Biology 211: Genetics & Development

Starfish Development - Unfertilized Egg & Zygote

Starfish Development - 2 & 4 Cell Stages

Starfish Development - Morula

Starfish Development - Blastula

Starfish Development - Gastrula

Starfish Development - Overview

Genetics Problem

Genetics Terms (click on a term for a brief definition)

mitosis & meiosis
tetrad & crossing over
chromosome, sister chromatids & homologous chromosomes
haploid & diploid
alleles
spermatogenesis (includes spermatogonia, primary & secondary spermatocytes, spermiogenesis & spermatids)
oogenesis (includes oogonium, primary & secondary oocytes, oocyte (ovum) & polar body)
fertilization & implantation

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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 I^A, I^B & 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)

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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

Click for Image

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

Example:

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)