PUNNETT SQUARES REVIEW Monohybrid cross is the cross
PUNNETT SQUARES
REVIEW • Monohybrid cross is the cross between two parent organisms and the inheritance patterns of only one trait are studied. • One allele from a female egg is joined with one allele from a male sperm.
PROBABILITY AND PREDICTING INHERITANCE • Since alleles for a trait separate during meiosis, we can use the laws of probability to predict genetic outcomes. • Probability is the chance that a certain event will occur.
PUNNETT SQUARES • A diagram that shows all possible gene combinations from a genetic cross. • Dominant alleles are capital letters • Recessive alleles are lowercase letters
USING PUNNETT SQUARES FOR MONOHYBRID CROSSES • Step 1: draw a Punnett square with four compartments • Step 2: take the genotype for one parent and put one letter outside the Punnett square to the left of each row • Step 3: take the genotype for the other parent and put one letter outside the Punnett square above each column. • Step 4: now you can fill in the grid. Take the letter over each column and copy it into the two squares below it • Step 5: Take the letter at the beginning of each row and copy it into the two squares to the right.
MATH REVIEW Description Ratio Fraction Percent One to three or one out of four 1: 3 1/4 25% Two to two or two out of four 2: 2 1/2 50% Three to one or three out of four 3: 1 3/4 75% Four out of four 100%
DIHYBRID CROSS • Studying 2 traits crossed from parent organisms • When two traits are studied at a time the illustrate the Law of Independent Assortment. • This means that inheriting one trait doesn’t affect the inheritance of another trait. • Ex. Having brown eyes has nothing to do with having freckles.
USING PUNNETT SQUARES FOR DIHYBRID CROSSES • Step 1: Pick a parent. • The mother is heterozygous for both traits, (Rr. Ff), so she has a dominant R gene and a recessive r gene. She also has a dominant F gene and a recessive f gene. She can pass on each of these traits independently to her children. • What are the four possible combinations of these traits?
CONTINUED • Step 2: Consider the father. • He is also heterozygous for both traits, so the combinations of traits are the same • RF, Rf, r. F, and rf
CONTINUED • Step 3: Now fill in a Punnett square with these combinations. • Since there are four combinations for each parent, the square is 4 X 4 instead of 2 X 2. • You fill it in the same way.
PUNNETT SQUARE Mother Father RF Rf r. F rf RF RRFf Rr. FF Rr. Ff Rf RRFf RRff Rr. Ff Rrff r. F Rr. Ff rr. FF rr. Ff rf Rr. Ff Rrff rr. Ff rrff
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