Planet Earth and Its Environment A 5000-million year Journey

Biology in Focus, HSC CourseGlenda Childrawi, Margaret Robson and Stephanie Hollis

Blueprint of LifeTopic 8: Solving Genetic Problems1DOT POINT(s)solve problems involving monohybrid crosses using Punnett squares or other appropriate techniquesIntroductionMendels laws can be directly applied to solve genetic problems. In this lesson we are going to explore this.www.hobart.k12.in.us

IntroductionThe model of Mendelian inheritance is used to make predictions of the possible outcomes when certain individuals are cross-bred. His laws provide a very good problem solving tool to calculate the probability of a particular genetic trait (such as blue eyes or dimples) being passed on. weheartit.com

IntroductionThis is particularly important because it can be used to predict the inheritance of genetic defects that arise as a result of a faulty gene, e.g. albinism, cystic fibrosis, Tay-Sachs disease, night blindness and short-sightedness. Some genetic defects are lethal in the homozygous recessive condition and so parents may be counselled on the probability of their having offspring inheriting this genotype if the genotypes of both parents are known.

www.cysticfibrosis.net Punnett SquaresGenetic problems may be solved using Punnett squares or branching diagrams. Both methods use standard genetic notation. Letters of the alphabet are used to represent genes:www.biography.com

Punnett SquaresTo represent a genotype that follows the law of dominance, only one letter is used to represent the gene (e.g. the letter T for height). The dominant allele is represented by a capital (upper case) letter (e.g. T ) and the recessive allele is represented by the lower case form of the same letter (e.g. t). www.dbriers.com

Punnett SquaresAt the start of a genetic problem, there should always be an outline of which letters will be used to represent the genotype. For example to investigate the probability of a person inheriting curly hair, one would begin by stating: Let C = curly hair and c = straight hair.www.ebay.com

Punnett SquaresThe different stages of the cross being investigated are outlined by clearly labelling the generations:P = the parent generationF1 = the first filial generationF2 = the second filial generation

Indicate at which stages in the cross segregation occurs, gametes are formed and fertilisation occurs.www.biography.com

Punnett SquaresStep 1: Designate letters that will represent the genes/traits. For example:

R = wrinkled seeds, r = round seedswww.dijitalimaj.com

Punnett SquaresStep 2: Write down the genotypes (genes) of each parent. These may be stated in the outline of the problem or you may be expected to determine the genotypes by logical deduction from other information that has been given.

P: Rr Rr (both heterozygous or hybrid)aalite.hubpages.com

Punnett SquaresStep 3: List the genes that each parent can contribute to the gametes.www.biography.com

Punnett SquaresStep 4: Draw a Punnett square to show all the possible combinations of genes when gametes fuse. A Punnett square consists of nine cells in a table (three cells in each row and each column). Write the possible genes in the gametes of one parent across the top and the genes from the gametes of the other parent along the side of the Punnett square see (blue letters below). R r R rPunnett SquaresStep 5: To show how genes recombine during fertilisation, complete each cell of the Punnett square by combining the letters above and beside the appropriate cell. As a general rule, the capital letter goes first and a lowercase letter follows (see red letters on the previous page).

Punnett SquaresStep 6: Write out the genotypic ratio and phenotypic ratios for the offspring (see orange text beneath the Punnett square).

Punnett SquaresAn alternative to using a Punnett square is to use a branching diagram. The use of these has been demonstrated in the explanation of Mendels laws. Steps 4 and 5 would be replaced by drawing branching lines from gametes to offspring.

Genetic ProblemsGenetic problems come in many forms. To solve a genetics problem, you need to use the information givenread it, analyse it, see what deductions you can make from it and then perform a cross to work out the information you do not know.www.seeitourway.org

Genetic ProblemsIt is important to determine which trait is dominantthis information may be given to you in the question, or you may be expected to work it out by deductive reasoning. Some common dominant and recessive traits are listed in Table 2.4.Handout Table

TaskUsing the table, identify the dominant and recessive traits evident in the photographs below.

Activity/Homework-Students to solve Problems 1-5