The Wareham approach to solving genetics word problems – V. 1
** In this packet, I am attempting to present how I approach genetics problems – a
superpower I gained after lots of PRACTICE!! Additionally, many problems can be
approached in different ways and solved in different ways – if you keep practicing
these and challenging yourself you will build the skills you need to solve them
QUICKLY and AVOID ERRORS whatever your approach (Hint: Writing out a full 16
box Punnett square is probably not the BEST approach!)
Solving Genetics problems
1. Read the problem CAREFULLY
a. What are they asking for? What answer are you trying to find?
i. As you have seen – these problems can ask for a lot of
different things. Make sure you know what to focus on.
b. What information are you given in the question?
i. The question must provide you with some information
required to solve the problem. Expect to see information
about parents when asked about possible outcomes in the
offspring OR information about outcomes in the offspring
when asked about geno/phenotypes of the parents.
ii. If you encounter a piece of information that seems odd
(strange ratios, more than 2 phenotypes) you are likely
dealing with an exception to Mendelian genetics – MORE
ON THIS LATER!
2. Provided parental Genotypes (a) or Phenotypes (b)
a. If a question provides you with the parental genotypes and wants
to know outcomes for the offspring of the cross – SMILE! That’s an
easy question – create a Punnett square or just calculate the
probability by looking at the parental genotypes.
i. Example - What phenotypic ratio would we expect from a
cross between parents with the genotypes AABb x AaBb?
1. Option 1 – create a Punnett Square, look at the
outcomes, determine how many outcomes would
correspond to each phenotype.
2. Option 2 – start with trait one and consider what
phenotypes are possible in the offspring and their
ratios (AA,Aa: 4/4 or 1; aa: 0.) Do the same for trait
two (BB, Bb: ¾; bb: ¼.) Then consider possible
combinations of those phenotypes in the offspring
and their probabilities: A_B_ (1 x ¾), A_bb (1 x ¼),
no other combinations are possible given the
parental genotypes. Phenotypic ratio: 3 dominant
for both traits: 1 dominant for trait 1 and recessive
for trait 2.
b. If a question provides you with information about parental
phenotypes you need to interpret the information, try to
determine the parental genotypes and proceed from there.
i. Determine which trait is dominant and which trait is
recessive.
ii. Remember, true breeding means homozygous and carrier
means heterozygous (individuals show the dominant
phenotype but are “carriers” of the harmful recessive
allele)
iii. My own observation: Many problems start out with a P
generation that consists a parents that are true breeding for
contrasting traits (AA x aa) creating F1 offspring that are all
heterozygous and all display the dominant phenotype, and
then crossing the F1 offspring (HETEROZYGOUS) produces
predictable genotypic and phenotypic ratios in the F2
offspring. (MORE ON PREDICTABLE RATIOS LATER)
3. Provided with information about the offspring
a. If you already have the genotypes of offspring remember the two
alleles of a genotype came from each of the parents – one from
mom and one from dad. Look for homozygous recessive outcomes
in the offspring!! That tells us that each parent has at least one
recessive allele in their genotype.
b. If you are given phenotypes for the offspring:
i. Determine which trait is dominant and which trait is
recessive
ii. Look at the number of phenotypes for each trait you are
dealing with in the offspring. This will help you determine if
each trait exhibits normal dominant recessive inheritance,
if one or both traits show incomplete dominance, if one
trait demonstrates epistasis over the other, or if you are
dealing with multiple alleles or polygenic inheritance
(quantitative traits.)
iii. Determine what genotypes could correspond to the
phenotypes present in the offspring. Use _ to identify where
either a dominant or recessive allele could be present.
iv. Look at the ratios of the phenotypes in the offspring.
Compare to the predictable ratios to try and interpret the
parents.
