Most nucleated cells in your body contain 23 pairs of chromosomes — 46 total. Twenty-two pairs are called autosomes and are the same in males and females. The 23rd pair — the sex chromosomes — helps determine whether you develop as male or female.

The Two Possibilities:

• XX: Two X chromosomes → Female development
• XY: One X and one Y chromosome → Male development

The Father Decides: The mother always contributes an X chromosome (since she has two X's). The father contributes either an X (making a girl) or a Y (making a boy). So biologically speaking, the father's sperm determines the baby's sex — about 50% chance each way.

The SRY Gene: The Y chromosome carries a crucial gene called SRY (Sex-determining Region Y). This gene acts as a master switch that triggers male development. Without SRY (in XX individuals), the default pathway is female development. The SRY gene initiates a cascade that leads to testes formation, which then produce testosterone and other hormones that drive male development.

Interesting Exceptions: Sometimes things don't follow this pattern perfectly. An XX individual with SRY (translocated to an X) develops as male. An XY individual with a non-functional SRY develops as female. This shows how important that single gene is!

Human development is an incredible journey from a single cell to a complete human being. Let's trace the major milestones:

Fertilization: A sperm penetrates an egg in the fallopian tube, combining 23 chromosomes from each parent into a single cell — the zygote. This is the moment when a unique genetic individual is created.

Week 1: The zygote divides repeatedly (cleavage), becoming a hollow ball of cells called a blastocyst. It travels to the uterus and implants in the uterine wall around day 6-7.

Embryonic Period (Weeks 2-8): This is the most critical period of development. All major organs form during this time. The heart starts beating around week 4. By week 8, the embryo looks recognizably human. This is also when the embryo is most vulnerable to teratogens — substances that cause birth defects.

Fetal Period (Week 9-Birth): Now called a fetus, the developing human grows dramatically. Organs mature and begin functioning. By about 24 weeks, the fetus has a chance of survival outside the womb (with intensive medical care). A term pregnancy begins at 37 weeks, but full term is 39-40 weeks; 41 weeks is late term and 42 weeks or more is post-term.

Birth: Hormonal changes trigger labor — coordinated uterine contractions that push the baby through the birth canal. This completes the 40-week journey from conception to independent life.

This final week covers the remaining pieces of human reproduction and development:

Lesson 2: Fertilisation, Implantation, In Utero Growth — We'll examine how sperm and egg meet, how the embryo implants, and how it grows in the womb. You'll learn about critical periods when teratogens are most dangerous.

Lesson 3: Birth and Maternal Changes — What triggers labor? How does the mother's body change during pregnancy and after birth? We'll cover the hormones of pregnancy and the mechanics of childbirth.

Lesson 4: Life Stages — Development doesn't stop at birth. We'll trace human development from infancy through old age, including puberty, adulthood, and the aging process.

Lesson 5: Genetics of Life — How do genes determine traits? What are inheritance patterns? We'll also explore epigenetics — how environment affects gene expression without changing DNA sequence.

This is the final week of the course! By the end, you'll have a complete picture of human biology from the cellular level all the way through the entire lifecycle.