Final answer:
The repeat number in Huntington's disease (HD) refers to the number of CAG nucleotide repeats in the huntingtin gene. As the repeat number increases, the risk of developing symptoms gradually rises. When the repeat number reaches approximately 36-39 repeats, individuals typically begin to manifest mild symptoms.
Step-by-step explanation:
In the context of Huntington's disease (HD), the repeat number refers to the number of CAG nucleotide repeats in the huntingtin gene. HD is caused by the expansion of this repeat, with longer repeat lengths associated with earlier onset and more severe symptoms.
Initially, individuals with a normal repeat number (less than 26 CAG repeats) are asymptomatic. As the repeat number increases, the risk of developing symptoms gradually rises. When the repeat number reaches approximately 36-39 repeats, individuals typically begin to manifest mild symptoms. As the repeat number continues to increase, the severity and age of onset of HD symptoms also increase.
It's important to note that the relationship between repeat number and symptom progression can vary, and other genetic and environmental factors can also influence the course of the disease.