Human Development

A comprehensive set of vocabulary flashcards covering genetics, fertilization, early embryology, gestation, placental development, germ layers, and postnatal development.

Genes

Sequence of DNA that provides instructions for making a polypeptide (or RNA) product.

Genotype

The genetic constitution of an organism—the set of genes and alleles it carries.

Phenotype

The observable expression of a genotype, i.e., the physical/biochemical traits produced.

Homologous chromosomes

Pairs of chromosomes that are similar in size and gene content, one from each parent.

Autosomes

Chromosomes 1-22 that are not sex chromosomes.

Sex chromosomes

Chromosomes X and Y; determine sex and often carry sex-linked genes.

X chromosome

A sex chromosome; carries many genes; in males it is the single X with no corresponding Y allele.

Y chromosome

A sex chromosome; carries genes that determine maleness and X-linked gene interactions.

Locus

The specific location of a gene on a chromosome.

Alleles

Different versions of a gene that can occupy the same locus.

Homozygous

Having two identical alleles for a gene.

Heterozygous

Having two different alleles for a gene.

Simple (monohybrid) inheritance

Inheritance determined by a single gene pair; phenotype influenced by one allele pair.

Polygenic inheritance

Phenotype determined by interactions among alleles of multiple genes.

Sex-linked inheritance

Inheritance involving genes on the sex chromosomes, often X-linked.

X-linked

Genes located on the X chromosome; often expressed differently in males and females.

Dominant

An allele that masks the expression of other alleles for a trait.

Recessive

An allele whose trait is masked by a dominant allele when present with it.

Codominance

Both alleles are expressed in the phenotype (e.g., AB blood type).

Incomplete dominance

Two alleles produce an intermediate phenotype rather than a dominant/recessive pattern.

Zygote

The diploid cell formed by the fusion of sperm and egg.

Cleavage

Mitosis of the zygote without growth in cytoplasmic volume.

Morula

A solid ball of cells (3-4 days after fertilization) formed during early development.

Blastocyst

A hollow sphere of cells with an inner cell mass and trophoblast; implants into the endometrium.

Trophoblast

Outer cell layer of the blastocyst that contributes to the placenta.

Embryoblast

Inner cell mass of the blastocyst; develops into the embryo.

Implantation

Attachment of the blastocyst to the endometrium, enabling pregnancy to proceed.

Placenta

An organ formed by embryonic (chorionic villi) and maternal tissues that exchanges nutrients, wastes, gases, and hormones between mother and fetus.

Chorionic villi

Finger-like projections from the chorion that invade the endometrium and allow fetal blood vessels to develop.

Amnion

The extraembryonic membrane that forms the amniotic cavity and protects the developing embryo.

Yolk sac

An early extraembryonic membrane source of nutrients and blood cell formation.

Allantois

An extraembryonic membrane that contributes to the umbilical cord and bladder formation.

Umbilical cord

Cord containing fetal vessels that connects the fetus to the placenta.

Preembryonic period

0–16 days after fertilization; includes zygote, cleavage, morula, blastocyst, and implantation.

Embryonic period

Weeks 3–8; organ and germ layer formation (differentiation and development of tissues).

Fetal period

Weeks 9–birth; growth and maturation of organs and systems.

Gastrulation

Formation of the three primary germ layers: ectoderm, mesoderm, endoderm.

Germ layers

Ectoderm (outer), mesoderm (middle), endoderm (inner) that give rise to all tissues.

Ectoderm

Germ layer that forms skin, nervous system, and related structures.

Mesoderm

Germ layer that forms bones, muscles, cardiovascular system, and more.

Endoderm

Germ layer that forms the lining of the digestive and respiratory tracts and associated glands.

Embryonic disc

Early embryonic structure; will differentiate into the body plan and germ layers.

Differentiation

Process by which cells become specialized in structure and function.

Induction

Signaling interactions that cause one group of cells to become something different.

Chorion

Fetal membrane that forms part of the placenta and surrounds the amniotic sac.

Amniotic cavity

Fluid-filled cavity within the amnion surrounding the developing embryo.

Placental exchange

Nutrients diffuse from maternal blood to fetal blood; wastes diffuse in the opposite direction.

Epigenetics

Heritable changes in gene expression not due to DNA sequence changes.

Imprinting

A type of epigenetic modification where gene expression depends on parental origin.

Nondisjunction

Failure of chromosome pairs to separate properly during meiosis, leading to abnormal chromosome numbers.

Disjunction

Normal separation of homologous chromosomes or sister chromatids during meiosis.

Turner syndrome

XO phenotype; female with a missing or structurally abnormal second sex chromosome.

Triple X syndrome (XXX)

Females with an extra X chromosome; often taller and may have developmental differences.

Dizygotic twinning

Fraternal twins from two separate fertilizations (two zygotes).

Monozygotic twinning

Identical twins from a single fertilized egg that splits.

Ovulation

Release of a secondary oocyte from the ovary prior to fertilization.

Capacitation

Maturation of sperm that enables fertilization, including increased motility and receptor exposure.

Acrosome reaction

Release of enzymes from the sperm acrosome that help penetrate the zona pellucida.

Corona radiata

Outer protective layer of cells surrounding the oocyte.

Zona pellucida

Glycoprotein layer surrounding the oocyte that sperm must penetrate during fertilization.

Fertilization

Fusion of sperm and oocyte nuclei forming a zygote.

Pronuclei

Male and female nuclei inside the zygote before they fuse to form the diploid genome.

Polyspermy prevention

Blockades (e.g., cortical reaction) that prevent more than one sperm from fertilizing the egg.

Epithelium

A type of tissue formed from germ layers; lines organs and structures.