Question

You find a “stunted” true-breeding line of corn plants that have a very short stalk height. Heterozygotes with wildtype are intermediate in height. You cross these stunted plants to a deletion line to make them hemizygous and unexpectedly find that the height is almost the same as wildtype.

A)What type of morph is the stunted allele?

B) Is this likely to be a gain-of-function or a loss-of-function mutation?

C) The stunted homozygous plants also have weird dark green colored corn kernels. What is this additional observation generally known as: (circle)

incomplete penetrance

pleiotropy

genetic heterogeneity

variable expressivity

Answer 1

A-----------------------Purkinje cells in the neurological mutants lurch and stagger exhibit a number of abnormal properties; Mutant ↔ wild-type chimeras have shown that these properties are direct effects of the mutant gene. What has not yet been investigated are the numerous dendritic abnormalities that show the two mutant Purkinje cells. In spread, Purkinje cells have lurcher in rudimentary, unbranched dendrites that do not have tertiary branched spikes before the Purkinje cells die, their dendrites remain short and underdeveloped. To determine if a system of healthy afferents (or other environmental factors) would change one of these phenotypes, we investigated young lurcher and adult spreading mouse chimeras using Golgi impregnation. On postnatal day 20 (P20) lurcher chimeras we found two different morphological classes of Purkinje cells. One derived from the wild type had a dendritic structure similar to the normal Purkinje cells in age-adjusted controls. The other consisted of cells with small somata, reduced dendritic prostheses and multiple dendritic processes, making them unpredictable from Purkinje cells in P20 lung mutants. We also investigated adult spread of chimeras. We found no evidence that the immature morphology of scattered Purkinje cells was saved in mosaics, but countless examples of medium to large neurons were observed that resemble atroic Purkinje cells from stagger mutants. These results suggest that the dendritic abnormalities described in both mutants reflect cell autonomous development genetic blocks in the cytological maturation of the Purkinje cell in the brain. The implication is that the action of wild-type alleles at these two loci is required to perform a normal program of dendritic development.

B-------------------It is loss of function mutation as no growth is seen.

C-------------------Plieotropy is one gene influences more than one phenotypic trait. Examples:
-Mutation of the tyr gene has two phenotypes: 1. albinism (lack of pigment in skin, hair, and eyes)
2. ocular malformation, reduced visual acuity
-Merle in dogs: Merle affects coat color but Merle homozygotes have deafness and vision problems.
-Sickle cell disease is due to a recessive allele of the hemoglobin gene (HbS)
-one phenotype (when homozygous): destruction of red blood cells leading to anemia
-another phenotype (when homozygous): accumulation of red blood cells in the spleen leading to enlargement and fibrosis
-another phenotype (when heterozygous): resistance to malaria

Genetic heterogeneity is mutations of different/more than one genes results in the same phenotype.

-the production of the same phenotype by more than one genotype

- trait that shows a simple pattern of inheritance, but it can be caused by mutation in any one of a list of genes.

Examples:
-polycystic kidney disease
-retinitis pigmentosa

Variable expressivity is the degree to which the trait is expressed is variable.
Examples:
-Marfan Syndrome: some people merely have long fingers and toes while others have the full-blown disease with dislocation of the lens and dissecting aneurysm of the aorta.
-piebald spotting: which is the presence of white patches on a colored coat. The presence of the spotting is due to a dominant allele of a single gene, but the size of the patches is variable.