Download Punnett Squares

Punnett
Squares
Kelly Coleman
High School Biology
Objective:
• After this lesson you will be able to
predict the outcome of genetic crosses
using a Punnett Square
How are traits inherited?
• We know offspring look like parents
• Early scientists thought offspring had an
average of the parents’ traits
– A tall parent and a short parent have
medium height offspring
Gregor Mendel
• Studied seven traits
in garden peas
• Various forms of
traits showed up in
the same ratio with
each cross 3:1
Gregor Mendel
• Mendel knew traits came from parents
• A plant has two pieces of information,
one from each parent
• Different combinations
of information lead to
different appearance
of the offspring
Genes and Alleles
• If one parent plant has Ss
on it’s genetic code for
pea shape, offspring
might get an S or an s
• To keep track of possible
combinations we use a
Punnett Square
Punnett Square Process
•
•
•
•
Read question and write parent’s genotypes
Draw Punnett Square with parent genotypes
Drop parent genotype down and across
Summarize possible offspring genotypes
Demonstration
• What will result from mating
two heterozygous tall pea plants?
Group Practice
• What will result from mating
a heterozygous tall pea plant and
a homozygous tall pea plant?
Individual Practice
• What will result from mating
two heterozygous round pea plants?
Extended Practice
• Having six fingers is a rare dominant trait (D).
Homozygous recessive people have five fingers(dd).
•
A.) What are the chances that a child will be
born with six fingers if
both parents are homozygous recessive?
•
B.) What are the chances that a child will be
born with six fingers if
one parent has five fingers and
the other parent is heterozygous?
Conclusion
• How could knowing what offspring might
come from two parents be useful?