Genetic question for AP Biology...?

Assume that a particular genetic condition in a mammalian species causes an inability to digest starch. This disorder occurs with equal frequency in males and females. In most cases, neither parent of affected offspring has the condition.





A. Describe the most probable pattern of inheritance for this condition. Explain your reasoning. Include in your discussion a sample cross (es) sufficient to verify your proposed pattern.





B. Explain how mutations could cause this inability to digest starch.





C. Describe how modern techniques of molecular biology could be used to determine whether the mutant allele is present in a given individual.

Genetic question for AP Biology...?
Breaking the first question down: Equal frequency in males and females tells you it is not an X linked gene. If more males that females had it that would say it was X linked. If neither parent has the problem then they must be heterozygotes and having one copy of the good gene is enough to function. So, we have a classic dominant recessive gene pattern. Lets call the gene D for digester. The parents would be Dd and if the offspring can't digest starch then it is dd. If the parent's genetics are put into a punnett square it would look like this





D d


D DD Dd


d dD dd





25% of the offspring would be homozygous dominant. dD and Dd are the same so 50% would be heterozygous and 25% would homozygous recessive.





As to mutations: The DNA coding fro the protein that digests starch makes a polypeptide that folds in certain ways so that starches fit into a pocket or groove of the protein so that it can be spit apart. Missense mutations would be a point mutation or replacement of one nucleic acid by another that results in the wrong amino acid being put into the protein so that the groove changes an starch no longer fits. (buying a new hammer that is missing the flat head side of it and trying to drive nails witrh it. Another mutation could be loss of a base so that the whole gene is shifted over by one nucleic acid. That would produce a nonsense mRNA and protein that didn't do anything or that wasn't processed by the ribosomes.





Modern molecular genetic testing uses a strategy where the suspected DNA is amplified in a PCR reaction and then mixed with known normal DNA. The mix is heated until all the duplex DNA falls apart and then is allowed to reanneal. The population of duplex DNA now will be a mix of normal-normal DNA, normal-mutated DNA and mutated-mutated DNA. The normal-mutated DNA has a bulge in it where the bases do not match up and migrates through a gel capillary tube slower than the normal-normal and mutated-mutated strands and comes out of the tube later than the others.


Check out the web site for DHPLC testing