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Anatomy and Physiology Genetic Unit

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Title: Anatomy and Physiology Genetic Unit


1
Anatomy and PhysiologyGenetic Unit
2
MENDEL'S GENETIC LAWS
  • Once upon a time (1860's), in an Austrian
    monastery, there lived a monk named Mendel,
    Gregor Mendel. Monks had a lot of time on their
    hands and Mendel spent his time crossing pea
    plants. As he did this over over over over
    over again, he noticed some patterns to the
    inheritance of traits from one set of pea plants
    to the next. By carefully analyzing his pea plant
    numbers (he was really good at mathematics), he
    discovered three laws of inheritance.
  • Mendel's Laws are as follows
  • 1. the Law of Dominance 2. the Law of
    Segregation 3. the Law of Independent Assortment
  • Now, notice in that very brief description of his
    work that the words "chromosomes" or "genes" are
    nowhere to be found.  That is because the role of
    these things in relation to inheritance
    heredity had not been discovered yet. What makes
    Mendel's contributions so impressive is that he
    described the basic patterns of inheritance
    before the mechanism for inheritance (namely
    genes) was even discovered.

3
Section 1
  • GENOTYPE the genes present in the DNA of an
    organism.  Use a pair of letters (ex Tt or YY or
    ss, etc.) to represent genotypes for one
    particular trait.  There are always two letters
    in the genotype because (as a result of sexual
    reproduction) one code for the trait from mom
    the other comes from dad, so every offspring gets
    two codes (two letters).
  • Now, turns out there are three possible GENOTYPES
    - two big letters (like "TT"), one of each
    ("Tt"), or two lowercase letters ("tt").
  • When we have two capital or two lowercase letters
    in the GENOTYPE (ex TT or tt) it's called
    HOMOZYGOUS ("homo" means "the same").  Sometimes
    the term "PURE" is used.
  • When the GENOTYPE is made up of one capital
    letter one lowercase letter (ex Tt) it's
    called HETEROZYGOUS ("hetero" means "other").  A
    heterozygous genotype can also be referred to as
    HYBRID.

4
  • PHENOTYPE how the trait physically shows-up in
    the organism.  What they look like! 
  • ALLELES (WARNING - THIS WORD CONFUSES PEOPLE
    READ SLOW) alternative forms of the same gene. 
    Alleles for a trait are located at corresponding
    positions on homologous chromosomes. Remember
    genotypes I said that "one code (letter) comes
    from ma one code (letter) comes from pa"? Well
    "allele" is a fancy word for what I called codes.
  • For example, there is a gene for hair texture
    (whether hair is curly or straight).  One form of
    the hair texture gene codes for curly hair.  A
    different code for of the same gene makes hair
    straight.  So the gene for hair texture exists as
    two alleles --- one curly code, and one straight
    code.

5
The Law of Dominance
  • In a cross of parents that are pure for
    contrasting traits, only one form of the trait
    will appear in the next generation.  Offspring
    that are hybrid for a trait will have only the
    dominant trait in the phenotype.
  • Cross pure yellow and pure green, yellow is
    dominate to green?
  • Cross 2 heterozygous yellow plants?

6
Let's revisit the three possible genotypes for
pea plant height
7
  • Note the only way the recessive trait shows-up
    in the phenotype is if the geneotype has 2
    lowercase letters (i.e. is homozygous recessive).
    Also note hybrids always show the dominant
    trait in their phenotype (that, by the way,  is
    Mendel's Law of Dominance in a nutshell).
  • ANY TIME TWO PARENT ORGANISMS LOOK DIFFERENT FOR
    A TRAIT, AND ALL THEIR OFFSPRING RESEMBLE ONLY
    ONE OF THE PARENTS, YOU ARE DEALING WITH MEDEL'S
    LAW OF DOMINANCE.

8
Here are the basic steps to using a Punnett
Square when solving a genetics question
  • BABY STEPS 1. determine the genotypes of the
    parent organisms 2. write down your "cross"
    (mating) 3. draw a p-square 4. "split" the
    letters of the genotype for each parent put
    them "outside" the p-square 5. determine the
    possible genotypes of the offspring by filling in
    the p-square 6. summarize results (genotypes
    phenotypes of offspring)

9
Step 1 Determine the genotypes of the parent
organisms.
  • "Cross a short pea plant with one that is
    heterozygous tall. Tall is dominant to short". 
     
