bicd 100 - lecture 8

bicd 100 - lecture 8 - LECTURE - 8 Basic Principles of...

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Unformatted text preview: LECTURE - 8 Basic Principles of Heredity DOMINANT RECESSIVE MENDEL STUDIED SEVEN OBSERVABLE CHARACTERISTICS OF THE PEA PLANT Homozygous Heterozygous PUNNETT SQUARE R r Rr R Rr R R RR r Rr r Rr Rr r Rr rr If both plants were homozygous, you would see only (100%) tall plants in the next generation t t TT: homozygous for the tall (T) allele T Tt Tt tt: homozygous for the short (tt) allele T Tt Tt TEST CROSS: An individual of unknown genotype is crossed with another individual with homozygous recessive phenotype. How does this help? If we consider a cross between two heterozygous plants (Tt): Gametes are of 2 types, T and t To use the multiplication rule to calculate probability of obtaining a certain genotype, say TT, you could multiply the probability of getting the T allele (1/2) in the gamete from each parent. ½x½= ¼ T t T TT Tt t Tt tt Hence the probability of getting a T allele from the first parent and a T allele from the second parent, (i.e. for the TT type of progeny) is: TT ½x½= ¼ homozygous dominant Similarly, for each of the other three types of progeny, Tt ½x½= ¼ tT ½x½= ¼ tt ½x½= ¼ heterozygous (two ways) homozygous recessive Now we can use the addition rule to calculate overall phenotypic ratios. T is dominant over t, hence tall plants can have the genotypes TT, Tt, and tT Using the addition rule, we add the probabilities of getting each type of tall progeny, ¼ + ¼ + ¼ = ¾ Tall Short progeny can only occur if a recessive allele is inherited from each parent, so the probability is ¼ (as per previous slide) Short Dihybrid Cross Rr R RR Rr r Rr rr Y y Y YY Yy y Yy yy Branch Diagrams Rr r Rr rr r Rr rr Y y y Yy yy y Yy yy Dihybrid Testcross INCOMPLETE DOMINANCE ...
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