This is an extension of the Human Traits survey activity designed to introduce students to genes, genotypes, and simple inheritance patterns. Using information from the Human Traits Survey, students make guesses about their own genotype, create gametes from their genotypes, then make “babies” with a partner. Along the way students discover answers to the questions: What are genes? How are genes (and traits) passed on? How are gametes different than other cells in our body? Why do I look like mom in some ways and dad in other ways and neither of them in still other ways? Why don’t siblings look alike?
Can explain the relationship between genotype and phenotype.
Can explain the inheritance of single gene traits using dominant/recessive relationships.
Can take genotype information from 2 parents, model the creation of gametes by independent assortment, and use those gametes to create offspring.
Individual initially then later in pairs. The teacher should devise a way to break the students into pairs (ramdomly or assign beforehand). Allowing students to pick their own partners is NOT a good idea for this activity. It is not necessary to have mixed gender pairs. In fact, the same gender pairs tended to be more mature about the whole thing.
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.
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.
Completion of the Human Traits activity.
- Who was Gregor Mendel?
- In one experiment, Mendel crossed a tall pea plant with a short pea plant. What kind of eggs and pollen are produced? What is the genotype of the baby plant? What is the phenotype of the baby plant?
- Next, he took these tall hybrids and bred them together. How many of these grandchild plants were tall? How many of these grandchild plants were short? Explain how it is possible for 2 tall pea plants to have a short baby.
- Why were Gregor Mendel’s experiments important?
- A brown eyed mom and a blue eyed dad have a blue eyed baby. What is the genotype of the baby? What is the genotype of the dad? What are two possible genotypes for the mom? Which genotype must she be to have a blue eyed baby? Explain why she must be this genotype.
This lesson was adapted from a lesson by Katie Ward of Aragon High School in San Mateo. After trying Katie’s version with my students, I found several other similar activities on the web. For instance, see the Making Babies lab written by Kevin Hartzog of Thurgood Marshall Academic High School. Kevin Hartzog's website Stars and Seas is extraordinary! With lots of other great lessons and ideas. Also recommended is this Making Babies lab from Friends Academy .
For information about Mendelian genetics, see:
For more information on the inheritance of human traits such as eye color, hair color, and tongue rolling, see:
2. A typical cell of any organism contains genetic instructions that specify its traits. Those traits may be modified by environmental influences. As a basis for understanding this concept:
b. Students know sexual reproduction produces offspring that inherit half their genes from each parent.
c. Students know an inherited trait can be determined by one or more genes.
d. Students know plant and animal cells contain many thousands of different genes and typically have two copies of every gene. The two copies (or alleles) of the gene may or may not be identical, and one may be dominant in determining the phenotype while the other is recessive.
2. Mutation and sexual reproduction lead to genetic variation in a population. As a basis for understanding this concept:
a. Students know meiosis is an early step in sexual reproduction in which the pairs of chromosomes separate and segregate randomly during cell division to produce gametes containing one chromosome of each type.
b. Students know only certain cells in a multicellular organism undergo meiosis.
c. Students know how random chromosome segregation explains the probability that a particular allele will be in a gamete.
d. Students know new combinations of alleles may be generated in a zygote through the fusion of male and female gametes (fertilization).
e. Students know why approximately half of an individual's DNA sequence comes from each parent.
f. Students know the role of chromosomes in determining an individual's sex.
g. Students know how to predict possible combinations of alleles in a zygote from the genetic makeup of the parents.
3. A multicellular organism develops from a single zygote, and its phenotype depends on its genotype, which is established at fertilization. As a basis for understanding this concept:
a. Students know how to predict the probable outcome of phenotypes in a genetic cross from the genotypes of the parents and mode of inheritance (autosomal or X-linked, dominant or recessive).
b. Students know the genetic basis for Mendel's laws of segregation and independent assortment.
c. * Students know how to predict the probable mode of inheritance from a pedigree diagram showing phenotypes.