  • T tall
  • t short
  • Parent 1 tt
  • Parent 2 Tt

10
Step 2 , 3 and 4
  • Step 2 Write down your "cross" (mating).  Write
    the genotypes of the parents in the form of
    letters (ex Tt x tt).
  • Step 3 Draw a p-square. 
  • Step 4"Split" the letters of the genotype for
    each parent put them "outside" the p-square.

t t
T Tall t short
T t
11
Step 5 Determine the possible genotypes of the
offspring by filling in the p-square.
T Tall t short
12
Step 6 Summarize the results (genotypes
phenotypes of offspring).
  • Simply report what you came up with.  You should
    always have two letters in each of the four
    boxes.
  • Genotype (what the genes look like) 2Tt and 2tt
  • Phenotype (what the offspring look like) 2 tall
    and 2 short

13
  • You know how, in Step 4, when we "split" the
    letters of the genotype put them outside the
    p-square?  What that step illustrates is the
    process of gametogenesis (the production of sex
    cells, egg sperm). 
  • Gametogenesis is a cell division thing (also
    called meiosis) that divides an organism's
    chromosome number in half. 
  • For example, in humans, body cells have 46
    chromosomes a piece.  However, when sperm or eggs
    are produced (by gametogenesis/meiosis) they get
    only 23 chromosomes each.  When the sperm egg
    fuse at fertilization, the new cell formed
    (called a zygote) will have 23 23 46
    chromosomes. 

14
Section 2The Law of Segregation
  • During the formation of gametes (eggs or sperm),
    the two alleles responsible for a trait separate
    from each other.  Alleles for a trait are then
    "recombined" at fertilization, producing the
    genotype for the traits of the offspring.

15
  • Now, when completing a Punnet Square, we model
    this "Law of Segregation" every time.  When you
    "split" the genotype letters put one above each
    column one in front of each row, you have
    SEGREGATED the alleles for a specific trait. In
    real life this happens during a process of cell
    division called "MEIOSIS". 
  • You can see from the p-square that any time you
    cross two hybrids, 3 of the 4 boxes will produce
    an organism with the dominant trait (in this
    example "TT", "Tt", "Tt"), and 1 of the 4 boxes
    ends up homozygous recessive, producing an
    organism with the recessive phenotype ("tt" in
    this example).

16
  • Any time two parents have the same phenotype for
    a trait  but some of their offspring look
    different with respect to that trait,  the
    parents must be hybrid for that trait. 

17
The Law of Independent Assortment
  • Alleles for different traits are distributed to
    sex cells ( offspring) independently of one
    another.
  • OK. So far we've been dealing with one trait at a
    time.  For example,  height (tall or short), seed
    shape (round or wrinkled), pod color (green or
    yellow), etc.  Mendel noticed during all his work
    that the height of the plant and the shape of the
    seeds and the color of the pods had no impact on
    one another.  In other words, being tall didn't
    automatically mean the plants had to have green
    pods, nor did green pods have to be filled only
    with wrinkled seeds, the different traits seem to
    be inherited INDEPENDENTLY.
  • Please note my emphasis on the word "different". 
    Nine times out of ten, in a question involving
    two different traits, your answer will be
    "independent assortment".  There is a punnet
    square that illustrates this law. It involves
    what's known as a "dihybrid cross", meaning that
    the parents are hybrid for two different traits.

18
  • The genotypes of our parent pea plants will be
  • RrGg x RrGg "R" dominant allele for round
    seeds "r" recessive allele for wrinkled seeds
    "G" dominant allele for green pods "g"
    recessive allele for yellow pods
  • Notice that we are dealing with two different
    traits (1) seed texture (round or wrinkled)
    (2) pod color (green or yellow).  Notice also
    that each parent is hybrid for each trait (one
    dominant one recessive allele for each trait).
  • We need to "split" the genotype letters come up
    with the possible gametes for each parent.  Keep
    in mind that a gamete (sex cell) should get half
    as many total letters (alleles) as the parent and
    only one of each letter. So each gamete should
    have one "are" and one "gee" for a total of two
    letters.  There are four possible letter
    combinations RG, Rg, rG, and rg. These gametes
    are going "outside" the p-square, above 4 columns
    in front of 4 rows.  We fill things in just
    like before --- "letters from the left, letters
    from the top". When we finish each box gets four
    letters total (two "are's" two "gees").