Saving time
4. Allelic combinations
a. When you have parental genotypes and need to set up a Punnett
square to see outcomes in the offspring you first figure out the
combination of alleles that could show up in the gametes produced
by each parent.
b. For AaBb there are four combinations: AB Ab aB ab
c. If the parent is homozygous (dominant or recessive) for either
trait or both traits, there are fewer combinations.
d. Only include one of each type of combination for each parent.
i. Example: AABb x AaBb
1. Possible combinations for AABb parent: AB or Ab
2. Combinations for AaBb parent: AB Ab aB ab
AB
Ab
aB
ab
AB
AABB AABb AaBB AaBb
Ab
AABb AAbb AaBb
Aabb
 no need to write out 16 boxes!!
 if one parent is AABB or aabb – There is only
one possible combination of alleles
5. Predictable Ratios
a. Some ratios we see over and over again and they can help us solve
problems quickly.
i. Two parents, heterozygous for one trait Aa X Aa
1. 3 dominant:1 recessive (phenotypic ratio)
2. 1:2:1 genotypic ratio
ii. Two parents, heterozygous for two traits AaBb X AaBb
1. 9 : 3 : 3 : 1
a. 9 dominant for both traits
b. 3 dominant for trait 1, recessive for trait 2
c. 3 recessive for trait 1, dominant for trait 2
d. 1 recessive for both traits
2. 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1 * probably won’t need this
a. 1 : 2 : 1 AABB AABb AAbb
b. 2 : 4 : 2 AaBB AaBb Aabb
c. 1 : 2 : 1 aaBB aaBb aabb
iii. Two parents, one heterozygous, one homozygous
(dominant or recessive)
1. 1:1 genotypic ratio
2. 1:1 phenotypic ratio if homozygous recessive
iv. Numbers of actual offspring may not EXACTLY fit the
predicted ratios – check to see if they are close.
6. Not-so predictable ratios AKA exceptions to Mendelian genetics
a. Lethal alleles – some recessive alleles are lethal so individuals who
are homozygous recessive die before birth or shortly after
i. Look for:
1. A question about one trait where there are fewer
than 4 outcomes, like 2:1
2. A question about two traits where there are 12
outcomes instead of 16 indicates – one of the two
traits is lethal in the recessive form
b. Codominance – heterozygotes display two versions of a trait at the
same time – this means heterozygotes won’t be grouped in with
c.
d.
e.
f.
the “dominant” phenotype in a phenotypic ratio. With
codominance there isn’t one dominant trait
i. Example: IA and IB alleles for human blood type are
codominant – individuals with one of each have type AB
blood
Incomplete dominance – heterozygotes have their own distinct
phenotype – this means heterozygotes won’t be grouped in with
the “dominant” phenotype in a phenotypic ratio
i. Look for:
1. A question about one trait where there are 3
different phenotypes
2. A question about two traits where there are 6
different phenotypes instead of 4
Epistasis – gene at one location controls expression of a gene at
another location
i. Look for:
1. A question about 2 traits where 3 different
phenotypes are discussed
Multiple Alleles – more than 2 alleles for a single trait
i. Write out the order of dominance!!
ii. Look for:
1. More than two possible phenotypes for a trait
2. Only as many phenotypes as there are types of
alleles
Polygenic traits – two or more genes have an additive effect on a
single trait
i. Look for:
1. More than two possible phenotypes for a trait
2. Quantitative traits – increase based one the number
of dominant alleles
ii. Remember: phenotypic classes – 1 = number of alleles in
the genotype.
Pedigrees
7. Dominant, recessive, or sex-linked inheritance?
8. Find the genotypes you know for sure.
9. Remember, actual outcomes in a family may not match predicted ratios
a. Example: Four children heterozygous parents COULD all be
recessive rather the 3 dominant and 1 recessive
CHECK YOUR WORK!!
This is all I can think of right now. Hopefully some of it is useful. We will go through
this together tomorrow prior to the OQQ and some examples. As much as I want this
to be totally comprehensive, I’m sure I neglected some things.