Exercícios sobre mapeamento genético

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BEV145	
  –	
  Genética	
  Básica	
  
GD	
  –	
  Mapeamento	
  Genético	
  	
   1) O	
  que	
  significa	
  recombinação?	
  Quais	
  são	
  as	
  duas	
  causas	
  de	
  recombinação?	
  2) Qual	
  é	
  o	
  efeito	
  do	
  crossing	
  over	
  sobre	
  a	
  ligação?	
  3) Qual	
   é	
   a	
  diferença	
   entre	
   genes	
  na	
   configuração	
  de	
   acoplamento	
   e	
   genes	
  em	
   repulsão?	
   Que	
   efeito	
   tem	
   o	
   arranjo	
   de	
   genes	
   ligados	
   (estejam	
   na	
  configuração	
  de	
  acoplamento	
  ou	
  em	
  repulsão)	
  sobre	
  os	
  resultados	
  de	
  um	
  cruzamento?	
  4) Nos	
   crossing	
   overs	
   simples,	
   a	
   frequência	
   de	
   gametas	
   recombinantes	
  corresponde	
  à	
  metade	
  da	
  frequência	
  de	
  crossing	
  overs	
  porque	
  a) um	
  cruzamento	
  teste	
  entre	
  um	
  homozigoto	
  e	
  um	
  heterozigoto	
  produz	
  ½	
  prole	
  heterozigota	
  e	
  ½	
  homozigota	
  b) a	
  frequência	
  de	
  recombinação	
  é	
  sempre	
  50%	
  c) cada	
  crossing	
  over	
  ocorre	
  só	
  entre	
  duas	
  das	
  quatro	
  cromátides	
  de	
  um	
  par	
  homólogo	
  d) os	
  crossing	
  overs	
  ocorrem	
  em	
  cerca	
  de	
  50%	
  das	
  meioses	
  	
  5) Explique	
  como	
  determinar	
  qual	
  dos	
  três	
  loci	
  ligados	
  é	
  o	
  locus	
  do	
  meio	
  da	
  prole	
  de	
  um	
  cruzamento	
  teste	
  de	
  três	
  pontos.	
  6) Faz-­‐se	
  um	
  cruzamento	
  teste	
  de	
  três	
  pontos	
  entre	
  genes	
  ligados.	
  As	
  proles	
  não	
   recombinantes	
   resultantes	
   são	
   s+	
   r+	
   c+	
   e	
   s	
   r	
   c	
   e	
   as	
   proles	
   com	
  
crossing	
  over	
  duplos	
  são	
  s	
  r	
  c+	
  e	
  s+	
  r+	
  c.	
  Qual	
  é	
  o	
  locus	
  do	
  meio?	
  7) Escreva	
  os	
  genótipos	
  de	
   toda	
  a	
  prole	
   recombinante	
  e	
  não	
   recombinante	
  esperada	
  do	
  seguinte	
  cruzamento	
  de	
  três	
  pontos:	
  	
   	
  	
   8) Uma	
  série	
  de	
  cruzamentos	
  de	
  dois	
  pontos	
  foi	
  realizada	
  entre	
  sete	
  loci	
  (a,	
  
b,	
  c,	
  d,	
  e,	
  f,	
  g),	
  produzindo	
  as	
  frequências	
  de	
  recombinação	
  abaixo.	
  Mapeie	
  os	
  setes	
  loci,	
  mostrando	
  seus	
  grupos	
  de	
  ligação,	
  a	
  ordem	
  dos	
  loci	
  em	
  cada	
  grupo	
  de	
  ligação	
  e	
  as	
  distâncias	
  entre	
  os	
  loci	
  de	
  cada	
  grupo:	
  	
  
Loci	
   Porcentagem	
  de	
  
recombinação	
  
Loci	
   Porcentagem	
  de	
  
recombinação	
  
a	
  e	
  b	
   50	
   c	
  e	
  d	
   50	
  
a	
  e	
  c	
   50	
   c	
  e	
  e	
   26	
  
a	
  e	
  d	
   12	
   c	
  e	
  f	
   50	
  
a	
  e	
  e	
   50	
   c	
  e	
  g	
   50	
  
a	
  e	
  f	
   50	
   d	
  e	
  e	
   50	
  
a	
  e	
  g	
   04	
   d	
  e	
  f	
   50	
  
b	
  e	
  c	
   10	
   d	
  e	
  g	
   08	
  
b	
  e	
  d	
   50	
   e	
  e	
  f	
   50	
  
b	
  e	
  e	
   18	
   e	
  e	
  g	
   50	
  
b	
  e	
  f	
   50	
   f	
  e	
  g	
   50	
  
b	
  e	
  g	
   50	
   	
   	
  	
