In the bread mold, a mutant strain (stp) exhibits erratic \"stop and go\" growth
ID: 92675 • Letter: I
Question
In the bread mold, a mutant strain (stp) exhibits erratic "stop and go" growth. The mutation in this strain is known to be in mitochondrial DNA. If the stp strains is used as the female parent in a cross with a normal strain as the male parent, what type of progeny can be expected? What progeny can be expected if the genotypes of the male and female parents are switched (the reciprocal cross)? Two plants are being studied, one that is resistant (R) to corn smut fungus and another that is susceptible (S) to corn smut fungus. Reciprocal crosses are made with the following results. S female x R male rightarrow all F1 are susceptible (S) R female x S male all F1 are resistant (R) What is responsible for this pattern of inheritance? How does this pattern of inheritance differ from sex-linked or sex-limited inheritance? In polygenic inheritance, how many different phenotypic classes, each corresponding to the different dose of gene products, are expected in self-crosses of: (each dominant allele = +1, each recessive allele = +0) a) strains with 3 different heterozygous polygenes (e.g., Xx Yy Zz)? b) strains with 4 different heterozygous polygenes (e.g., ww xx Yy zz)? c) strains with 6 different heterozygous polygenes (e.g., Uu Vv Ww Xx Zz)?Explanation / Answer
1. It is given that the mutation is present in mitochondrial DNA. It has been established that mitochondrial DNA is inherited from female parent.
If the stp strain is used as the female parent in cross with a normal male, the the resulting progeny will also be mutant, as it will inherit its mitochindrial DNA from the female parent.
If the reseverse of above is done, i.e. stp strain is used as a male parent and the female parent is normal, the the resulting progeny will also be normal.
2. It can be observed that F1 progeny is expressing the trait of the female parent. This is an example of maternal inheritance. Usually, mitochondrial DNA (mtDNA) is inherited from the female parent. This is a non-mendelian type of inheritance.
This differs from sex-linked inheritance in the sense that the alleles being expressed are present in the mtDNA, whereas in sex-linked inheritance, the alleles are exclusively present on either the X or Y chromosome.
3. a) Since nothing is mentioned in the question, let us assume that the genes have additive effects.
So XxYyZz will have a value of 3. Similarly, XXYYZZ will have a value of 6 and xxyyzz will have value of 0.
On self-crossing these 3 polygenes, we will obtain phenotypic having all values between 0 and 6.
Thus we will have 7 phenotypic classes.
b) Solving same as above,
WWXXYYZZ will have a value of 8.
wwxxyyz will have a value of 0.
All phenotypes between values 0 and 8 are possible. Thus total phenotypic classes will be 9.
c) There are 6 polygenes. So UUVVWWXXYYZZ will carry a value of 12.
uuvvwwxxyyzz will carry a value of 0.
So total phenotypic classes posible are 13.
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