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Mendelian Genetics




 
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A series of free High School Biology Video Lessons. Understanding genotype used in Mendelian genetics. The description of phenotype. How to generate and use the Punnet squares. The concept of codominance of incomplete dominance. The concepts of chromosome crossing over and linked genes.

Genotype
An organism's genotype is its pairing of alleles for a specific gene. Genotypes, unlike phenotypes, can be homozygous or heterozygous. If an organism is heterozygous for a gene, or possesses one of each allele, then the dominant trait is expressed. A recessive allele is only expressed if an organism is homozygous for that trait, or posses two recessive alleles.
Understanding genotype used in Mendelian genetics.
Phenotype
An organism's phenotype is the form of a gene which is expressed. Phenotypes disregard the allele pairing for a gene and focuses only on the trait. We can figure out an organism's phenotype from physical tests, but its genotype can only be determined with genetic analysis.
The description of phenotype.



Punnett Square
Punnett squares are useful in genetics to diagram possible genotypes of the offspring of two organisms. The Punnett square for a monohybrid cross tracks the inheritance of a single trait and consists of four boxes, each of which represents a possible genotype. The Punnett square for a dihybrid cross tracks two genes and consists of sixteen boxes.
How to generate and use the Punnet squares.
Codominance - Incomplete Dominance
In both codominance and incomplete dominance, both alleles for a trait are dominant. In codominance a heterozygous individual expresses both simultaneously without any blending. An example of codominance is the roan cow which has both red hairs and white hairs. In incomplete dominance a heterozygous individual blends the two traits. An example of incomplete dominance is the pink snapdragon, which receives a red allele and white allele.
The concept of codominance of incomplete dominance.


 
Chromosomes Crossing Over - Linked Genes
When two genes are located on the same chromosome they are called linked genes because they tend to be inherited together. They are an exception to Mendel's law of Segregation because these genes are not inherited independently. When chromosomes cross over, two different chromosomes trade pieces of genetic information during prophase I of meiosis. If the linked genes are far apart on the chromosome, it is more likely that crossing over will separate them.
The concepts of chromosome crossing over and linked genes.


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