2. Making Babies

Summary
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?

Objectives
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.

Vocabulary
Trait
Phenotype
Genotype
Gene
Allele
Dominant
Recessive
Incomplete dominance
Homozygous
Heterozygous
Gamete
Zygote

Attachment Size
babies_handout.doc 60.5 KB
2making_babies.doc 61 KB

2. Making Babies - Logistics

Time
50 minutes

Grouping
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.

Materials

  • Completed “Human Traits Surveys” (download from the bottom of the Human Trait lesson)
  • Copies of the “Making Babies” handout
  • Pennies
  • Colored pencils

Setting
Classroom

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.

2. Making Babies - Getting Ready

Getting Ready

  1. Make copies of the “Making Babies” handout.
  2. Set out pennies and colored pencils.

2. Making Babies - Lesson Plan

Lesson Plan

  1. Make sure each student has their “Human Traits Survey” completed and available.
  2. Pass out the “Making Babies” handout and assign partners for the activity.
  3. Read through the first page together, answering any questions students may have. Work through a couple examples of going from genotype to phenotype and back again with different traits. “If you cannot roll your tongue, what is your genotype?” “If your genotype is Rr for tongue rolling, what is your phenotype?”
  4. Help students fill in the table on page 2 with their phenotype and genotype. If the student has the dominant trait, assume that they are heterozygous unless they know for SURE that no relative has the recessive trait.
  5. Next, make gametes. Between the partners, one should make sperm, the other should make eggs. Each person will make 2 gametes by flipping a coin for each gene. We are assuming that the genes are not linked (even though hair color and eye color genes are, in fact, linked).
  6. Finally, students can create 2 babies by combining sperm #1 with egg #1 and combining sperm #2 with egg #2. Beside the genotype/phenotype descriptions, students should draw a picture of their children in color as they would appear in middle school.

2. Making Babies - Assessment

Assessment

  1. There are conclusion questions at the end of the handout.
  2. For homework, I assigned my students several pages from the book The Cartoon Guide to Genetics, by Lary Gonick and Mark Wheelis. Pages 37-55 deal with Gregor Mendel and inheritance patterns. The questions I asked include:
  • 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.

Going Further

  1. Try the Dragon Genetics project.
  2. Have students model the mitosis and meiosis. My favorite mitosis/meiosis lesson is written by the Biology Lessons site from San Diego State University. The mitosis activity models the stages of mitosis with plastic utensils as chromosomes. Continue using the plastic utensil chromosomes to model meiosis OR you can add in the idea of crossing over by switching to modeling clay and yarn (see crossing over activity). Download the handout at the bottom of this page for an outline of the mitosis and meiosis process on which I had my students take notes.
  3. Make plant babies! Raise Wisconsin Fast Plants in the classroom and cross-fertilize different strains (hairy x hairless or tall x short). Study the inheritance of plant traits over several generations. For information on how to grow Fast Plants in the classroom, see the Raising Plants project in the Physiology Box.

2. Making Babies - Sources and Standards

Sources
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:

  • The Wikipedia articles on Gregor Mendel and Mendelian inheritance are both excellent short summaries.
  • An excellent genetics tutorial is available from Dr. Dennis O’Neil of Palomar College.
  • MIT has a good online genetics textbook chapter targeted for college level genetics courses.
  • The Biology Project at the University of Arizona also had a good series of online genetics resources and tutorials.
  • To go right to the source, Mendel’s original 1865 article with reference materials and commentary from classroom teachers can be found at MendelWeb.

For more information on the inheritance of human traits such as eye color, hair color, and tongue rolling, see:

  • The Eye Color Calculator from the Tech Museum of Innovations’ online genetics exhibit is very fun with good genetics background information. In addition, their “Ask a geneticist” area contains great Q&A about other human traits such as hair color, addiction, ADHD, genetic diseases and more. Do your students have a question that you can’t answer? Post a question!
  • For a searchable database of human genes and relevant scientific studies, John Hopkins has a fantastic resource, the Online Mendelian Inheritance in Man website.
  • The Wikipedia article on dominance relationships including a short list of traits governed by simple dominance.
  • Bill Kendrick created a very fun “Gene Machine” that predicts your genotype based on inputting the phenotypes of yourself, your mother and your father.

Standards
Grade 7
Genetics
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.

Grades 9-12
Genetics
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.