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| [Carrier] | a person who has one normal gene and one non-working gene for a recessively inherited disease,or a person with a balanced chromosomal rearrangement. carriers do not usually develop disease but can pass on the non-working copy or an unbalanced chromosome rearrangement to their children.
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| [Blastomere] | the first cells resulting from divisions of a fertilized egg cell
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| [Blastocyst] | a very early stage of embryonic development. about 4-5 days after conception the embryo consists of a circle of cells with fluid in the center and a clump of cells at one end with a thinner layer of cells at the other end.
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| [Birth defects] | an abnormality of structure, function, or body metabolism which often results in a physical or mental handicap. the causemay be genetic or environmental.
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| [Autosome] | any chromosome that is not a sex chromosome. in humans, the autosomes are the numbered chromosomes, numbered from 1 - 22. chromosome 1 is the largest and chromosome 22 is the smallest.
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| [Angelman syndrome] | a conditioncharacterized by severe mental deficiency, developmental delay and growth deficiency and frequent laughter unconnected to emotions of happiness.
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| [Aneuploidy] | the gain or loss of one chromosome compared to the typical chromosome complement, 46.
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| [Anaphase lag] | an error which can occur during cell division where one chromosome simply fails to get incorporated into the nucleus of a daughter cell. the mechanism for trisomic rescue. |
| [Amniotic sac] | the membrane-bound compartment that holds the baby and the amniotic fluid in the uterus. |
| [Amniotic fluid] | the fluid that surrounds the baby, held inside the amniotic sac. |
