* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download QUANTITATIVE INHERITANCE
Genetic drift wikipedia , lookup
Gene desert wikipedia , lookup
Essential gene wikipedia , lookup
Hybrid (biology) wikipedia , lookup
Pharmacogenomics wikipedia , lookup
Site-specific recombinase technology wikipedia , lookup
Polymorphism (biology) wikipedia , lookup
Nutriepigenomics wikipedia , lookup
Genetic engineering wikipedia , lookup
Public health genomics wikipedia , lookup
Genome evolution wikipedia , lookup
Hardy–Weinberg principle wikipedia , lookup
Dominance (genetics) wikipedia , lookup
Genetically modified crops wikipedia , lookup
Minimal genome wikipedia , lookup
Ridge (biology) wikipedia , lookup
Artificial gene synthesis wikipedia , lookup
Gene expression programming wikipedia , lookup
Population genetics wikipedia , lookup
Genomic imprinting wikipedia , lookup
Epigenetics of human development wikipedia , lookup
Behavioural genetics wikipedia , lookup
Human genetic variation wikipedia , lookup
Gene expression profiling wikipedia , lookup
Genome (book) wikipedia , lookup
Biology and consumer behaviour wikipedia , lookup
History of genetic engineering wikipedia , lookup
Designer baby wikipedia , lookup
Microevolution wikipedia , lookup
QUANTITATIVE INHERITANCE Many traits, especially those that are measured in a "quantitative" manner, are affected by many genes. Examples include" Height Weight Yield (bu/A) in crops Growth rate in farm animals IQ In addition to quantitative inheritance, inheritance of these traits is often referred to as "cumulative gene action" or "polygenic-inheritance Genetic Analysis- contributing genes Inheritance of quantitative traits can best be examined using simplified models. As an example, lets consider plant height in grain sorghum (often called milo). Wild sorghums brought from Africa are often 10' tall; those grown for grain in Texas are more nearly 2' tall, which greatly simplifies harvest. The "combine" types are sometimes referred to as "4-dwarf" sorghums; they are produced by the Texas Ag Exp Station "conversion program" at the Vernon station. The model for sorghum height is as follows: There are 4 genes involved Each gene has 2 alleles, one of which adds about one foot of height, the other is "null". The alleles that contribute to height are called "contributing alleles", and are designated by primes, A', B' C' etc. Non-contributing (null) alleles are designated with A, B, C etc. (Sometimes a, b, c may be used) A plant with no contributing alleles is 2' tall (AA, BB, CC, DD) Each gene is equivalent and shows incomplete dominance; that is, A'A' adds 2 feet, A'A adds 1 foot and AA adds 0 feet to the base height. The same is true for the B, C, and D gene loci. A plant with the genotypes A'A', BB, CC, DD or AA, BB C'C', DD will be 4' tall. A'A', BB, CC, DD (4') X AA, BB, CC, DD (2') A'A, BB, CC, DD (3') If we allow the 3' tall F1 progeny to self pollinate, which is normal in sorghum, we expect to see a 1:2:1 ratio of F2 plants that are 4': 3' :2' tall, respectively. This is true because only one gene with incomplete dominance is "segregating" (heterozygous) in the F1 hybrid. (A'A X A'A gives 1/4th A'A' : 2/4th A'A : 1/4th AA) One Gene Segregating 2 N u m 1 b e r 0 2' 3' 4' Height The same would be true if AA, B'B', CC, DD or any other 4' tall truebreeding plant (homozygous at all 4 gene loci) was crossed to the AA, BB, CC, DD parent. What if we cross A'A' BB, CC, DD to AA, B'B', CC, DD ? Both parents are 4' tall as is the F1, but the F1 is heterozygous for 2 genes (A'A, B'B, CC, DD). Now in the F2, we can predict that 1/4th of the progeny will be A'A' and that 1/4th of these will also be B'B'; thus 1/16th of the progeny should be 6' tall! Likewise, 1/16th should be AA, BB, CC, DD or only 2' tall having no primes. When the range in the F2 progeny goes beyond the original parents, we see "transgressive segregation". Transgressive segregation implies that the parents donate contributing alleles from different genes to the hybrid. There should also be plants in the F2 generation that have 1 contributing allele (A'A, BB, CC, DD or AA, B'B, CC, DD), These should represent 1/4th (1/8th + 1/8th) of the progeny, and they will be 3' tall. The same is true for plants with 3 contributing alleles (A'A',B'B,CC, DD and A'A, B'B', CC, DD). Four foot tall F2s can occur several ways: A'A', BB, CC, DD; A'A, B'B, CC, DD; and AA, B'B', CC, DD. (P= 1/16th + 1/4th + 1/16th = 6/16th). 2 Genes Segregating 6 5 N 4 u m 3 b e 2 r 1 0 2' 3' 4' 5' 6' Height On your own, make the following crosses and predict the genotype, the height of the F1 and the range of heights expected in the F2: 1) A'A', B'B', C'C', D'D' X AA, BB, CC, DD 2) A'A', B'B', CC, DD X AA, BB, C'C', D'D' Generalized formulas can be used to calculate the number of classes expected in the F2 and the frequency of progeny that fell in the extreme classes: If N genes are heterozygous in the F1 hybrid, there will be: 2N +1 phenotypic classes in the F2 (0, 1, 2, 3, etc. up to 2 N contributing alleles). The fraction of individuals falling into each of the extreme classes (0 or 2N contributing alleles) will be 1/4th to the N power (1/4)N Genes that contribute to quantitative traits are referred to as polygenes, or QTLs which stands for for Quantitative Trait Loci. The model we have used where all the genes have