Genes and Inheritance
Fertilization: New life begins at fertilization, when the sperm and egg combine their genetic material. Genetic material is located in the nucleus.
Chromosome ploidy: Haploid (1N) = one complete set of all the different chromosomes in one cell. Diploid (2N) = two copies of each chromosome. Mitosis = Duplication and division of 2N diploid cells to produce more diploid cells.
Meiosis: Human somatic cells (body cells):
23 pairs (46 total) of homologous chromosomes (2N,
22 pairs are autosomes
1 pair is sex chromosomes - X & Y
Females are XX
Males are XY
Human germ line cells (eggs and sperm):
23 chromosomes (1N, haploid)
Dominant versus recessive: Dominant allele - An allele that expresses its trait regardless of the other allele, usually designated with an upper-case letter. Recessive allele - An allele that cannot express its phenotype when a dominant allele is present, usually designated with a lower-case letter.
Laws of Segregation: During the gamete formation, the two alleles of one gene segregate independently without mixing with each other.
Independent assortment: During the gamete formation, genes from different chromosomes assort independently and combine randomly.
Punnett Square: Punnett Squares simplify genetic problem-solving to predict genotypes of progeny.
Linkage and recombination: Some traits (genes) can be linked, linked = genes on the same chromosome that are likely to be inherited together. Linked genes can cross-over and recombine at a certain frequency called the linkage ratio.
Early development: Day 0 = Ovulation of an egg from the ovary into the oviduct, Day 1 = Fertilization of the egg by a sperm, Days 2-5 = Cleavage and blastocyst formation and Day 6 or 7 = Implantation into uterus endometrium.
Blastocyst formation: Blastocyst = ball of cells with a center cavity, first evidence of cell differentiation is formation of trophoblast and inner cell mass. Blastocoel = fluid- filled cavity in the center of the blastocyst. Trophoblast = outer layer of cells, contributes to placenta. Inner cell mass = inner group of cells, contributes to embryo.
Implantation: Around Day 6-7: Blastocyst fuses to uterine endometrium and embeds itself into the tissue.
Fetal development: Trimesters = the 9-10 month pregnancy is divided into 3 trimesters. First Trimester: early embryogenesis, organogenesis begins, limb formation, heartbeat detected. Second Trimester: organ systems become more complex, fetal movement felt by mother (quickening), lung surfactant first produced. Third Trimester: largest increase in size and weight, most organ systems functional, fetus able to survive outside mother if born prematurely.
Labor induction: Labor = delivery of fetus from uterus to external environment Marked by regular time intervals between contractions. Dilation stage (lasts 6-12 hours), from labor onset, to complete cervical dilation, there are contractions of the uterus and rupture of amniotic sac (“my waters broke”). Expulsion stage (lasts minutes to hours) - From cervical dilation to delivery, the baby travels out through birth canal. Placental stage (lasts 5-30 minutes) - From delivery to placenta elimination Placenta must be removed from mother, called the “afterbirth”.