19
  • The results from a dihybrid cross are always the
    same for 2 heterozygous parents 9/16 boxes
    (offspring) show dominant phenotype for both
    traits (round green), 3/16 show dominant
    phenotype for first trait recessive for second
    (round yellow), 3/16 show recessive phenotype
    for first trait dominant form for second
    (wrinkled green), 1/16 show recessive form
    of both traits (wrinled yellow).
  • So, as you can see from the results, a green pod
    can have round or wrinkled seeds, and the same is
    true of a yellow pod.  The different traits do
    not influence the inheritance of each other. 
    They are inherited INDEPENDENTLY.
  • Interesting to note is that if you consider one
    trait at a time, we get "the usual" 31 ratio of
    a single hybrid cross (like we did for the LAw of
    Segregation). For example, just compare the color
    trait in the offspring 12 green 4 yellow (31
    dominantrecessive).  Same deal with the seed
    texture 12 round 4 wrinkled (31 ratio).  The
    traits are inherited INDEPENDENTLY of eachother
    --- Mendel's 3rd Law.  

20
In a dihybrid crossIf two heterozygous round,
yellow plants are crossedyellow is dominate to
green and round is dominate to wrinkled what are
the genotype phenotype? 1st write your
letters Y yellow y green S spherical/round s
wrinkled 2nd write your parents YySs YySs
21
Pair up the letters 1st pairs with 3rd , 1st with
4th, then 2nd letter with 3rd, 2nd with 4th
22
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  • Next drop in your letters bring both letters down
    separate them by letter S or Y and always put the
    capitol letter first

24
  • Now count up the genotype (letter combination
    square by square)
  • Genotypes
  • SSYY 1
  • SSYy 2 SsYY 2SsYy 4
  • SSyy 1 Ssyy 2
  • ssYY1ssYy 2
  • ssyy 1

25
  • Then do the Phenotype what they look like
  • Phenotype
  • 9 yellow, round
  • 3 yellow, wrinkled
  • 3 green, round
  • 1 green, wrinkled

26
Summarize Mendel's Laws by listing the cross that
illustrates each.
27
  • Mendels work has stood the test of time, even as
    the discovery understanding of chromosomes
    genes has developed in the 140 years after he
    published his findings.  New discoveries have
    found "exceptions" to Mendel's basic laws, but
    none of Mendel's things have been proven to be
    flat-out wrong.

28
Section 3IncOMpleTe COdominANce
  • In many ways Gregor Mendel was quite lucky in
    discovering his genetic laws.  He happened to use
    pea plants, which happened to have a number of
    easily observable traits that were determined by
    just two alleles.  And for the traits he studied
    in his peas, one allele happened to be dominant
    for the trait the other was a recessive form. 
    Things aren't always so clear-cut "simple" in
    the world of genetics, but luckily for Mendel (
    the science world) he happened to work with an
    organism whose genetic make-up was fairly
    clear-cut simple.
  • INCOMPLETE DOMINANCE
  • If Mendel were given a mommy black mouse a
    daddy white mouse asked what their offspring
    would look like, he would've said that a certain
    percent would be black the others would be
    white.  He would never have even considered that
    a white mouse a black mouse could produce a
    GREY mouse!  For Mendel, the phenotype of the
    offspring from parents with different phenotypes
    always resembled the phenotype of at least one of
    the parents.  In other words, Mendel was unaware
    of the phenomenon of INCOMPLETE DOMINANCE.  

29
  • I remember Incomplete Dominance in the form of an
    example like so RED Flower x WHITE Flower ---gt
    PINK Flower 
  • With incomplete dominance, a cross between
    organisms with two different phenotypes produces
    offspring with a third phenotype that is a
    blending of the parental traits. It's like
    mixing paints, red white will make pink.  Red
    doesn't totally block (dominate) the white,
    instead there is incomplete dominance, and we end
    up with something in-between.
  • We can still use the Punnett Square to solve
    problems involving incomplete dominance.  The
    only difference is that instead of using a
    capital letter for the dominant trait a
    lowercase letter for the recessive trait, the
    letters we use are both going to be capital
    (because neither trait dominates the other).  So
    the cross I used up above would look like this

30
INCOMPLETE DOMINANCE
  • R allele for red flowers W allele for white
    flowers
  • red x white ---gt pink RR x WW ---gt 100 RW

31
  • The trick is to recognize when you are dealing
    with a question involving incomplete dominance. 
    There are two steps to this 1) Notice that the
    offspring is showing a 3rd phenotype.  The
    parents each have one, and the offspring are
    different from the parents. 2) Notice that the
    trait in the offspring is a blend (mixing) of the
    parental traits.

