Risks+and+Inheritance

Spinal Muscular Atrophy is a recessive disorder, meaning that one needs two alleles in order for it to be active in the body. Each individual gets one allele from each parent, and they code for some trait. If the individual inherits one dominant allele and one recessive allele, then the dominant trait will be part of their phenotype, or observable trait of an organism.

This is what is called a punnet square. It visually shows the risk of passing on the SMA allele (labeled Y/y). The capital "Y" is the dominant trait, meaning that SMA would not be passed down. The lowercase "y" represents the recessive allele, which does code for SMA disorder. Both of Sam's parents are heterozygous, meaning that they each have one recessive and one dominant allele. Because the dominant allele exists in both parents and SMA is a recessive disorder, the problem is not active in the parent's bodies, but they are carriers. The letters outside of the box code for the parents. Inside the box are the chances of having a child with SMA. The top left corner (YY) is a child with no SMA. The top right corner (Yy) is a child without SMA, but who is a carrier for the SMA gene. The lower left corner (Yy) is also a carrier. Finally, the top right corner (yy) represents Sam. He, like anyone else, inherited one allele from his mother, as well as from his father. With every child that the couple has, there is an equal chance (shown above) of them inheriting each type. For Sam, there was a 25% chance that he would inherit two recessive alleles, which he has, hence his disorder.

Otherwise, for every child in the future, there is the same 25% chance of them acquiring the disorder, a 50% chance that they would be a carrier, and a 25% chance of the disorder not being anywhere in their DNA.