Chapter 2: Extensions to Mendel: Complexities in Relating Genotype to Phenotype. please read pages 38-47; 49-55;57-63. Slide 1 of Chapter 2 1
Extension sot Mendelian Behavior of Genes Single gene inheritance where: dominance is not complete more than two alleles at a gene can exist one gene may determine more than one trait (pleiotropy) multi-factoral inheritance where : more than one gene affects a single trait, and/or traits are also affected not only by genes but also by environment, chance or all three. Slide 2 of Chapter 2 2
Different forms of dominance Slide 3 of Chapter 2 3
genetic terminology reviewed Chromosome: cellular organelle found in the nucleus, comprised of tightly coiled/packed linear molecule of dsdna (double stranded) and bound proteins. Chromosomes immediately after mitosis have a single dndna molecule of great length Locus: a position on the dsdna molecule within a chromosome. A locus may be a gene, but most are not. Gene: a subsequence of DNA that has an affect on phenotype; generally transcribed into mrna and translated into protein Allele- one version of a locus, where different versions have different sequences of bases Slide 4 of Chapter 2 Mendel worked with only two alleles of each of the genes he studied. There is, however, no particular limit on the number of alleles that can exist for a locus. alleles arises by mutation of preexisting alleles. Mutations are changes in the base sequence of a subsequence of DNA (or locus, or gene) But a zygote and resulting plant can have only two alleles for each locus (except in the case of polyploidy which we will cover later) the dominance properties (i.e., gene action) of an allele is a function of the second allele under consideration. one allele may be dominant to a second allele, but co-dominant to a third allele. allele frequencies (sometimes called gene frequencies) are calculated by counting how many alleles of a given type exist in a population of plants, and dividing that by the number of plants x 2 (since each plant carries two alleles) Example- if you have 100 plants, 99 of which are homozygous AA, and one of which is heterozygous Aa, the frequency of the a allele is 1/(100x2) or 1/200. You can ignore the discussion in the book about the definition of wild type alleles- just regard those as alleles commonly found in nature. 4
Incomplete dominance Slide 5 of Chapter 2 The heterozygous F1 exhibits a third phenotype different from either homozygous parent. F2 segregation with incomplete dominance is therefore 1 : 2 : 1 for both genotype and phenotype 5
Codominance Slide 6 of Chapter 2 both parental phenotypes are visible in the heterozygous F1. Like partial dominance, genotypic and phenotypic ratios are 1 : 2 : 1 blood type in humans exhibits co-dominance variation on complete dominance does not negate Mendel s law of segregation. 6
Genes may have more than 2 alleles Lentils Slide 7 of Chapter 2 See figure 2.6 for an example of 5 alleles at a single locus in lentils alleles are alternative versions of a subsequence of DNA. Alleles arise by changes in the sequence of bases within a given subsequence of DNA Genes are detectable only if at least two alleles exist AND the alleles have detectably different affects on phenotype (now, we are only considering visual, external phenotype, but later we will consider differences in DNA sequence to also represent phenotype) The only limit on the number of alleles at a locus (locus is a point on a chromosome where a subsequence of DNA is found- a locus may or may not be a gene) is the number of permutations of the sequence, which is n 4, where n is the number of nucleotides in the subsequence. blood type in humans is multi-allelic 7
incompatability gene systems are multi-allelic Slide 8 of Chapter 2 petunia and wild tomato plants have a self-incompatibility system that precludes self pollination. pollen carrying either allele present in the female plant cannot germinate and therefore do not create a pollen tube and cannot effect fertilization. as many as 92 allelic version have been identified at incompatibility loci 8
Pleiotropy: alleles of gene at one locus may influence more than one trait the Rht genes in wheat reduce plant height, increase resistance to lodging, and increase yield potential. Vp1 in maize is a gene that regulates the expression of genes controlling synthesis of anthocyanins (pigments) synthesis AND genes controlling seed dormancy. Seed coat color genes in wheat appear to control both seed dormancy and seed coat pigmentation. Slide 9 of Chapter 2 9
One trait can be controlled by more than one gene (i.e., two or more loci) Slide 10 of Chapter 2 pg 49: Extensions to Mendel for multifactor inheritance: two genes can interact to determine one trait crosses between pure-breeding tan and gray seed coat lentils generates seed coat (you have to grow the F1 seed to see its phenotype) phenotypic ratio in the F2 of this cross between two pure-breeding lentils is 9:3:3:1 Brown: tan: gray: green this rules out single gene control since two alleles at one locus can only generate 3 genotypes and therefore a maximum of 3 phenotypes. self crosses of F2 plants demonstrated that: green seed coat was homozygous aabb tan seed coat was conditioned by the two genotypes represented by A-bb gray seed coat was conditioned by the two genotypes represented by aab- brown occurred with the four genotypes represented by A-B- 10
2 gene control of color in sweet peas Slide 11 of Chapter 2 Figure 2.12- This is a situation similar to the one in your homework this week- two loci must have at least one dominant allele in order for pigment to be produced. the book refers to this as complementary gene action- note that a cross of two homozygotes with the same phenotype yielded an F1 with a second phenotype, and segregation in the F2 The 9:7 ratio is easily derived from knowledge that only the genotypes represented by A-B- make pigment: both A- and B- have a probabilities of 3/4, so their joint probability is 9/16. All other genotypes are white, and their frequency combined is 1-9/16 = 7/16. 11
Epistasis: the phenotypes of genotypes at one locus depend on the genotype at 1 or more other loci.: i.e., loci interact with each other with complete dominance at one locus, the dominant and recessive alleles interact with each other in the previous example (fig2.12), the genotypes AA or Aa only generate pigment if the B locus has one or more dominant alleles. With Mendel s dihybrids, the effects of genotypic change at one locus did not influence the relationships between phenotype and genotype at the second locus. Epistasis can occur when two or more genes control one trait, but it can also occur when more than two traits are involved. Slide 12 of Chapter 2 your text discusses several specific types of epistasis. Some genetic instructors expect students to be able to recognize those different types from the ratios that they generate in F2 progeny. So recessive epistatsis generates a 9:3:4 F2 ratio, and dominant epistasis generates 12 : 3 : 1 ratios in the F2. Read this material (pages 51-53), but don t worry about memorizing the ratios associated with these specific instances of gene interactions. They have great historical significance because they were surprises at the time of their discovery- but nature hasn t bothered to conform to just a few gene interaction systems and basically anything in the way of gene interactions that you can imagine can and probably does occur. 12
Summer squash fruit color Slide 13 of Chapter 2 figure 2.14- another example of interaction between loci. In this case, the dominant B allele causes white color even if the A locus has a dominant allele present. With out the dominant B, the A locus is yellow if a dominant is present (i.e. A-), and green if not (aabb). 13
The same genotype does not always produce the same phenotype penetrance: % of individuals with the same genotype that exhibit the expected phenotype small or large differences in environment may influence expression of the trait example-siamese cats: a recessive gene for dark pigment is temperature sensitive. So only the extremities show dark pigment. Cats in colder environments will have less pigment- so expressivity varies. Slide 14 of Chapter 2 14
expressivity: degree or intensity that a given phenotype is expressed. small or large differences in environment may influence expression of the trait Example: Terinoblastoma: eye cancer in humans is condition by a dominant mutation of a single gene, but only 75% of humans with that mutation develop cancer: penetrance varies. Slide 15 of Chapter 2 15
Slide 16 of Chapter 2 16