32
CODOMINANCE
  • First let me point out that the meaning of the
    prefix "co-" is "together". Cooperate work
    together.  Coexist exist together.  Cohabitat
    habitat together.
  • The genetic gist to codominance is similar to
    incomplete dominance.  A hybrid organism shows a
    third phenotype --- not the usual "dominant" one
    not the "recessive" one ... but a third,
    different phenotype.  With incomplete dominance
    we get a blending of the dominant recessive
    traits so that the third phenotype is something
    in the middle (red x white pink).
  • In COdominance, the "recessive" "dominant"
    traits appear together in the phenotype of hybrid
    organisms.

33
  • I remember codominance in the form of an example
    like so
  • red x white ---gt red white spotted
  • With codominance, a cross between organisms with
    two different phenotypes produces offspring with
    a third phenotype in which both of the parental
    traits appear together. 
  • When it comes to punnett squares symbols, it's
    the same as incomplete dominance.  Use capital
    letters for the allele symbols.  My example cross
    from above would look like so

34
CODOMINANCE
  • R allele for red flowers W allele for white
    flowers
  • red x white ---gt red white spotted RR x WW
    ---gt 100 RW

35
  • The symbols you choose to use don't matter, in
    the end you end up with hybrid organisms, and
    rather than one trait (allele) dominating the
    other, both traits appear together in the
    phenotype.  codominance.
  • A very very very common phenotype used in
    questions about codominance is roan fur in
    cattle.  Cattle can be red (RR all red hairs),
    white (WW all white hairs), or roan (RW red
    white hairs together).  A good example of
    codominance.
  • Another example of codominance is human blood
    type AB, in which two types of protein ("A"
    "B") appear together on the surface of blood
    cells.

36
MULTIPLE ALLELES
  • It makes absolutely no sense whatsoever to
    continue if we don't know what the word "allele"
    means.
  • allele (n) a form of a gene which codes for
    one possible outcome of a phenotype
  • For example, in Mendel's pea investigations, he
    found that there was a gene that determined the
    color of the pea pod.  One form of it (one
    allele) creates yellow pods, the other form
    (allele) creates green pods.
  • Get it? Two possible phenotypes of one trait (pod
    color) are determined by two alleles (forms) of
    the one "color" gene.

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  • If there are only two alleles involved in
    determining the phenotype of a certain trait, but
    there are three possible phenotypes, then the
    inheritance of the trait illustrates either
    incomplete dominance or codominance.
  • In these situations a heterozygous (hybrid)
    genotype produces a 3rd phenotype that is either
    a blend of the other two phenotypes (incomplete
    dominance) or a mixing of the other phenotypes
    with both appearing at the same time
    (codominance).

39
THE DEALS ON MULTIPLE ALLELES
  • Now, if there are 4 or more possible phenotypes
    for a particular trait, then more than 2 alleles
    for that trait must exist in the population.  We
    call this "MULTIPLE ALLELES".
  • Let me stress something.  There may be multiple
    alleles within the population, but individuals
    have only two of those alleles.
  • Why?
  • Because individuals have only two biological
    parents.  We inherit half of our genes (alleles)
    from ma, the other half from pa, so we end up
    with two alleles for every trait in our
    phenotype.
  • An excellent example of multiple allele
    inheritance is human blood type. Blood type
    exists as four possible phenotypes A, B, AB,
    O.
  • There are 3 alleles for the gene that determines
    blood type. (Remember You have just 2 of the 3
    in your genotype --- 1 from mom 1 from dad).

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  • As you can count, there are 6 different genotypes
    4 different phenotypes for blood type.
  • Note that there are two genotypes for both "A"
    "B" blood --- either homozygous (IAIA or IBIB) or
    heterozygous with one recessive allele for "O"
    (IAi or IBi).
  • Note too that the only genotype for "O" blood is
    homozygous recessive (ii).
  • And lastly, what's the deal with "AB" blood? 
    What is this an example of?  The "A" trait the
    "B" trait appear together in the phenotype. 
    Think think think ....

42
  • Lubey, Steve. Lubey's Biohelp! - Mendel's Genetic
    Laws. Aug. 26, 2005lthttp//www.borg.com/lubehawk
    /mendel.htmgt
  • Lubey, Steve. Lubey's Biohelp! Incomplete
    Codominance. Aug. 26, 2005lthttp//www.borg.com/l
    ubehawk/inccodom.htmgt
  • Lubey, Steve. Lubey's Biohelp! Multiple
    Alleles. Aug. 26, 2005lthttp//www.borg.com/lubeh
    awk/multalle.htmgt
  • Mendel Image. http//www.micro.utexas.edu/courses/
    levin/bio304/genetics/genetics.html
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