A stroke occurs when blood flow to a part of the brain stops. A stroke is sometimes called a brain attack.. If blood flow is cut off for longer than a few seconds, the brain cannot get nutrients and oxygen. Brain cells can die, causing lasting damage.
A cell is said to be homozygous for a particular gene when identical alleles of the gene are present on both homologous chromosomes. The cell or organism in question is called a homozygote. True breeding organisms are always homozygous for the traits that are to be held constant.
Homozygous plants have two identical copies of a single gene. Heterozygous plants have two copies of a single gene, but those copies are alleles, or different versions of that gene. Example of alleles controlling a trait for susceptibility to a lesion-causing disease.