Activity: You’ll Flip for a Tortie!

Adapted from C. Moertel, Mayo HS, Minnesota

Needed:1 coin for flipping; a brown and orange colored pencil; copy of tortoishell cat figure.

Background

In cats as in people, females are XX and males are XY. Do females get twice as many “doses” of genes on the X as males? No! In every cell of an embryo, one X chromosome is randomly “shut off” and stays shut off for the rest of the cell’s life (and the lives of its mitotic descendants). And when an X is turned off, so are all of its genes.

The Symbolism

The cat in the figure has genotype XB Xb. The “B” allele results in brown fur and the “b” allele in orange fur.

Modeling the genetics

The 64 squares on the cat represent the hair cell patches descended from each of the 64 cells of the cat embryo that were present when X inactivation occurred.* Model which X chromosome (and its genes) were shut off in each cell by flipping a coin. In our model, HEADS means the XB chromosome was shut off, leaving the “b” allele to turn that patch of fur ORANGE. TAILS means the Xb chromosome was shut off, leaving the “B” allele to turn that patch of fur BROWN. Use the data table below as a model. Write your results on a separate piece of paper before you color your cat.

Flip Number / Heads or tails? (H or T) / Fur patch color / Flip Number, cont’d / Heads or tails? (H or T) / Fur patch color
1 / 33
2 / 34
3 / 35
4 / 36
5 / 37
6 / 38
7 / 39
8 / 40
9 / 41
10 / 42
11 / 43
12 / 44
13 / 45
14 / 46
15 / 47
16 / 48
17 / 49
18 / 50
19 / 51
20 / 52
21 / 53
22 / 54
23 / 55
24 / 56
25 / 57
26 / 58
27 / 59
28 / 60
29 / 61
30 / 62
31 / 63
32 / 64

The resulting unique coat color of your cat is the orange-and-brown blotching pattern known as ‘tortoiseshell” or “tortie” for short.

Hint: for the cutest tortie, make your coloring “furry” by blurring the edges between fur patches. After all, the daughter cells descended from a given cell wouldn’t spread out into the shape of a perfect square, right?

*X inactivation happens at about the 32-cell stage embryo, but we took a little “artistic license” with 64 cells on this drawing.

NAMES ______Per. ______DATE ______

Activity: You’ll Flip for a Tortie!- Post Activity Questions

FYI: Cats have 38 chromosomes.

1. Why are the alleles “B” and “b” written as a superscript to an “X?”

2. What do we call the “X” that condenses and is “shut off?”

3. a) What would be the genotype for a male cat with one of these condensed structures?

b) What would be his diploid number?

4. a) What would be the genotype for a female cat with two of these condensed structures?

b) What would be her diploid number?

5. Why does your cat look different than all the other cats in the room?

6. What is the problem with Meiosis in cats from Questions # 3 & 4?