Human Reproductive System and Sexual Maturation
Senior 1 · Biology
Human Reproductive System and Sexual Maturation
Lesson 2 of 5
25 min
Mukamana and her husband have three daughters. At a family gathering, a neighbour whispers that Mukamana "can only give girls" and that her husband should worry. Mukamana's eldest daughter, who is in Senior 1, knows this is not true, and by the end of this lesson you will be able to explain why, using chromosomes and a simple diagram.
In some communities in many countries, mothers have been wrongly blamed for the sex of their children. Biology shows that this blame has no basis at all.
Definition
Chromosome: a thread-like structure in the nucleus of a cell, made of DNA, that carries genes (the instructions for the body's features). A human body cell usually has 46 chromosomes, in 23 pairs.
Meiosis: a special type of cell division, found only in the testes and ovaries, that makes gametes with half the number of chromosomes: 23 instead of 46.
Gametogenesis: the making of gametes: spermatogenesis (making sperm) and oogenesis (making eggs).
In both sexes, gametes are made from special cells in the gonads that divide by meiosis. The two processes differ in timing and in how many gametes they produce.
| Spermatogenesis (males) | Oogenesis (females) | |
|---|---|---|
| Where | tiny coiled tubes inside the testes | the outer layer of the ovaries |
| When it starts | at puberty | before birth: a baby girl is born with about one to two million immature eggs |
| How long it lasts | continues throughout life | from puberty, usually one egg matures and is released each month |
| Gametes from each dividing cell | four small sperm cells | one large egg (plus tiny cells that die), so the egg keeps nearly all the food store |
| Number | millions of sperm every day | usually one egg a month |
Figure 1: A sperm cell and an egg cell (not to scale: the egg is many times wider than the sperm's head).
| Cell | Feature | How it helps |
|---|---|---|
| Sperm | tail (flagellum) | lets the sperm swim towards the egg |
| many mitochondria in the middle piece | release energy for swimming | |
| acrosome | holds enzymes that digest a way through the jelly coat of the egg | |
| small and streamlined; nucleus with 23 chromosomes | moves easily; carries the father's half of the genes | |
| Egg | large cytoplasm with a food store | feeds the zygote in its first days, before it settles in the uterus |
| jelly coat | protects the egg; after one sperm enters, it changes so that no other sperm can | |
| nucleus with 23 chromosomes | carries the mother's half of the genes |
Definition
Adaptation: a feature of a cell or organism that makes it well suited to its job or its surroundings. Example: the sperm's tail is an adaptation for swimming.
Worked example 1: why must gametes have half the chromosomes?
A human body cell has 46 chromosomes. Show why sperm and eggs must carry 23, not 46.
Answer: meiosis halves the number in the gametes, so that fertilisation restores it: .
Of the 23 pairs of chromosomes in a body cell, 22 pairs are the same in males and females. The 23rd pair are the sex chromosomes. Females usually have two X chromosomes (XX); males usually have one X and one much smaller Y chromosome (XY). A gene on the Y chromosome switches on the development of testes in the early embryo; without it, ovaries develop.
Figure 2: Sex determination. Each box is one equally likely way an egg and a sperm can combine.
Definition
Sex chromosomes: the pair of chromosomes that determines whether a person develops as male or female: usually XX in females and XY in males. The other 22 pairs are called autosomes.
Sex determination: the process that decides whether a fertilised egg develops as male or female.
Punnett square: a grid that shows all the equally likely ways the gametes of two parents can combine.
Worked example 2: the chance of a girl or a boy
Use the Punnett square in Figure 2 to find the probability that a child is a girl, and the ratio of girls to boys.
Answer: for every pregnancy the chance of a girl is and of a boy is . Across a whole country, roughly equal numbers of girls and boys are born.
Worked example 3: answering the neighbour
Mukamana has three daughters. (a) Who decided the sex of each daughter? (b) What is the probability that her next child is a boy? (c) What was the probability, before any of them were born, that her first two children would both be girls?
Conclusion: the neighbour is wrong. No mother "gives" only girls; the sex of a child is a matter of chance, and nobody is to blame. Every child, girl or boy, has equal value.
Important
Nobody is to blame for the sex of a child. The mother's egg always carries X; the X or Y comes from the sperm, and which sperm fertilises the egg is chance. Blaming or mistreating a woman because she has no sons, or no daughters, is scientifically wrong and unjust.
In pairs, write three calm, respectful sentences that Mukamana's daughter could say to the neighbour, using the words chromosome, sperm and chance. Then discuss: why might wrong ideas like the neighbour's still be common, and how can science education help?
Warning
Common mistakes. (1) "The mother decides the sex": she always gives an X. (2) "After three girls, a boy is due": each birth is an independent event with probability . (3) "Sperm and eggs have 46 chromosomes": they have 23, because they are made by meiosis. (4) Drawing the Y chromosome as big as the X: the Y is much smaller.
Tip
Memory hook: "Mum gives X; Dad gives X or Y." Then read the Punnett square like a times table: row letter plus column letter.
Practice: gametes and sex determination
Key points
Sources and credits
Figures 1 and 2, the story, the tables, the worked examples, the discussion and the practice questions are written by Ganzaa.
