Development and Inheritance

Development and Inheritance

Human Development

  • Developmental biology: The study of events from fertilization to death.
  • Embryology: The study of pre-natal development from fertilization to birth.
  • Conceptus: Term identifying products of conception from fertilization up to birth.

Developmental Biology Stages

  • Gametogenesis: Production of gametes (oocytes and sperm).
  • Fertilization: Fusion of oocyte (n) and sperm (n) to form a zygote (2n).
  • Pre-Embryonic Period (Weeks 1-2):
    • Cleavage: Cell division of the zygote.
    • Blastocyst Formation: Development into a blastocyst.
  • Embryonic Period (Weeks 3-8):
    • Gastrulation: Formation of germ layers.
    • Organogenesis: Development of organs.
  • Fetal Period (Weeks 9-38):
    • Maturation: Continued development and growth.
  • Birth

Pre-Natal Development Periods

  1. Pre-embryonic period: First two weeks after fertilization.
  2. Embryonic period: Week three through week eight.
  3. Fetal period: Ninth week through birth (about 30 weeks).
  4. Post-natal development: From birth until death.

Pre-Embryonic Period Details

  1. Prep steps:
    • Oocytes' corona cells release progesterone, attracting sperm.
    • Spermatocytes undergo capacitation, enabling penetration of corona radiata.
  2. Fertilization:
    • Sperm (haploid) and oocyte (haploid) fuse into zygote (diploid).
  3. Spermatocyte and oocyte nuclei join, forming the zygote.
  4. Cleavage occurs.
  5. Blastocyst formation.

Phases of Fertilization

  1. Sperm penetrates corona radiata.
  2. Sperm undergoes acrosome reaction and penetrates zona pellucida.
  3. Sperm and oocyte plasma membranes fuse; pronuclei of ovum and sperm fuse.

Pre-Embryonic Stages

  1. Cleavage of zygote.
  2. Blastocyst formation.
  3. Blastocyst cells differentiate into:
    • Trophoblast: Gives rise to the placenta.
    • Embryoblast: Gives rise to the embryo.

Blastocyst in Uterus

  1. Implantation: Occurs in the body/fundus of the uterus (placenta previa is a complication).
  2. Trophoblasts differentiate into:
    • Syncytiotrophoblast:
      • Digests endometrium.
      • Secretes hCG (human chorionic gonadotropin).
    • Cytotrophoblast.
  3. Embryoblast cells differentiate into:
    • Epiblast: Gives rise to three germ layers.
    • Hypoblast.

Hormones During Pregnancy

  • hCG levels peak early, then decline.
  • Estrogen and progesterone rise steadily, with the placenta becoming the main producer.

Changes in Epiblast and Hypoblast

  1. Epiblast and hypoblast make up the embryonic disc.
  2. Epiblast:
    • Separates from trophoblast creating amniotic cavity; the roof is called the amnion.
    • Develops a groove in the midline called the primitive streak.
      • Primitive streak establishes bilateral symmetry.
    • Epiblast cells differentiate into three germ layers (gastrulation):
      • Ectoderm.
      • Mesoderm: Contributes to the placenta (extraembryonic mesoderm).
      • Endoderm.
  3. Hypoblast cells form the yolk sac, enclosing a space.

Epiblast Cells Invaginating - Gastrulation

  • Epiblast cells invaginate through the primitive streak to form endoderm and mesoderm.

Embryonic Stage

  1. Placenta begins development:
    • Functions include:
      • Exchange of nutrients, gases, and waste between mother and conceptus.
      • Hormone production: estrogen, progesterone, inhibin, and relaxin.
  2. The trilaminar disc differentiates into all organs and organ systems (organogenesis).
  3. Embryonic folding occurs.

Embryonic Stage - Continuation

  1. Embryonic Folding:
    • Occurs in week 4.
    • Cephalic and caudal ends curve, almost touching.
    • Lateral walls curve.
    • Mesoderm splits into two layers creating cavities called the coelom:
      • Thoracic and peritoneal cavities.
  2. Mesoderm differentiates into 5 categories:
    • Notochord.
    • Paraxial mesoderm.
    • Intermediate mesoderm.
    • Lateral Plate mesoderm.
    • Head mesenchyme.
  3. Neural tube appears, giving rise to brain and spinal cord.

Five Categories of Mesoderm

  1. Notochord:
    • Formed by tightly packed midline group of mesodermal cells.
    • Basis for central body axis and axial skeleton.
    • Induces formation of neural tube.
  2. Paraxial mesoderm:
    • Found on both sides of neural tube.
    • Forms somites, blocklike masses:
      • Sclerotomes (Vertebral column).
      • Myotomes (Trunk muscles).
      • Dermatomes (Dermis).
  3. Intermediate mesoderm
    • Lateral to paraxial mesoderm.
    • Forms most of kidneys, ureters, and reproductive system
  4. Lateral plate mesoderm
    • Most lateral layers of mesoderm
    • Forms spleen, adrenal cortex, and cardiovascular system
    • Serous membranes and connective tissue of limbs
  5. Head mesenchyme
    • Forms connective tissues and musculature of face

Embryonic Folding Details

  • Illustrations showing cephalocaudal and transverse folding during weeks 3 and 4.
  • Formation of gut tube, neural tube, and body cavities.

