Inheritance Because Darwin knew nothing about genetics, he had no idea how varia
ID: 151199 • Letter: I
Question
Inheritance Because Darwin knew nothing about genetics, he had no idea how variations are passed to offspring (see Charlesworth and Charlesworth 2009). Biologists did not understand inheritance until Mendel's work with peas was rediscovered and verified, 35 years after its original publication. Mendel's laws of segregation and independent assortment confirmed the mechanism behind Darwin's postulate 2, which states that some of the variation seen in populations is heritable -MSH MSH receptor (MC1R) Eumelanin Nucleus Before Mendel's laws became known, many biologists thought inheritance worked like pigments in paint. Advocates of this hypothesis, called blending in heritance, argued that favorable variants would merge into existing traits and be lost. In 1867, Scottish engineer Fleeming Jenkin published a mathematical treat- ment of blending inheritance along with a thought experiment. If a dark-skinned sailor were stranded on an equatorial island inhabited by light-skinned people, Jenkin's model predicted that no matter how advantageous dark skin might be (in, say, reducing skin cancer), the population would never become dark skinned. If the dark-skinned sailor had children with a light-skinned woman, the kids would be brown-skinned. If they, in turn, had children with light-skinned people, their children would be light-brown-skinned, and so on. Conversely, if a light-skinned sailor were stranded on a northern island inhabited by dark-skinned people, blending inheritance argued that, no matter how advantageous light skin might be (in, say, facilitating the synthesis of vitamin D with energy from UV light), the population would never become light. Under blending inheritance, new variants are diluted away. For populations to evolve by natural selection, Melanocyte (b a-MSH MSH receptor Pheomelanin Nucleus Melanocyte MSH receptor (MC1R) Agouti signaling protein (ASP) vorable new variations have to be passed to offspring intact and remain discrete Pheomelanin We now understand that phenotypes blend in some traits, including skin color, but genotypes do not. Flgure 3.22 shows why for skin color. Color is determined mainly by pigments produced in cells called melanocytes (Figure 3.22a). Mela nocytes make eumelanin, a brownish-black pigment, when alpha melanocyte- stimulating hormone (-MSH) binds to their melanocortin 1 receptors (MCI Figure 3.22b). Melanocytes make pheomelanin, a reddish-yellow pigment, when their MC1Rs are dysfunctional or when they are blocked by agouti signaling skin color In humans Color is protein (ASP; Figure 3.22c). Variation in human coloration has been tied to al- determined largely by pigments lelic variation in both the gene for MC1R and the gene for ASP (Harding et al made by cells called melanocytes 2000; Schaffer and Bolognia 2001; Kanetsky et al. 2002). For example, homo- The organelles that produce and zygotes for the Arg151Cys allele of the gene for MC1R almost always have red store the pigments are called me- hair and fair skin (Smith et al. 1998). The effects of alleles in determining phe- notype may blend. An individual with just one copy of the Arg151Cys allele, for ment, when alpha melanocyte- instance, may have intermediate coloration. But the alleles themselves are passed stimulating hormone (a-MSH) on intact to offspring, and two Arg151Cys heterozygotes can have a homozygous binds to its receptor (MC1R). (b red-haire Melanocyte Flgure 3.22 Regulatlon of lanosomes. (a) Melanocytes make eumelanin, a dark brown pi d offspring. Inheritance is thus particulate, not blending and c) Melanocytes make phe- omelanin, a reddish yellow Jenkin's hypothetical population would, in fact, become increasingly darker or lighter skinned if selection were strong and mutation continually added darker- or lighter-skinned variants to the population via changes in the genes that regu- late the production of melanins ment, if MC1R is dysfunctional or blocked by agouti signaling protein (ASP). After Schaffer and Bolognia (2001)Explanation / Answer
A) Only statements i and ii convey the accurate information. According to the first statement, the population would never become dark pigmented when a dark pigmented mammal were swept by the typhoon onto an island because according to the blending inheritance, the variants would mix with the existing traits and ultimately be lost.
According to the second statement, the island population can become teh dark pigmented because the inheritance of the skin color is particulate, not blended. Hence, option A) is the correct answer.