  
( ). Three types of crossover events can take place
between these three genes: two types of single crossovers (see
Figure 5.12a and b) and a double crossover (see Figure
5.12c). In each type of crossover, two of the resulting chro-
mosomes are recombinants and two are nonrecombinants.
Notice that, in the recombinant chromosomes resulting
from the double crossover, the outer two alleles are the same
as in the nonrecombinants, but the middle allele is different.
This result provides us with an important clue about the
order of the genes. In progeny that result from a double
crossover, only the middle allele should differ from the alle-
les present in the nonrecombinant progeny.
Constructing a Genetic Map 
with the Three-Point Testcross
To examine gene mapping with a three-point testcross, we will
consider three recessive mutations in the fruit fly Drosophila
melanogaster. In this species, scarlet eyes (st) are recessive to
red eyes (st!), ebony body color (e) is recessive to gray body
color (e!), and spineless (ss)—that is, the presence of small
bristles—is recessive to normal bristles (ss!). All three muta-
tions are linked and located on the third chromosome.
We will refer to these three loci as st, e, and ss, but keep
in mind that either the recessive alleles (st, e, and ss) or the
dominant alleles (st!, e!, and ss!) may be present at each
locus. So, when we say that there are 10 m.u. between st and
ss, we mean that there are 10 m.u. between the loci at which
these mutations occur; we could just as easily say that there
are 10 m.u. between st! and ss!.
To map these genes, we need to determine their order on
the chromosome and the genetic distances between them.
First, we must set up a three-point testcross, a cross between
a fly heterozygous at all three loci and a fly homozygous for
recessive alleles at all three loci. To produce flies heterozy-
gous for all three loci, we might cross a stock of flies that are
homozygous for normal alleles at all three loci with flies that
are homozygous for recessive alleles at all three loci:
The order of the genes has been arbitrarily assigned
because, at this point, we do not know which is the middle
gene. Additionally, the alleles in these heterozygotes are in
coupling configuration (because all the wild-type dominant
alleles were inherited from one parent and all the recessive
mutations from the other parent), although the testcross can
also be done with alleles in repulsion.
In the three-point testcross, we cross the F1 heterozy-
gotes with flies that are homozygous for all three recessive
mutations. In many organisms, it makes no difference
whether the heterozygous parent in the testcross is male or
P
st + e+ ss+
st + e+ ss+
* st e ss
st e ss
T
F1
st + e+ ss+
st e ss
a b c female (provided that the genes are autosomal) but, in
Drosophila, no crossing over takes place in males. Because
crossing over in the heterozygous parent is essential for
determining recombination frequencies, the heterozygous
flies in our testcross must be female. So we mate female F1
flies that are heterozygous for all three traits with male flies
that are homozygous for all the recessive traits:
The progeny produced from this cross are listed in
Figure 5.13. For each locus, two classes of progeny are pro-
duced: progeny that are heterozygous, displaying the domi-
nant trait, and progeny that are homozygous, displaying the
recessive trait. With two classes of progeny possible for each
of the three loci, there will be classes of phenotypes
possible in the progeny. In this example, all eight phenotypic
classes are present but, in some three-point crosses, one or
more of the phenotypes may be missing if the number of
progeny is limited. Nevertheless, the absence of a particular
class can provide important information about which com-
bination of traits is least frequent and, ultimately, about the
order of the genes, as we will see.
To map the genes, we need information about where and
how often crossing over has taken place. In the homozygous
recessive parent, the two alleles at each locus are the same, and
so crossing over will have no effect on the types of gametes
produced; with or without crossing over, all gametes from this
parent have a chromosome with three recessive alleles
( ). In contrast, the heterozygous parent has differ-
ent alleles on its two chromosomes, and so crossing over can
be detected. The information that we need for mapping, there-
fore, comes entirely from the gametes produced by thehet-
erozygous parent. Because chromosomes contributed by the
homozygous parent carry only recessive alleles, whatever alle-
les are present on the chromosome contributed by the het-
erozygous parent will be expressed in the progeny.
As a shortcut, we often do not write out the complete
genotypes of the testcross progeny, listing instead only the
alleles expressed in the phenotype, which are the alleles
inherited from the heterozygous parent. This convention is
used in the discussion that follows.
st e ss
23 = 8
st + e+ ss+
st e ss
 Female * st e ss
st e ss
 Male
122 Chapter 5
Concepts
To map genes, information about the location and number of
crossovers in the gametes that produced the progeny of a cross is
needed. An efficient way to obtain this information is to use a
three-point testcross, in which an individual heterozygous at three
linked loci is crossed with an individual that is homozygous reces-
sive at the three loci.
✔ Concept Check 4
Write the genotypes of all recombinant and nonrecombinant 
progeny expected from the following three-point cross:
m+ p+ s+
m p s 
*
m p s 
 m p s
9) Os	
  genes	
  a	
  e	
  b	
  distam	
  20	
  cM.	
  Um	
  individuo	
  a+	
  b+/a+	
  b+	
  foi	
  cruzado	
  com	
  um	
  individuo	
  a	
  b/a	
  b.	
  a) diagrame	
  o	
  cruzamento	
  e	
  mostre	
  os	
  gametas	
  produzidos	
  por	
  cada	
  genitor	
  e	
  o	
  genótipo	
  da	
  F1	
  b) que	
  gametas	
  a	
  F1	
  pode	
  produzir	
  e	
  em	
  que	
  proporções?	
  c) Se	
  a	
  F1	
  cruzada	
  com	
  indivíduos	
  a	
  b/a	
  b,	
  que	
  prole	
  seria	
  esperada	
  e	
  em	
  que	
  proporções?	
  d) Este	
  é	
  um	
  exemplo	
  de	
  ligação	
  em	
  fase	
  de	
  acoplamento	
  ou	
  repulsão?	
  	
