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 |
|||
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 (cv cv,ct ct) are crossed with flies homozygous (v+cv+, ct+ct+). Female progeny that are cvcv+, ctct+ are then test crossed with the doubley recessive males ( cv cv, ct ct)
| Phenotype | Genotype | ||
|
wild type |
cv+ct+ |
674 |
|
|
crossveinless, cut |
cv ct |
681 |
|
|
crossveinless |
cv ct+ |
50 |
|
|
cut |
cv+ct |
43 |
|
|
1448 |
|||
| Description | Geneotype | expected independant assortment |
Expected complete linkage |
Actual percentage |
|
|
Parental |
cv+ct+ | 25% |
50% |
46.55% |
|
|
Parental |
cv ct | 25% |
50% |
47.03% |
|
|
Recombinant |
cv ct+ | 25% |
0 |
3.45% |
|
|
Recombinant |
cv+ct | 25% |
0 |
2.97% |
|
| % recombination = map distance |
centimorgans | 50 cm | 0 cm | 6.42 cm | |
If male progeny that are cvcv+, ctct+ are then test crossed with the doubley recessive females ( 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?
by Kenneth G. Wilson,
Professor of Botany
Miami University
wilsonkg@muohio.edu
