X chromosome to their
    offspring
  • Males have both an X and Y chromosome.
  • If Dad donates an X chromosome, the offspring
    will be female. If Dad donates a Y chromosome,
    the offspring will be male.
  • A father cannot pass a sex-linked Y-chromosome
    trait to his daughter OR an X-chromosome trait to
    his son.

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Sex-Linked Traits in Humans
  • A female will not have any gene located on a Y
    chromosome
  • What if a gene is located on the X chromosome?
  • Males only have one X chromosome. They will only
    have one allele for that trait, dominant or
    recessive.
  • Females have two X chromosomes, so they will have
    two alleles for the trait.
  • Because of this, it is harder for females to have
    a recessive X-linked phenotype than it is for
    males.
  • Phenotype ratios for sex-linked traits are
    different depending on the gender of the
    offspring

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  • In punnett squares, sex-linked traits are
    expressed as an X or a Y.
  • There is also a superscript to describe which
    allele is being represented
  • Fruit flies XRRed eyes. Xrwhite eyes. YY
    chromosome, so no gene is present on the
    chromosome

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Recessive sex-linked trait
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Human X-linked disorders
  • Muscular Dystrophy
  • Muscles are weak, to the point where the victim
    loses almost all use of their muscles
  • Death usually results by age 20
  • Color Blindness
  • Color-blind people have difficulty distinguishing
    colors, particularly in the red/green spectrum
  • Hemophilia
  • Hemophilia is an absence of the ability to clot
    blood.
  • Fragile X syndrome
  • A form of mental retardation, but victims are
    able to live to become a grandfather

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Sex-Determined Traits in Humans
  • Sometimes organisms have a gene for a specific
    trait, but the trait is not expressed because of
    the organisms gender.
  • A sex-determined trait is when a trait only
    appears in a certain gender
  • Hormones produced by other genes block
    sex-determined genes from expressing in an
    organism
  • Reason Parents of one gender may not express the
    same traits as children of the opposite gender.
    However, parents still need to pass all necessary
    genes to their child regardless of age.

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Sex-Determined Trait in Humans
  • Both men and women produce testosterone and
    estrogen.
  • Around puberty, the body begins to produce higher
    amounts of one or the other
  • Your gender determines which structures your body
    will form (testes or ovaries), and these
    structures produce high quantities of
    testosterone and estrogen, respectively
  • Therefore, even though you have the gene to
    produce both hormones, your gender decides which
    you will produce more of.

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Changes in Chromosome number
  • Chromosomal mutations occur when the gametes
    formed during meiosis contain unusual numbers of
    chromosomes
  • After meiosis I, you should have two cells, each
    with two copies of each chromosome
  • After meiosis II, you should have four cells,
    each with one copy of each chromosome
  • Its always possible that homologous chromosomes
    fail to separate or sister chromatids fail to
    break

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Polyploidy
  • Polyploidy is a chromosomal mutation where a cell
    has more than two copies of each chromosome
  • Triploid 3n
  • Tetraploids 4n
  • Pentaploids 5n
  • Polyploidy occurs often in plants, and is one
    reason for huge evolutionary diversity of flower
    designs and plant species
  • A set of gametes may go through meiosis and form
    polyploids, but only 1 out of these 4 gametes
    will be fertilized.

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Monosomy and Trisomy
  • If an organism has one less or one more of a
    specific chromosome, it is referred to as
    monosomy or trisomy
  • Monosomy and Trisomy is a result of
    nondisjunction
  • Both members of a homologous chromosome enter the
    same daughter cell during meiosis I
  • Both sister chromatids fail to separate in
    meiosis II
  • Down syndrome is the most common set of human
    trisomy, when the individual has a third 21
    chromosome.

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Sex Chromosome number
  • It is possible to inherit an abnormal number of
    sex-chromosomes as well.
  • XOTurner syndrome, female
  • Short, broad chested and extra folds of skin.
  • Do not undergo puberty, menstruation, but are
    capable of giving birth and can lead fairly
    normal lives
  • XXYKlinefelter syndrome, male
  • The male sexual organs are underdeveloped, but
    the female organs (breasts, in particular,) are
    slightly overdeveloped
  • Very large hands, feet and arms
  • XXXPoly-X female
  • Usually, poly-x females lead normal lives. Only
    slight physical abnormalities are evident
  • XYYJacobs syndrome, male
  • Taller than average, persistent acne, and
    communication problems

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Chromosome structure
  • Some chromosomes are more fragile than others,
    and damage is always possible
  • Most of the time damage is repaired by the cell.
    If not, it can result in more chromosome
    mutations
  • Deletion
  • In a deletion, a chromosome breaks, usually at
    the end of the chromosome
  • Williams syndrome is a loss of the end of 7
    chromosome
  • Williams children are academically and physically
    awkward, but show excellent communication and
    musical skills
  • Cru de Chats syndrome is a loss of the 5
    chromosome end
  • Individuals are mentally retarded with facial
    abnormalities, and an enlarged larynx results in
    an infant cry similar to a cats

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Chromosome Structure
  • A translocation is when a chromosome section
    moves from one chromosome to another
  • Down syndrome (Trisomy 21) is a result of a
    section of the 21 chromosome breaking and
    attaching to a 14 chromosome, resulting in three
    21 chromosomes
  • Alagille syndrome, when a translocation occurs
    between 20 and 2 chromosomes
  • The missing section of chromosome 20 results in
    severe itching and irritation of eyes, skin, and
    genital areas

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Chromosome structure
  • Other types of chromosome break mutations
  • Duplication
  • A chromosome section breaks and reattaches to
    its sister chromatid, resulting in a repeated
    section
  • Inversion
  • A chromosome section breaks and reattaches, but
    backwards

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Gene Linkage
  • Because crossing over and/or chromosome structure
    mutations are so common, geneticists study the
    locations of genes on chromosomes
  • The centromere is the structure of a chromosome
    where two sister chromatids are held to each
    other
  • The closer a gene is to a centromere, the less
    likely it will crossover or break from the
    chromosome
  • To study this, geneticists make gene-linkage maps

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Gene Linkage
  • There is a direct relationship between the
    frequency of crossing over (the percentage of
    recombinant phenotypes) and the distance between
    alleles.
  • A recombinant is a section of offspring DNA that
    contains multiple, recessive genes
  • The further apart genes are, the more likely a
    crossing over will occur between these two genes.
  • If 6 of organisms are recombinant organisms,
    then the rate of crossing over is 6.
  • A map unit is the unit used for measuring
    relative distance on a chromosome
  • 1 of crossing over rate 1 map unit

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Gene Linkage
  • Example An experiment is conducted on fruit
    flies to find the of recombinants for certain
    traits.
  • Black-body and purple-eyes recombinants occurred
    6 of the time, so these genes were 6 map units
    apart.
  • Purple-eyed and vestigial-wing recombinants
    occurred 12.5 of the time, so these genes were
    12.5 map units apart.
  • Black-body and vestigial-wing recombinants
    occurred 18.5 of the time, so these genes were
    18.5 map units apart.
  • What is the order of alleles on a chromosome, and
    how far apart is each one from each other?

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Gene Linkage
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