Mitosis: the nuclear division preceding cell division in all dividing cells of the body
during mitotic cell division, one cell divides to form 2 identical cells
Meiosis: two nuclear divisions that lead to formation of the gametes (sperm cell & egg cell (ovum))
during meiotic cell division, one diploid (2n) cell divides to form 4 non-identical haploid (n) cells
Tetrad: the structure formed by the close association of homologous chromosomes during prophase of meiosis I (usually preceding exchange of chromosomal material)
Crossing over: the exchange of chromosomal material (may include genes) between (nonsister) chromatids of homologous chromosomes (during prophase I of meiosis)
chromosome: a threadlike structural form of DNA in the nucleus of a cell (composed of DNA & histone proteins)
the nucleus of a normal diploid human cell contains 46 chromosomes
sister chromatids: genetically identical chromosomes joined at the centromere resulting from duplication of a single chromosome
homologous chromosomes: maternal & paternal chromosomes that belong to a numbered pair (e.g.: maternal chromosome #1 & paternal chromosome #1)
haploid: half the number of chromosomes found in a normal somatic (diploid) cell
the haploid number (n) of chromosomes in humans is 23
diploid: the number of chromosomes characteristically found in a normal somatic (diploid) cell
the diploid number (2n) of chromosomes in humans is 46
alleles: alternate forms of a gene that control the same trait (& are located at the same position on homologous chromosomes)
for example, the IA, IB & i alleles for ABO blood groups
spermatogenesis: meiotic cell division in the seminiferous tubules of the testes to produce 4 haploid sperm cells from a diploid spermatogonium
spermatogonia: precursor stem cells that occupy the outer wall of the seminiferous tubules & begin spermatogenesis
primary spermatocytes:diploid cells resulting from differentiation of type B cells, which are derived from mitotis of spermatogonia in the first step of spermatogenesis... primary spermatocytes are committed to meiotic cell division & enter meiosis I
secondary spermatocytes:haploid cells resulting from meiosis I during spermatogenesis
spermatids:haploid cells resulting from meiosis II during spermatogenesis
spermiogenesis: differentiation of spermatids to sperm cells (spermatozoa)... includes addition of an acrosome with enzymes to penetrate the secondary oocyte at the head region & a flagellum for locomotion at the tail region
spermatozoa: the mature male gametes (i.e.: haploid sperm cells)
oogenesis: meiotic cell division beginning in the ovary & completing in the uterine tube to produce one haploid egg cell from a diploid oogonium
oogonia: precursor stem cells that occupy the cortex of the ovary & begin oogenesis
primary oocytes:diploid cells resulting from mitosis & differentiation of oogonia during fetal development... primary oocytes begin meiosis I but stop at prophase I... several are stimulated to complete meiosis I at the beginning of each ovarian cycle
secondary oocytes:haploid cells resulting from completion of meiosis I during oogenesis... secondary oocytes begin meiosis II but stop at metaphase II prior to ovulation
ovum (mature egg cell):haploid cell resulting from completion of meiosis II following sperm entry during fertilization... the ovum is the mature female gamete
polar body: nonfunctional cell resulting from meiosis I and meiosis II of oogenesis... during each division, the secondary oocyte & egg cell receive most of the cytoplasm & polar bodies are cells containing mostly nuclear material that eventually degenerate
the first polar body may divide again before it degenerates to make a total of 3 polar bodies formed during complete oogenesis
fertilization: the process whereby a mature sperm cell penetrates a secondary oocyte, resulting in completion of meiosis II of the secondary oocyte to form an ovum, and subsequent union of sperm & egg nuclei to form the zygote
implantation: the attachment of the blastocyst to the endometrium of the uterus (about 6 days after fertilization)
heredity: the genetic transmission of traits from parent to offspring
genetics: the science that studies mechanisms of heredity
dominant inheritance: one copy of an allele is sufficient for expression of a trait
both homozygous dominant (e.g.: AA) & heterozygous (e.g.: Aa) individuals express the trait
recessive inheritance: two copies of an allele are necessary for expression of a trait
only homozygous recessive (e.g.: aa) individuals express the trait
sex-linked inheritance: expression of a traits determined by genes located on the sex chromosomes (X or Y chromosomes)
traits inherited on the X chromosome are said to be X-linked
several genetic disorders are characterized by sex-linked recessive inheritance (e.g.: hemophilia & color blindness), involving recessive alleles on the X chromosome
x-linked recessive traits are more frequently expressed in males, since males normally only have 1 copy of the x chromosome
Punnett square: a means for predicting the possible offspring resulting from the mating of parents of known genotypes
autosomes: every chromosome except the sex chromosomes (chromosome pairs #1-22 in humans)
sex chromosomes: the X and Y chromosomes (chromosome pair #23 in humans)
homozygous dominant: an individual with 2 copies of the dominant allele for a trait (e.g.: AA)
heterozygous: an individual with 1 copy of the dominant allele & 1 copy of the recessive allele for a trait (e.g.: Aa)
homozygous recessive: an individual with 2 copies of the recessive allele for a trait (e.g.: aa)
incomplete dominance: neither allele for a trait is completely dominant
the heterozygous individual has a phenotype intermediate in appearance between homozygous dominant & homozygous recessive phenotypes
examples are four o'clock flowers (pure-breeding red & white-flowered plants produce pink-flowered offspring) & sickle-cell disease in humans (the heterozygous condition is called sickle-cell trait)
gametes: the haploid cells resulting from meiosis
the female gamete is the egg cell (ovum); the male gamete is the sperm cell
phenotype: the appearance of an individual for a trait (e.g.: white squash or yellow squash)
genotype: the genetic makeup (alleles) of an individual for a trait (e.g.: WW, Ww or ww)
