Dihybrid Crosses: Punnett squares for two traits Genes on Different Chromosomes If two genes are on different chromosomes, all four possible alleles combinations for two different genes in a heterozygous cross are _____________ due to independent assortment. If parents are RrYy (heterozygous for both traits) Equally R r likely to R r OR line up either way y Y Y y when dividing Ry rY RY ry Gametes: ___________________________________ Setting up a Dihybrid Cross: RrYy x RrYy Each side of a Punnett Square represents all the possible allele combinations in a gamete from a parent. Parent gametes always contain one allele for___ _______ .(____________-R or r & Y or y in this case). Four possible combinations of the alleles for the two genes are possible if heterozygous for both traits. (For example: ___________________) Due to independent assortment, each possible combination is equally likely if genes are on separate chromosomes. Therefore Punnett squares indicate probabilities for each outcome. Discuss with your table partner: A. R r Y y Which is correct for a R dihybrid cross of two r heterozygous parents RrYy x RrYy? Y y B. RR Rr rr rR C. RY Ry rY ry YY RY Yy Ry yy rY yY ry Explain how the correct answer relates to the genes passed down in each gamete (egg or sperm) Dihybrid Cross of 2 Heterozygotes 9 3 3 1 Heterozygous Dihybrid Cross Dominant Dominant Recessive Recessive for both 1st trait 1st trait for both traits Recessive Dominant traits 2nd trait 2nd trait Round Round Wrinkled Wrinkled Yellow Green Yellow Green __ __ __ __ ______ = ______ = ______ = _______ = ___ ____ ____ _____ Heterozygous cross __________ratio if independent assortment Mendel developed the Law of Independent Assortment because he realized that the results for his dihybrid crosses matched the probability of the two genes being inherited independently. Exceptions to the Law of Independent Assortment: Linked Genes Studying fruit flies, Thomas Hunt Morgan identified dihybrid crosses that did not result in the phenotype ratios expected through independent assortment. Review of Recombinants Recombinants have new combinations of the genes. If the parental generation of cats are white furred with short tails (SS bb) and brown fur with long tails (ss BB), Parental Genotypes the F1 will be SsBb. In the gametes for the next generation, ________________ are the parental combinations for the gametes (egg or sperm) and ________________ are the recombinant gametes. ___________ ____________ Combinations gametes Linked Genes When two genes are linked (relatively close together on the same chromosome), it ________ ___________________________ __________________________. Recombinants do occur due to crossing over. The further apart the genes are the more likely recombinants are because crossing over is more likely to occur. One possible gamete from SsBb x ssbb homozygous recessive parent. sb heterzygote SB SsBb Short, Brown Sb Ssbb Short, White sB ssBb Long, Brown sb ssbb Long, White Test cross (heterozygote crossed with a recessive) results if independently assorting: ________ Four Four possible from gametes Equally likely if independently Equally likely assorting. (_____ of each of 4 possible combinations.) Parental combinations: Gray with normal wings and Black with vestigal wings Cross of the heterozygous offspring with a double recessive. Expect 1:1:1:1 if independently assorting. Results show more of ____________________ because genes are on the same chromosome. Discuss with your table partner: Mythbuster Many people assume that dominant traits are always more common. It is true that if both parents are heterozygous, it is more likely that the offspring will have the dominant trait. However, a recessive trait can be more common in the population. For example, Achrondoplasia dwarfism is caused by a dominant allele, and yet the trait is very rare. Explain how and why this is possible. __________________________________ __________________________________ __________________________________ Genetics and ______________ _________________can change the allelic frequency. (Allelic frequency = how common an allele is in the gene pool.) Bb BB bb bb Bb Bb bb BB Bb Bb BB Bb Initial allelic frequency of b = 50% (12/24 alleles) If the white rabbits are more likely to _________ , the frequency of b allele will increase in the gene pool. bb bb bb bb bb Bb bb bb bb bb bb bb bb bb Allelic frequency of b after natural selection= 96% (23/24) Discuss with your table partner: Review the meaning of the words genotype and phenotype. Do you think natural selection acts upon the genotype or phenotype of an organism? Explain your reasoning. ________________________________ ________________________________ ________________________________ ________________________________ ________________________________ ________________________________ ________________________________ .