Overview of Topics in Quantitative Genetics Multilocus Inheritance 1 Many Traits are influenced by many loci Unlike the examples used to teach basic genetics most characteristics are not influenced by alleles at a single locus Instead they are often influenced by many loci each of which has a relatively small effect on the trait and the effects of the alleles at these loci are additive In a simple example there are two loci A and B each with two alleles A1 A2 B1 B2 Both loci affect tail length in a lizard species The A1 and B1 loci cause tails to be shorter and the A2 and B2 alleles cause tails to be longer Thus an A1A1B1B1 genotype produces a very short tail and a A2A2B2B2 genotype a very long tail Both a A1A1B2B2 and a A1A2B1B2 genotype produce an intermediate phenotype The phenotype is just the sum of the effects of the alleles 2 The greater the number of loci involved the larger the number of possible phenotypic classes Using the example in the previous section you cross a long tailed A2A2B2B2 lizard to a short tailed A1A1B1B1 lizard and get an intermediate F1 A1A2B1B2 If you cross the F1s you will get a 4X4 Punnet s square and 1 4 6 4 1 ratio of individuals with allele values adding up to 4 5 6 7 and 8 respectively A1A1B1B1 4 A2A2B2B2 8 Thus in a two locus case there are five possible phenotypic classes Your text book shows you a three locus case with seven phenotypic classes The more loci are involved the greater the number of possible discrete phenotypes 3 The environment affects most quantitative traits Quantitative traits vary continuously and do not form discrete phenotypic groups Instead they are measured The reason for continuous variation is the effect of the environment Almost all quantitative traits are affected by the environment to a greater or lesser degree 4 The early 20th century was a period of scientific conflict about the nature of inheritance In the late 19th century a group of English biologists began studying inheritance by measuring the values of traits in parents and offspring in a wide variety of species They developed statistical techniques to describe the relationships between parents and offspring These techniques were the start of modern statistical analysis These scientists were known as the biometricians They focus on quantitative traits that could be measured In 1900 Mendelian inheritance was rediscovered independently by three different researchers A group of scientists intensively studied Mendelian inheritance over the next 15 years The two groups appeared to be studying completely different phenomena and each thought they were studying the true nature of inheritance while the other group was studying unimportant aberrant cases Around 1915 this issue was resolved several different ways Theoretical geneticist R A Fisher proved mathematically that many loci acting under Mendelian inheritance could show the pattern of phenotype seen by the biometricians Plant biologists Ehle and East showed that plant crosses for quantitative traits could be explained by many loci acting together 5 The inheritance of quantitative traits is measured by partitioning the variation in a trait into subgroups Because genes and the environment act over prolonged periods to produce a phenotype it is not possible or at least not practical to try and determine for an individual what proportion of a trait is due to genes and what proportion is due to the environment Instead quantitative geneticists divide up the variation in a trait among individuals in a population The variance for a trait e g height can be measured in a straight forward manner by measuring the trait in everyone and then calculating the variance using the standard formula We will call this value VP We can then write this simple equation VP VG VE VG is the variance in the trait that is due to variation in genes among individuals VE is the variance in the trait that is due to variation in the environment experienced by individuals 6 Heritability measures the similarity between relatives Using the values in the previous section we can calculate a value called heritability or h2 h2 VG VP or the proportion of the total variation in the trait that is due to variation in genes Heritability is a ratio and as VP is always going to be as large or larger than VG then h2 is going to range between 0 and 1 Heritability is a measure of the similarity among relatives for the trait in question If the heritability of a trait is high 0 8 1 0 then relatives will have similar values for the trait If heritability is low 0 0 2 then relatives will only slightly resemble one another 7 Measuring heritability in human populations is difficult In plants and animals heritability can be estimated by performing selection experiments or controlled breeding experiments in which the environment is randomized for members of a family None of these techniques are possible in human studies Comparisons of human family members are always subject to the problem that families share a common environment as well as genes Twin studies are one technique that is used to attempt to get around these problems Monozygotic twins share 100 of their genes while regular siblings share 50 of their genes Traits in which monozygotic twins are much more similar than other siblings could be considered to be highly influenced by genes However twin studies are limited by the fact that monozygotic twins share a complete genome with identical genetic interactions e g dominance This property of monozygotic twins will make genes seem more important that they actually are among other individuals 8 Heritability is an easily misunderstood value Heritability is a ratio and is influenced by both the numerator and the denominator Traits with a small h2 may still have large amounts of genetic variation but even larger amounts of environmental variation Similarly a trait may be strongly influenced by genes that do not vary in the population Thus heritability can be zero for a trait with no genetic variation but strong genetic influence Finally heritability is a measure of variance proportions in a particular population under particular circumstances It doesn t tell you anything about the degree to which phenotypic differences among populations are influenced by genes As we saw in the plant species grown at different elevations the relative effect of genotypes may vary unpredictably across environments Plants are shorter when grown at higher elevations This is true for all
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