Download 1 Trait Punnet Squares

Name: __________________________________________________________________ Single Trait Punnett Squares Background: Punnett squares shoes the genes (represented by letter – capital (dominant) or lower case (recessive) that could be present in the parents’ gametes. The possibilities for the male and female gametes are then placed along the side and the top of the square. Then by looking at possible gene combinations in the zygote, you can predict what is expected in the offspring by calculating the genotypic and phenotypic rations. Remember that the genotype is the combination of the alleles and the phenotype is the actual physical trait that is expressed. Example: A homozygous green pea plant (GG) is crossed with a heterozygous green pea plant (Gg). G G G GG GG Genotypic ratio: 2 GG : 2 Gg : 0 gg Phenotypic ratio: 4 green pea plants : 0 other color g Gg Gg Practice Problems: 1. Tall (T) is dominant to short (t) in pea plants. What are the genotypic and phenotypic ratios of the offspring with parents that are both heterozygous tall (Tt)? Tall (dominant) = Short (recessive) = genotypic ratio: _____________________________ phenotypic ratio: ____________________________ 2. Cross a homozygous tall plant (TT) with a short plant (tt). What are the genotypic and phenotypic ratios of the offspring? Parent 1 genotype: ______________________ Parent 2 genotype: ______________________ Gametes of parent 1: ____________________ Gametes of parent 2: ____________________ genotypic ratio: _____________________________________ phenotypic ratio: ____________________________________ 3. When another tall plant is crossed with a short plant, some of the offspring are short. a. What must be the genotypes of the parents? ____________________________________________ b. What are the genotypes of the offspring? ________________________________________________ c. What is the phenotypic ratio in the offspring? ___________________________________________ d. Show the punnett square below. 4. Yellow pod color (Y) is dominant to green pod color (y) in pea plants. Cross a heterozygous plant (Yy) with a green plant (yy). What are the genotypic and phenotypic ratios of the offspring? genotypic ratio: _____________________________________ phenotypic ratio: ____________________________________ 5. What cross would result in half of the offspring having green pods and half of the offspring having yellow pods? Genotypes of parents: _______________________________________ 6. Let’s say that in seals, the gene for the length of the whiskers has two alleles. The dominant allele (W) codes for long whiskers and the recessive allele (w) codes for short whiskers. What precentage of offspring would be expected to have short whiskers from the fross of two long-­‐whiskered seals, a female that is homozygous dominant and a male that is heterozygous? % with long whiskers = _____________________ % with short whiskers = ____________________ 7. If one parent seal is homozygous long whiskered and the other is homozygous short whiskered, what percent of the offspring would have short whiskers? % with short whiskers = _____________________ 9. In mice, black hair color (B) is dominant over brown hair color (b). Cross a homozygous black mouse with a heterzygous black mouse and determine the genotypic and phenotypic ratios of their offspring. Genotypic ratio: _____________________________________ Phenotypic ratio: ____________________________________ 10. Two black mice are mated several times. Out of 100 offspring, 25 of them have brown hair. What are the probable genotypes of the parents? ________________________________ Show the cross to prove it.