Pedigree ChartsGoals:• Be able to construct a pedigree chart• Be able to trace the inheritance of selected traits• Understand the use of pedigree charts
• Because we cannot tell whether a person showing a dominant phenotype is homozygous or heterozygous for that trait, we try to work it out by looking at the phenotypes of their relatives.• This is known as a pedigree
Why would we want to know?• To determine if particular traits are genetically inherited• To work out if individuals are heterozygous for particular traits• And therefore, predict the likelihood of passing on the trait• All this information can be used by genetic counsellors and animal breeders
So, how do we do it?• Pedigree charts have a universally accepted format
Marfan’s Syndrome: An Example• Expressed in both sexes. – Thus, autosomal.• Expressed in every generation. – Thus, dominant.
Albinism: An Example• Expressed in both sexes at approximately equal frequency. – Thus, autosomal.• Not expressed in every generation. – Thus, recessive.
Hairy Ears: An Example• Only males are affected.• All sons of an affected father have hairy ears.• Thus, hairy ears is Y-linked.
Questions:1. Do the shaded symbols represent individuals who suffer from cystic fibrosis or those who do not?2. Give the genotypes of all members of the pedigree3. Using an example from the pedigree, explain how you can determine that cystic fibrosis is caused by a recessive gene. Answers are on the next slide – try to work it out first
Let C = no cystic fibrosis And c = cystic fibrosis Cystic fibrosis will beHere we have two parents present in a person with ccwho do not show the trait,producing offspring that doesshow the trait. This suggeststhat the parents are Cc Ccheterozygous and that thetrait is recessive Cc cc cc Genotypes cc cc Only the genotypes shown on the pedigree can be determined with certainty, the others Shaded individuals are those could be either CC or with cystic fibrosis Cc
1. Assign genotypes to the parents2. Is short sightedness dominant or recessive? Justify your answer.3. Predict the possible phenotypes of the individuals with a ?4. Calculate the chance of any individual in generation II having the recessive genotype.
From this pedigree it is not possible to conclusively say whether short-sightedness isrecessive or dominant because we need to see a cross where two parents display thedominant allele and produce an offspring with a different phenotype – ie a double recessive.To answer this question you need to look up whether short sightedness is dominant orrecessive – It is recessive. we will assign S for normal vision and s for shortsightedness. ss Ss ss ss Ss ss Ss ss ss ss ss Ss Ss S s s Ss ss Analysing the punnett square, we can see that there is a 50/50 chance of offspring from these parents being s Ss ss short-sighted