Biology Forums - Study Force

•5. Junctional epidermolysis bullosa (JEB) is a severe skin disorder that results in  blisters over the entire body.  The disorder is caused by an autosomal recessive mutation on chromosome 1. A case with a male newborn who was born with JEB and died at 2 months of age. The child had healthy unrelated parents.  Chromosomal analysis revealed that the infant had 46 normal –appearing chromosomes. Analysis of DNA showed his mother was heterozygous for the mutation and the father had 2 normal alleles.  Paternity testing confirmed the assumed father was the real father.

•a. Assuming no new mutations occurred in the family, explain the presence of an autosomal recessive disease in a child when the mother is heterozygous and the father is homozygous normal. (2pts)



•b. If the karyotype had been 47, 1+ , would the severity of the phenotype be any different? Explain. (2pts)

A. One possibility is that the infant inherited both copies of chromosome 1 from the mother. A nondisjunction in meiosis II could result in a gamete with two copies of maternal chromosome 1 bearing the recessive JEB allele. Fertilization by a normal male sperm would normally result in trisomy 1, with early embryonic lethality. In this case, the paternal chromosome 1 may have been lost in the first mitotic division, resulting in an embryo with a normal karyotype, but carrying two maternal copies of chromosome 1 (disomy) and homozygosity for the JEB
allele.

I would like to answer your homework letter A which is all about genes. Many people with autosomal dominant PKD live for several decades without developing symptoms. The cysts grow out of nephrons, the tiny filtering units inside the kidneys. The cysts eventually separate from the nephrons and continue to enlarge. So I don't think it could be happened because of the genes of the parents.