Chapter 4 of 8

All 8 chapters
  1. 1DNA, genes and chromosomes, readunderstood
  2. 2The structure of DNA, readunderstood
  3. 3Meiosis and sex cells, readunderstood
  4. 4Boy or girl: X and Y, readunderstood
  5. 5Dominant and recessive, readunderstood
  6. 6Family trees and genetic disorders, readunderstood
  7. 7Variation, readunderstood
  8. 8Mutation and genetic engineering, readunderstood

S4.2.4 Chapter 4

Boy or girl: X and Y

What decides whether a baby is a boy or a girl?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Use a Punnett square to show how X and Y chromosomes decide sex
  • Explain why the chance of a boy or a girl is about even

Of your 23 pairs of chromosomes, 22 pairs are the same kind in everyone. They are numbered 1 to 22. The 23rd pair is the sex chromosomes. They are called X and Y.

The two sex chromosomes side by side: a long X chromosome and a much shorter Y chromosome. They are labelled X chromosome and Y chromosome.X chromosomeY chromosome
The Y chromosome is much smaller than the X chromosome and carries far fewer genes.

In most people, a pair of two X chromosomes (XX) develops as female. A pair of an X and a Y (XY) develops as male. A gene on the Y chromosome starts the changes that make a body male. Without it, the body develops as female.

A table with the columns Sex pair and Usually. The pair XX usually develops as female. The pair XY usually develops as male.Sex pairUsuallyXXfemaleXYmale
Usually means develops as. This is the usual pattern. A few people have other patterns, which are described below.

Most women have two X chromosomes in each body cell. Meiosis shares them out, so every egg gets one X. In most men, a body cell has an X and a Y. Meiosis gives half of his sperm an X and half a Y.

Three boxes joined by arrows, from top to bottom: a man's body cell: X and Y, meiosis shares them out, two kinds of sperm: one with X and one with Y.a man's body cell: X and Ymeiosis shares them outtwo kinds of sperm: one withX and one with Y
A man makes two kinds of sperm in equal numbers. A woman makes one kind of egg, with an X.

A Punnett square shows every way an egg and a sperm can meet. Put the egg's chromosomes down the side and the sperm's across the top. Each box is one possible child, called an offspring.

Draw it first

Cross an egg with a sperm

  1. A square of four boxes. The egg, with X and X, is down the side, and the sperm, with X and Y, is across the top. In the first picture the boxes are empty. In the second picture the boxes show XX, XY, XX and XY.spermeggXYXX

    1 Write the two eggs, X and X, down the side. Write the two kinds of sperm, X and Y, across the top.

  2. A square of four boxes. The egg, with X and X, is down the side, and the sperm, with X and Y, is across the top. In the first picture the boxes are empty. In the second picture the boxes show XX, XY, XX and XY.spermeggXYXXXXXYXXXY

    2 Fill each box with the letter from its row and the letter from its column. Two boxes read XX and two read XY.

Two boxes read XX and two read XY. So each baby has a 1 in 2 chance of being a girl. It has a 1 in 2 chance of being a boy. The sperm decides. A sperm with an X gives a girl. A sperm with a Y gives a boy.

Worked example

Use the Punnett square. What is the chance that a baby is XY?

  1. The square has 4 boxes, and each is equally likely.
  2. The number of boxes that read XY is 2.
  3. The chance is 2 out of 4, which is 1 in 2.

Answer: 1 in 2

Each birth is a fresh chance. The sperm that wins is not affected by earlier babies. A family with three girls still has a 1 in 2 chance of a boy next time. Chance has no memory.

A ratio is a chance, not a promise. A family with four children may have four girls. But among very many births, the numbers come close to half and half. Across the world, slightly more boys are born than girls: about 105 boys for every 100 girls.

Worked example

About 105 boys are born for every 100 girls. A region has 2 050 births in a year. About how many are boys?

  1. In every group of 105 + 100 = 205 births, about 105 are boys.
  2. The number of groups of 205 births is 2 050 ÷ 205 = 10.
  3. About 1 050 are boys, because 10 × 105 = 1 050.

Answer: About 1 050 boys

A bar chart of 205 births. The axis is labelled number of babies, with a scale from 0 to 120 in steps of 20. The bar for boys reaches 105. The bar for girls reaches 100.020406080100120boysgirlsnumber of babies
Out of 205 births, about 105 are boys and 100 are girls. That is close to even, with a few more boys.

