BackHuman Development and Heredity: Study Guide
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Human Development
Sex Determination
The sex of an offspring is determined by the combination of sex chromosomes contributed by each parent at fertilization.
Key Point: The mother always contributes an X chromosome, while the father can contribute either an X or a Y chromosome.
Key Point: An XX combination results in a female, while an XY combination results in a male.
Example: If a sperm carrying a Y chromosome fertilizes the oocyte, the offspring will be male (XY).
Oocyte at Ovulation
At ovulation, the oocyte is released from the ovary and is typically arrested in metaphase II of meiosis.
Key Point: The oocyte is surrounded by a protective layer called the zona pellucida and a group of follicular cells known as the corona radiata.
Key Point: It will only complete meiosis II if fertilization occurs.
Amphimixis
Amphimixis is the fusion of the male and female pronuclei during fertilization, resulting in the formation of a diploid zygote.
Key Point: This process restores the diploid number of chromosomes (46 in humans).
Major Changes in Each Trimester
Human gestation is divided into three trimesters, each marked by distinct developmental milestones.
First Trimester: Major organ systems begin to form; embryo becomes a fetus by the end of this period.
Second Trimester: Organs and structures continue to develop; fetus grows rapidly in size.
Third Trimester: Most organ systems become fully functional; fetus gains weight and prepares for birth.
From Zygote to Morula
After fertilization, the zygote undergoes a series of mitotic divisions called cleavage, resulting in a solid ball of cells known as the morula.
Key Point: Cleavage increases the number of cells without increasing the overall size of the embryo.
From Morula to Blastocyst
The morula develops a fluid-filled cavity, transforming into a blastocyst.
Key Point: The blastocyst consists of an outer layer (trophoblast), an inner cell mass, and a central cavity (blastocoel).
Layers of the Blastocyst
Trophoblast: Outer layer that will contribute to the placenta.
Inner Cell Mass: Cluster of cells that will develop into the embryo.
Blastocoel: Fluid-filled cavity.
Implantation
Implantation is the process by which the blastocyst attaches to and embeds within the endometrial lining of the uterus.
Key Point: The trophoblast cells invade the endometrium, initiating the formation of the placenta.
Formation of the Blastodisc
The blastodisc forms from the inner cell mass and gives rise to the embryonic germ layers.
Key Point: The blastodisc is a flattened, two-layered structure (epiblast and hypoblast).
The Neural Tube
The neural tube is a structure that forms from the ectoderm and will develop into the central nervous system (brain and spinal cord).
The Three Germ Layers and Their Derivatives
During gastrulation, three primary germ layers form:
Ectoderm: Forms the nervous system, skin, and related structures.
Mesoderm: Forms muscles, bones, cardiovascular system, and other organs.
Endoderm: Forms the lining of the digestive and respiratory tracts, and associated organs.
Extraembryonic Membranes and Their Functions
Membrane | Function |
|---|---|
Amnion | Encloses the embryo in amniotic fluid, providing protection |
Chorion | Contributes to the placenta and facilitates nutrient/gas exchange |
Yolk Sac | Early site of blood cell formation |
Allantois | Involved in early fluid exchange and forms part of the umbilical cord |
Trophoblast, Chorion, and Placenta
Trophoblast: Outer layer of the blastocyst; initiates implantation and forms part of the placenta.
Chorion: Develops from the trophoblast; forms the fetal part of the placenta.
Placenta: Organ that facilitates exchange of nutrients, gases, and wastes between mother and fetus.
Decidua of the Uterus
The decidua refers to the modified endometrial lining of the uterus during pregnancy, which supports the developing embryo and placenta.
Maternal System Changes During Gestation
Cardiovascular: Increased blood volume and cardiac output.
Respiratory: Increased respiratory rate and tidal volume.
Renal: Increased glomerular filtration rate.
Endocrine: Hormonal changes to support pregnancy.
Three Stages of Labor
Dilation Stage: Cervix dilates, contractions begin.
Expulsion Stage: Baby is delivered.
Placental Stage: Placenta is expelled from the uterus.
Placenta After Birth
After the fetus is born, the placenta detaches from the uterine wall and is expelled as the "afterbirth."
Monozygotic vs. Dizygotic Twins
Monozygotic (Identical) Twins: Develop from a single fertilized egg that splits; genetically identical.
Dizygotic (Fraternal) Twins: Develop from two separate eggs fertilized by two different sperm; genetically similar as siblings.
Neonatal Period Changes
Respiratory: Lungs begin to function; first breath inflates alveoli.
Circulatory: Closure of fetal shunts (foramen ovale, ductus arteriosus).
Thermoregulation: Newborn must regulate its own body temperature.
Heredity and Genetics
Gene, Allele, Locus
Gene: A segment of DNA that codes for a specific protein or trait.
Allele: Different forms of a gene found at the same locus.
Locus: The specific physical location of a gene on a chromosome.
Dominant, Recessive, Homozygous, Heterozygous
Dominant: An allele that is expressed in the phenotype even if only one copy is present.
Recessive: An allele that is only expressed when two copies are present.
Homozygous: Having two identical alleles for a gene.
Heterozygous: Having two different alleles for a gene.
Monohybrid Cross
A monohybrid cross examines the inheritance of a single trait.
Example: Crossing two heterozygous individuals for a trait (Aa x Aa) yields a 3:1 ratio of dominant to recessive phenotypes.
Sex-Linked Cross
A sex-linked cross involves genes located on the sex chromosomes, often the X chromosome.
Example: Color blindness inheritance in humans, where the gene is on the X chromosome.
Multiple Alleles Cross
Some genes have more than two possible alleles in the population.
Example: Human ABO blood groups (alleles: IA, IB, i).
Basic Genetics of Offspring
The genetic makeup of offspring is determined by the combination of alleles inherited from each parent, following Mendelian principles.
Key Equation:
