posted by brandon .
A child is born witrh cystic fibrosis-a recessive trait. However, both his parents do not exhibit the trait. How is this possible?
I suspect his parents are both carriers, carrying the recessive gene, but because neither is homozygous, neither have the disease.
In case you are not familiar with bobpursley's terms, here is a more detailed explanation.
With recessive traits, a person must have both genes for the recessive condition (homozygous) to show the physical trait. If one of the gene pair are dominant, it will prevent the recessive gene from being expressed in the physical traits (phenotype). That person will be a carrier for that trait, having the genes in the genotype but not showing the trait in the phenotype. However, there is a 25% chance that both carrier parents will contribute the recessive gene to an offspring, making the offspring homozygous for that trait. This means that the child will show the trait in the phenotype.
This is true for any recessive trait.
I hope this helps a little more. Thanks for asking.