Menkes disease in humans is caused by a mutation in a copper-transport protein that is coded for by an X-linked gene. Individuals with the recessive form of this mutant gene MNK have a copper deficiency which results in kinky hair, growth failure and nervous system deterioration. Carl has Menkes disease, but his parents both have normal copper transport. In his family history, he knows that his grandfather and uncle on his mother’s side both had Menkes disease. None of his female relatives are known to have Menkes disease.If Carl marries Felicia, who has normal copper transport. Felicia’s mother is not a MNK carrier but her father has Menkes disease. What are the expected phenotypic and genotypic ratios of their children (state your answers as percentages of sons and daughters)? (Show your work using a Punnett Square)

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Asked Oct 3, 2019
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Menkes disease in humans is caused by a mutation in a copper-transport protein that is coded for by an X-linked gene. Individuals with the recessive form of this mutant gene MNK have a copper deficiency which results in kinky hair, growth failure and nervous system deterioration. Carl has Menkes disease, but his parents both have normal copper transport. In his family history, he knows that his grandfather and uncle on his mother’s side both had Menkes disease. None of his female relatives are known to have Menkes disease.

If Carl marries Felicia, who has normal copper transport. Felicia’s mother is not a MNK carrier but her father has Menkes disease. What are the expected phenotypic and genotypic ratios of their children (state your answers as percentages of sons and daughters)? (Show your work using a Punnett Square)

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Step 1

X-linked genes are present on X chromosome (one of the sex chromosomes in humans) and X-linked recessive traits are the genes present on the X-chromosomes and causes disease when present in homozygous (both the alleles of the genes are same) condition in females (because females have ‘XX’  chromosomes) and if only one recessive gene containing chromosome is present in males, that male will be affected (because males has ‘XY’ chromosomes). Occurrence of X-linked recessive disease is very rare in females instead most of the times they are the carrier (XMX) for the disease.

Step 2

To determine the genotypes of crossing parents in the given question, pedigree analysis will be done using the information provided. A pedigree chart shows the occurrence of a phenotype in the individual and in its ancestors.

Making the pedigree chart using the information given as follows:

Key
I
Normal male
II
3 4
Normal female
1
Affected individuals
Ш
1
Individuals and
generations
Carrier female
Oa
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Key I Normal male II 3 4 Normal female 1 Affected individuals Ш 1 Individuals and generations Carrier female Oa

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Step 3

The genotypes of all the individuals in the pedigree is as follows:

I1 & I2: unknown genotype (grandparents of Carl from maternal side and their genotype has not been given)

I3: XMX or XX (grandmother of Carl from paternal side who may or may not be the carrier of the disease)

I4: XMY (grandfather of Carl from paternal side who is affected by the disease)

II1: XMY (uncle of Carl, who is affected)

II2: XMX (carrier mother of Carl and not a...

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