ANTH 101 Chapter 3 Genetics: From Genotype to Phenotype
ANTH 101 Chapter 3 Genetics: From Genotype to Phenotype ANTH 101 - 001
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I. Genetics A. INTRODUCTION 1. William Bateson (1861-1926), had concept of gene before anyone knew what DNA was or how it worked 2. Genetics vitally important for understanding human evolution B. FROM GENOTYPE TO PHENOTYPE 1. Gregor Mendel (1822-1884), had sense of how genes worked ~90 yrs before Watson & Crick figured out structure & function of DNA 2. Genes that contain info to make proteins are structural genes a) Surrounded by regulatory regions sequences of bases important in initiating, promoting, or terminating transcription; if regions altered/missing, expression of gene can be affected 3. Genes that further guide the expression of structural genes are regulatory genes a) Critical in determining the form an organism or species takes b) A single change in this gene can result in the synthesis of protein being shut down in one species while it is maintained in another can lead to differences b/t two species in anatomy, physiology, or behavior 4. Estimated that DNA sequences b/t humans & chimps 95- 96% identical a) Difference b/t two species accounted for by variety of deletions, insertions, inversions, & gene copy numbers b) Overall similarity indicates physical & behavioral differences result primarily from regulatory (not structural) genes 5. Wilhelm Johannsen introduced two more terms (along with “gene”): a) Genotype set of specific genes (alleles) an organism carries; genetic constitution of organism b) Phenotype observable physical feature of an organism which is under some form of genetic control or influence (1) Sometimes relationship b/t the two types is direct: observed phenotype is direct product of underlying alleles; Other times the genotype interacts w/ factors in environment to produce phenotype (ex: ABO blood type system & obesity) C. THE ABO BLOOD TYPE SYSTEM 1. Illustrates straightforward relationship b/t genotype & phenotype 2. Refers to the genetic system for one of the proteins found on the surface of red blood cells; consists of one gene w/ three alleles: A, B, & O a) A & B stand for two versions of the protein modified by enzymes from common precursor; O stands for absence of the protein (has only the precursor version of the protein) 3. Allele that must be present on both chromosomes to be expressed is called recessive (O is recessive) 4. Allele that must be present at only one chromosomal locus to be expressed is called dominant (both A& B are dominant to O and co-dominant w/ each other) a) Co-dominant In a diploid organism, two different alleles of a gene that are expressed in a heterozygous individual 5. AA, BB, & OO homozygous; AO, BO, & AB heterozygous (AB is co-dominant &will be AB blood type. D. OBESITY: A COMPLEX INTERACTION 1. Complex example of interaction b/t genes, environments, and phenotypes a) Some with obese phenotype are in some way genetically predisposed to condition; specific genes critical to regulating appetite important factor in overall body development b) Some have alleles for genes which make it difficult to regulate appetite tend to become obese at young age 2. Genes that regulate fat storage, metabolism, etc also critical in development of obese phenotype a) May be able to identify combinations of alleles that could make an individual especially prone to obesity if we could look at all genes underlying development of body size and shape 3. Obesity and ABO blood type both phenotypes observation of phenotype does not necessarily provide understanding of underlying genotype II. Mendelian Genetics th th A. 19 & early 20 centuries, scientists embrace ideas about heredity that were ill-conceived or later proved wrong 1. Blending inheritance supported by Darwin; based on two assumptions: a) Each parent contributes to offspring equally valid b) Contributions halved at each successive generation only appears valid (see Figure 3.4, pg 71) B. Gregor Mendel’s experimental work demonstrated non- blending, genetic nature of heredityparticulate inheritance 1. Concept of heredity based on the transmission of genes (alleles) according to Mendelian principles C. 1856-68, conducted plant breeding experiments on diff varieties of common garden pea; involved series of hybridizations 1. Recorded transmission of several characteristics across generations 2. Best feature of garden pea displays two alternative phenotypes (dichotomous variation) for several different & independent traits; appear in one distinct form or the other w/ no apparent blending (Fig 3.6, pg 72) D. From basic observations, Mendel developed series of postulates (laws/principles) E. MENDEL’S POSTULATES 1. Hereditary characteristics are controlled by particulate (genetic) unit factors that exist in pairs in individual organisms a) Unit factors genes; in pairs b/c in diploid organisms, chromosomes are in pairs. Individual receives one copy of e/ chromosomes from e/ parent; individual receives one of e/ of his/her pair of unit factors from e/ parent; May have two of the same or two that are diff. (homozygous vs heterozygous) 2. When an individual has two diff unit factors responsible for a characteristic, only one is expressed and is dominant to the other, which is recessive (i.e. blood type) 3. During formation of gametes, the paired unit factors separate randomly so e/ sex cell receives one or the other w/ equal likelihood a) Mendel’s law of segregation; reflects fact that in diploid organisms, chromosomes in a pair segregate randomly into sex cells during meiosis. b) Punnett square illustrates parental genetic contribution to offspring (fig 3.9, pg 75) 4. During gamete formation, segregating pairs of unit factors assort independently of each other a) Mendel’s law of independent assortment (fig. 3.8, pg 74) F. LINKAGE AND CROSSING OVER 1. Law of independent assortment applies only to genes on diff chromosomes a) Linkage genes found on same chromosomes are said to be linked. The closer together two genes are on chromosome, the greater the linkage