Mesoderm Differentiation Details

  • Superior and cross-sectional views showing the differentiation of mesoderm into different structures.

Extraembryonic Membranes and Other Structures

  1. Extraembryonic membranes:
    • Chorion.
    • Amnion.
    • Allantois.
    • Yolk sac.
  2. Other structures:
    • Umbilical cord: Connects embryo/fetus to placenta; begins as connecting stalk.
    • Placenta: For exchange; consists of embryonic and maternal tissues.

Amnion Details

  1. Transparent membrane.
  2. Develops from epiblast.
  3. Grows to enclose the embryo.
  4. Only penetrated by the umbilical cord.
  5. Filled with amniotic fluid.
  6. Protects the embryo/fetus from trauma, temperature changes, and infections.

Yolk Sac Details

  1. Develops from hypoblast.
  2. Suspended from the ventral side of the embryo.
  3. Becomes part of the digestive tract.
  4. Makes the first blood cells.
  5. Makes future sex cells.

Allantois Details

  1. Begins as an out pocket of the yolk sac, protruding into the connecting stalk.
  2. Foundation for the umbilical cord.
  3. Becomes continuous with the urinary bladder.

Chorion Details

  1. Outermost layer.
  2. Encloses all other membranes and embryo.
  3. Begins as an outgrowth of the trophoblast - chorionic villi - all around the blastocyst.
  4. As it develops, villi will only be present at the placental side; all the other villi degenerate.
  5. Attaches the placenta to the uterine wall.

Chorion - Illustration

  • Diagrams showing the development of the chorion, amnion, yolk sac, and placenta across weeks 3 and 4.

Fetal Period Details

  1. Conceptus growth from 2.5 cm to 53 cm (1 inch to 21 inches).
  2. Time of rapid growth.
  3. Tissues and organs mature.
  4. By weeks 17-20:
    • Body is covered with lipids (vernix caseosa) for protection and hair (lanugo).
    • Movements are felt by the mother.

Fetal and Newborn Differences

  1. Circulatory system differences:
    • Ductus venosus: Bypasses the liver.
    • Foramen ovale in the heart and ductus arteriosus between the aorta and pulmonary trunk: Bypasses the lungs.

Pregnancy or Gestation

  1. Time when one or more offspring develop inside a woman.
  2. 40 weeks long (from last menstrual period).
  3. Divided into trimesters:
    • First trimester (first 13 weeks = 3 months): Most critical stage; most malformations occur during this time.
    • Second trimester (from 4th month through the 6th month): Organ systems are complete.
    • Third trimester (from 7th month through the 9th month): Rapid growth; most organ systems are functioning.
  4. 26% of all pregnancies end in miscarriage (26 out of 100).

Teratogens

  1. Exposure to certain environmental factors can damage/cause death to the embryo/fetus.
  2. Most common teratogens:
    • Chemicals/drugs:
      • Alcohol: Fetal alcohol syndrome (defective heart, malformed limbs, genital abnormalities).
      • Cocaine: Increases chances of SIDS, problems with attention, missing organs.
      • Marijuana: Low birth weight and attention deficit disorders.
      • Cigarette smoking: Low birth weight, cleft lip, cleft palate, SIDS, heart abnormalities.
      • Aspirin.
    • Infections: TORCH syndrome (Toxoplasmosis, Rubella, Cytomegalovirus, and Herpes simplex) – microcephaly, blindness, mental retardation.
    • Irradiation: X-rays or radioactive isotopes – microcephaly, mental retardation.

Fetal Alcohol Syndrome

  1. Most common cause of mental retardation.
  2. Most common preventable cause of birth defects.
  3. Symptoms: slow growth, thin upper lip, defective heart, hyperactivity disorder, inability to concentrate, inability to measure cause-effect relationship.

Prenatal Diagnostic Tests

  1. Ultrasound: Commonly used; non-invasive.
  2. Amniocentesis: To detect genetic disorders at 14-18 weeks; invasive.
  3. Chorionic villi sampling: To detect genetic disorders as early as 8 weeks; invasive.
  4. Maternal Blood tests: Non-invasive to the developing embryo/fetus.
    • Maternal alpha-fetoprotein level: Detects neural tube defects in the fetus.
    • Quad AFP Plus: To screen for Down syndrome, trisomy 18, and neural tube defects.

