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Consider the gene for eye color. The gene could be written as BB, Bb, or bb. The different letters represent the different forms of a gene. Different forms of a gene are called alleles. A gene is always comprised of two letters or alleles. This is because each parent gives an allele to make the gene.
Johann Gregor Mendel
Mendel's Law of Dominance
Mendel's Law of Dominance
Johann Gregor Mendel bred pea plants with different traits and studied their pea plant babies. He observed that some forms of a gene dominated other forms. For instance, in the example to the right, the yellow pea pod color was found to be dominant to green pea pods. He said that yellow was the dominant factor.
The parents are outside the Punnett Square: Yy and yy. Their children are inside the boxes. Note that Y overpowers y.
Let's consider eye color! In the case of eye color, brown is dominant over blue.
Let B = The dominant eye color allele: Brown
Let b = The recessive eye color allele: blue
This means that a person with the Bb genotype would have brown eyes. The blue allele is recessive and is masked by the brown allele.
B_
B_
"BB" and "Bb" alleles would show the dominant phenotype, brown eyes. There is no difference between "Bb" and "bB".
bb
bb
"bb" alleles result in the recessive phenotype, blue eyes.
Question: Why are letters always found in pairs in the developing baby?
Why do genes look like this: HH, Tt, pp?
Answer: Each parent gives a letter to make up that trait. 2 factors, or 2 alleles, control every trait (hair color, number of fingers) - one letter comes from each parent.
Mendel also performed more elaborate experiments. This diagram summarizes the results. No matter the trait, those patterns always remained the same.
P: The original generation:
Tall pure-bred pea plants (TT) * short pure-bred pea plants (tt)
F1: The next generation:
Tall, hybrid pea plants (Tt)
When hybrids inbreed:
Tall hybrid pea plants (Tt) X Tall hybrid pea plants (Tt)
F2: The next generation:
75 % tall pea plants
25% short pea plants
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