BIOL 224 Lecture Slides: Independent Assortment - PDF

Lecture 02 Independent assortment BIOL 224 Jan. 20th, 2025 1 Learning objectives 1. Review Mendel’s law of independent assortment. 2. Know how to work with inheritance problems involving 2+ genes. 3. Describe the chromosomal basis of independent assortment....

Lecture 02 Independent assortment BIOL 224 Jan. 20th, 2025 1 Learning objectives 1. Review Mendel’s law of independent assortment. 2. Know how to work with inheritance problems involving 2+ genes. 3. Describe the chromosomal basis of independent assortment. 2  Mendel’s law of independent assortment Key Concept describes inheritance patterns involving 2+ genes 3 The law of independent assortment For many of his pea crosses, Mendel examined individuals that varied in two different traits For this, he performed dihybrid crosses = cross involving individuals heterozygous for two genes 4 The law of independent assortment He formed two hypotheses on how these traits would be inherited: Dependent assortment = alleles from different genes are inherited together Independent assortment = alleles from different genes are inherited independent from one another Practice: what would be expect for phenotype ratios under each scenario? 5 The law of independent assortment 6 The law of independent assortment The law of independent assortment states that each pair of alleles segregates independently during gamete formation Only applies to genes on different chromosomes OR far apart on same chromosome Genes located near each other on same chromosome tend to be inherited together (dependent assortment or linkage) Mendel found all his pea traits followed the law of independent assortment 7 Concept check Pea plants heterozygous for flower position and stem length (AaTt) are allowed to self-pollinate, and 400 of the resulting seeds are planted. Draw a Punnett square for this cross. How many offspring would be predicted to have terminal flowers and be dwarf? (both recessive traits) List all gametes that could be made by a pea plant heterozygous for seed colour, seed shape, and pod shape (YyRrIi). How large a Punnett square would you need to draw to predict the offspring of a self-pollination of this “trihybrid”? 8 Key Concept Probability laws govern Mendelian inheritance 9 Probability laws govern Mendelian inheritance Predicting outcomes of crosses becomes exponentially more complicated with increasing numbers of genes Monohybrid cross = 2 different gametes each, 4 zygotes Dihybrid cross = 4 gametes each, 16 zygotes Trihybrid cross = 8 gametes each, 64 zygotes Tetrahybrid cross = 16 gametes each, 256 zygotes 10 Probability laws govern Mendelian inheritance Mendel’s laws of segregation and independent assortment reflect rules of probability When tossing a coin, outcome of one toss has no impact on outcome of next toss In the same way, alleles of one gene segregate into gametes independently of another gene’s alleles 11 Probability laws govern Mendelian inheritance Probability scale of an event occurring ranges from 0 (no chance of occurring) to 1 (certain to occur) Outcome of one event has no impact on outcome of second event E.g.) Flipping a coin p = 0.5 p = 0.5 p = ½ = 0.5 12 Probability laws govern Mendelian inheritance The rule of multiplication (product rule) calculates the probability of two independent events both occurring E.g.) What is the probability of flipping two heads? 1st 2nd heads heads 1 1 1 × = 2 2 4 13 Probability laws govern Mendelian inheritance The rule of addition calculates the probability of an event that can occur in alternative ways E.g.) What is the probability of flipping one heads and one tails? H/T T/H 𝟏 𝟏 𝟐 𝟏 + = = 𝟒 𝟒 𝟒 𝟐 14 Solving complex genetics problems with the rules of probability We can apply multiplication and addition rules to predict the outcome of crosses involving multiple characters A dihybrid+ cross same as two+ independent monohybrid crosses In calculating the chances for various genotypes, each character is considered separately, and then individual probabilities are multiplied 15 Solving complex genetics problems with the rules of probability What fraction of offspring from the following cross will exhibit at least two recessive phenotypes? PpYyRr x Ppyyrr Five possible genotypes with 2+ recessive phenotypes: Ppyyrr, PPyyrr, ppYyrr, ppyyRr, ppyyrr 16 Solving complex genetics problems with the rules of probability Probability of Pp = ½ Probability of PP/pp = ¼ Probability of Yy/Rr = ½ Probability of yy/rr = ½ 17 Concept check A couple wants to have a large family of five children. What is the probability all the children will be boys? (Hint: use the product rule) 18 Concept check You perform the following cross of involving two plants with genotypes of: Aa;bb;Cc;Dd;Ee x Aa;Bb;Cc;dd;Ee What proportion of progeny would you expect to have a fully recessive phenotype (aa;bb;cc;ee;ff)? (Hint: use the product rule) 19  Genes located on different Key Concept chromosomes are inherited independently 20 The chromosomal basis of independent assortment The mechanical basis of equal segregation and independent assortment of alleles is the anaphase segregation of chromosomes at meiosis The randomness of spindle attachment throughout the chromosome set accounts for Mendel’s second law 21 The chromosomal basis of independent assortment Meiosis I Meiosis II 22 Next class DNA structure and function (Chapter 6) 23