This phenotypic effect produces abnormal fingernails, toenails and kneecaps. From the pedigree, can you tell if the syndrome is controlled by a dominant or a recessive gene?
[Note: The shaded individuals are afflicted with the syndrome. For the moment disregard the letter symbols inside the figures. They represent blood group genotypes. If you wish to review blood group data, click on the appropriate button below.]
Is the patella syndrome dominant or recessive? Is the patella syndrome X-linked or autosomal? Think about this relationship and come to a decision before taking the quiz because this pedigree appears there also. Write your answer below and then scroll down the window for the answer.
Your Comments:
If the gene were recessive, then the female in the P1 generation is homozygous. If it were X-linked, then every male would have an unmasked patella gene in his genome. However, two out of four males in the F1 do not display the phenotype so the gene is probably not recessive and X-linked. It could be dominant and X linked. If so, the female in the P1 could be heterozygous and thus explain the phenotypic spread in the F1. However, the #1 male would transmit this X chromosome to his daughter in the F2 and she should display the patella phenotype then. She does not so it is a safe bet that the gene is not X-linked dominant either. This leaves us with an autosomal gene which is either dominant or recessive. If dominant, then the P1 female could be either homozygous or heterozygous. Since several offspring in the F1 are free of the phenotype, then the P1 female cannot be homozygous. Certainly the P1 male is homozygous for the normal allele if the gene is dominant. If the gene were recessive, then the P1 female is homozygous and the P1 male must be heterozygous in order to produce the patella phenotypes in the F1.
