Friday, June 13, 2008
Gentics: Online Lab #2
The human genome is made up of 15,000 genes that exist on 23 pairs of chromosomes. Each parent contributes one of each chromosome to their offspring. It is our genes that dictate so much of who we are today. Everything from the length of our fingers, to the color of our eyes, to whether or not certain protein receptors in our plasma membrane work correctly is determined by our genes. The variations that exist for any given gene are called alleles. Although both parents pass on to us one of every gene, not every allele that we receive is expressed as a trait in our phenyotype. In order for a recessive allele to be expressed, a person must receive the recessive allele from both their mother and father. If one dominant allele is received, the dominant phenotype will prevail.
When looked at from an evolutionary perspective, the genes we have today were "selected" for by nature. They provided our ancestors some advantage over others, such that they were able to reproduce and pass on those genes.
The purpose of this lab was to confirm my understanding of how genes are inherited and how those genes affect the adult phenotype.
First, I will start with the definitions.
1. Genotype - the genes of an individual. When looking at one trait that is coded for by one set of alleles, the genotype is represented by 2 letters. One letter stands for the allele received from the mother, the other letter represents the allele received from the father. For instance, in the case of the fruitfly, the 3 genotypes in scenario 5 were Ll, LL, and ll.
2. Phenotype - the physical appearance of the trait that is expressed through the genotype. Back to the fruitfly, the 2 phenotypes that could be expressed by the 3 genotypes are as follows: LL=long-winged, Ll=long-winged, ll=vestigial-winged.
3. Allele - variations of the same gene. A few examples of alleles from dragon lab are horns vs no horns, wings vs no wings, scales vs no scales. There are dominant alleles and recessive alleles. If an individual has at least one copy of the dominant allele, the dominant allele will be expressed. For the recessive allele to be expressed, the individual must have both copies of the recessive allele. (Unless it is an x-linked allele in males) The alleles for a given trait occur at the same loci of homologous pairs.
4. Cross - refers to taking the genotype of a set of parents and determing the possible genotype/phenotype of their offspring through the use of a Punnett Square. For example, in scenario 5, we crossed a heterozygous long-winged fly (genotype=Ll) with a heterozygous long-winged fly (genotype=Ll).
5. Dominant - when referring to genes and alleles, the dominant allele is one that will express itself any time it is present. In other words, the dominant allele will express itself in the heterozygous individual (1 copy of dominant allele, 1 copy of recessive allele) or in the homozygous dominant (2 copies of dominant allele) individual. In the scenario 5, the dominant allele is long wings. Both parents were heterzygous for this trait. Both had long wings because they each had one copy of the dominant long-wing allele.
6. Recessive - when referring to genes and alleles, the recessive allele is one that will only be expressed in the individual if both copies are present. In the fruitfly lab, the vestigial-wing was the recessive allele. The vestigial-winged fruitfly has 2 copies of the recessive allele for the vestigial-wing.
Here is the screen shot from the dragon lab. In this lab I had to change the genotype (to the right of the dragons) of the second dragon so that its phenotype (physical apperance) matched that of the first dragon.
Here is the screen shot from the Punnett Square lab. This shows the offspring that could result from the crossing of two heterozygous long-winged parents. This visuals shows both the genotype and the phenotype for each individual.
Hair color. Hairline. Eye shape. Eyelash length. Freckles. Ability to roll your tongue. All of these things are human traits that are coded for in an individuals genes, all of which are passed down from the parents. Each parent supplies half of an individual's genome. This is possible through the process of meiosis. Meiosis includes 2 phases of nuclear division. It is the 2 phases of nuclear division that allow the gametes to end up with half the genetic information of the parent cell. The first phase separates the homologous pairs into two daughter cells, and the second separates the sister chromatids. In this way, the gamete ends up with only half the genetic information of the parent cell. The full genome is restored once a sperm cell and egg cell join during fertilization. It is the combination of genes that make up who we are. The combination is also what provides for so much variation between individuals.