  10) 	
  Uma	
   fêmea	
   fenotipicamente	
   tipo	
   selvagem	
  de	
  mosca	
  das	
   frutas	
  que	
  era	
  heterozigota	
  para	
  os	
  genes	
  que	
   controlam	
  a	
   cor	
  do	
   corpo	
  e	
   tamanho	
  da	
  asa	
   foi	
   cruzada	
   com	
   um	
   macho	
   mutante	
   homozigoto	
   com	
   corpo	
   preto	
  (alelo	
   b)	
   e	
   asas	
   vestigiais	
   (alelo	
   vg).	
   O	
   cruzamento	
   produziu	
   a	
   seguinte	
  prole:	
   corpo	
   cinza,	
   asas	
   normais	
   126;	
   corpo	
   cinza,	
   asas	
   vestigiais	
   24;	
  corpo	
   preto,	
   asas	
   normais	
   26;	
   corpo	
   preto,	
   asas	
   vestigiais	
   124.	
   Estes	
  dados	
  indicam	
  ligação	
  entre	
  os	
  genes	
  para	
  cor	
  do	
  corpo	
  e	
  tamanho	
  da	
  asa?	
  Qual	
  a	
  frequência	
  de	
  recombinação?	
  Diagrame	
  o	
  cruzamento,	
  mostrando	
  o	
  arranjo	
  dos	
  marcadores	
  genéticos	
  nos	
  cromossomos.	
  	
  11) 	
  Fêmeas	
   heterozigotas	
   de	
   Drosophila	
   para	
   três	
   mutações	
   recessivas	
   e	
  (corpo	
  ébano),	
  st	
  (olhos	
  escarlate)	
  e	
  ss	
  (cerdas	
  curtas)	
  foram	
  submetidas	
  a	
  cruzamentos	
  teste,	
  e	
  a	
  seguinte	
  prole	
  foi	
  obtida:	
  	
  
Fenótipo	
   Genótipo	
   Número	
  Tipo	
  selvagem	
   e+	
  st+	
  ss+	
   235	
  Escarlate,	
  ébano,	
  cerdas	
  curtas	
   e	
  	
  	
  st	
  	
  	
  ss	
   270	
  Ébano,	
  cerdas	
  curtas	
   e	
  	
  st+	
  	
  ss	
   62	
  Escarlate	
   e+	
  st	
  	
  	
  ss+	
   60	
  Cerdas	
  curtas	
   e+	
  st+	
  ss	
   40	
  Escarlate,	
  ébano	
   e	
  	
  st	
  	
  ss+	
   48	
  Ébano	
   e	
  st+	
  ss+	
   7	
  Escarlate,	
  cerdas	
  curtas	
   e+	
  st	
  	
  ss	
   4	
  
Total	
   	
   726	
  	
  a) O	
  que	
  indica	
  que	
  os	
  genes	
  estão	
  ligados?	
  b) Qual	
  foi	
  o	
  genótipo	
  das	
  fêmeas	
  heterozigotas	
  originais?	
  c) Qual	
  é	
  a	
  ordem	
  dos	
  genes?	
  d) Qual	
  a	
  distância	
  de	
  mapa	
  entre	
  e	
  e	
  st?	
  e) Entre	
  e	
  e	
  ss?