Biology · Introductory biology · Worked example

Trace nondisjunction through meiosis

In a woman’s egg-making cell, the two copies of chromosome 21 fail to separate in meiosis I. How many chromosomes can the egg carry, and how many does the fertilized egg have after a normal sperm joins it? What changes if the failure happens in meiosis II instead?

Start from a normal division

Normally meiosis gives each gamete half of the $46$ chromosomes.

\frac{46}{2} = 23

One cell gets both copies

Both copies of chromosome 21 go to the same pole in anaphase I. That cell, and every product it makes, has one chromosome extra.

23 + 1 = 24

The other cell gets none

Why one short? Chromosome $21$ went entirely to the other pole, so this cell has no copy of it at all.

23 - 1 = 22

Fertilize the egg with the extra copy

A normal sperm adds $23$ . The embryo has three copies of chromosome 21, trisomy $21$ , which causes Down syndrome.

24 + 23 = 47

Fertilize the egg with no copy

The embryo has one copy of chromosome 21, monosomy $21$ , which rarely survives.

22 + 23 = 45

Compare a failure in meiosis II

Suppose meiosis I runs normally and the sister chromatids of chromosome 21 fail to part in one of the two cells. That cell’s products carry $24$ and $22$ , while the other cell’s two products are normal, with $23$ each. Half the products are affected, not all four. In a woman only one product becomes the egg, so which one it is decides the outcome.

Result

After a meiosis I failure, every product carries $24$ or $22$ chromosomes, so the egg does too. With a normal sperm the fertilized egg has $47$ , trisomy $21$ , or $45$ , monosomy $21$ . A meiosis II failure in one cell leaves half the products normal, with $23$ .

Your turn

1. One like the example

In a man’s sperm-making cell, the X and Y chromosomes fail to separate in meiosis I. Some sperm carry both X and Y, and some carry neither. How many chromosomes does each kind give when it fertilizes a normal egg with $23$ , one of them an X?

Show the answer and explanation

The sperm with both give $47$ chromosomes, XXY. The sperm with neither give $45$ , with a single X.

The XY sperm has $23 + 1 = 24$ , and $24 + 23 = 47$ . The sperm with neither has $22$ , and $22 + 23 = 45$ . XXY is Klinefelter syndrome, and $45$ with one X is Turner syndrome.

\begin{array}{l} 23 + 1 = 24 \\ 24 + 23 = 47 \\ 22 + 23 = 45 \end{array}

2. A variation

Chromosome $21$ fails to separate in meiosis I in one cell. In another cell meiosis I is normal, and it fails in meiosis II in only one of the two second-division cells. What fraction of each set of four products is abnormal?

Show the answer and explanation

All four after the meiosis I failure, and $2$ of $4$ , one half, after the meiosis II failure.

A meiosis I failure leaves both cells unbalanced, so all four products are too. A meiosis II failure in one cell affects only that cell’s two products, $2 / 4 = 1 / 2$ .

\frac{2}{4} = \frac{1}{2}

3. Somewhere new

A triploid plant has three copies of every chromosome, $3 n = 21$ , in seven sets of three. Assume each set sends one or two copies into a gamete, with equal chance and independently of the other sets. What fraction of gametes is balanced, with one copy of every set or two of every set?

Show the answer and explanation

$2 / 2^{7} = 1 / 64$ , about $1.6 %$ .

A gamete is balanced only if all seven sets send one copy, chance ${(1 / 2)}^{7}$ , or all send two, also ${(1 / 2)}^{7}$ . Together that is $2 / 2^{7} = 1 / 64$ . Almost every gamete is unbalanced, which is why triploid plants, such as seedless watermelons, make almost no seeds.

\frac{2}{2^{7}} = \frac{1}{64}

Keep exploring

Trace the counts in Math starts from $46 / 2 = 23$ . Write $23 + 1 = 24$ under it, and each row is checked.

Return to the concept →

Sources and scope

Authored study material. Tool results depend on the stated inputs and model assumptions.

After reading

Study it, then try it

Practice the key terms as flashcards, then open the example in its tool, change a value and keep a useful result on your board.

Study the 11 key terms Open the cell in Chromosomes & meiosis Trace the counts in Math Open worked example on a board Genetics in Biology Reference

Your existing work stays on this device. Examples open as editable copies.

Start from a normal division

One cell gets both copies

The other cell gets none

Fertilize the egg with the extra copy

Fertilize the egg with no copy

Compare a failure in meiosis II

Result

Your turn

1. One like the example

The sperm with both give 4747 chromosomes, XXY. The sperm with neither give 4545, with a single X.

2. A variation

All four after the meiosis I failure, and 22 of 44, one half, after the meiosis II failure.

3. Somewhere new

2⁢/27=1⁢/642/27=1/64, about 1.6⁢%1.6%.

Keep exploring

Study it, then try it

The sperm with both give $47$ chromosomes, XXY. The sperm with neither give $45$ , with a single X.

All four after the meiosis I failure, and $2$ of $4$ , one half, after the meiosis II failure.

$2 / 2^{7} = 1 / 64$ , about $1.6 %$ .