AP Biology Scientific Journal
Nimisha Jain
Mendel’s Genetics
For a long time now, farmers and herders have been selectively breeding their plants and animals to produce more useful hybrids. Gregor Mendel, a Central European Monk, actually researched and experimented on common pea plants to discover the basic principles of heredity.
He discovered 3 main things:
1. The inheritance of each trait is determined by "units" or "factors" that are passed on to descendents unchanged (these units are now called genes)
2. An individual inherits one such unit from each parent for each trait
3. A trait may not show up in an individual but can still be passed on to the next generation.
Probability of Inheritance
Studying genetics helps us predict the likelihood of inheriting particular traits. Such knowledge can help plant and animal breeders in developing varieties that have more desirable qualities. Such knowledge also helps explain and predict patterns of inheritance in family lines.
One way to calculate the mathematical probability of inheriting a specific trait was invited by an early 20th century English geneticist named Reginald Punnett. He created the Punnett square which graphically shows the potential combinations of genotypes that can occur in children, given the genotype of their parents. Such techniques of this graphical tool can be used to determine whether parents will give birth to healthy children or not, depending on dominant and recessive alleles.
Exceptions to Simple Inheritance
Since Mendel’s time, we have made some other discoveries that do not fit in his rules. Such as the fact that inheriting one allele can, at times, increase the chance of inheriting another or can affect how and when a trait is expressed in an individual's phenotype. Likewise, there are degrees of dominance and recessiveness with some traits. The simple rules of Mendelian inheritance do not apply in these and other exceptions. They are said to have non-Mendelian inheritance patterns.
Polygenetic Traits
Traits that are determined by the combined effect of more than one pair of genes.
Intermediate Expression
When there is incomplete dominance, apparent blending can occur in the phenotype, which results in the intermediate expression of a trait in heterozygous individuals.
Codominance
For some traits, two alleles can be codominant. That is to say, both are expressed in heterozygous individuals.
Incomplete Penetrance
Some genes are incompletely penetrant. That is to say, their effect does not normally occur unless certain environmental factors are present.
Sex Related Genetic effects
Sex-limited genes are ones that are inherited by both men and women but are normally only expressed in the phenotype of one of them.
Sex-controlled genes are expressed in both sexes but differently.
Some genes are known to have a different effect depending on the gender of the parent from whom they are inherited- genome imprinting.
Pleiotropy
A single gene may be responsible for a variety of traits.
Stuttering Alleles
Some genetically inherited diseases have more severe symptoms each succeeding generation due to segments of the defective genes being doubled in their transmission to children. These are referred to as stuttering alleles or unstable alleles.
Enviroomental Influences
Genes are influnced by their surrounding environment.
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