Chapter 14 - Mendel Genetics

u Introduction to Genetics Concept 14.1 Mendel used scientific approach to identify two Laws of Inheritance Inheritance Terms Inheritance Terms Concept 14.2 Probability governs Mendelian inheritance Concept 14.3 Inheritance patterns are often more complex than predicted by Mendelian inheritance Complex patterns in inheritance occur because of 1. Allele Interactions - (i) 2 alleles of 1 gene (ii) multiple alleles of 1 gene 2. Gene Interactions 1. Deviations in Inheritance due to Allele Interactions (i) Simple 2 Allele Interactions 2. incomplete dominance 3. codominance occurs when both alleles are expressed 1. Deviations in Inheritance due to Allele Interactions 1. Deviations in Inheritance due to Allele Interactions 1. dominance series (with complete dominance) 2. pleiotropy 2. Deviations in Inheritance due to Gene Interactions 1. Epistasis - a gene alters expression of second gene 1. Epistasis - complementary genes work together two genes are mutually dependent expression of each depends on the alleles of the other gene ( complementary genes) 2. Polygenes 2. Polygenes - control skin colour in humans 3 genes with multiple alleles control human skin colour - dominant alleles result in more melanin darker skin Environment also Affects Phenotype Concept 14.4 Many human traits follow Mendelian inheritance cannot do genetic experiments on humans use pedigree analysis to see human inheritance patterns some disorders determined by single alleles but most are multi-allelic or multifactorial conditions Click to edit Master title style Click to edit Master text styles Second level Third level Fourth level Fifth level Click to edit Master text styles Second level Third level Fourth level Fifth level Gregor Mendel and peas alternative forms of a gene alleles - show different traits allele composition of an organism its genotype physical appearance of an organism its phenotype Homozygous Purple (PP) Homozygous White (pp) GENOTYPE PP X pp Pp PHENOTYPE Purple White Parent Parent Heterozygous Purple (Pp) Purple (P) allele dominant White (p) allele recessive Figure 14.4 Figure 14.3 F2 Generation (offspring of F1 cross) F1 Generation (hybrids) P Generation (true-breeding parents) well-organized experiments allowed Mendel to observe traits of each generation in sufficient quantity to explain relative proportions of different kinds of progeny true-breeding individuals with two copies of same allele ( homozygous) - true-breeding purple (PP) or true-breeding white (pp) some individuals are not true-breeding because they have only one copy of each allele (Pp) ( heterozygous) SS F1 ss F2 1 character seed shape 2 traits round or wrinkled 1 gene 2 alleles S (dominant) s (recessive) Figure 10.3 in Purves et al. (2001) all spherical seeds all Ss 3 round 1 wrinkled Monohybrid heterozygous for 1 character 1 SS 2 Ss 1 ss Table 14.1 Flower Colour Flower Position Seed Colour Seed Shape Pod Shape Pod Colour Stem Length 705 224 651 207 6022 2001 5474 1850 882 299 428 152 787 277 3.15 1 3.14 1 3.01 1 2.96 1 2.95 1 2.82 1 2.84 1 1. Alternative versions of a gene (alleles) account for variation in inherited characters 2. For each character, organism inherits 2 alleles (1 allele from each parent) 3. If 2 alleles differ, the dominant allele determines appearance while the recessive allele has no effect 1 SS 2 Ss 1 ss 3 round 1 wrinkled Figure 10.4 in Purves et al. (2001) Punnett Square showing all allelic combinations for F2 F1 F2 ss Ss Ss SS Figure 10.5 in Purves et al. (2001) Diploid F1 Parent (Ss) 2 alleles of gene for seed shape pair of homologous chromosomes Chromosome DNA Replication 4 Haploid Gametes in F2 Generation Meiosis I Meiosis II F1 X F1 Monohybrid Cross involved additional characters with heritable traits dihybrid SsYy produces four possible gametes that have one allele of each gene SY, Sy, sY and sy random fertilization of gametes results in offspring with phenotypes in a 9 3 3 1 ratio Dihybrid heterozygous for 2 