Slide 1
Human pedigrees describe the interrelationships between parents
and children, over generations, regarding a specific trait.
This is in contrast to a Punnett square that allows you to make predictions about future offspring
Slide 2
Because relationships between individuals (whether we are talking
about humans or pedigreed dogs) can become quite complicated - pedigrees can
also become quite complicated. There are symbols to indicate the
different types of relationships that can exist.
Slide 3
For this class, we will restrict our use of symbols to those that
are circled.
Squares refer to males and circles refer to females. If the square or circle is empty - the individual is termed "normal" and does not exhibit the trait that is being analyzed in the pedigree. If the square or circle is filled in - the individual is termed as "affected" with the trait.
Please be aware that the term "normal" has a very specific meaning in genetics that derives from statistics - it means the "norm" or average phenotype and has no connotation about weirdness or strangeness.
If a shape is half-filled in that indicates that the indivdual is a carrier. A carrier is an individual who "carries" a recessive allele but is phenotypically normal. In other words, a carrier is an individual who is heterozygous for a recessive condition. Carriers will not always be indicated on a pedigree - questions on the quiz may require you to determine who the carriers are.
Horizontal lines between a circle and square means that the two are mating partners. Vertical lines that descend from this horizontal line indicate offspring.
Slide 4
We see these features in this simple pedigree of a recessive
trait. The circle is the female and the square is the male. The
line connecting them indicates that they are partners. They have two
children a daughter and a son. The daughter is the first born (birth
order is indicated left to right). The son is affected by the trait.
But how can we tell that this trait is recessive? Because neither mom nor dad has the condition but the son does. This means that both parents must be carriers for the condition - they are both heterozygous.
Slide 5
We can use a Punnett square to reinforce this idea of
carriers. If the trait is recessive, both parents can "carry" the allele
and not express it - but they can have a child with the condition.
Slide 6
This pedigree shows a trait that most likely has dominant
inheritance. How can we tell? Well, when we look at the pedigree -
there is no evidence of carriers. All affected children have at least
one affected parent. So for this pedigree - the most likely mode of
inheritance is dominant.