In genetics’ problems, you will often be given the results of a backcross and sometimes the results of a selfing F1*F1 to find out information about the localization of the considered genes.the following golden paragraph will help you in this mission!
“The variety and the proportions of the results of the test cross reflect the gametic variety and proportions of the female F1 since the male contribute with 100% recessive alleles.
- The given results do not show 2 different phenotypes this means that the studied genes are not absolutely linkedon the same chromosome.
- The given results do not show 4 different genotypes of equal proportions which mean that there is no independent disjunction of the studied genes; the genes are not found on independent chromosomes.
- The given results of the test cross show 4 different phenotypes with 2 high proportions and 2 low proportions which means that there is partial linkage or (a linkage followed by a crossing over) which is produced during the formation of gametes of the hybrid female F1.
- And since we have 2 dominant alleles are found together in the phenotypes of high proportions and the 2 recessive alleles are also together in the high proportions this means the genes are found in the Cis position.
- And since we have 1 dominant allele and one recessive allele found together in the phenotypes of high proportions this means the genes are found in the Trans position.”
The following statements are very helpful in justifying a specific fact in genetics’ problems.
Statement 1: “The recessive trait is not expressed unless in the homozygous state.”
Statement 2: “ The normal couple gave birth to a diseased child, this means that the allele of the disease is hidden in the genotypes of parents , masked by the normal allele. Thus, the allele of the disease is recessive and the normal allele is dominant.”
Statement 3: “This is a case of intermediate inheritance since two pure flowers , one red and one white gave rise to a pink flower, a new phenotype that is an intermediate color between red and white colors, this means that both alleles responsible for red and white colors are dominant and expressed incompletely.”
Statement 4: “ Both parents are of blood group A gave birth to a child of blood group O. The child of blood group O having for genotype oo ( since allele o is recessive with respect to allele “A”) so he should have taken 1 allele “o” from his mother and 1 allele “o” from his father, so the parents should have allele “o” in their genotypes. And since they are of blood group A, each will also have an allele “A” in his/her genotype. Thus, the parents are hybrid for the blood group genotype Ao .”
to be continued……