Changes During Pregnancy

  1. End of the third month: The uterus occupies most of the pelvis; frequent urination, nausea/vomiting (hCG) – morning sickness.
  2. End of the third trimester: Uterus fills most of the abdominal cavity.
  3. Signs and symptoms:
    • Heartburn.
    • Increased urination.
    • Weight gain (extra water, amniotic fluid, fetus weight, placenta, uterus enlargement).
    • Breast enlargement.
    • Lower back pain.

Progression of Pregnancy

  • Illustrations showing the position of the uterus and fundus at different months of pregnancy.

Labor – Giving Birth

  1. Hormonal changes:
    • Increased CRH.
    • Increased estrogens.
    • Decreased progesterone.
  2. Stages of labor:
    • Stage of dilation: 6-12 hours.
    • Stage of expulsion: 10 min - hours.
    • Placental stage: 3-5 minutes.

Stages of Labor Details

  • Diagrams illustrating the stages of labor: early dilation, late dilation, expulsion, and placental stage.

Inheritance

  • Inheritance is the passage of hereditary traits (genes) from one generation to the next – parent to child.
  • Genetics: Branch of biology that deals with inheritance.

DNA and Genes

  1. Genes are nucleotide sequences on DNA.
  2. Genes code for proteins.
  3. Mutations: change in nucleotide sequence.
  4. Mutations may lead to an abnormal protein.
  5. Genes are responsible for different traits.

Chromosomes and Chromatids

  1. Somatic cells have 23 pairs of chromosomes:
    • 22 autosomes pairs.
    • 1 pair of sex chromosomes (XX / XY).
  2. Chromosomes of a pair carry the same genes – homologous.
  3. Sex cells (gametes) have 23 chromosomes:
    • 22 autosomes.
    • 1 sex chromosome (X or Y).

Abnormal Karyotype

  • Image illustrating an abnormal karyotype with monosomy and loss of the second sex chromosome (Turner Syndrome).

Genotype and Phenotype

  1. Genotype is the genetic makeup (genes that a person has).
  2. Phenotype: Outer or physical expression of a genotype.
  3. Genotype determines phenotype.

Alleles – Dominant and Recessive

  1. Traits may have more than 2 varieties – the genes that code for different varieties of a trait are called alleles.
  2. An individual can only carry 2 different alleles for a particular trait.
  3. Alleles of a pair may be:
    • Homozygous - identical.
    • Heterozygous - different.
    • Dominant – are always expressed.
    • Recessive – expressed when both alleles of a pair are recessive.
  4. Heterozygous individuals are carriers of the recessive gene.

Traits That Follow a Strict Dominant-Recessive Inheritance

  • Table listing dominant and recessive traits such as dimples, cleft chin, widow's peak, and blood type.

Punnett Squares

  • Diagrams used to predict the allele composition of an offspring between individuals of known genetic makeup.

Straight Hairline - Recessive Trait

  • Punnett square example showing inheritance of widow's peak (dominant) vs. straight hairline (recessive).

Dominant Recessive Inheritance Example

  • Example of a Punnett square for cystic fibrosis, where both parents are carriers (Cc).
  • Outcomes: 50% carrier, 25% have cystic fibrosis, 75% no disease, 50% homozygous, 50% heterozygous.

Incomplete Dominance

  1. Neither allele is dominant over the other.
  2. The heterozygous have an intermediate phenotype.
  3. Example: Sickle Cell Disease - Half of the hemoglobin is normal, and half is not – individuals have some anemia and are carriers.

Codominance Inheritance

  1. Genes for a trait may have more than two alleles.
  2. For codominance, there is more than one dominant allele. These alleles are codominant.
  3. ABO blood groups alleles: I^A, I^B, i
  • Punnett square example showing inheritance of ABO blood types.

Complex Inheritance

  1. Polygenic inheritance is seen when a trait is controlled by the combined effects of two or more genes.
  2. Complex inheritance is the combined effects of many genes controlling one trait and the environment.
  3. Complex traits:
    • skin color
    • hair color
    • eye color
    • height
    • metabolic rate
    • body type

Sex Link Inheritance

  1. Sex chromosomes determine the sex of the offspring, but more importantly, they are responsible for the transmission of many nonsexual traits.
  2. Most traits are present on the X chromosomes because they have more genes unrelated to female development.
  3. Women are usually carriers, and males express the affected genes.

Red-Green Color Blindness

  1. Individuals lack either green or red cones.
  2. Normal is dominant – C.
  3. Color blind is recessive – c.

Hemophilia

  1. Conditions in which blood fails to clot or does so very slowly.
  • Punnett square example showing inheritance of Hemophilia A, a sex-linked trait.
  • X^HX^H: Non-affected daughter
  • X^HX^h: Carrier daughter
  • X^HY: Non-affected son
  • X^hY: Son with hemophilia