In the diagram below, Squares represent Male phenotypes and
Circles represent female phenotypes; if a condition has been inherited the
shape is coloured, by condition we generally mean diseases, but not necessarily.
Reading the diagram
Initially the diagram initiates with an “affected” female
carrying a condition and a “normal” male. The children of these parents
are shown by the vertical line below the parents, a male child has two children
with another female... these become the grandchildren, one of the granddaughters
has a four children, and two of which can be seen as affected. So two
great-grandchildren (a male and a female) are infected in this pedigree diagram
Analysing the diagram
What we now should do is to determine whether the affected
condition is caused by a dominant allele or a recessive allele.
If the condition is caused by a dominant allele then the
following statement is true: ‘All individuals with genotypes DD and Dd will be
affected’. The alternative hypothesis is that ‘All individuals with the
genotype dd will be affected’.
If we examine the granddaughter and her infected son we have two
unaffected parents and an infected child. If our first hypothesis was correct
then the child must contain a “D” allele, which means at least one of
the parents must have the “D” allele, but if that where the case then the
parent would also be “affected” so we can safely make the assumption that the
condition in this case is caused by a the Homozygous recessive genotype.
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