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2. Making Babies - Background
Mendel worked with pea plants, carefully characterizing their traits and cross-breeding them over many generations. He observed that many traits occur in only two different forms (short or long stems, purple or white flowers, round or wrinkled seeds). When a short plant is crossed with a tall plant, the offspring are all either tall or short, not of middle stem length. This observation countered the “blending theory” that was generally accepted at the time.
He established many pure-breeding lines – for instance short plants that when cross-bred always had short offspring and tall plants that always had tall offspring. Interestingly, when a pure-bred short plant is crossed with a pure-bred tall plant, the first offspring (f1 generation) are all tall! If these tall f1 plants are cross-bred to one another, the next generation (f2) consistently have a 3:1 ratio of tall to short plants.
With these ratios and careful breeding experiments, Mendel discovered the basic laws of inheritance. He came to several conclusions:
Thus, what happened in Mendel’s tall x short plant experiment was this… The pure breeding tall plant can be represented by TT. The pure breeding short plant can be represented by tt. Each letter (T or t) represents one factor or gene. Since each individual has 2 copies of every factor or gene, each plant has 2 letters to represent its combination of genes (its genotype). The different forms of the gene (alleles) are represented by capital versus lower case letters.
When the tall and short plants were crossed, each parent gave the offspring one of its two genes. The result is that all the offspring inherit the combination Tt. All of these f1 plants are tall. Thus, the tall T allele covers up the short t allele resulting in tall f1 offspring. The tall T allele is therefore said to be dominant over the short t allele and the short t allele is recessive to the tall T allele.
These tall f1 plants with the genotype Tt are then crossed to one another. Since the offspring can be given a tall T allele or a short t allele, there are 4 possible combinations that may occur, each are equally likely: TT, Tt, tT or tt. Only tt would produce a short plant since in all the other cases, the dominant tall T allele is present to cover up any recessive short t alleles that might be present. Thus, there is a 3:1 ratio of tall to short plants in this f2 generation.
This can be graphically shown in what is known as a Punnett square which resembles a multiplication table as shown at left.
This inheritance pattern is simplest of all possibilities. It gets a whole lot more complex when you consider incomplete dominance (where the heterozygotes that have two different alleles like Tt have an intermediate phenotype), X linkage (what happens with genes on the sex chromosomes), polygenetic traits (traits determined by more than one gene), linked genes (genes that often go together because they are located close to one another on the same chromosome), and more.
In this activity, we examine some human facial traits that are assumed to be single gene traits. The actual genetics is MUCH more complex. However a brief run-down is provided here (for photos of the traits listed below, see attachment at the bottom of the Human Traits activity):
There are a few traits to be cautious about using with students because people have a difficult time accurately recording traits. For instance, nearsightedness is a single gene trait with normal vision dominant to nearsightedness. However, kids often aren’t diagnosed with nearsightedness until late in adolescence. This leads to awkward questions such as, “Both my parents are nearsighted but I’m not. Does that mean I’m adopted?” Often teachers use round versus square shaped faces in which round faces are dominant to square faces. Both me and my students have difficulty categorizing faces as round or square, leading to much confusion. Other traits that are often difficult to categorize include: lip shape, eye spacing, eyelash length, eye slant, nose shape, and eyebrow thickness. Finally, be sure to warn students that in categorizing freckles, DO NOT COUNT sun freckles. Freckles means a very large number of freckles all over the nose and cheeks, whether you’ve been in the sun or not.