Human Development and Genetics Overview

Chapter Overview

• Human development from meiosis to aging is covered, encompassing gene activity, anatomy, physiology, and environmental influences
• Genetic conditions may affect individuals throughout life
• Sexual reproduction (meiosis and fertilization) maintains the diploid chromosome number and recombines alleles, protecting against environmental changes
• Developing humans are vulnerable to teratogens and detrimental environmental agents, possibly leading to birth defects
• Genome studies are elucidating the genetic basis of longevity

Chapter Outline

3.1 The Reproductive System

• Genes control growth and development from fertilization; mutations can affect health
• Male and female reproductive systems have paired gonads, tubes for transport, and glands whose secretions enable gamete function

The Male

• Sperm develop in seminiferous tubules within the testes
• Sperm mature in the epididymis, which connect the testes to the ductus deferentia which meet at the urethra in the penis
• Prostate gland, seminal vesicles, and bulbourethral glands produce secretions for semen
• Approximately 200-600 million sperm are released per ejaculation

The Female

• Oocytes develop in the ovaries
• One oocyte is released monthly from an ovarian follicle and captured by uterine tube projections
• Uterine tubes transport the oocyte to the uterus
• The uterus nurtures a fertilized ovum (zygote)
• The cervix connects the uterus to the vagina
• Hormones regulate oocyte development and release, and uterine preparation

3.2 Meiosis

• Meiosis is cell division producing haploid (1n) gametes from diploid (2n) germline cells
• Meiosis involves two stages: Meiosis I and Meiosis II
• Meiosis I reduces chromosome number
• Meiosis II is an equational division of the two cells from Meiosis I, resulting in four haploid cells
• Crossing over and independent assortment (random alignment of chromosomes during Metaphase I) generate genetic diversity
• More than 70 trillion combinations are possible when a sperm fertilizes an oocyte

3.3 Gametes Mature

Sperm Form

• Spermatogenesis begins with a diploid spermatogonium, producing a stem cell and a primary spermatocyte
• Meiosis I halves genetic material, producing two haploid secondary spermatocytes
• Meiosis II produces four equal-sized spermatids
• Spermatids mature into spermatozoa with a tail, body (midpiece), and head (with acrosome)
• Acrosome enzymes allow sperm to penetrate the oocyte's protective layers
• Sperm with mutations or damage have difficulties fertilizing an oocyte

Oocyte Form

• Oogenesis starts with a diploid oogonium and development to a primary oocyte
• Meiosis I produces a large haploid secondary oocyte and a small polar body
• Meiosis II results in a mature ovum and another polar body
• Millions of oocytes are present at birth, but most do not mature
• Only about 400 oocytes are ovulated during a woman's reproductive life
• Meiosis arrests at prophase I until ovulation

3.4 Prenatal Development

• Prenatal development encompasses the embryo (first 8 weeks) and fetus (remaining period)
• Embryogenesis establishes organ development
• Sperm and oocyte meet in fertilization
• Sperm (six-day survival) capacitated and chemically attracts to oocyte
• Acrosome enzymes penetrate the oocyte's protective layer
• Oocyte's surface changes to prevent multiple sperm entry
• Fertilized egg (zygote) unites pronuclei.

3.5 Birth Defects

• Teratogens are substances causing birth defects, which contain chemicals or agents
• Genetic variants influence teratogen effects
• Environmental factors can be passed to future generations, whereas environmentally caused birth defects cannot
• Critical period is when prenatal structures are vulnerable to damage

3.6 Maturation and Aging

• Aging is a progression through life with declining cellular and bodily functions
• Later-onset diseases are often complex, influenced by both genes and environment
• Adult-onset inherited disorders: Genetically programmed, potentially affecting health later in life
• Aging syndromes help understand genetics of normal aging