Inheritance of the Ac element
The Ac or activator element is necessary for the expression
of the Ds element. The Ac factor is inherited in typical
Mendelian segregation ratios.
See McClintock, B. 1953. Induction of instability at
selected loci in maize. Genetics 38:579-599.
To see the influence of the Ac factor some testcross
progeny were scored.
Male: I Ds ac/I Ds ac
X Female: C ds Ac/C ds ac --- segregating
for Ac-ac
The resulting progeny segregated 1 yellow:1 purple
variegated.
 |
Male:
ac |
Male:
ac |
Female:
Ac |
|
Female:
ac |
The progeny with the I Ds ac/C ds ac genotype are yellow
endosperm because there is no Ac element to cause dissociation
of the short arm of chromosome 9. The Ac locus is located
on a non-homologous chromosome to chromosome 9. The
progeny that are I Ds ac/C ds Ac have variegated kernels
because the Ac element activates the Ds element and
the I Ds segment of chromosome 9 dissociates. After
dissociation these progeny are dicentric for chromosome
9. The I Ds segment is acentric and thus the I allele
is not expressed. Variegated purple sectors on a yellow
background result from the fact that the Ds element
does not cause a chromosome break in every cell. The
timing of the chromosome break also varies among cells
undergoing mitosis, which results in variegated purple
sectors or different shapes and sizes.
