Anatomy and Physiology Genetic Unit

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Anatomy and PhysiologyGenetic Unit
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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.

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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.

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• 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.

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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?

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Let's revisit the three possible genotypes for
pea plant height
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• 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.

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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)

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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

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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
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Step 5 Determine the possible genotypes of the
offspring by filling in the p-square.
T Tall t short
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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

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• 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. 

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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.

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• 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).

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• 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. 

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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.

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• 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").

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• 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.  

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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
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Pair up the letters 1st pairs with 3rd , 1st with
4th, then 2nd letter with 3rd, 2nd with 4th
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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

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• 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

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

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Summarize Mendel's Laws by listing the cross that
illustrates each.
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• 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.

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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.  

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• 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

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INCOMPLETE DOMINANCE

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

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• 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.

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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.

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• 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

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CODOMINANCE

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

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• 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.

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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).

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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 ....

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• 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