A few people have another pattern of sex chromosomes, such as a single X, or XXY. Some are born with a body that develops in a way that does not match the usual XX or XY pattern. This is part of the natural range of human bodies. The pages above describe the usual pattern.

Try it

Cross two parents with paper

You need: an old sheet of paper, a pencil, two cups or bowls.

  1. Tear four small pieces of paper. Write X on two of them for the egg. Write X on the third and Y on the fourth for the sperm.
  2. Put the two X pieces for the egg in one cup. Put the other two in the second cup.
  3. Shake both cups. Pick one piece from each, without looking. The two together are one child, such as XY.
  4. Write down boy for XY or girl for XX. Put the pieces back. Do this 20 times and count the boys and the girls.

What you will see: You may not get exactly 10 and 10. That happens only about 1 time in 6. The count is usually close to half and half, and sometimes it is not. That is what a 1 in 2 chance means for a small number of tries.

If you cannot try it, this is what happens.
A table with the columns Sperm, Egg and Child. A sperm with X and an egg with X make a child with XX. A sperm with Y and an egg with X make a child with XY.SpermEggChildXXXXYXXY

A wrong idea many people have

The mother decides whether the baby is a boy or a girl.

Why it feels right: The baby grows inside the mother, so it seems that her body must decide.

The test that breaks it: Look at the Punnett square. Both eggs carry an X. Only the sperm differs, so only the sperm can change the result.

What is true: Every egg carries an X. The sperm carries an X or a Y. So the father's sperm decides.

Key words

sex chromosomes
the pair of chromosomes called X and Y.
Punnett square
a square of boxes that shows every way two parents' chromosomes or genes can combine.
offspring
a child or young one of a parent.

Back to your guess

The answer is C: The father's sperm decides, because it carries either an X or a Y chromosome.

The sperm decides. Every egg carries an X chromosome, and a sperm carries either an X or a Y. Food and the mother's body do not change which one fertilises the egg.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Which sex chromosomes can one sperm cell carry?

    Tap your answer.

    1. Yes, that is right. A man's body cell has an X and a Y. Meiosis gives one to each sperm, so half the sperm carry X and half carry Y.

    2. Not this one. Every egg carries an X, but a man's sperm come in two kinds, X and Y. Try another.

    3. Not this one. The sperm already carries its X or Y before it meets the egg. Try another.

    4. Not this one. A sex cell gets only one chromosome of each pair, so a sperm carries one sex chromosome. Go to S4.2.3. Try another.

    See the answer

    Either an X or a Y

    A man's body cell has an X and a Y. Meiosis gives one to each sperm, so half the sperm carry X and half carry Y.

    • If you chose B: Every egg carries an X, but a man's sperm come in two kinds, X and Y.
    • If you chose C: The sperm already carries its X or Y before it meets the egg.
    • If you chose D: A sex cell gets only one chromosome of each pair, so a sperm carries one sex chromosome. Go to S4.2.3.

  2. Which sex chromosome does every egg cell carry?

    Tap your answer.

    1. Yes, that is right. A woman's body cells usually have two X chromosomes. Whichever one goes into an egg, it is an X.

    2. Not this one. That is true for sperm. A woman has two X chromosomes, so every egg gets an X. Try another.

    3. Not this one. A woman usually has no Y chromosome, so her eggs carry an X. Try another.

    See the answer

    An X

    A woman's body cells usually have two X chromosomes. Whichever one goes into an egg, it is an X.

    • If you chose B: That is true for sperm. A woman has two X chromosomes, so every egg gets an X.
    • If you chose C: A woman usually has no Y chromosome, so her eggs carry an X.

  3. What decides whether a baby gets XX or XY?

    Tap your answer.

    1. Not this one. Food matters for health, but it does not change which sperm joins the egg. Try another.

    2. Not this one. Every egg carries an X, so the egg cannot make the difference. Try another.

    3. Yes, that is right. The egg always gives an X. An X sperm makes XX and a Y sperm makes XY.

    See the answer

    Whether the sperm that joins the egg carries an X or a Y

    The egg always gives an X. An X sperm makes XX and a Y sperm makes XY.

