Mendel's Second Law is Blemished

The chromosome theory of inheritance is born in two decades following the discovery of Mendel's work.

Bateson and Punnett encounter a strange dihybrid ratio in using purple vs white flower color and round vs long pollen grains in Sweet Pea (Lathrus).

P1 - Purple Long x Red Round

F1- all Purple Long
  Number of Progeny

Phenotype

Genotype

Expected Ratio

Observed

Expect

Purple, Long

P_,L_

9/16

284

215

Purple, Round

P_,ll

3/16

21

71

Red, Long

pp,L_

3/16

21

71

Red, Round

pp,ll

1/16

55

24

     

381

381

Why are there any recombinants (purple-round and red-long)?

Thomas Hunt Morgan investigates Fruit Flies ! Wins Nobel Prize in 1933

In Drosophila the following wild type alleles are used for analysis cv (cross-veinless) and ct (cut or snipped wing edges). Flies doubly recessive and homozygous (cvcv,ctct) are crossed with flies homozygous (cv+cv+, ct+ct+). Female progeny that are cvcv+, ctct+ are then test crossed with the doubley recessive males ( cvcv, ctct)

Phenotype   Genotype  

wild type

cv+ct+

674

crossveinless, cut

cv ct

681

crossveinless

cv ct+

50

cut

cv+ct

43

Total  

1448

 

Description Geneotype Observed expected
independant assortment		 Expected
complete
linkage		 Actual
percentage

Parental

cv+ct+

674

25%

50%

46.55%

Parental

cv ct

681

25%

50%

47.03%

Recombinant

cv ct+

50

25%

0

3.45%

Recombinant

cv+ct

43

25%

0

2.97%

% recombination =
map distance		 centimorgans   50 cm 0 cm 6.42 cm

If female progeny that are cvcv+, ctct+ are then test crossed with the doubley recessive males ( cvcv, ctct)

Phenotype Genotype results

wild type

cv+ct+

715

crossveinless, cut

cv ct

733

crossveinless

cv ct+

0

cut

cv+ct

0

   

1448

How can you interpret these results?

Text iGenetics by Peter J. Russell

This web site is provided for instruction in Botany and Zoology 342

by Kenneth G. Wilson,
Professor of Botany
Miami University
wilsonkg@muohio.edu