Genes and Basic Definitions
Genes: A small strip of DNA that occupies a position (also called a locus) on the chromosomes. They control specific characteristics about an organism, such as the color of a pea. They are also inherited.
Alleles: A specific form of a gene. If a gene controls the color of a pea, one allele (G) could be the form that produces green peas and another allele (g) could be the form that produces yellow peas. Can either be dominant or recessive.
Homozygous: Having two of the same exact alleles for a gene. For example, if a pea was homozygous for green, the allele combination would be GG. Or if the allele was homozygous for yellow, the combination would be gg.
Heterozygous: Having two different alleles for a gene.
Dominant Allele: An allele that has the same effect on the phenotype when present in a heterozygous or homozygous state. For example, if green (G) was the dominant allele then a pea would be green if it's genotype for that trait was either GG or Gg.
Recessive Allele: An allele that only affects the phenotype when present in the homozygous form. In this case, in order for the pea to be yellow its genotype must be gg.
Carrier: An individual that is heterozygous for a recessive trait, usually a disease. The trait is not manifested in them as a result but they still have that recessive allele.
Test Cross: When one is unsure about the genotype of an individual's trait (doesn't know if the trait is homozygous or heterozygous), one can cross the unkown individual with a known recessive homozygous individual to discover the unknown genotype.
Genome: The whole genetic information of an organism. According to the human genome project, finished in 2001, and other research there are roughly 19,000 - 20,000 genes in the human genome.
Genotype: The allele combination in an organism.
Phenotype: The physical manifestation of the genotype.
Alleles: A specific form of a gene. If a gene controls the color of a pea, one allele (G) could be the form that produces green peas and another allele (g) could be the form that produces yellow peas. Can either be dominant or recessive.
Homozygous: Having two of the same exact alleles for a gene. For example, if a pea was homozygous for green, the allele combination would be GG. Or if the allele was homozygous for yellow, the combination would be gg.
Heterozygous: Having two different alleles for a gene.
Dominant Allele: An allele that has the same effect on the phenotype when present in a heterozygous or homozygous state. For example, if green (G) was the dominant allele then a pea would be green if it's genotype for that trait was either GG or Gg.
Recessive Allele: An allele that only affects the phenotype when present in the homozygous form. In this case, in order for the pea to be yellow its genotype must be gg.
Carrier: An individual that is heterozygous for a recessive trait, usually a disease. The trait is not manifested in them as a result but they still have that recessive allele.
Test Cross: When one is unsure about the genotype of an individual's trait (doesn't know if the trait is homozygous or heterozygous), one can cross the unkown individual with a known recessive homozygous individual to discover the unknown genotype.
Genome: The whole genetic information of an organism. According to the human genome project, finished in 2001, and other research there are roughly 19,000 - 20,000 genes in the human genome.
Genotype: The allele combination in an organism.
Phenotype: The physical manifestation of the genotype.
The Infamous Punnett Square
In order to find out what genotype and thus phenotype an offspring of two organisms might have, we can perform a test cross. To do so, we need the alleles of a trait from both parents. Going along with our pea theme, say that both parents are heterozygotes for the green color. (Gg)
Make a square with four boxes, then line the alleles of each parent on the outer sides of the square to look like this:
Make a square with four boxes, then line the alleles of each parent on the outer sides of the square to look like this:
Then, fill in each square according to the example. One can also foil Gg x Gg like in algebra to get the results. The results are the different genotypes that the offspring could have. According to the punnet square there is a 75% chance that the offspring will be green and a 25% that the offspring will be yellow.
The results can be made into ratios.
Phenotype ratio: 3:1 (3 green, 1 yellow)
Genotype ratio: 1:2:1 (1 homozygous dominant combo, 2 heterozygous dominant combo and 1 homozygous recessive combo).
The results can be made into ratios.
Phenotype ratio: 3:1 (3 green, 1 yellow)
Genotype ratio: 1:2:1 (1 homozygous dominant combo, 2 heterozygous dominant combo and 1 homozygous recessive combo).
Dihybrid Crosses
Dihybrid crosses are similar to punnett squares except instead of testing for one specific gene, the dihybrid cross determines the possible allele combinations for two genetic traits that are unliked.
Note
F1 refers to the first generation of offspring. When it comes to alleles, the dominant allele is always capitalized while the recessive allele is always a lower case letter. The letters must be the same however ex. Gg.
F1 refers to the first generation of offspring. When it comes to alleles, the dominant allele is always capitalized while the recessive allele is always a lower case letter. The letters must be the same however ex. Gg.