Embryonic Development and Principles of Heredity

Fertilization

Male and female pronuclei are in the process of fusing - about 24 hours - in fallopian tube.

Zygote

Fertilized egg with one nucleus containing (normally) 46 chromosomes.

Embryo

Weeks 3-8 of development.

Fetus

Week 9 to birth.

Differentiation

Cellular 'decision' to become a particular cell type.

Amnion

Surrounds growing embryo, filled with fluid (amniotic fluid), ruptures at birth.

Allantois

Becomes umbilical cord.

Yolk sac

Provides nutrients very early in development then placenta takes over, creates embryonic blood cells, becomes part of the digestive system.

Embryoblast

Inner cell mass.

Implantation

By the time implantation is complete, gastrulation is also complete.

Chorionic villi

Reach into endometrium into blood-filled sinusoids.

Decidua basalis

Endometrium beneath embryo, will become part of the placenta.

Decidua capsularis

Endometrium on the luminal side of uterus.

Genotype

Genes.

Phenotype

Expression of the gene (what you see).

Allele

Version of the gene (may be dominant or recessive).

Locus

Specific 'address' of the gene on the chromosome (loci - plural).

Dominant

The version of the allele that is expressed (you see this one - the phenotype is seen).

Recessive

The version that can be 'hidden' by the expression of the dominant gene.

Homozygous

Both alleles are the same (either both dominant or both recessive).

Heterozygous

Alleles are different; one allele is dominant and one is recessive.

Punnett square

Given genotype of parents, predict percentage of genotype and phenotype of offspring (assuming total dominance).

ABO blood group

A case of codominance; Mom - ii (genotype) O (phenotype), Dad - IAi.