Thrifty genotype and Thrifty phenotype.

kuzawa-chapter-for-evolutionary-anthropology

The reading for next week is posted above. Please get started early since it is a book chapter.

Writing assignment (read especially carefully pages 1-4):

Kuzawa points out that humans have the fattest babies of any mammalian species (p. 3-4, look also at Figure 1, p. 15. As he explains, there are several adaptive explanations for why this might be so:

#1. Insulation (p. 3). Human fat might help keep babies warm, since they have immature metabolism and high surface area (and lack of fur)  and thus have have difficulty regulating body temperature.

#2. Surplus energy that can be spent on brain development. This is Kuzawa’s idea, that fat is like a bank account of extra energy that permits human to have large brains that have large energetic demands. As Kuzawa points out, energy use by the brain cannot be turned off, unlike in other tissues like muscle.

For your assignment argue for #1 or #2.

For full credit, the writing assignment will be due in hardcopy in class next tuesday 11/8/11.

For extra credit (1 point), write a paragraph describing the thrifty genotype idea originally described by James Neel, that Kuzawa cites in his introduction. (no more than 1/3 to 1/2 page, maximum. You can look this up).

If you have too much time on your hands, you may watch a video lecture by Chris Kuzawa that he delivered at Yale in 2008. I recommend you watch this. It will really help you with the writing assignment. However, I am going to recap many of his major points in class, so this video is optional. If you want to watch it, you will need Real Player or Media Player to watch the videos. Click here and scroll down on this site and click on Chris Kuzawa’s Talk. As an extra bonus you can also watch David Haig’s excellent lecture on genetic imprinting on Prader Willi syndrome and Angelman syndrome. 

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I promised some additional information on 11/1/11’s topic: genomic imprinting and reproductive conflict.We discussed a variety of esoteric concepts and some phenomena that seem hard to explain via imprinting, e.g. gradual increase in birth weight with birth order, for instance. I should have pointed out that not all these phenomena require an imprinting explanation. Changes in birth weight can occur entirely because of maternal gene expression in the mother, which might serve to increase the delivery of nutrients to the developing fetus differently in different pregnancies. The “maternal provisioning” can occur outside of any effect from the fetus, and thus does not require imprinting to occur.

A student raised the question of imprinting as an all or nothing phenomenon, for example the IGFR pattern of inheritance that Haig discussed in his admittedly long paper. It turns out that partial expression of genes can also occur. This was recently shown in mice.

However, classic imprinting is usually thought of as a process that either turns on gene expression or completely silences it.

That process is thought to occur in the gene duplication/deletion events that can lead to human developmental disorders:

These disorders include Beckwith-Wiedeman syndrome (caused by either duplication of paternal IGF2 or inactive maternal CDKN1C), Prader Willi syndrome (caused by deletion of paternal 11p15.5 portion of chromosome 15 or duplication of maternal chromosome 15), among others.

The example that I tried to explain in class is that of Prader Willi syndrome. The chromosome that is involved in this syndrome is pictured here:

The genetic region at 15q11-q13 are usually expressed only when inherited from the father. Prader Willi syndrome occurs when that region is deleted in paternally derived genes, or when the maternally derived gene is doubly expressed, which happens when the baby inherits a two copies of chromosome 15 from the mother. In both cases, paternal imprinting is lost. Because of this maternal influence in prenatal fetal development and postnatal behavior is unopposed. This unmasking of maternally influence/imprinting makes these babies undemanding of maternal resources, they grow slowly in utero, they have a poor suck mechanism at birth, and they are remarkably un-picky when it comes to eating food (and non-food items) so that early weaning is the default condition. You can read more about this, and its opposite, Angelman’s Syndrome in this paper by David Haig.

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