pairing, with more or less obvious results. VARIA-
TIONalso occurs through MUTATION, in which DNA
sequences change during replication. Polymor-
phisms and mutations may have positive, neutral,
or negative effects, which differ among individuals
based on circumstance, lifestyle, and other factors.
See also ALLELE; CELL STRUCTURE AND FUNCTION;
GENETIC PREDISPOSITION; INHERITANCE PATTERNS; PHENO-
TYPE.
zygote The fertilized ovum (egg) before it begins
to divide. The spermatozoon (SPERMcell) and the
ovum are each haploid cells (gametes); they con-
tain half the complement of chromosomes neces-
sary to create an organism. When two gametes
join they form a single diploid cell that contains
the full complement of chromosomes. The zygote
then divides as a haploid cell, becoming a blasto-
cyst and eventually forming an EMBRYO.
For further discussion of zygotes within the
context of the structures and functions of genetics,
please see the overview section “Genetics and
Molecular Medicine.”
See also ASSISTED REPRODUCTIVE TECHNOLOGY(ART);
CELL STRUCTURE AND FUNCTION; CHROMOSOME; CONCEP-
TION; GAMETE; OVULATION.144 Genetics and Molecular Medicine