Lecture 3: Cell Division and Genetic Traits

Cell Division - Mitosis and Meiosis

  • Chromosomes are cellular structures, DNA wrapped around protein. They are only found in the cell nuclei. We can only see them during the cell division phase under a light microscope where they condense and thicken while getting ready to undergo cell division. One DNA molecule is made up of 10's of 1000's of genes. It is like one big ball of string which unwraps as one long string. It stores a lot of information. In the shape of a double helix.


Chromosome Sets

  • There are 46 non identical chromosomes in somatic cells, since each pair comes from a different parent. Each pair codes for the same trait, (ex. blood type or eye color), but it won't be identical information. (Ex. one is coded for blue eyes while the other is coded for brown eyes). It might also be same.
  • In gametes the 22 pairs are called autosomes, which code for the same trait, but are non identical. The 23'd pair determines sex. Females have XX and males have XY chromosomes. Have to have both pairs to get by, or organism won't survive, except for sex chromosomes, in which you can have one. In these situations, the organism survives and usually develops as a female.

0308 DNA genes chromos

Mitosis

  • Figure 3.9 and 3.11. Occurs in somatic cells. Every cell produces two identical daughter cells with 46 chromosomes each. Produces new cells for organism to grow, develop or repair itself.

Meiosis

  • Figure 3.10 and 3.11. Occurs in sex cells. One cell with 46 chromosomes replicates and splits twice. The end product is four daughter cells which are all genetically different from the parent cells. They have 23 chromosomes each, one from each pair. Produces eggs and sperm cells. The process of meiosis creates diversity, since it promotes genetic variation which evolution acts on. Sexually reproducing organisms have an evolutionary advantage. Diversity is good because the organisms are better resisted to disease.
  • During cell division, the chromosomes replicate. They are joined by a centromere.

0311 mitosis meiosis

Evolutionary Importance

  • Recombination, sexual reproduction, mutation. Mutation is a course of variation, but recombination and sexual reproduction are much greater sources of population variation. Mutation is important. Ex. the process of recombination aka crossing over is a mutation. As the pairs of chromosomes are spitting, they get tangled up so that a piece of one crosses over with the other one. The information ends up different, so that not all the traits from one parent pass on. The organism won't function if one whole part disappears, but usually only a small piece crosses. Its OK if happens in gametes, because it creates diversity. Mutations result in failure of the reproductive or replication process. Nothing that is too harmful and cannot be fixed, but sometimes there are extreme which effect the life of the organism.

Genetics - Monogenic (Mendelian Traits)

  • Monogenic traits occur at a single locus. There are only a few traits which are monogenic in humans, such as blood types and sickle cell anemia.


  • Principle of Segregation


Trait Expression - Genotypes and Phenotypes

  • Alleles are one of the alternative forms of a gene. Figure 3.15.
  • Genotypes are the genetic makeup of an organism. Figure 3.16
  • Phenotypes are the physical or detectable traits in an organism. Figure 3.14.
  • The dominant trait is the trait which is expressed or physical when present. It doesn't mean it's the best trait. Table 3.2 shows that some dominant traits can be deadly diseases, but they might help on an evolutionary scale. Ex. people who live in regions which are highly affected by malaria are more likely to get anemia. Anemia turns normal healthy blood cells, which the malaria thrives on, into sickle cells which are resistant to malaria.
  • The recessive trait is the hidden one which is not expressed when present.
  • Homozygote means there are two copies of the same allele, ex. AA.
  • Heterozygote means there are two different alleles, where the dominant trait is expressed. Ex. lactose intolerance or blood types.
  • Codominance occurs when both alleles are expressed phenotypically. Ex. blood type AB.
  • Fitness and dominance.


  • The traits are discrete meaning that the phenotypic expressions don't overlap. Ex. there are only four discrete blood groups. You have to be either A (AA or AO), B (BB or BO), O (OO) or AB. Ex. pea plants are either short or tall, no medium height.


  • Principle of independent assortment

Genetics - Polygenic Traits

  • Have a continuous, addictive effect meaning the phenotypic expressions overlap. Each allele present adds up to contribute to the final phenotype.
  • Are governed by genes at more than one locus.
  • These traits can be metrically analyzed. Can't do punnett squares for these.
  • These include most of the traits which we see. Ex. eye colour, hair colour, fingerprints, height, body and face shape etc.
  • These traits seem to be more influenced by environmental factors. Ex. height is mostly predetermined, but the amount of nutrition and physical activity an individual gets contributes to the final height. Ex. your skin colour is mostly predetermined, but the amount of vitamin D you get and your exposure to the sun influences the final phenotype.


Monogenic vs. Polygenic Traits

Pea Plants: Dominant and Recessive. Genotypes and Phenotypes.


0316 pea Punnett

0315 alleles


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