Development and Genetics

Human Development Overview

• Human development describes the growth of a fertilized egg into a human
• Sperm undergo final maturation in the female reproductive tract, a process that culminates in fertilization.
• During fertilization, the sperm nucleus enters the ovum and egg and sperm nuclei fuse.
• The product of fertilization is a zygote, which contains 46 chromosomes, consisting of 23 from the egg and 23 from the sperm
• The zygote contains 22 pairs of autosomes and a pair of sex chromosomes.

Stages of Early Development

• The zygote begins mitotic divisions known as cleavages, growing from 2-cell to 4-cell to 8-cell stages
• The morula forms a solid sphere of cells before dividing into a hollow sphere called a blastocyst
• The blastocyst consists of the trophoblast, an outer layer of cells and the embryoblast, an inner cell mass containing a potential embryo

Blastocyst and Implantation

• Embryoblasts are stem cells that aren't yet specialized.
• Surface proteins on the blastocyst are compatible with docking molecules on the endometrium.
• The trophoblast secretes enzymes, creating a depression in the endometrium for implantation.
• Implantation takes place 5-8 days after fertilization.
• After implantation, the trophoblast turns into the chorion and develops into the fetus' portion of the placenta

Embryonic Development

• An embryo develops from a zygote for eight weeks of gestation.
• Three primary germ layers develop: ectoderm, mesoderm, and endoderm.
• Organ systems have formed by the eighth week of gestation

Primary Germ Layers

• Ectoderm structures include the epidermis, hair, nail follicles, sweat glands, and nervous system.
• Other ectoderm features are the pituitary gland, adrenal medulla, lens, cornea of the eye, the internal ear, and the mucosa of oral and nasal cavities, and salivary glands.
• Mesoderm structures include the dermis, bone, cartilage, skeletal muscles, and cardiac and smooth muscle.
• Additionally, kidneys, adrenal cortex, bone marrow, blood, lymphatic tissue, and lining of blood vessels are derived from the mesoderm.
• Endoderm structures include the mucosa of the esophagus, stomach, intestines, epithelium of the respiratory tract, lungs, liver, mucosa of the gallbladder, thyroid gland and the pancreas.

Embryonic Membranes

• The yolk sac forms the first blood cells and cells that become spermatogonia or oogonia
• The Amnion surrounds the fetus, contains amniotic fluid, and absorbs shock.
• The Chorion develops chorionic villi that contains blood vessels for the fetal portion of the placenta

Fetal Development

• The fetus develops from weeks 9-40 of gestation.
• During this period, organ systems grow and mature.
• The growing fetus leads to structural and functional changes in the mother

Umbilical Cord and Placenta

• The umbilical cord connects the fetus to placenta.
• The placenta, formed by the chorion of the embryo and uterine endometrium, functions as the exchange site between maternal and fetal blood and produces hormones to maintain pregnancy.
• The fetal blood does not mix with maternal blood, but the fetal capillaries are within maternal blood sinuses to allow exchanges between maternal and fetal blood.
• There are two umbilical arteries that carry blood from the fetus to the placenta

Exchange in Maternal Blood

• Fetal carbon dioxide and waste products diffuse into the maternal blood.
• Oxygen and nutrients enter fetal blood.
• The umbilical vein returns blood from placenta to fetus.

Aspects of Development by Month

• Month 1: A heart begins to beat, limb buds form, the backbone forms, facial features are not distinct, and the neural tube closes
• Month 2: Calcification of bones begins, fingers and toes appear on limbs, facial features become more distinct, and body systems are established
• Month 3: Facial features are distinct but eyes are still closed, fingers and toenails develop; ossification of the skeleton continues and the fetus is distinguishable as male or female
• Month 4: The head is still quite large compared to the body, the arms and legs lengthen, hair appears on the head, and body systems continue to develop
• Month 5: Skeletal muscles become active and the body grows more rapidly than the head, and body is covered with fine hair (lanugo)
• Month 6: Eyelashes and eyebrows form, eyelids open, skin is wrinkled and pulmonary surfactant is first produced
• Month 7: The head and body approach normal infant proportions, with deposition of subcutaneous fat making skin less wrinkled
• Month 8: The testes of the male fetus descend into the scrotum, subcutaneous fat is deposited, and production of pulmonary surfactant is adequate to permit breathing if the birth is premature
• Month 9: Lanugo is shed, nails become fully developed, cranial bones ossify with fontanels present and lungs become more mature

Genetics Basics

• Genetics studies inheritance
• Each cell in the human body has 46 chromosomes, in 23 homologous pairs.
• Of these, 22 pairs are autosomes and 1 pair are sex chromosomes, either XX or XY
• DNA is the hereditary material of the chromosomes, and a gene is the genetic code for one protein.
• Each individual possesses two genes for each protein or trait, one maternal and one paternal.

Genotype and Phenotype

• Alleles are versions of a gene that determine gene expression.
• Genotype is the actual genetic makeup, including the alleles present.
• Phenotype is the appearance, or how alleles are expressed, based on dominance, recessiveness, and inheritance patterns.
• A gene has two or more alleles.
• Alleles can be dominant or recessive

Heterozygous vs Homozygous

• Heterozygous individuals have one dominant allele and one recessive allele.
• In heterozygotes, the dominant allele appears in the phenotype, while the recessive allele is not expressed but can be passed to children
• Homozygous individuals have two identical alleles, which can be homozygous dominant or recessive.
• Expression of recessive alleles requires homozygous recessive genotype

Inheritance

• In dominant-recessive inheritance, the dominant gene appears in the phenotype of heterozygotes.
• The recessive gene appears in the phenotype only if the individual is homozygous recessive
• When multiple alleles are present, there are more than two possible alleles for each gene, but each person only has two alleles, same or different

Sex-Linked Traits

• Genes for sex-linked traits are found only on the X chromosome.
• There are no corresponding genes on the Y chromosome.
• Women with one gene for the trait and one without it are called carriers
• Men cannot be carriers, they either have the trait or do not have it.

Inheritance Pattern Example

• Autosomal recessive inheritance includes disorders like sickle cell anemia and cystic fibrosis
• Examples of autosomal dominant inheritance include Huntington’s disease, polycystic kidney disease, and Marfan syndrome
• Sex-linked inheritance patterns include Duchenne’s muscular dystrophy, hemophilia A and B, and red-green color blindness