2. Making Babies - Background

Teacher Background
One of the greatest mysteries – how we inherit traits from our parents – was solved in the 1800s by the Austrian monk Gregor Mendel. Although he published his work in 1866, it went almost entirely unrecognized until the 1900s, long after his death.

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.

Mendels peasMendels peasHe 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:

  1. That traits are determined by “factors” (now called genes) that are passed from parent to offspring in an unchanged, undiluted, unblended form.
  2. Everyone has 2 copies of a “factor” for each trait, one from each parent.
  3. Even if a “factor” does not show up physically in an individual, that “factor” can still be passed on to the next generation.

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):

  • Brown vs. non brown eyes – There are in fact at least 3 genes for eye color. The best understood is the brown vs. non brown gene in which brown in dominant to non-brown.
  • Freckles vs. no freckles – Freckles is dominant to no freckles by a single gene. Be sure to emphasize that this means have a LOT of PERMANENT freckles, not just a couple sun freckles that come and go depending on how much sunbathing you do.
  • Tongue rolling vs. non rolling – The ability to roll ones tongue into a U shape is dominant to not being able to.Dimples vs. no dimples – Cheek dimples that appear when a person smiles is dominant to not having dimples.
  • Attached vs. unattached earlobes – Unattached earlobes are dominant to attached earlobes.
  • Widow’s peak vs. straight hairline – Having a widow’s peak is dominant to have a straight hairline.
  • Cleft chin vs. smooth chin – Cleft chin is dominant to smooth chin.
  • Dark vs. light hair – Like eye color, hair color is a polygenetic trait. However, dark hair (black or brown) is dominant to light hair (blond or red) due to a single gene.
  • Curly vs. wavy vs. straight hair – Hair texture is a case of incomplete dominance. Heterozygotes end up with wavy hair.

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.

Student Prerequisites
Completion of the Human Traits activity.