Mon Spr W5 - Complex genetic traits and variation (Lec6) Understand and explain complex traits Complex traits - traits influenced by multiple genes and the environment e.g height, rice grain shape or susceptibility to disease - These are more prevalent than single gene traits (e.g mendel's peas) Describe and explain the evidence that interactions between the genotype and environment determine the phenotype a Zhenshan 97 NIL (ZS07) Zhonghua 11 H94 NIL (H94) Minghui 83 - We can identify single gene traits via mutagenesis - For complex traits phenotypes are usually measured (e.g size, number, shape,yield etc) - These are known as quantitative traits - because are measured along continuum w/ small intervals between similar individuals Complex trait expression depends on lifestyle choices and the environment - In "true-breeding" strains (inbred lines/homozygous) such as crops even though seeds are genetically identical, as have same genotype, there is still variation due to the environment (e.g sunlight, water, wind, nutrients etc) Herman Nilsson - Ehle found out that complex traits are subject to the same laws for single-gene traits! -
Two studies were done on relationship between env and complex gene trait phenotype: Wheat seed casing colour 1) Looked at 3 unlinked genes each with 2 alleles that influenced intensity of red coloration in wheat seed casings 2) When crossed 7 phenotypes shown from 0 (colourless) to 6 (dark red) this was made possible by 64 total allele combinations from the original 3 genes 3) Dominant alleles were inherited and shown by the colours of the seeds 4) Whilst analysing the progeny the phenotype appeared as a normal distribution Two plants heterozygous for each of three unlinked genes affecting the color of seed casing are crossed. Aa Bb Cc X Aa Bb Cc The genes act cumulatively to determine the intensity ofred coloration; in this example, each ABC abc ABC 3 abC Female gametes aBC AbC 2 2 4 aBc Male 4 Abc gametes Abc 2 3 3 aBC aBc 1 3 AbC abC 7 2 ABC abc 1 2 2 ABC 0 2 2 2 1 2 7 2 1 3 2 2 3 2 3 The bar graph of color phenotypes among the progeny approximates a bell-shaped curve. Relative probability of phenotype - From this it was concluded that phenotypes of many complex traits (including height) adhere to a normal distribution! Corn strains 0 1 2 3 4 5 6 Intensity of red coloration Figure 18.4 Biology: How Life Works 2014 W. H. Freeman and Company 5000 Corn strain 1 Corn strain 2 4000 3000 2 corn strains studied in 2 different environments with varying Nitrogen enrichment Grain yield (kg/hectare) 2000 1000 Strain 1 - little variation in yield from high to low N conc Strain 2 - higher N conc = higher yield 0 20 40 60 80 100 120 Nitrogen enrichment (kg/hectare) From this we can infer that the effect of a certain genotype may be at times specific to the environment it's in, and other times it is