    • If you chose A: Food matters for health, but it does not change which sperm joins the egg.
    • If you chose B: Every egg carries an X, so the egg cannot make the difference.

  4. A couple has three girls. What is the chance that their next baby is a boy?

    Tap your answer.

    1. Not this one. Chance has no memory. Earlier babies do not change which sperm wins. Try another.

    2. Yes, that is right. Each pregnancy is a fresh chance. The sperm is X or Y with equal chance, so the chance of a boy is still 1 in 2.

    3. Not this one. The father's sperm are half X and half Y whatever has happened before. Try another.

    See the answer

    1 in 2

    Each pregnancy is a fresh chance. The sperm is X or Y with equal chance, so the chance of a boy is still 1 in 2.

    • If you chose A: Chance has no memory. Earlier babies do not change which sperm wins.
    • If you chose C: The father's sperm are half X and half Y whatever has happened before.

  5. Why are about as many boys as girls born?

    Tap your answer.

    1. Not this one. Many families have more of one than the other. Only among very many births do the numbers come close. Try another.

    2. Yes, that is right. A man makes X and Y sperm in equal numbers. So each baby has about a 1 in 2 chance of being a boy or a girl.

    3. Not this one. Chance has no memory. It stays about 1 in 2 for every baby. Try another.

    See the answer

    Half of sperm carry an X and half carry a Y, so each baby has a 1 in 2 chance of each.

    A man makes X and Y sperm in equal numbers. So each baby has about a 1 in 2 chance of being a boy or a girl.

    • If you chose A: Many families have more of one than the other. Only among very many births do the numbers come close.
    • If you chose C: Chance has no memory. It stays about 1 in 2 for every baby.

  6. Look at the empty Punnett square. When it is filled in, how many of the four boxes read XX?

    An empty square of four boxes. The egg, with X and X, is down the side and the sperm, with X and Y, is across the top.spermeggXYXX
    Tap your answer.

    1. Not this one. Both eggs carry an X, so each of them gives XX with an X sperm. That makes two boxes. Try another.

    2. Yes, that is right. The two X sperm boxes read XX, one in each row. The two Y sperm boxes read XY.

    3. Not this one. Half of the sperm carry a Y, so half of the boxes read XY. Try another.

    See the answer

    2 boxes

    The two X sperm boxes read XX, one in each row. The two Y sperm boxes read XY.

    • If you chose A: Both eggs carry an X, so each of them gives XX with an X sperm. That makes two boxes.
    • If you chose C: Half of the sperm carry a Y, so half of the boxes read XY.

  7. From an earlier chapter

    A woman's body cell has 46 chromosomes. How many chromosomes does one of her egg cells have?

    Tap your answer.

    1. Not this one. That is the number in a body cell. Meiosis halves it for sex cells. Go to S4.2.3. Try another.

    2. Yes, that is right. Meiosis gives each sex cell one chromosome from each pair, so half of 46. 46 ÷ 2 = 23.

    3. Not this one. Cells cannot have double the number, or it would double again each generation. Go to S4.2.3. Try another.

    See the answer

    23 chromosomes

    Meiosis gives each sex cell one chromosome from each pair, so half of 46. 46 ÷ 2 = 23.

    • If you chose A: That is the number in a body cell. Meiosis halves it for sex cells. Go to S4.2.3.
    • If you chose C: Cells cannot have double the number, or it would double again each generation. Go to S4.2.3.

  8. Each baby has a 1 in 2 chance of being a boy. In 400 births, about how many boys do you expect?

    See the answer

    About 200 boys

    A chance of 1 in 2 means half. 400 ÷ 2 = 200. The real number will be near 200, but not exactly 200.

  9. About 105 boys are born for every 100 girls. A town has 410 births. About how many are boys?

    See the answer

    About 210 boys

    Each group of 105 + 100 = 205 births has about 105 boys. 410 ÷ 205 = 2 groups, and 2 × 105 = 210.

Keep it

Copy this into your exercise book.

XX usually develops as female and XY as male. Every egg carries X. A sperm carries X or Y, so the sperm decides. Each baby has about a 1 in 2 chance of each. Chance has no memory.

Draw: A Punnett square with X and X down the side, X and Y across the top, and XX, XY, XX, XY inside.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

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