Class 16 - Oct 20 - 10/18/2009 IB35ac Class 16: What we...

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Unformatted text preview: 10/18/2009 IB35ac Class 16: What we look like We'll talk about Mendel and his pea plants, and then apply those concepts to variation in human height, hair color, eye color, and a couple of other aspects of our phenotype. We'll also revisit and elaborate on Bergmann and Allen's Rules, and that MC1R gene that came up in the discussion of skin color and Neanderthals. IB35 Human Biological Variation Lecture #16 Darwin's black box "The laws governing inheritance are for the most part unknown." (Origin of Species, 3rd ed 1869 , p.14) Topics for today: What we look like (What Mendel can tell us about eye color, hair & our ears) "Some naturalists have maintained that all variations are connected with the act of sexual reproduction; but this is certainly an error..." (Origin of Species, 3rd ed 1869, p.11) Epigamic Features ->Phenotypic features that contribute to sexual selection ->E.g. Ornaments, coloration etc Gregor Mendel 1822-1884 Punnett Squares Mendel's Laws of Inheritance 1865 Inheritance is particulate; it operates through the transmission of discrete bits of self-producing alleles matter. 1. Offspring get their `factors' equally from both parents 2. `factors' are inherited independently of each other Allele an alternative form of a gene at a particular locus Locus A particular place on a chromosome 1 10/18/2009 alleles at a particular gene locus on homologous chromosomes E.g. GG, gg Homozygous: having the same Dominant: the allele or phenotype that is expressed in a heterozygote Heterozygous: having two different alleles at the same gene locus on homologous chromosomes E.g. Gg Recessive: an allele that is expressed when homozygous but not expressed when heterozygous Genotype - Phenotype not 1-1 relationship Earlobes exception to the rule Phenotype: the observed attribute(s) of a cell or individual, brought about by the interaction o genotype and environment of ge o ype a d e o e Genotype: the precise allelic makeup of an organism or cell Genotype frequency can refer to individual alleles or pairs of alleles Ear Lobes E = dominant allele (unattached) e = recessive allele (attached) EE = unattached Ee = unattached ee = attached E E E E e E e e E e EE = 25% Ee = 50% ee = 25% ee Prediction: Autosomal Dominant ->Only need 1 allele to express phenotype 2 10/18/2009 Inheritance of Huntington disease (an autosomal dominant trait) Autosomal Recessive ->Need 2 alleles to express phenotype Albinism ->Autosomal Recessive ->Except for occular albinism (affects eyes only) which is dominant ->3 Base pairs (CAG) repeated on chromosome 4 -> Neurological degenerative disorder Red Hair Autosomal recessive 13% of people in Scotland have red hair 4% of the European 4% of the European population MC1R mutation & pheomelanin Medieval Europe perceptions... Reddish pigmentation Inheritance of albinism (an autosomal recessive trait) Full expression for each allele E.g. AB Blood Type MC1R sits in a family of genes related to pain receptor in the brain Redheads need 20% more anaesthetic than other hair colors Codominance Incomplete Dominance Lower levels of expression for each allele Curly or straight hair? C = curly hair allele s = straight hair allele CC = curly hair ss = straight hair Cs = wavy hair Both alleles expressed in a heterozygote Incomplete dominance 3 10/18/2009 The Caveat... ->Inheritance is usually not this simple. For example, eye color... How many genes influence the phenotype? Monogenic (one gene) g g Oligogenic (a few genes) Polygenic (many genes) E.g. Height Variation (46 Genes that influence height) Chr 15 bey1 bey2 Chr 19 gey Br/BI Br/BI BI/gr Different genes responsible for height variation in different populations Body Shapes Bergmann's Rule 1847 In warmblooded organisms, populations with less massive individuals are often found individuals are often found in warm climates near the equator, and populations with more mass are found farther from the equator in colder regions. Allen's Rule 1877 The length of the arms and legs and other appendages also have an affect on the amount of heat lost to the surrounding environment. surrounding environment Populations in warm climates near the equator tend to have longer limbs than do populations living farther away from the equator in colder environments. Neanderthals fit Bergmann's and Allen's Rule since they lived in Europe Body Shape in Humans ->Body breadth increases as latitude increases Hair Variation D. Hrdy (1973) American Journal of Physical Anthropology Seven variables Average diameter Medullation Whether the hair has an Scale count empty cavity within the hair Kinking Average curvature Ratio of maximum to minimum curvature Crimp Ration of natural to straight length Ratio of natural to straight length 5 4.5 4 3.5 3 2.5 25 2 1.5 1 0.5 0 1 2 3 4 5 6 7 Shared Ancestry 4 10/18/2009 Haplotype Map Average Hair Diameter 140 120 The HapMap Project Fujimoto et al. (2008) They found an Asianspecific allele of EDAR 186 people from Indonesia and Thailand Found that the Asianspecific allele is associated with thicker hair 100 80 60 40 20 0 1 2 3 4 5 6 7 mtDNA 150-200 kya CONCEPT: Genes that are responsible for genetic disorders can also underlie normal variation. Ectodermal Dysplasia Abnormal development of teeth, hair & eccrine sweat glands Area in blue at the extremes ->Pathology or Affected Genotype http://www.ectodermaldysplasia.org/ Leads to problems with temperature regulation Unshaded area all fall within normal human variation, which means not all mutations lead to abnormal mutations The HapMap Project Fujimoto et al. (2008) They found an Asianspecific allele of EDAR 186 people from Indonesia and Thailand Found that the Asian specific allele is associated Found that the Asianspecific allele is associated with thicker hair The allele appears to have undergone positive selection Warmth in colder climates Sideeffect of selection for another phenotype (Teeth? Sweat glands?) Conclusions for Today Various epigamic phenotypes are influenced by one, several, or many different genes Be cautious when newspapers report "a gene for..." Mutations in these genes can result in variation we consider normal But sometimes mutations in these genes can result in more extreme variation (i.e., a genetic disorder) 5 ...
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