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Dihybrid Crosses 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 came up with 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. Linked Genes Studying fruit flies, Thomas Hunt Morgan identified dihybrid crosses that did not result in the phenotype ratios expected through independent assortment. Recall that expected ratios are: 9:3:3:1 for heterozygous crosses 1:1:1:1 for cross of a heterozygote with a recessive . One possible gamete from YyRr x yyrr homozygous recessive parent. yr heterzygote YR YyRr Yellow Round Yr Yyrr Yellow Wrinkled yR yyRr Green Round yr yyrr Green Wrinkled Test cross results if independently assorting: Four Four possible from gametes Equally likely if independently Equally likely assorting. 1:1:1:1 (25% of each of 4 possible combinations.) 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 Then ____________ 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. 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 parental combinations because genes are on the same chromosome. 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 parental combinations because genes are on the same chromosome. 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 (75% probability). However, a recessive trait can be more common _____ ___________________________________________. For example, Achrondoplasia dwarfism is caused by a dominant allele, and yet the trait is very rare in the population. 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 survive, 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) Natural Selection and Genetic Natural selection acts upon ____________ (expression of trait) rather than genotype. Most harmful genetic disorders are recessive. Carriers can have a hidden copy of the harmful gene that does not affect their own survival. .
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