and the less likely they are to be separated during crossing over III. Mutation A. An error that occurs in replication of DNA; any change in DNA sequence that becomes established in a daughter cell 1. Mutations in sex cells important can be passed to next generations & will be present in all cells of bodies in offspring 2. Can occur in any part of DNA; those occurring in structural or regulatory genes much more critical than those occurring in noncoding regions or introns B. POINT MUTATION & SICKLE CELL DISEASE 1. Point mutation occurs when single base in a gene is changed a) Number of diseases can be attributed to specific point mutations b) Sickle cell disease one of most well-known & anthropologically important (Comparision b/t normal v. sickle cell: fig 3.11, pg 77) (1) Caused by an abnormal form of the protein hemoglobin transports oxygen throughout body in red blood cells; makes up 95% of protein found in a red blood cell; hemoglobin molecules normally exist separately from r.b.c , e/ binding to oxygen molecule (2) Hemoglobin consists of 4 polypeptide chains (two alpha, two beta) (a) Beta chains 146 amino acids 2. Autosomal recessive disease caused by a (homozygous) recessive allele; one copy of allele must be inherited from e/ parent for disease to develop C. TRINUCLEOTIDE REPEAT DISEASE 1. Insertion mutation change in base sequence of a gene that results from the addition of one or more base pairs in DNA 2. Deletion mutation change in base sequence of a gene that results from loss of one or more base pairs of DNA 3. Trinucleotide repeat disease family of autosomal dominant diseases caused by the insertion of multiple copies of a 3-base pair sequence, which codes for the amino acid glutamine; typically, the more copies inserted into the gene, the more serious the disease a) Most well-known Huntington disease: neurological disorder caused by a dominant allele an autosomal dominant disease: disease caused by dominant allele; Only one copy need to be inherited from either parent for disease to develop 4. X-linked disorders Genetic conditions that result from mutations to genes on X chromosome; almost always expressed in males, who only have one copy of X; in females, the second X containing the normally functioning allele protects them from developing x-linked diseases 5. Pedigree diagram used in the study of human genetics; shows the transmission of a genetic trait over several generations of a family; show typical pattern in X-linked affected families disorder appears to skip a generation D. MENDELIAN GENETICS IN HUMANS 1. Hundreds of human disorders & diseases have been catalogued (over past century) can be explained in terms of Mendelian genetic transmission (table 3.2, pg 82) IV. Genetics beyond Mendel A. Scientists have gained significant understanding of many other and more complex biological phenomena by studying Mendelian genetics 1. Important a single-gene, dominant-recessive model of heredity cannot explain much of biological world around us; only provides foundation for understanding heredity B. Qualitative variation Phenotypic variation characterized as belonging to discrete, observable categories (ex: albinism – absence of pigmentation in the skin, hair, & iris of eyes C. Quantitative variation Phenotypic variation characterized by the distribution of continuous variation (expressed using numeric measure) within a population (ex: fig 3.17, pg 83) D. Polygenic traits Phenotypic traits that result from the combined action of more than one gene; most complex traits (i.e. stature, timing of puberty, skin color, & body composition) 1. Expression depends on the action of multiple genes, e/ of which may have more than one allele; the more genes & alleles that contribute to polygenic trait, the more genotypes & phenotypes are possible E. One gene can have multiple phenotypic effects pleiotropy (fig 3.19, pg 84) F. POLYGENETIC TRAITS, THE PHENOTYPE, & THE ENVIRONMENT 1. Polygenic inheritance may produce bell-curve distribution for the expression of a trait; expression of many traits is result of multiple genes & an interaction b/t those genes & the environment (in which individual was raised) 2. Heritability proportion of total phenotypic variability observed for a given trait that can be ascribed to genetic factors; statistical concept used by scientists to investigate the relative contribution of genes & environment in production of phenotype a) Twin method method for estimating the heritability of a phenotypic trait by comparing the concordance rates of identical and fraternal twins (1) Concordant for trait = if twins share common phenotype or if both get certain disease (2) Discordant = dissimilar G. HERITABILTY & IQ TEST SCORE PERFORMANCE 1. Most well-known and controversial use of heritability statistics 2. Almost all studies of the heritability of IQ test scores agree that genetics is important factor in producing the variation observed within populations 3. Heritability scores apply w/in a population or environment, not between populations V. Phenylketonuria (PKU): Illustrating Mendelian & Post- Mendelian Concepts A. PKU autosomal recessive condition that leads to the accumulation of large quantities of the amino acid phenylalanine, which causes mental retardation and other phenotypic abnormalities 1. The result of a deficiency of an enzyme, phenylalanine hydroxylase converts phenylalanine to another amino acid, tyrosine. 2. Transmission appears to follow classic Mendelian rules VI. Genes & Environments A. Environment from a gene’s perspective is made up mainly of other genes. B. Concepts (i.e. pleiotropy & polygenic inheritance) emphasize that the genetic environment is just as critical to the production of phenotypes as any other kind of environment C. Mendelian concepts (independent assortment & segregation) useful in establishing activities of genes in isolation from e/o 1. Essential for doing away w/ “blending inheritance” concept 2. Challenge of 21 c. will be to determine how genes work together to produce complex phenotypes in the context of complex evironments *Visual summary of Chapter on pgs 90 & 91