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Mendel noticed that certain plant traits were dominant; that is, usually present and passed on from parent to offspring. For example, the flower position was mostly along the axis of the stem. But sometimes, Mendel saw flowers in a non-axial position. So, he pondered how these positions were inherited. He concluded correctly that there was a genetic code for an axial position and another for non-axial position. Let us call the inheritable code for axial position  "P" and that for non-axial a lower-case "p".  But since he already knew that the axial position was dominant, he correctly guessed that the only way he could get non-axial flowers was if both members of the coding pair were lower-case "p". He called these non- dominant coding controls "recessive."

 

What are alleles?

Alleles are alternate forms of the same gene, like P and p described above. For example, the gene that codes for eye color has several different alleles, blue, green, and brown. "Dominant" means that if only ONE of the two alleles for a trait is present that trait will be visible in the organism. For example, if a pea seed has only one Smooth-trait allele, that pea will be smooth, because the Smooth allele just happens to be dominant. "Recessive" means that BOTH alleles must be present in order for the trait to be present.  For example, both alleles must be for Wrinkled-trait allele in order for the pea to be wrinkled.

In general, the inheritance of a given individual trait is independent of other traits. For example, inheritance of genes for the position of flower along the plant stem is independent of the other traits such as smoothness of the peas. These ideas were summarized in what we now call Mendel's laws:

Mendel's laws, stated in modern terms are:
  • The law of segregation states that members of each pair of alleles of a gene become separated when sex cells (gametes) are produced. 
  • The law of independent assortment states that pairs of alleles separate independently of each other during formation of sex cells. We now know that this is only true for genes located on different chromosomes. The term "independent assortment" refers to the behavior of genes for a given trait on different chromosomes.

When sex cells (reproductive cells or gametes) are made, the chromosomes segregate independently, so that each gamete cell has one half of the chromosomes found in all other cells. For example, P is a dominant allele and p is a recessive allele. When sex cells are created, one sex cell will have the P allele and the other the p allele. When one gamete combines during fertilization with a gamete from the opposite sex to make a new baby plant, the baby plant will get both alleles (in this case P and p).

Click here for an advanced explanation of Mendelian genetics and also information on what happens during division of sperm and ova.

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