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Transcript
Dihybrid Crosses Involve two sets of genes, for two different characteristics Can be displayed as a punnet square as shown in the following example: What are the genotypes of a cross between pure breeding round, yellow peas with pure breeding wrinkled, green peas? Round and green are dominant Let G = green and g = yellow Let R = round and r = wrinkled The parent plants become RRgg x rrGG (Gametes are Rg x rG) First filial generation produces all RrGg (round, green) rG rG Rg RrGg RrGg Rg RrGg RrGg Second filial generation produces combinations of all genotypes and phenotypes Gametes: RG, rG, Rg, rg Offspring: round green : round yellow : wrinkled green : wrinkled yellow 9 3 3 1 RG Rg rG rg RG RRGG RRGg RrGG RrGg Rg RRGg RRgg RrGg Rrgg rG RrGG RrGg rrGG rrGg rg RrGg Rrgg rrGg rrgg 1. Coat colour and tail length in mice has been studied. Black coat is dominant to brown coat, while long tail is dominant to short tail Let Let B= b= T= t= Phenotype: Black, long (pure breeding) Gametes: F1 x F1: F1 gametes: x Brown short Therefore: F2 phenotypes: ___ black long, ___ black short, ___brown long, ___brown short Ratio: 2. A test cross is performed whereby one of the F1 offspring is crossed with a homozygous recessive parent: BbTt x bbtt What phenotypic ratio would you expect to see in the offspring of this test cross? 3. In guinea pigs, rough coat (R) is dominant to smooth coat (r) and black coat (B) is dominant to white (b). If a homozygous rough, black animal is crossed with a smooth white one: a. What will the F1 look like? b. What phenotypes would you expect in the F2 and in what ratio? (Show punnett working) c. Predict the ratios of phenotypes in the offspring of the following crosses: RrBb x rrbb Rrbb x RrBb Rrbb x rrBB 4. Consider two genes each with two alternative alleles. A gene with the alleles B (abnormally high cholesterol) and b (normal cholesterol levels) is on chromosome 19. Another gene controls Rhesus blood type (D – Rhesus positive and d – Rhesus negative). Its locus is on chromosome 1. a. A couple are both heterozygous for the two genes. What is the expected phenotypic ratio amongst their offspring given that they produce a huge number of children? b. Use the background information from the previous question. In another couple, the father is heterozygous for both characteristics and the mother has low cholesterol levels and is Rhesus negative. Calculate the expected phenotypic ratios. Solutions: 3.
