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.
1 When both alleles are the same, the condition is called the homozygous condition; if the alleles differ, then it is the heterozygous condition. 2  When heterozygous, one allele is dominant (A), the other is recessive (a).  Homozygous dominant = AA, homozygous recessive = aa, and heterozygous = Aa.