IB31Lecture+outline10+QuantGenII+Sp08 - IB 31 Sp 2008...

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IB 31, Sp, 2008 February 25 (and first part of Feb. 27), 2008 VI. If you think about it, there are situations where we can easily separate out genetic and environmental components of variation by studying differences and similarities between individuals who share different proportions of their genes. One of the oldest attempts was the use of various types of twins by Galton. While this is an animal behavior course, I'm going to use human examples since they are somewhat controversial. A. Two types of twins: Monozygotic (identical) and Dizygotic (fraternal). 1. Monozygotic twins result from one egg fertilized by one sperm. The zygote splits at an early stage of cell division to produce two identical genetic compliments. There will be minor environmental differences in utero due to position in the uterus, circulation, etc., but all genes are identical in the two offspring. (They will be the same sex.) 2. Dizygotic twins result from two eggs ovulated at one time, fertilized by two sperm giving rise to two zygotes. If the sperm are from the same father, they are no more alike genetically than are normal sibs, although they share the uterine environment. They can be same or different sex. They will share on average 50% of their genes. Note: It is not uncommon for fraternal twins to have different fathers. In some populations, this happens up to 7% of the time or more. This used to be a problem, but with genetic fingerprinting, we can now be certain of the genetic relationship between twins. Using these new molecular fingerprinting techniques, we are discovering that there are occasionally intermediates between monozygotic and dizygotic twins – Chimaeras – their cells are not genetically uniform. Recent case where “identical twins” were found to result from one egg fertilized by two sperm. The zygote split producing two, semi-identical twins. Tissues were a mixture of XX and XY. (One sperm was X and one Y.) 4. One in every 83 births are twins. Two-thirds of the twins in the U.S. are dizygotic; one-third are monozygotic. Interestingly, dizygotic twinning is heritable, monozygotic twinning is not.
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B. This sets up a natural experiment. MZ twins are genetically identical, so differences between them are due to environmental effects. DZ twins only share, on average, 1/2 of their genes (explain – chances are 50/50 which of two alleles you get from your mother and from your father, so on average, sibs will have the same allele from a parent half of the time) - so differences could be due both to genes and environment. However, there is a known relationship between the two types; MZ twins have a two-fold greater genetic similarity due to descent. If we study a trait for many pairs of both types of twins (same sex) and assume that they are reared in a similar environment, we can look at how similar they are to one another.
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  • Spring '08
  • Twin, MZ twins, dizygotic twins

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