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3.4: Inheritance

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Describe conclusions drawn from Mendel's pea plant experiments.

Understanding: Mendel discovered the principles of inheritance with experiments in which large numbers of pea plants were crossed.
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Through selective breeding of pea plants, Mendel discovered that certain traits show up in offspring without blending of the parent's characteristics. Mendel observed seven traits: flower color, stem length, seed color, pod color, flower position, seed shape and pod shape.

Mendel concluded:
1. genetic "units" of inheritance are passed from parents to offspring

2. the offspring inherits one "unit" from each parent for each trait.

3. the "unit" may be masked or hidden (i.e. recessive) in an individual but can still be passed on to the next generation.
Many genes have multiple alleles within the population. For example, in ABO blood typing there are three common alleles for the Isoagglutinogen gene: I^A, I^B and i.

Within a diploid individual there may only be a combination of two of the alleles:
I^A, I^A (type A)
I^A, I^B (type AB)
I^A, i (type A)
I^B, I^B (type B)
I^B, i (type B)
i, i (type O)
Since there isn't a true dominant allele, a lowercase letter is NOT used when alleles are codominant. Rather, two different capital letters are used and places as superscript next to a common letter that represents the name of the gene. For example, type A and type B alleles of the Isoagglutinogen gene.

I^A and I^B are codominant.