Variation in Chromosome Number

Aneuploidy
Development of Aneuploids
Phenotypic Effects of Aneuploidy
Genetics of Aneuploidy
Monoploidy
Euploidy
Euploidy and Plant Speciation
Variation in Chromosome Number WWW Links
Genetic Topics

Genetics of Aneuploids
Monosomic and trisomics are usually inviable in Drosophila. The exception are those aneuploids involving chromosome IV. Therefore the effects of aneuploidy on inheritance has been investigated using stocks with altered chromosome IV. The terms for specific Drosophila stocks are haplo-IV (monosomic for chromosome IV; 2n-1); diplo-IV (normal; 2n) and triplo- IV (trisomic for chromosome IV; 2n+1). Located on chromosome IV is the gene for eyeless (ey) that is recessive to normal eye. The following crosses illustrate several genetic principles of aneuploids. The first is a cross between a diplo-IV eyeless female (ey ey) and a haplo-IV normal eye male (ey⁺).

Female Gametes Phenotype

ey⁺

Male Gametes
ey⁺

ey⁺ey
diplo-IV normal eye

no IV
ey
haplo-IV eyeless

The next cross will be between a diplo-IV eyeless male and a triplo-IV normal eye that was homozygous for the three ey⁺ alleles.

Female gametes

ey⁺ey⁺ ey⁺ey⁺ ey⁺ey⁺ ey⁺ ey⁺ ey⁺

Male gamete ey ey⁺ey⁺ey ey⁺ey⁺ey ey⁺ey⁺ey ey⁺ey ey⁺ey ey⁺ey

Phenotype
normal
triplo-IV

normal
triplo-IV
normal
triplo-IV
normal
diplo-IV

normal
diplo-IV

normal
diplo-IV

The genotypic ratio of the progeny is ½ diplo-IV ey⁺ey and ½ triplo-IV ey⁺ey⁺ey.
The last cross will be between a normal female triplo-IV progeny from the above cross and an eyeless diplo-IV male.

Female gametes

ey⁺ey⁺ ey⁺ey ey⁺ey ey⁺ ey⁺ ey

Male gamete ey ey⁺ey⁺ey
ey⁺eyey

ey⁺eyey

ey⁺ey

ey⁺ey

eyey

Phenotype
normal
triplo-IV

normal
triplo-IV
normal
triplo-IV
normal
diplo-IV

normal
diplo-IV

eyeless
diplo-IV

The phenotypic ratio in this cross is 5 normal to 1 eyeless. The is the normal trisomic ratio. The genotypic ratio is ½ triplo-IV normal eye and 1/3 normal diplo-IV and 1/6 eyeless.
S egregation ratios for crosses involving aneuploids in plants are affected by the fact that male gametes carrying an extra chromosomes are inviable. Therefore, only normal haploid gametes are produced in the expected ratio. The example that we will use is for purple vs. white leaf color in poinsetta. At this locus the purple allele (p+) is dominant to the white allele (p). The cross that will be analyzed involves trisomic males and females that had the genoytpe p+p+p. The following Punnett square illustrates the results.

Female Gamete

p⁺p⁺ p⁺p p⁺p p⁺ p⁺ p

Male Gamete p⁺ p⁺p⁺p⁺ p⁺pp⁺ p⁺pp⁺ p⁺p⁺ p⁺p⁺ pp⁺

p⁺ p⁺p⁺p⁺ p⁺pp⁺ p⁺pp⁺ p⁺p⁺ p⁺p⁺ pp⁺

p p⁺p⁺p p⁺pp p⁺pp p⁺p p⁺p pp

The phenotypic ratio among the progeny from this cross is 17 purple to 1 white leaves. (The white leaf individual is in bold in the table. All others have a single p+ and are purple.) This ratio of 17:1 is typical for a cross in plants between trisomic individuals.
Copyright © 1997. Phillip McClean