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White mouse genetics: Albino mice and other white mice

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Table of contents
  1. How albinism works
  2. Why lab mice are white
  3. Albino hides every other color gene
  4. More worked crosses
  5. Two carriers: C/c × C/c
  6. Albino × colored carrier: c/c × C/c
  7. What was the albino hiding? b/b c/c × B/B C/C
  8. Other ways a mouse can be white
  9. Are albino mice blind?
  10. Common questions
  11. Are all white mice albino?
  12. Does B/b C/c × B/b C/c give 9:3:3:1?
  13. Does an albino mouse have no color genes?
  14. Is a C/c mouse partly pale?
  15. Practice
  16. Where the model stops
  17. Related
  18. Sources

Level: School to high school · You’ll need: dominant and recessive alleles and a dihybrid cross · Part of mouse genetics

Albino mouse genetics is a single recessive allele. A mouse with two copies of the albino allele c of the Tyr gene (genotype c/c) makes a tyrosinase enzyme that doesn’t work, so it can’t make any melanin pigment. Its fur is white and its eyes look pink, because the blood vessels show through the unpigmented eye. Most white laboratory mice are albino.

Assumptions for the crosses on this page: the genes named in each cross, other genes held the same, independent inheritance, and enough pups for the ratios to show.

How albinism works

Tyrosinase is the first enzyme in making melanin, the pigment behind black, brown and yellow fur. The classic albino allele has a single-letter DNA change in which one amino acid, a cysteine, is replaced by a serine, and the enzyme stops working. The pigment cells are still there, but without working tyrosinase they can’t make melanin.

C (full color) is dominant: one working copy makes enough enzyme. A C/c mouse looks like any other colored mouse but carries albino.

Albinism is very old in mice. White mice were known in ancient Greece and Rome, and albino mice were among the first animals used to test Mendel’s laws after 1900.

Why lab mice are white

Many of the best-known laboratory strains, such as BALB/c, and many outbred “Swiss” white stocks are albino (c/c). Albinism was common among the fancy mice that early geneticists collected, and once a strain is inbred, every mouse in it carries the same genotype, so the white coat stays.

Black lab mice exist too. The most widely used strain, C57BL/6, is black (a/a, with full color at C). Researchers pick a strain for its other genes, not its coat.

Albino hides every other color gene

An albino mouse can carry alleles for black, brown, agouti or yellow, but you can’t see any of them, because there’s no pigment to show them. When one gene’s genotype hides the effect of another gene, it’s called epistasis. Because c has to be homozygous to hide the others, this is recessive epistasis.

A classic way to see it is a dihybrid cross. Two black mice, both carrying brown (b) and albino (c):

B/b C/c × B/b C/c

PupGenotypesChance
BlackB/- C/-9/16
Brownb/b C/-3/16
Albino-/- c/c (any color genes)4/16 = ¼

Instead of Mendel’s 9:3:3:1, you get 9:3:4. The “1” and one of the “3”s merge, because every c/c mouse looks the same: white. Check it in the calculator.

More worked crosses

Two carriers: C/c × C/c

¾ colored, ¼ albino. Two colored mice can have albino pups if both carry c. Check it.

Albino × colored carrier: c/c × C/c

½ albino, ½ colored. Check it.

What was the albino hiding? b/b c/c × B/B C/C

An albino mouse that secretly carries brown (b/b), crossed with a black mouse (B/B C/C): every pup is B/b C/c, so all pups are black. Cross two of those pups, and the 9:3:4 ratio above reappears, with brown pups revealing what the albino grandparent carried. Check the first cross.

Other ways a mouse can be white

Not every white mouse is albino. Eye color is the quickest clue:

White mouseEyesCause
AlbinoPink (red)c/c at Tyr: no melanin anywhere
Black-eyed whiteBlackPigment cells missing from the skin but not the eyes, from very extensive white spotting (for example piebald s/s with strong modifier genes, or dominant spotting genes)
Very pale, pink-eyedPinkPink-eyed dilution (p/p), sometimes with other dilutions, can make a coat look nearly white

Black-eyed white and albino mice can look alike at a glance, but their genetics are completely different. Two albino parents can only have albino pups. Two black-eyed white parents can have colored or spotted pups, depending on the genes involved.

Are albino mice blind?

No, but their vision is weaker than that of pigmented mice. Pigment in the eye matters for how the eye develops: in albino mice, fewer nerve fibers from each eye connect to the same side of the brain, and the unpigmented eye is more sensitive to bright light. Researchers take this into account when choosing strains for vision or behavior studies.

Common questions

Are all white mice albino?

No. Check the eyes: black-eyed white mice have pigment-making cells in their eyes, so they aren’t albino.

Does B/b C/c × B/b C/c give 9:3:3:1?

No. Albino masks color, so the ratio is 9 black : 3 brown : 4 albino.

Does an albino mouse have no color genes?

No. It has a full set of color genes; they just can’t show without pigment.

Is a C/c mouse partly pale?

No. One working copy of C is enough for normal color.

Practice

  1. Two albino mice have pups. What colors are possible?
  2. A black mouse and an albino mouse have 8 pups: 4 black and 4 albino. What is the black parent’s genotype at Tyr?
  3. In a B/b C/c × B/b C/c cross, what fraction of all pups are brown?
Answers
  1. Only albino. Both parents are c/c, so every pup is c/c.
  2. C/c. Albino pups need a c from each parent, and a 1:1 split is what C/c × c/c predicts.
  3. 3/16: they need b/b (¼) and at least one C (¾), so ¼ × ¾ = 3/16.

Where the model stops

  • The albino gene has in-between alleles. Chinchilla and Himalayan alleles make less pigment rather than none; Himalayan mice, like Siamese cats, have heat-sensitive tyrosinase and darker points.
  • Linked genes. Albino and pink-eyed dilution are close together on mouse chromosome 7, so crosses using both won’t follow independent assortment exactly.

Across species: the same gene in cats and people. TYR, the gene behind albino mice, is also the colorpoint gene in cats: a heat-sensitive TYR allele gives Siamese points. In people, TYR variants cause oculocutaneous albinism type 1. White coats from missing pigment cells, like black-eyed white mice, have their own parallel in dominant white cats.

Sources

  • Mouse Genome Informatics, Tyrc (albino) allele: mutation, phenotype and history.
  • Wolfe HG, Coleman DL. “Pigmentation.” In Biology of the Laboratory Mouse, Chapter 21. The Jackson Laboratory, via Mouse Genome Informatics. informatics.jax.org/greenbook
  • Beermann F, et al. “The Tyr (albino) locus of the laboratory mouse.” Mammalian Genome 15, 749–758 (2004). doi:10.1007/s00335-004-4002-8
Try it: work out any cross with the mouse coat color calculator, or a general one with the Punnett square calculator.