Unformatted text preview: show the trait (e.g., round/wrinkled peas),
penetrance is complete.
Retinoblastoma is the most common form of eyecancer. Although it is a dominant trait, only 75% of
the individuals carrying the allele are affected. In this
instance, penetrance is incomplete.
instance, January 31, 2007 BIS101001, Spring 2013—Genes and Gene Expression, R.L. Rodriguez ©2013
BIS101001, Spring 2013—Genes and Gene Expression, R.L. Rodriguez 38 Incomplete Penetrance
In a certain breed of plants, thorns (T) are determined by a dominant
allele and thornless (t) iis determined by the recessive allele for the thorny
gene. However, T is only 80% penetrant in the heterozygotes (i.e., 20%
of the heterozygotes will appear thornless). Starting with the parental
cross TT x tt, what would be the expected number of phenotypes
TT tt what
observed in a population of 500 F2 plants?
t TT Tt 125
Tt T t
tt 125 125 100% Penetrance T
t t TT Tt 125
tt +Tt 100 175 80% Penetrance Total thorny = 250Tt x 0.8 = 200Tt + 125TT = 325Tt+TT
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- Spring '08
- R.L. Rodriguez