Exam 4: Modifications of Mendelian Ratios

Alleles that are masked by an epistatic locus are said to be hypostatic to the genes at that locus.

Many of the color varieties of summer squash are determined by several interacting loci: AA or Aa gives white, aaBB or aaBb gives yellow, and aabb produces green. Crosses among heterozygotes give a 12:3:1 ratio. What type of gene interaction would account for these results?

The white -eye gene in Drosophila is recessive and sex -linked. Assume that a white -eyed female is mated to a wild -type male. What would be the phenotypes of the offspring?

With incomplete dominance, a likely ratio resulting from a monohybrid cross would be ________.

Assume that a mutation occurs in the gene responsible for the production of hexosaminidase A, such that only about 50 percent of the enzyme activity is found in the heterozygote compared with a homozygous normal individual. If heterozygotes are phenotypically normal, we would say that the mutant allele is recessive to its normal allele.

If an X -linked disorder is lethal to the affected individual prior to the age at which one reaches reproductive maturation, the lethality will be expressed only in males. Why is this so?

With both incomplete dominance and codominance, one expects heterozygous and homozygous classes to be phenotypically identical.

A conditional mutant is one whose expression is influenced by some environmental condition.

A particular cross gives a modified dihybrid ratio of 9:7. What phenotypic ratio would you expect in a testcross of the fully heterozygous F1 crossed with the fully recessive type? Diagram the testcross using A,a,B,b as symbol sets.

"Gain of function" mutations are generally dominant because one copy in a diploid organism is sufficient to alter the normal phenotype.

Assume that a dihybrid cross (AaBb × AaBb) is made in which the gene loci are autosomal, independently assorting, and incompletely dominant. What phenotypic ratio would you expect from such a cross? Just provide the ratio, not the phenotypes.

The trait for medium -sized leaves in iris is determined by the genetic condition PP'. Plants with large leaves are PP, whereas plants with small leaves are P'P'. The trait for red flowers is controlled by the genes RR, pink by RR', and white by R'R'. A cross is made between two plants each with medium -sized leaves and pink flowers. If they produce 320 seedlings, what would be the expected phenotypes, and in what numbers would they be expected? Assume no linkage.

Expressivity is the term used to describe the balanced genetic output from a hemizygous condition.

Because of the mechanism of sex determination, males of many species can be neither homozygous nor heterozygous. Such males are said to be _.

A cross was made between homozygous wild -type female Drosophila and yellow -bodied male Drosophila. All of the resulting offspring were phenotypically wild type. Offspring of the F2 generation had the following phenotypes: Based on this information: a() Is the mutant gene for yellow body behaving as a recessive or dominant gene? b() Is the yellow locus on an autosome or on the X chromosome?

Assume that a dihybrid cross is made in which the genes' loci are autosomal, independently assorting, and incompletely dominant. How many different phenotypes are expected in the offspring?

The following F2 results occur from a typical dihybrid cross: If a double heterozygote (AaBb) is crossed with a fully recessive organism (aabb), what phenotypic ratio is expected in the offspring?

Which types of ratios are likely to occur in crosses (F2) when one is dealing with two interacting, epistatic gene pairs?

Many of the color varieties of summer squash are determined by several interacting loci: AA or Aa gives white, aaBB or aaBb gives yellow, and aabb produces green. Assume that two fully heterozygous plants are crossed. Give the phenotypes (with frequencies) of the offspring.

State a significant difference between X -linked and sex -influenced inheritance.