recombination and gene linkage

Recombinants  and  Gene  Linkage  Polygenic  Inheritance  

IB  Biology  Topic  10.2  &  10.3  

Typical  Dihybrid  cross  

9:3:3:1  phenotypic  raDo  when  2  dihybrid  heterozygous  parents  are  crossed.  

Two  traits  are  considered-­‐    •  seed  color  

(yellow  vs.  green)    •  seed  texture  

 (wrinkled  vs.  round)  

Independent Assortment"

Independent assortment

An Interpretation from the Dihybrid cross    •  During gamete formation, segregating pairs of unit

factors assort independently."•  In other words, segregation of 2 alleles at one genetic

locus has no effect on the segregation of 2 alleles at another locus."

•  For example, the assortment of yellow and green alleles has no effect on the assortment of round and wrinkled alleles, and vice versa."

•  Results have expected, typical ratios"

Discovery  of  Linkage  

•  William  Bateson  and  R.C.  PunneP  were  working  with  several  traits  in  sweet  peas,    

•  a  gene  for  purple  (P)  vs.  red  (p)  flowers  •  a  gene  for  long  pollen  grains  (L)  vs.  round  pollen  grains  (l).    

Bateson and Punnett pea study:

Flower Color: P = purple p = red Pollen seed shape: L = long l = round

True Breeding lines: PPLL x ppll

PpLl F1

What is going on????

Crosses  produced  a  deviaDon  from  the  predicted    Mendelian  independent  assortment  raDos.    

Phenotype   Observed   Expected  Ra5o  

Expected  Number  

Purple  long   284   9   215  

Purple  round   21   3   71  

Red  long   21   3   71  

Red  round   55   1   24  

Results:   There  are  4  expected  genotypes   There  are  4  recombinant  genotypes  but  cannot  be  observed  from  phenotypes   There  are  2  recombinant  phenotypes  which  would  be  actually  observed  during  the  experiment   Results  did  not  show  a  typical  9:3:3:1  therefore  not  dihybrid  unlinked.   Results  did  not  show  a  totally  linked  raDo  either  (3:1)   Therefore,  there  must  be  something  else  happening  with  the  genes.  

•  Because the parental phenotypes reappeared more frequently than expected, the researchers hypothesized that there was a coupling, or connection, between the parental alleles for flower color and pollen grain shape

•  This coupling resulted in the observed deviation from independent assortment.

Linkage without Recombination  

Thomas  Morgan’s  work  with  Drosophila  lead  to  the  discovery  of  linked  genes  and  recombina5on  due  to  crossing  over  •  Morgan  proposed  that  the  

chiasmata  visible  on  chromosomes  were  regions  of  crossing  over.  

•  Occurs  between  non-­‐sister  chromaDds.  

EvoluDon,  Natural  SelecDon  and  Crossing  Over  

•  From an evolutionary point of view, the purpose of sex is to re-shuffle the combinations of alleles so the offspring receive a different set of alleles than their parents had.

•  Natural selection then causes offspring with good combinations to survive and reproduce, while offspring with bad combinations don’t pass them on.

•  Genes are on chromosomes. Meiosis is a mechanism for re-shuffling the chromosomes: each gamete gets a mixture of paternal and maternal chromosomes.

•  However, chromosomes are long and contain many genes. To get individual genes re-shuffled, there needs to be a mechanism of recombining genes that are on the same chromosome. This mechanism is called “crossing over.

RecombinaDon  due  to  crossing  over  

Recombinants  

RecombinaDon=    

 the  reassortment  of  alleles  into  combinaDons  different  from  those  of  the  parents,  as  a  result  of:  independent  assortment,  crossing  over  and/or  ferDlizaDon.  

Example-­‐  recombinants  due  to  crossing  over  

Linked  genes:  A  with  B          AB  high  frequency  a  with  b          ab  high  frequency  Recombinant  a  with  B    aB  low  frequency  Recombinant  A  with  b    Ab  low  frequency   Recombinants  due  to  

crossing  over  will  always  have  a  lower  frequency  

Cross  with  2  linked  genes  

Frequency  of  crossover  exchange...                            of  chroma5ds  of  a  homologous  pair  at  synapsis  forming  a  

chiasmata...      

•  Frequency  is  GREATER  the  FARTHER  apart  2  genes  are    is  propor5onal  to  rela5ve  distance    between  2  linked  genes  

Rela5ve  distance  is  established  as...    

•  1%  crossover  frequency  =    1  map  unit  of  map  distance  

•  1%      CrossOver    Freq      =                    1    cen5Morgan  (cM)  

Genetic Map of Drosophila melanogaster"

Polygenic  Inheritance  

•  Polygenic  inheritance-­‐  more  than  one  gene  contributes  to  a  phenotype.    A  characterisDc  that  is  controlled  by  more  than  one  gene.  

•  Allows  for  conDnuous  variaDon  •  Examples  include:  human  height,  human  skin  color,  wheat  seed  color  

Polygenic  Traits  –  Skin  Color    About  5  genes  are  involved  in  human  skin  

color  which  creates  a  conDnuous  range  of  skin  color  in  the  populaDon  from  very  light  to  very  dark.      

  Each  gene  has  2  melanin-­‐producing  alleles  that  are  codominant.  

  Therefore,  many  genes  contribute  to  the  color  of  skin  

  EvoluDon  has  selected  for  greater  melanin-­‐producDon  in  areas  where  there  is  more  intense  sunlight  to  protect  against  UV  radiaDon  and  prevent  skin  cancer.  

  Lower  light  intensity  regions  selected  over  Dme  for  less  melanin  to  allow  for  more  Vitamin  D  producDon  

  SelecDve  advantage  of  skin  color  is  now  compensated/overcome  by  the  use  of  sunblock  and    vitamin  D  supplements.  

 Using  whitener  on  your  skin  is  a  terrible  idea!