Lecture 9: Populations

  1. Mendelian Concepts
    Gregor Mendel performed a hybridization experiment with pea plants. He found that when he crossed purple flowered plants with white flowered plants, the first filial, or F1 generation, produced purple flowers. He called the purple trait dominant and white trait recessive. When Mendel self-pollinated the F1 generation plants, the F2 generation expressed both the dominant and recessive traits in a 3 to 1 ratio, now referred to as the Mendelian ratio.

    The expression of a trait is the phenotype, and an individual's genetic make up is the genotype. The phenotype is expressed through the action of enzymes and other structural proteins. Which are encoded by genes. In complete dominance, exhibited by the flowers in Mendel's experiment, for any one trait, a diploid individual will have two chromosomes each containing a separate gene that codes for that specific trait. These two chromosomes are homologous. Their corresponding genese are located at the same locus or position on respective chromosomes. Each gene contributes an allele to the genotype.

    An individual with a genotype having two dominant or two recessive alleles is said to be homozygous for that trait. An individual with a genotype having one dominant and one recessive allele is said to be heterozygous for the trait, and is called a hybrid.

    The first law of heredity, the law of segregation, states that alleles segregate independently of each other when forming gametes. Any gamete is equally likely to posess any allele. The phenotypic expression of the alleles is not a blend of the two, but an expression of the dominant allele.

    Mating relatives is called inbreeding and does not change the frequency of alleles, but does increase the number of homozygous individuals within a population. Outbreeding is mating of nonrelatives which produces hybrids.

    A punnett square is used to predict genotypic ratios of offspring.

    The second law of heredity is the law of independent assortment, which states that genes located on different chromosomes assort independently of each other. Genes that code for different traits when located on different chromosomes do not affect each other during gamete formation. If two genes are located on the same chromosome, the likelihood that they will remain together during gamete formation is indirectly proportional to the distance separating them.

    A dihybrid cross involves 4 traits and has a ratio of 9:3:3:1.

    In human, the 23rd pair of chromosomes establishes the sex of the individual, and each partner is called a sex chromosome. In males, instead of appearing as two X's in a karyotype (map of the chromosomes), they appear as an X and a Y. When a gene is found on the sex chromosome it is called sex-linked. Generally, the Y chromosome does not carry the allele for the sex-linked trait; thus, the allele that is carried by the X chromosome in the male is expressed whether it is dominant or recessive. Since the female has two X chromosomes, her genotype is found through the normal rules of dominance.

    A female may carry a recessive trait on her 23rd pair of chromosomes without expressing it. If she does, she is said to be a carrier for that trait.
  2. Evolution
    The gene pool is the total of all alleles in a population. Evolution is a change in the gene pool.

    Kingdom, Phylum, Class, Order, Family, Group, Species.

    Since organisms in the same group have similar genetic structures, they share similar phylogenies (evolutionary history). For isntance all mammals belong to the class Mammalia and the phylum Chrodata

    There are new superkingdoms calld domains. The three domains are Bacteria, Archaea, and Eukarya. The kingdoms of protista, fungi, plantae, and animalia into eukarya.

    • Species is loosely limited to, but not inclusive of, all organisms that can reproduce fertile offspring with each other. Organisms which normally reproduce selectively fit offspring in the wild.
    • -The way in which a species exploits its environment is called its niche. Survival of the fittest predicts that one species will exploit the environment more efficiently then another, leading to the extinction of the other with the same niche. The definition of the fittest organism in this theory is the organism which can best survive to reproduce offspring which will, in turn reproduce.

    There are two opposing reproductive strategies, r-selection and k-selection. R-selection involves producing large numbers of offspring that mature rapidly with little or no parental care. R-strategists generally have a high brood mortality rate.

    K-selection involves small brood size with slow maturing offspring and strong parental care. K-strategists tend to have a sigmoidal growth curve which levels off at carrying capacity. The carrying capacity is the maximum number of organisms that an environment can maintain.

    Speciation is the process by which new species are formed. When gene flow ceases between two sections of a population, speciation begins. Factors which cause this include geographical, seasonal, and behavioral isolation. Adaptive radiation occurs when several separate species arise from a single ancestral species.

    A species may face a crisis so severe as to cause a shift in the allelic frequencies of the survivors of the crisis. This is called an evolutionary bottleneck.

    Divergent evolution
    exists when two or more species evolving from the same group maintain a similar structure from the common ancestor. However, two species may independently evolve similar structure in convergent evolution.

    A symbiosis is a relationship between two species. The relationship can be beneficial to both (mutualism), beneficial to one and neutral to the other (commensalism), or beneficial for one and detrimental to the other (parasitism).
  3. Hardy-Weinberg Equilibrium
    Statistically there should be no change in the gene pool of a sexually reproducing population possesing the five following conditions.

    1) large population

    2) mutational equilibrium

    3) immigration or emigration must not change the gene pool

    4) random mating

    5) no selection for the fittest organism.

    A population with these 5 features are said to be in Hardy-weinberg equilibirum. No real population ever possesses these characteristics completely. Small populations are subject to genetic drift where one allele may be permanently lost due to the death of all members having that allele.

    The binomial theorem: p^2 + 2pq + q^2 predicts the genotype frequency of a gene with only two alleles in a population.

    Basically, square the %dominant and square the percentage recessive. Subtract from one and you have the percentage heterozygous.
  4. Origin of Life
    Early earth probably had an atmosphere made mainly from nitrogen and hydrogen gas, and very little oxygen gas. The urey-miller experiment was one of the early experiments to make such an attempt to recreate this atmosphere.

    The first cells are thought to have evolved from coacervates, lipid or protein bilayer bubbles. Organisms may have initially assimilated carbon from methane and carbon dioxide in the early atmosphere.

    The earliest organisms were probably heterotrpohs, and fossils have been dated 3.6 billion years ago. Around 2.3 billion years ago, the ancestors of cyanobacteria evolved. They were able to use sunlight and water to reduce carbon dioxide. These were the first oxygen producing, photosynthetic bacteria. The atmosphere began to fill with oxygen.
  5. Chordate features
    Chordata is the phylum containing humans. All chordates have bilateral symmetry. They are deuterostomes, meaning their anus develops from or near the blastopore. Chordates have a coelom (a body cavity with mesodermal tissue). At some stage of development, they possess a notochord (an embryonic axial support, not the backbone), pharyngeal slits, a dorsal, hollow nerve chord, and a tail.

    Members from the subphylum vertebrata have their notochord replaced by a segmented cartilage or bony structure.

    The taxonomy of homo sapiens:

    Domain: Eukarya

    Kingdom: animalia

    Phylum: Chordata

    Subphylum: Vertabrata

    Class: mammalia

    Order: Primata

    Family: Homididae

    Genus: Homo

    Species: Sapiens
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Lecture 9: Populations
Lecture 9: Populations