Download PUNNETT SQUARE PROBLEMS (or how to do a cross): STEP 1

PUNNETT SQUARE PROBLEMS (or how to do a cross):
STEP 1: Find the genotypes of both parents
A genotype is expressed in letters, where each letter represents a different gene. Because
parents are diploid, they will have 2 of every letter.
A typical diploid genotype for a monohybrid cross might be Aa, or AA, or aa
(monohybrid means you are working with only 1 gene, or 1 kind of letter).
A typical diploid genotype for a dihybrid cross might be RRYY, or RRYy, or RRyy, or
RrYY, or rrYy, etc. (dihybrid means you are working with 2 genes, represented by 2
different letters).
So how do you find genotypes? Sometimes genotypes may be given to you. Other times
you may have to figure them out based on descriptive terms, like:
homozygous dominant = big-big
homozygous recessive = little-little
heterozygous = big-little
STEP 2: List all possible gametes (egg cells or sperm cells) each parent genotype
can produce
When you are doing a monohybrid cross, finding possible gametes is easy. All you have
to do is give one allele to half the gametes, and the other allele to the other half of the
gametes (remember gametes are haploid, so they get only 1 of each gene or letter!). Then
cross out any duplicates. So a AA parent gives “A” gametes only, an aa parent gives “a”
gametes only, and an Aa parent gives both “A” and “a” gametes. I like to draw little egg
or sperm cells with the (single) letter inside – it keeps me from making mistakes during
step 3.
When you are doing a dihybrid cross, finding possible gametes becomes a little more
complicated. Obviously, haploid gametes will still need 1 of every gene or letter. But
will they have big or little letters, that is the question. To find out you must FOIL each
genotype. Again, it’s a good idea to draw little egg and sperm cells with 1 of each letter
inside. Then cross out duplicate gametes. See if you can verify the following gamete
lists for each genotype:
RRYY gives RY gametes only
RRYy gives RY and Ry gametes
RrYY gives RY and rY gametes
RrYy gives RY, Ry, rY, and ry gametes.
Note that GAMETES NEVER HAVE DUPLICATE LETTERS. So gametes CANNOT
look like Aa, or YY, or rr.
STEP 3: List all possible gametes of one parent on one side of the square, and all
possible gametes of the other parent on an adjacent side of the square
It really helps if you continue to draw egg cells and sperm cells on the outside of the
square, so you are not tempted to combine them inappropriately.
STEP 4: Fuse each egg cell with each sperm cell to make a (diploid) baby inside the
square
This is the easy part. Just remember to be consistent, especially when doing a dihybrid
cross. If you started out grouping R’s before Y’s, keep doing it that way. And within
each letter group, make sure you continue to list big before little.
STEP 5: Tally up baby genotypes for a genotypic ratio, and tally up baby
phenotypes for a phenotypic ratio
SAMPLE MONOHYBRID PROBLEM:
In pea plants, T=tall, and t=short. If you cross a homozygous tall plant with a
heterozygous tall plant, what will be the genotypic and phenotypic ratios in the F1?
STEP 1:
homozygous tall x heteroxygous tall = TT x Tt
STEP 2:
TT gives T gametes only (remember to cross out the duplicates)
Tt gives T gametes and t gametes
STEP 3:
T
t
T
STEP 4:
T
STEP 5:
T
t
TT
Tt
half the babies are TT, and half are Tt, so the genotypic ratio is 1 TT : 1 Tt
All babies are tall, so the pheonotypic ratio is 1 tall : 0 short
SAMPLE DIHYBRID PROBLEM:
In pea plants, R=round seeds, and r=wrinkled seeds, while Y=yellow seeds, and y=green
seeds. You want to cross a plant that is heterozygous for both traits with a plant that has
wrinkled green seeds. What will be the genotypic and phenotypic ratios in the F1?
STEP 1:
heterozygous round and yellow x wrinkled green = RrYy x rryy
STEP 2:
You will have to foil to find all possible gametes.
RrYy gives RY, Ry, rY, and ry gametes
rryy gives ry gametes only (remember to cross out the duplicates)
STEP 3:
RY
Ry
rY
ry
ry
STEP 4:
ry
STEP 5:
RY
Ry
rY
ry
RrYy
Rryy
rrYy
rryy
F1 genotypic ratio = 1 RrYy : 1 Rryy : 1 rrYy : 1 rryy
F1 Phenotypic ratio = 1 round yellow : 1 round green : 1 wrinkled yellow
: 1 wrinkled green