How to Solve Genetic Problems Click on the Punnet Square.

Review - Alleles Most genes exist in at least 2 forms. Each form of a gene is called an allele. During anaphase II, each gamete receives only ONE allele for each gene. The Law of Random Segregation states that which of the two alleles are passed on is random and purely a matter of chance.

What makes an allele dominant or recessive? Commonly, the dominant allele codes for a working protein while the recessive allele does not. For example, M represents a dominant allele and codes for the production of melanin (a skin pigment). The recessive m allele is unable to code for the production of melanin. Genotype Phenotype MM Able to produce melanin Mm mm Albino (unable to produce melanin)

Procedure for Solving Genetic Problems There are five steps you will follow when solving genetic problems. Show the letter that represents the genes. Show the genotypes of the parents. Show the gametes produced by each parent. Draw a Punnet Square using the gametes produced by each parent. A Punnett Square is a diagram that summarizes every possible combination of each allele from each parent. Show the genotypic and phenotypic summary of the offspring.

Monohybrid Cross: considering only one trait when crossing/breeding 2 organisms Example 1: Flower colour in pea plants Let P represent the gene for purple flowers. (dominant) Let p represent the gene for white flowers. (recessive) P: PP x pp (homozygous dominant x homozygous recessive) PP x pp P P p p 4. P p Pp Each parent produced two possible gametes each containing one of the alleles. The probability of producing either of the gametes is ½. The probability of any particular offspring inhertiing a specific combination of the alleles is therefore ½ x ½= ¼. The four square of Punnett saure represent the four possible combinations and each as a ¼ or 25% chance of occuring. 5. F1 Summary: Genotype: 100% Pp Phenotype: 100% purple flowers

F1: Pp x Pp Example 2: F2 generation P p P p And the F2 summary is: Genotype Phenotype 25% PP 50% Pp 25% pp 75% purple flowers 25% white flowers breeding of 2 organisms with different traits is called a cross

Example 3: Determining parent genotypes using offspring phenotypic ratios In one cross of tomatoes, 1821 red tomatoes and 615 yellow tomatoes were produced. Determine which allele is dominant and the probable genotypes of the parents. Assume that the trait is influenced by only two alleles and follows the laws of Mendelian inheritance. Use R and r to represent the alleles. How should we start?

Since this is approximately a 3:1 ratio, it matches the cross of 2 heterozygous parents. Therefore, red (R) is dominant to yellow (r) and both parents are heterozygous, Rr. Draw a Punnett square to check.