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Working Genetics Problems solving problems with Punnett squares © 2006 Plano ISD, Plano, TX Always follow these 3 steps when working genetics problems. © 2006 Plano ISD, Plano, TX 1 Write the key. Information needed to write the key will either be stated in the problem or given. 2 Determine the genotypes of both parents. 3 Solve the problem using a Punnett square. © 2006 Plano ISD, Plano, TX Possible Each reproductive cell contributes combinations of half of theare genes parent’s shown in genes a Punnett to its square. offspring. Each parent Purple Both White One parent color is dominant parents flower contributes color a a purple over contribute is purple recessive white. flower a flower white to gene; purple; one gene. flower therefore, parent gene. the contributes a offspring white flower is white. gene. © 2006 Plano ISD, Plano, TX Possible Each reproductive cell contributes combinations of half ofare genes its shown genes to aitsPunnett in offspring. square. Yellow seed color is dominant over green. © 2006 Plano ISD, Plano, TX Substitute letters for genes. Purple flower color is dominant over white. P- purple p - white © 2006 Plano ISD, Plano, TX P p PP Pp Pp pp P p Substitute letters for genes. Yellow seed color is dominant over green. Y - yellow y - green © 2006 Plano ISD, Plano, TX Y Y y Yy Yy y Yy Yy How are these flowers different? PP Pp Both flowers have the same physical appearance or phenotype: they are both purple. The flowers have different genetic makeup or genotype. © 2006 Plano ISD, Plano, TX Describe the phenotype of these peas. © 2006 Plano ISD, Plano, TX A purple flower can be pure (homozygous) PP or hybrid (heterozygous). Pp The phenotype is the same: both flowers are purple. The genotype is different: one flower is PP, one is Pp. © 2006 Plano ISD, Plano, TX A white flower can be only be pure (homozygous). Why? © 2006 Plano ISD, Plano, TX pp Follow these steps for every genetics problem. 1. Write the key. 1 upper case = dominant trait P = purple lower case = recessive trait p = white 2. Determine both genotypes (parents). 2 Cross a pure purple flower with a white flower. PP x pp 3. Work the problem with Punnett square. 3 © 2006 Plano ISD, Plano, TX