punnett squares step-by-step overview. genetics problems: punnett squares when we have enough...
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Punnett Squares
Step-by-Step Overview
Genetics Problems: Punnett Squares
• When we have enough information about two parent organisms, we can predict the genotype and phenotype of their offspring
Genetics Problems: Punnett Squares
• Remember,– Genotype: genetic composition of an organism,
the actual alleles of each gene– Phenotype: expression of the gene as a physical
characteristic– Dominant: If dominant allele is present, it is
always expressed– Recessive: Allele only expressed when
homozygous recessive
Punnett Squares: Step by Step
1. Determine the genotype of the parents
2. Write down the “cross”
3. Draw the Punnett square
4. “Split” the parents’ genotypes into gametes on the
outside of the Punnett square
5. Determine genotypes of offspring
6. Summarize results (genotypes and phenotypes)
Punnett Squares: Step by Step
1. Determine the genotypes of the parents from the question
In humans, brown eyes are dominant over blue eyes. What type of offspring would you expect if you crossed a heterozygous brown-eyed person to a
heterozygous brown eyed-person?
B = brown eyes (dominant) b = blue eyes (recessive)
Punnett Squares: Step by Step
1. Determine the genotypes of the parents from the question
In humans, brown eyes are dominant over blue eyes. What type of offspring would you expect if you crossed a heterozygous brown eyed-person to a
heterozygous brown-eyed person?
B = brown eyes (dominant) b = blue eyes (recessive)
Punnett Squares: Step by Step
1. Determine the genotypes of the parents from the question
In humans, brown eyes are dominant over blue eyes. What type of offspring would you expect if you crossed a heterozygous brown-eyed person to a
heterozygous brown-eyed person?
B = brown eyes (dominant) b = blue eyes (recessive)
Parent 1 (Mom) = Bb Parent 2 (Dad) = Bb
Punnett Squares: Step by Step
2. Write down the “cross”
In humans, brown eyes are dominant over blue eyes. What type of offspring would you expect if you crossed a heterozygous brown-eyed person to a
heterozygous brown-eyed person?
Cross: Bb x Bb
Punnett Squares: Step by Step
3. Draw the Punnett Square
Cross: Bb x Bb
Punnett Squares: Step by Step
4. Split the parent’s genotypes into gametes and put them on the outside of the Punnett square
Cross: Bb x Bb
B b
B
b
Punnett Squares: Step by Step
5. Determine the genotypes of the offspring by filling in the Punnett Square
Cross: Bb x Bb
B b
B
b
Punnett Squares: Step by Step
5. Determine the genotypes of the offspring by filling in the Punnett Square
Cross: Bb x Bb
B b
B BB Bb
b Bb bb
Punnett Squares: Step by Step
5. Summarize results (genotype and phenotype)Cross: Bb x Bb
B b
B BB Bb
b Bb bb
In humans, brown eyes are dominant over blue eyes. What type of offspring would you expect
if you crossed a heterozygous brown eyed person to a heterozygous brown eyed person?
Punnett Squares: Step by Step
5. Summarize results (genotype and phenotype)Cross: Bb x Bb
B b
B BB Bb
b Bb bb
In humans, brown eyes are dominant over blue eyes. What type of offspring would you expect
if you crossed a heterozygous brown eyed person to a heterozygous brown eyed person?
The genotypes of the offspring included: 1 homozygous dominant (BB), 2 heterozygous
(Bb), and 1 homozygous dominant (bb).
You would expect the phenotypes of the offspring to be 3 with brown one with blue
eyes.