WEEK 3 Discussion Forum--Anthropology 1 (need high quality assignment)

Heredity and Evolution

 A monk living in what is now the Czech Republic

 Crossed different strains of purebred plants and studied their progeny.

 His work illustrates the basic rules of inheritance.

 Discrete units, or genes, occur in pairs because chromosomes occur in pairs.

 During meiosis each gene pair separate (segregate) so each gamete contains one member of a pair.

 During fertilization, the full number of chromosomes is restored and members of a gene or allele pairs are reunited.

 The distribution of one pair of alleles into gametes does not influence the distribution of another pair.

◦ The genes controlling different traits are inherited independently of one another.

◦ This rule can be broken by genes that are closely located on the same chromosome

 The chance distribution of chromosomes to daughter cells during meiosis; along with recombination, a source of genetic variation (but not new alleles) from meiosis.

 Alternate forms of a gene.

 Alleles occur at the same locus on a pair of chromosomes and influence the same trait.

 However, because they are slightly different, their action may result in different expressions of that trait.

 The term is sometimes used synonymously with gene.

 The position on a chromosome where a given gene occurs.

 The term is sometimes used interchangeably with gene, but this usage is technically incorrect. ◦ The locus is the where not the what

 Having the same allele at the same locus on both members of a pair of chromosomes.

◦ Think both parents gave the same form of a gene to their offspring

 Having different alleles at the same locus on members of a pair of chromosomes.

◦ Think each parent gave a different form of this gene to their offspring

 Dominant traits are governed by an allele that can be expressed in the presence of another, different allele.

◦ Dominant alleles prevent the expression of recessive alleles in heterozygotes.

◦ They are not “better” or “stronger”

 Recessive traits are not expressed in heterozygotes.

◦ For a recessive allele to be expressed, there must be two copies of the allele.

 Demonstrates how alleles can be combined from F1 generation to F2 generation.

 Shows that 1/4 of the F2 plants should be homozygous dominant (TT), 1/2 heterozygous (Tt), and the remaining 1/4 to be homozygous recessive (tt).

 The genetic makeup of an individual.

 Genotype can refer to an organism’s entire genetic makeup or to the alleles at a particular locus.

 The observable or detectable physical characteristics of an organism; the detectable expressions of genotypes, frequently influenced by environment.

 Characteristics that are influenced by alleles at only one genetic locus.

 Examples include many blood types, such as ABO.

 Many genetic disorders, including sickle- cell anemia and Tay-Sachs disease, are also Mendelian traits.

 Over 19,000 human trains are known to be inherited according to Mendelian principles

 The human ABO blood system is an example of a simple Mendelian inheritance. ◦ The A and B alleles are dominant to the O allele. ◦ Neither the A or B allele are dominant to one

another; they are codominant and both traits are expressed.

 The expression of two alleles in heterozygotes.

 In this situation, neither allele is dominant or recessive; thus, both influence the phenotype.

 Shows discontinuous distribution of ABO blood type in a hypothetical population.

 The expression of the trait is described in terms of frequencies.

 Polygenic traits, or continuous traits, are governed by alleles at two or more loci, and each locus has some influence on the phenotype.

 Hair, eye and skin color are polygenic traits

 Coloration is determined by pigment produced by specialized cells called melanocytes

 The amount of melanin produced determines how dark or light skin will be

 Melanin production is influenced by interactions between several different loci

 Represents the continuous expression of height in a large group of people.

 Phenotypic variation in humans is controlled by polygenic traits

 The social construction of race is based on polygenic traits

Evolution, as it has been considered from the middle of the twentieth century, is a two-stage process:

1. The production and redistribution of variation (inherited differences among organisms).

2. Natural selection acting on this variation, whereby inherited differences, or variation, among individuals differentially affect their ability to successfully reproduce.

 From a modern genetic perspective, we define evolution as a change in allele frequency from one generation to the next.

 Allele frequencies are indicators of the genetic makeup of a population, the members of which share a common gene pool.

 In a population, allele frequencies refer to the occurrence of a specific allele at a specific locus within the members of that population.

 Mutation

 Gene Flow

 Genetic Drift and Founder Effect

 Recombination

 Mutation is a molecular alteration in genetic material: ◦ For a mutation to have evolutionary

significance it must occur in a gamete.

◦ Mutation rates for any given trait are usually low

◦ When combined with natural selection, evolutionary changes can occur and can occur more rapidly.

◦ Only way to produce new genes

 The exchange of genes between populations.

 If individuals move temporarily and mate in the new population (leaving a genetic contribution),

they don’t necessarily stay there.

 Consistent feature of human evolution ◦ Example: The offspring of U.S. soldiers and Vietnamese

women represent gene flow, even though the fathers returned to their native population.

 Genetic drift occurs solely because the population is small:

◦ Alleles with low frequencies may not be passed to offspring and eventually disappear from the population.

 Occurs when a small band of “founders” leaves its parent group and forms a colony elsewhere.

 A new population is established and as long as mates are chosen within this population, all the members will be descended from the founders.

 A once rare allele that was carried by even one of the founders can eventually become common.

 Natural selection provides directional change in allele frequency relative to specific environmental factors.

 If the environment changes, selection pressures also change.

 If there are long-term environmental changes in a consistent direction, then allele frequencies should also shift gradually each generation.

 X linked recessive traits are the most common

◦ Women have some protection due to have two X chromosomes and therefore a “backup”

◦ Men only have one so they will express whatever is on their X Chromosome

 Y linked traits are responsible for the normal development of male reproductive organs and fertility

 A severe inherited hemoglobin disorder in which red blood cells collapse when deprived of oxygen.

 It results from inheriting two copies of a mutant allele.

 This mutation is caused by a single base substitution in the DNA.

Understanding your genetic makeup can determine significant life decisions:

 Do you carry a genetic mutation for a malady?

 Do you want to know if you will get sick/pass it on to your children?

 Would you change yourself/offspring if you could?