characters characters seed shape seed colour traits round or wrinkled shape yellow or green colour alleles S (dominant) or s (recessive) Y (dominant) or y (recessive) Figure 10.7 in Purves et al. (2001) F1 SSYY SsYy Gametes F2 Generation 9 3 3 1 ratio ssyy Figure 10.8 in Purves et al. (2001) F1 X F1 Dihybrid Cross Diploid F1 Parent (SsYy) 4 Haploid Gametes in F2 Generation SY sy 2 pairs of homologous chromosomes leads to 22 or 4 possible combinations of alleles in gametes Sy sY http //highered.mcgraw-hill.com/sites/0072437316/ Figure 13.5 from Freeman (2005) inheritance deviates from simple Mendelian patterns when 2 alleles of 1 gene are not completely dominant/recessive Figure 10.13 in Purves et al. (2001) in snapdragons, heterozygotes may show an intermediate phenotype which suggest blending theory F2 Generation demonstrates Mendelian genetics - for self-fertilizing F1 pink individuals - blending theory would predict all pink F2 offspring - whereas the F2 offspring actually show Mendelian phenotypic ratio of 1 white 2 pink 1 red test cross confirmation - 2 pink 2 white F1 F2 in cattle - hair colour is a codominant character heterozygotes for hair colour have phenotype where both parental phenotypes are distinctly expressed in the offspring P red (RR) X white (WW) (ii) Complex Multiple Allele Interactions 1. dominance series (with complete dominance) 2. pleiotropy ii) Complex Multiple Allele Interactions in rabbits, coat color is determined by one gene with four different alleles (C, cch, ch, c) - five colours result from combinations of these alleles C cch ch c even if more than two alleles exist in a population, any given individual can have no more than two of them - one from the mother and one from the father Figure 10.12 in Purves et al. (2001) pleiotropic alleles single alleles that have more than one distinguishable phenotypic effect example - coloration pattern and crossed eyes of Siamese cats - these unrelated characters are caused by same protein produced by same allele in fact - several genes may interact to determine trait - also environment plays a role - Gene Interactions 1. epistasis - a gene alters expression of second gene - complementary genes 2. polygenes Figure 10.11 Figure 10.15 in Purves et al. (2001) occurs when alleles of one gene stop, cover up or alter expression of alleles of another gene for example - coat color in mice gene at first locus determines pigment colour (B black / b brown) gene at second locus determines if pigment is deposited on hair (C makes colour pigment / c allele makes no pigment) for example - two genes code for two different enzymes that are both required for purple pigment to be produced in a flower recessive alleles code for non-functional enzymes if plant is homozygous for either a or b, no purple pigment will form Enzyme A Enzyme B AA or Aa BB or Bb If aa If bb X X colourless precursor colourless intermediate examples - height - continuous gradation from short to tall (not just 2 discrete choices - short or tall) Figure 14.12 AaBbCc AaBbCc aabbcc Aabbcc AaBbcc AaBbCc AABbCc AABBCc AABBCC Fraction of progeny aabbcc aabbCc aaBbcc aaBbCc Aabbcc AabbCc AaBbcc AaBbCc abc aabbCc aabbCC aaBbCc aaBbCC AabbCc AabbCC AaBbCc AaBbCC abC aaBbcc aaBbCc aaBBcc aaBBCc AaBbcc AaBbCc AaBBcc AaBBCc aBc aaBbCc aaBbCC aaBBCc aaBBCC AaBbCc AaBbCC AaBBCc AaBBCC aBC Aabbcc AabbCc AaBbcc AaBbCc AAbbcc AAbbCc AABbcc AABbCc Abc AabbCc AabbCC AaBbCc AaBbCC AAbbCc AAbbCC AABbCc AABbCC AbC AaBbcc AaBbCc AaBBcc AaBBCc AABbcc AABbCc AABBcc AABBCc ABc AaBbCc AaBbCC AaBBCc AaBBCC AABbCc AABbCC AABBCc AABBCC ABC abc abC aBc aBC Abc AbC ABc ABC phenotype of some characters depend on environmental conditions as well as on the genotype of the individual organism shows that both Nature and Nurture impact the phenotype expressed in an individual organism 1. Inherited disorders based on recessive traits - Tay-Sachs disease - cystic fibrosis - sickle cell disease can test for a growing number of genetic disorders x