Punnett Squares – Dominance, Incomplete Dominance, Co-dominance
Review- Vocabulary needed to know when working with genetics
1. Allele – Different form of a trait
2. Genotype – The gene make-up of a trait expressed as a set of Capital and lower case letters
3. Phenotype – The physical presentation of the genetic expression
4. Dominant – The trait that expresses itself over another
5. Recessive – The trait that is masked or hidden by the dominant trait.
6. Homozygous – Having the same two alleles for a genetic trait.
7. Heterozygous – Having two different alleles for a genetic trait.
8. Homozygous Dominant – FF
9. Homozygous Recessive – ff
10. Heterozygous - Ff
11. True breed – Offspring having the same homozygous trait as the parent
12. Hybrid – Offspring having one allele from each homozygous parent
13. Carrier – Offspring having a recessive trait masked by a dominant trait that may express itself in
future generations
14. P – Parent generation – The first generation crossed
15. F – Filial generation – The generations that follow the parent generation. Subscripts show which
generation F1; F2; F3
Punnett Square
1. Punnett Square – a visual presentation of possible traits of offspring
a. First you make a 2 X 2 box
b. Put the two set of parental alleles above and next to the punnett square. (one pair from
mom and one pair from dad)
c. Drop and cross the alleles from each parent.
d. Pay attention to what each question asks.
2. To find the percentage of a phenotype or genotype:
a. Give a value of 25% to each box.
b. Add the totals in each box.
i. Ques: What is the total percentage of arched footed
children?
Ans: 75% = (25% + 25% + 25%)
ii. Ques: What is the percentage of children that are
homozygous recessive?
Ans: 25%
3. To find the ratio in a question:
a. Use colons and count boxes.
b. Write the phenotype or genotype with the number of boxes each is found in.
c. The numbers must add up to 4
i. Question. What are the possible genotypic ratios for children of heterozygous
parents?
Ans: 1 AA : 2 Aa : 1 aa (1+2+1=4)
ii. Question. What are the possible phenotypic ratios for children of heterozygous
parents?
Ans: 3 with arches : 1 without arches (3+1=4)
Complete Dominance
1. This is the type of Dominance Mendel studied.
2. The traits he studied were either/or traits.
a. Either dominant (capital letter) or recessive (lowercase letter)
3. The dominant trait will show
4. The recessive trait is hidden
5. The person that is heterozygous is a carrier of the trait
Incomplete Dominance
1. Unlike Complete dominance, there are NO lowercase letters.
2. NEITHER trait has dominant control
3. A mixed color or a blending is made.
a. To remember a mixed color is made, think of the {m} in
inco{m}plete.
4. Dominant red is written as {RR} (a purebred - homozygous)
5. Dominant white is written as {WW} (a purebred - homozygous)
6. The mixed color they make is pink {RW} (a hybrid heterozygous)
Codominance
1. Like Incomplete dominance, there are NO lowercase letters.
a. This time, both traits express themselves in different
locations
2. The punnet square will be written exactly as in incomplete
dominance.
a. One dominant color is written as {RR} (a purebred - homozygous)
b. The other dominant white is written as {WW} (a purebred - homozygous)
c. The both colors showing will be written as {RW} (a hybrid - heterozygous)
d. A cow or horse with both colors (usually brown and white) showing is called a “roan”
horse or cow.