Transmission Genetics

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					Transmission Genetics

n How traits are passed down from
  generation to generation.
n Transmission of genes and the
  phenotypes which come from those genes
n The phenotype determines how the
  individual interacts with the world, and
  it is the phenotype that is subject to
  natural selection.

                                         1
    Gregor Mendel
n   1856 -1863
n   monk, Czech Republic
n   Studied 7 traits in pea plants, Pisum sativum
     ¨ Established basic rules of transmission genetics
        n Good science, but ignored for >30 years

n   Why peas?
     ¨ Many varieties with contrasting traits
     ¨ Self-pollinating, with true-breeding varieties
     ¨ easy to snip parts to cross pollinate
     ¨ Need little space, produce lots of offspring

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3
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His experiments would not have worked out
  except:
  1) He chose traits that were all dominant
     or recessive (“contrasting traits”)
  2) He chose traits that were all located on
     different chromosomes (pea plants
     have 7 chromosomes)
Pretty amazing since he had no idea how
  these traits were passed on – he called
  them “unit factors”

                                                5
n Mendel’s unit factors we now call genes.
n The two versions of each gene are called
  alleles.
n If an organism has two copies of the same
  allele is it said to be homozygous.
   ¨True breeding
n If an organism has different alleles of the
  same gene it is said to be heterozygous.



                                                6
q An allele is dominant, if it is expressed
  whenever that allele is present.
q A recessive trait is one that is hidden by
  another.
  q Itis only expressed when the allele is
    homozygous, that is 2 copies of it and none of
    another allele.
  q A dominant allele masks, or hides a recessive
    allele.




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n   The Genotype is all the genetic
    information the individual has for a
    particular trait.
    ¨ In   a diploid organism, that means two copies.
n   Those traits that are expressed: can be
    seen (physical traits) or measured
    (chemical traits) are the individual’s
    phenotype.
    ¨ An individual heterozygous for a trait may only
      show the effect of one allele (a dominant one)
      and not the other recessive one.

                                                        8
n Breeding two different, genetically distinct
  organisms is called cross breeding or
  crossing.
n The offspring of such crosses are called
  hybrids.
n The parents are called the parental or
  P generation
n The offspring of these parents are called
  the F1 generation (first filial)


                                                 9
n   Dominant traits are written with capital letters.
n   Recessive traits are written with small letters.

P = purple pigment (Purple flowers)
p = no pigment (white flowers)

PP = ?
Pp = ?
pp = ?

PP and Pp = purple flowers
pp = white flowers
                                                        10
n Homozgous individuals are true breeding
  (produce same trait when self-pollinated)
n What would we get if we crossed a
  homozygous purple flower (PP) with a
  homozygous white flower (pp)?

n   To find out, we can use a Punnett square
    – named after Reginald Punnett



                                               11
              Parent 1     (PP)
P
          P            P
a
r
e    p   Pp           Pp
n
t    p   Pp           Pp

2
              Gametes made; each has a
pp            50% chance of getting an
              allele from either homolog.
                                        12
Results: the offspring are all
heterozygous
n Genotype of all is Pp
n P is dominant to p, so all have purple
  flowers; the white flower phenotype is
  masked (p, white, is recessive)




                                           13
n   What would happen if we crossed
    members of the F1 generation?

                P        p

           P

           p



                                      14
Genotypes: 1: 2 : 1   PP : Pp : pp



                                     15
If you come upon a plant with purple flowers,
   it could be because the plant has a
   genotype of PP, homozygous for purple;
   OR it could be heterozygous, Pp, with the
   white allele masked.

How do we find out if it is homozygous or
 heterozygous ?

We could do a back cross or test cross –
 breed the individual with a homozygous
 recessive individual.
                                            16
n   Why breed it with a homozygous
    individual?
    ¨ Because    it is the only parent that we can be
      sure of its genotype from its phenotype!
    ¨ PP is purple, but so is Pp (that’s our question)
    ¨ Because the only way a recessive trait can be
      seen is if there is no dominant trait hiding it,
      then a white flower MUST have the genotype
      pp which means you can tell what the
      genotype is from the phenotype.