equal and additive effects is simplified, but even so, there are many traits where the model applies. Examples include wheat kernel color, ear length in rabbits, skin color and the number of fingerprint ridges in humans, the number of rows of seeds on an ear of corn, etc. When this model applies, the hybrid will always show a phenotype that is the average of the parents, and there will be more variation among the F2 progeny than in either P1 or F1 progeny. It would not be difficult to imagine cases where some genes made larger or smaller contributions than others, or where one or more genes may be dominant. Other than skewing the expected frequencies somewhat, neither change from the model significantly alters the analysis of polygenic traits. Environmental effects Most or all quantitative traits are also influenced by environmental factors. In the case of sorghum height, plants will not reach their genetic potential without water, nutrients (fertilizer) and sunlight! Environmental effects means that some differences can be seen within a purebred parent or in an F1 population, where all of the plants have the exact same genotype. Environmental effects will also "smooth out" the stair-step effect expected for genotypes in the F2 that differ in the number of contributing alleles. standard deviation mean The smoothing of the binomial distribution seen for segregating genes converts the binomial distribution to a "Normal distribution", often called a "bell-curve". The normal curve is defined by 2 parameters: The mean or average locates the "center" of the curve The standard deviation is a measure of breadth of the curve and is a measure of the variation within the population. It is the square root of the variance. If we measure height of many sorghum plants all with the genotype A'A'B'B' CC DD, the average will be about 6' and the variation will be small; If we measure height the F2 generation from selfing an A'A B'B C'C D'D parent, the variation and standard deviation will be large. Since it is not possible to "count" the number of classes in an F2 population when environmental effects smooth away the genotypic differences, or to identify individuals in the extremes, the number of genes that contribute to the trait cannot be simply estimated. Partitioning sources of variation; the concept of heritability Normal curves can be used in other ways to analyze the contributions of genes and environment to a trait. The concept is to partition the sources of variation that lead to differences among individuals in the sample, and to identify the portion of variation that results from segregating genes. Total variation (Vt) which is also often called phenotypic variation (Vp) arises from differences in genotype (Vg), the environment (Ve) and may also result from interactions (Vgxe)where some genotypes do better in one environment and others in another. Vt = Vp = (Vg + Ve + V gxe) In many cases, the interaction component cannot be measured, so is ignored or handled by working within a specific environment or only working with a specific "breed" or cross' Heritability (H2 or broad-0sense heritability) is the fraction of variation due to genetic differences, ie Vg. H2 = Vg/Vt It is relatively simple to make H2 estimates in plants, since pure-breeding, completely homozygous parents can be maintained. Any variation within a purebreeding homozygous parent, or in the F1 progeny of a cross between 2 purebreeding parents must result from Ve, since all plants within each of the populations have the exact same genotype. Thus, these plants can be used to estimate Ve. Variation among F2 plants arises both from differences in genotype and from local environments, so the variation in the F2 is a measure of Vt. In the example below, two true breeding corn parents, one with an average row number of 6 is crossed to another with an average of 14 rows. The F1, as expected, is right between the parents, having an average of 10 rows. Variance (V) for row number in the parents and F1 is low (arbitrarily measured as 1) Variance in the F2 is 8. (We will not learn how to calculate variances in Gene 310) Vt (8) = Vg + Ve (1); ignoring Vgxe since all plants were grown in the same environment, Vg must be 7. Therefore, H2 = 7/8 Heritability for row number in maize ears from a cross between a 6 rowed parent by a 14 rowed parent P1 P2 V=1 V=1 6 rows 14 rows F1 V=1 10 rows F2 V=8 Ave = 10 rows It is critical to realize that: 1) heritability measures are only valid for the population that was measured. 2) genetic differences will not be measured unless the parents have different alleles 3) environment by gene interactions may be important, but are generally ignored Plant and animal breeders are interested in heritability, because it allows them to predict if selective breeding can be used to "improve" a trait. For example, if rate of weight gain in nursing Hampshire pigs is highly h heritable, saving those that grow the fastest for breeding purpose will lead to improved weight gain in future generations. If H2 is low, changes in the diet may be more important. Selection will make progress so long as genetic differences can be combined to make an improved genotype; Selection for one trait may be balanced by loss in another; for example selection for increased egg size in Leghorns is successful, but the hens lay fewer eggs.