Human Reproductive Biology and Genetics Essentials

Posted on May 22, 2026 in Biology

Reproductive System Functions

  • Produce gametes
  • Transport and maintain gametes
  • Female supports embryo/fetus
  • Produce sex hormones
    • Male: testosterone
    • Female: estrogen, progesterone

Meiosis

  • Produces haploid gametes (23 chromosomes)
  • Meiosis I: homologous chromosomes separate; crossing over in Prophase I
  • Meiosis II: sister chromatids separate
  • Result: 4 haploid cells
  • Oogenesis: 1 ovum + polar bodies
  • Spermatogenesis: 4 sperm

Male Reproductive System

Testes

  • Seminiferous tubules: sperm production
  • Sertoli cells: nourish developing sperm
  • Leydig cells: testosterone production

Sperm Pathway

Seminiferous tubules → rete testis → epididymis → vas deferens → ejaculatory duct → urethra

Accessory Glands

  • Seminal vesicles: fructose-rich alkaline fluid; majority of semen
  • Prostate: activates and protects sperm
  • Bulbourethral glands: mucus; neutralizes urethra

Sperm Structure

  • Head: nucleus + acrosome
  • Midpiece: mitochondria
  • Tail: flagellum

Spermatogenesis

Spermatogonia → Primary spermatocyte → Secondary spermatocyte → Spermatids → Spermatozoa

Hormonal Control

GnRH → FSH + LH

  • FSH: stimulates spermatogenesis
  • LH: stimulates Leydig cells → testosterone

Testosterone Functions

  • Male reproductive organ development
  • Testicular descent
  • Secondary sex characteristics
  • Muscle and bone growth

Female Reproductive System

Ovaries

  • Cortex: follicles
  • Medulla: blood vessels and nerves

Uterine (Fallopian) Tubes

  • Site of fertilization
  • Fimbriae guide oocyte

Uterus

  • Regions: fundus, body, cervix
  • Layers: endometrium, myometrium, perimetrium

Vagina

  • Birth canal
  • Receives penis

External Genitalia (Vulva)

  • Labia majora
  • Labia minora
  • Clitoris
  • Vestibule

Oogenesis

  • Before birth: primary oocytes arrested in Prophase I
  • Puberty: completes Meiosis I → secondary oocyte + polar body
  • Ovulation: arrested in Metaphase II
  • Fertilization completes Meiosis II

Follicle Development

Primordial → Primary → Secondary → Mature (Graafian) → Ovulation → Corpus luteum → Corpus albicans

Female Hormones

  • FSH: follicle growth
  • LH: ovulation and corpus luteum formation
  • Estrogen: secondary sex characteristics; endometrial growth
  • Progesterone: maintains uterine lining

Menstrual Cycle

  • Days 1–5: Menstrual
  • Days 6–13: Follicular/Proliferative
  • Day 14: Ovulation (LH surge)
  • Days 15–28: Luteal/Secretory

Mammary Glands

  • Lobes → lobules → alveoli
  • Colostrum: antibody-rich first milk

Birth Control

  • Condoms
  • Diaphragm
  • Spermicides
  • Oral contraceptives
  • Depo-Provera
  • IUDs
  • Vasectomy
  • Tubal ligation

STIs

  • Chlamydia and gonorrhea may cause PID
  • HIV causes AIDS
  • Many infections are asymptomatic

Fertilization

Occurs in uterine tube:

  1. Acrosome reaction
  2. Sperm enters oocyte
  3. Cortical reaction blocks polyspermy
  4. Oocyte completes Meiosis II
  5. Pronuclei fuse → zygote (46 chromosomes)

Prenatal Period (38 Weeks)

  • Pre-embryonic: Weeks 1–2
  • Embryonic: Weeks 3–8
  • Fetal: Week 9 to birth

Cleavage and Implantation

Zygote → Morula → Blastocyst

  • Inner cell mass = embryo
  • Trophoblast = placenta
  • Implantation occurs days 6–12

Extraembryonic Membranes

  • Chorion: forms fetal placenta
  • Amnion: amniotic sac and fluid
  • Yolk sac: early blood formation; germ cells
  • Allantois: forms umbilical vessels

Placenta Functions

  • Gas exchange
  • Nutrient delivery
  • Waste removal
  • Hormone production (hCG, estrogen, progesterone)

Embryonic Stage

Germ Layers

  • Ectoderm: skin, nervous system
  • Mesoderm: muscle, bone, blood
  • Endoderm: digestive and respiratory linings

Teratogens

Drugs, alcohol, radiation, infections. Highest risk: Weeks 3–8

Fetal Stage

  • Sex identifiable by 12 weeks
  • Organs mature, especially lungs and brain

Fetal Circulation

  • Ductus venosus: bypasses liver
  • Foramen ovale: right atrium → left atrium
  • Ductus arteriosus: pulmonary trunk → aorta

Newborn Changes

  • Foramen ovale → fossa ovalis
  • Ductus arteriosus → ligamentum arteriosum
  • Ductus venosus → ligamentum venosum
  • Umbilical vein → ligamentum teres

Parturition (Birth)

↓ Progesterone → ↑ prostaglandins and oxytocin → uterine contractions

Genetics Basics

  • Gene: DNA sequence coding for a trait
  • Genome: complete genetic information
  • Karyotype: chromosome display
  • Alleles: different forms of a gene

Key Terms

  • Homozygous: AA or aa
  • Heterozygous: Aa
  • Genotype: genetic makeup
  • Phenotype: observable trait

Mendelian Inheritance

  • Dominant alleles mask recessive alleles
  • Punnett squares predict probabilities
  • Pedigrees track inheritance patterns

Extensions of Mendelian Genetics

  • Codominance: AB blood type
  • Incomplete dominance: pink flowers
  • Multiple alleles: ABO system
  • Polygenic inheritance: height, skin color
  • Pleiotropy: Marfan syndrome

Sex-Linked Traits

  • X-linked: more common in males
  • Y-linked: father passes to all sons
  • SRY gene triggers male development

Gene Expression Terms

  • Penetrance: whether a trait appears
  • Expressivity: severity of trait
  • Genetic heterogeneity: same phenotype from different genes

Hormone Effects on Traits

  • Sex-limited: expressed in one sex only
  • Sex-influenced: dominance differs by sex

Multifactorial Traits

Genes + environment. Examples: height, diabetes, heart disease, cancer