12.. Phenylketonurics cannot convert phenylalanine to tyrosine Why don\'t these
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12.. Phenylketonurics cannot convert phenylalanine to tyrosine Why don't these individuals exhibit a deficiency of tyrosine? 13. Phenylketonurics are often more lightly pigmented than are normal individuals. Can you suggest a reason why this is so? 14. Early detection and adherence to a strict dietary regime have prevented much of the mental retardation that used to occur in those afflicted with phenylketonuria (PKU). Affected individuals now often lead normal lives and have families. For various reasons, such individuals adhere less rigorously to their diet as they get older. Predict the effect that mothers with PKU who neglect their diets might have on newborns. 15. The synthesis of flower pigments is known to be dependent on enzymatically controlled biosynthetic pathways. For the crosses shown here, postulate the role of mutant genes and their products in producing the observed phenotypes:Explanation / Answer
12. Both phenylalanine and tyrosine are non-essential amino acids which can be obtained from the food sources. Most natural proteins contain these amino acids.
13. The light pigmentation is due to a lack of melanin, which normally colors the hair, skin and eyes. Melanin is made from the amino acid tyrosine, which is lacking in untreated cases of PKU. Physiologically, PKU patients show high levels of phenylalanine and low levels of tyrosine in the blood.
14. When an expectant mother returns to consumption of phenylalanine in her diet, she subjects her baby to higher than normal levels of phenylalanine throughout its development. Since increased phenylaline is toxic, many (~90%) newborns are severely and irreversibly retarded at birth. Expectant mothers (who are genetically phenylketonurics) should return to a low phenylalanine intake during pregnancy.
15.
a. If two genes are involved in a specific pathway and functional products from both are required for expression, then one recessive allelic pair at either allelic pair would result in the mutant phenotype. This is called duplicate recessive epistasis.
b. Recessive alleles at one locus mask the phenotypic expression of other gene loci such epistasis is called recessive epistasis. The alleles of ‘B’ locus express themselves only when epistatic locus ‘A’ has dominant allele like AA or Aa. The phenotypic ratio is 9: 3: 4.
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