Download A MOUSE`S TAIL… Introduction: When you start to determine the

A MOUSE’S TAIL…
Introduction:
When you start to determine the patterns of inheritance, it becomes important to understand a few
concepts and key words. The inheritance of the characteristics or traits that you have depend on
genes that you inherit from your father and mother. Many times only two genes are involved in
the expression of a trait. The gene usually has two types of expression or alleles. Examples may
be tall or short ears, black or blue eyes, etc. By looking at the phenotypes or external
characteristics, it is sometimes possible to determine the genotype or combination of alleles. If
the phenotype is inconclusive, a DNA test can be used. One of the two alleles of a gene is usually
stronger, which means it is more likely to be expressed. This is the dominant allele. The other
allele is the weaker allele and is called the recessive allele. All characteristics are a mixture of
dominant and recessive alleles – this is called genetic variability and explains why no two people
look exactly alike (except identical twins—can you explain that?).
Often these characteristics are put into a probability table called a Punnett Square.
The following conventions have been used in this story intended to teach you about basic
genetics and heredity.
Dominant traits are always represented by capital letters, recessive traits by small letters.
B = brown fur
b = white fur
1.
R = black eyes
r = red eyes
Basic Punnett Squares
Esmeralda was born one sunny day in spring. Her father was a handsome heterozygous
brown mouse and her mother was a beautiful white mouse. Esmeralda’s phenotype is brown.
Use a Punnett Square to determine Esmeralda’s genotype.
2. Is it possible that Esmeralda had a white-furred brother? Why or why not?
3. Esmeralda spent a rather uneventful childhood, until one day she met and married—as
mice are likely to do—a homozygously brown-furred stranger named Tom. What are the
possible genotypes and phenotypes of Esmeralda’s and Tom’s first litter?
Genotypes _______________________
Genotypic Ratio __________________
Phenotypes ______________________
Phenotypic Ratio _________________
4. A month later, Esmeralda’s cousin, Minnie, comes to visit—she even brought her litter with
her—six adorable little mouselings, half with red eyes and half with black eyes. She is
very upset because she does not know whether Sylvester, a red eyed cat, is the father, or
Felix, a black eyed cat. Minnie herself is heterozygously black eyed. Can you help her out
by calculating the percentages of eye color for her offspring? (You might need both
squares below for this one—those paternity back-cross determinations can be rough!)
The father is ___________________________
5. Esmeralda also has some other cousins—Pinky and Brain—who are neither brown furred
or white furred, but are gray like a squirrel! This is called incomplete dominance. For
example, when a mouse like that is heterozygous for two colors, both colors are expressed
as a mix. What happens when Brain (who is always dreaming of world domination) has a
litter with a female gray mouse? What would their litter darlings look like?
Genotypes _______________________
Genotypic Ratio __________________
Phenotypes ______________________
Phenotypic Ratio _________________
6. Some mouse (and human) characteristics are carried on the sex determining genes, and
are said to be sex linked. BUT, before we get on with the story, you should know what
Experalda’s sex chromosomes look like. (Big hint—she is a girl!) How about Tom’s?
Esmeralda
Tom
__________
__________
OK, so here is the deal. There is a skeleton in Tom’s family closet. There is a rare gene
which is sex-linked and causes affected male mice to fall madly in love with hamsters. No, I
suppose it’s not the most threatening of all genetic diseases, but if you were a mouse…well, let’s
not go there. Anyway, as far as we know, Esmeralda’s family checks out OK, so what are the
chances that if Esmeralda’s and Tom’s first litter is half boys and half girls, that
a) One of the boys has this fatal attraction?
b) One of the girls is a carrier for the disease?
FYI—Tom likes hamsters! Use XX and XY to mark the parents. The defective gene is only carried
on the X chromosome. You can mark a defective X chromosome as Xham.
7. Let’s say a female carrier for the hamster illness, Speedy, wants to mate with a hamsterobsessed character named Fievel. How many of their offspring have the disease or carry
it?
8. Last but not least, some traits, even in our mice friends, are available in more than two
allele forms; they are said to be multiple allele genes. Any one mouse can only have any
two of the alleles, but more than two exist. Get it? An example for this would be mouse
(and human!) blood types. There are four basic blood types: A, B, AB, and O, controlled
by three alleles: IA, IB, and i. The i allele is recessive to the big I ones. Let’s try this out. If
Esmeralda’s second cousin thrice removed (Mickey) has type AB blood, and his fiancée,
Minnie, has type O blood, what are the possible blood types of their offspring?
IA
i
i
IB