Punnett squares and Pedigree Charts

The Basics of Genetics…the passing of
• Our genes determine our traits or
• Your genotype (Dd) or genetic makeup for a
particular trait determines your phenotype
(dimples) or physical appearance for that
• Alleles are different forms that a gene may
have for a single trait. For example, having
dimples or not having dimples.
• Alleles can be represented by letters. You get one
allele from each parent; that’s why there are 2
• Alleles can be dominant, represented by a capital
letter or recessive, represented by a lowercase
• Homozygous (pure) = 2 of the same alleles for a
trait; for example: dd or DD
• Heterozygous (hybrid) = 2 different alleles for a
trait; for example: Dd
Punnett squares and Pedigree Charts
Punnett squares
• A chart that shows the chances or
probability of a particular trait
being expressed in offspring.
• Each box of the Punnett square
represents a 25% chance for that
trait being expressed.
• Example: Cross Tt and tt
• T = dominant allele = tall
• T = recessive allele = short
• The Tt and tt along the top and
sides of the Punnett square are
the genotypes for the parents.
• The genotypes for the offspring
are inside of the Punnett square.
Interpreting Punnett squares
• Remember that T is dominant
over t, which is recessive.
• Parent 1’s genotype = tt;
phenotype = short
• Parent 2’s genotype = Tt;
phenotype = tall
• There is a 50% chance of
producing short offspring = tt
because there are 2 boxes
with that trait.
• There is a 50% chance of
producing tall offspring = Tt.
Pedigree Charts (also see Pedigree notes)
• There are four generations
shown on this pedigree
• Squares = males and circles
= females; if the shape is
totally shaded then he/she
has the condition; if the
shape is shaded ½ way
then he/she is a carrier for
that condition.
• Generation I parents had 2
boys and 3 girls of which
only 1 has the condition.

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