                                                     17
Genotypic ratio 1:1 Pp : pp
If the purple plant was homozygous, the F1
generation would all be purple - Pp

                                             18
But, not all traits show simple dominant-
  recessive relationships. There is also partial
  dominance where both traits are expressed.
Some traits show incomplete dominance.
Snap dragons have genes for red flowers (R1)
  and white flowers (R2).
A heterozygous flower (R1R2) would be
  Pink!
  This type of trait gave early scientists the idea
  that traits blended in offspring of different
  individuals.

                                                19
20
Notice that both
traits are given
capital letters, and
the F2 generation
shows a 1:2:1 ratio
of phenotypes as
well as genotypes.




                  21
Other traits show Codominance where both
  alleles are equally expressed.
Blood types: A B O blood groups
Glycolipids on the cell membrane of RBCs.
  IA, IB and i (A and B are codominant; O is
  recessive)
  IAIA = Type A IBIB = Type B
  IAi = Type A      IBi = Type B
  IAIB = Type AB      ii = Type O

                                               22
Mendel’s Crosses Showed:
n Principle of segregation: each sexually
  reproducing organism has two genes for
  each characteristic, and these two genes
  segregate or separate during the
  production of gametes.
n Principle of independent assortment:
  traits which lie on different chromosomes
  are passed on independently of each
  other.

                                              23
Mendel’s Law of Segregation
                    Parent 1      (PP)
   P
                P             P
   a
   r
   e    p      Pp           Pp
   n
   t    p     Pp            Pp

   2
                    Gametes made; each has a
  pp                50% chance of getting an
                    allele from either homolog.
                                              24
Mendel’s Independent Assortment




                                  25
With these rules in mind, we can cross
 individuals that have two different traits.
Dihybrid (vs. monohybrid) cross.
We can cross peas that have green pods (G)
 which are inflated (I) with peas that have
 yellow pods (g) which are constricted (i).

 GG II X gg ii = Gg Ii
   parents       F1


                                          26
The only “trick” to a dihybrid cross is setting
 up the Punnett square.
 GgIi →
            GI, Gi, gI, gi

  Like making a snack – take one of each
     Chips: Fritos or Doritos
     Fruit: apple or a peach
     candy bar: snickers or butterfinger
                                                  27
      GI     Gi     gI    gi

GI   GGII   GGIi   GgII   GgIi

Gi   GGIi   GGii   GgIi   Ggii

gI   GgII   GgIi   ggII   ggIi

gi   GgIi   Ggii   ggIi   ggii

                                 28
Genotypes :
1 GGII : 2 GgII : 2 GGIi : 4 GgIi
1  GGii : 2 Ggii 1 ggII : 2 ggIi
1 ggii
Phenotypes:    9 Green inflated
               3 Green constricted
               3 Yellow inflated
               1 Yellow constricted
              16

                                      29
Sex-Linked traits
n Some genes carried on the X chromosome
  are missing from the Y chromosome.
n These traits show up in different ratios in
  males and females and are called sex-
  linked traits
n Males are said to be hemizygous for these
  traits since they can only have one gene
  and a recessive gene will always be
  expressed.

                                           30
Color blindness is carried on the X
 chromosome (X’) Normal color vision (X)

     X      Y        The females all have
X    XX    XY      normal color vision, but
                   half the males are color
X’   XX’   X’Y     blind.




                                           31
     X’      Y      Here half the females
X    XX’    XY    and half the males are
X’   X’X’   X’Y   color blind.




                                        32
What happens when two traits are
located on the same chromosome?
They tend to be passed on together – this is
 called genetic linkage.
Can these two traits be inherited separately?
The likelihood that two genes on the same
 chromosome will be inherited separately
 depends on the distance between them.
They can be separated during “crossing
 over” that occurs during Prophase I of
 meiosis.
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posted:6/27/2014
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