What is on our Genes?

A gene is a piece of information passed from parents to offspring, and genes often are in different forms called alleles. For example, the gene for pea plant height has two alleles, tall and short.


Genes

As we saw in the previous chapter, every gene on a chromosome in our DNA makes a different protein. There are 20,500 of these genes in every human. Almost every single one of these genes are the same from person to person, which means that the vast majority of these genes contain information for our lungs, heart, liver, kidneys, bones, brain and more. There is only a handful of genes that contain information for the color of our skin, hair, eyes, and the shapes of our faces, hands and feet.

Allele from mother Allele from father Gene that turns into a protein
Dominant Dominant Dominant
Dominant Recessive Dominant
Recessive Dominant Dominant
Recessive Recessive Recessive

A Punnett square showing a cross between two pea plant

Since we have two copies of (almost) every gene in our body, we call these copies alleles. We get one allele from our father and one from our mother. Since only one of those alleles can turn into a protein, it is the more dominant allele that gets turned into a protein by our cells. If both alleles are dominant, then it is clear that the dominant protein is made. If one allele is dominant, then the other trait, the recessive trait, is ignored and the dominant protein is made. Only if both alleles are recessive then the recessive protein is made.

In order to try and figure out what the chances are of having a child with a dominant or recessive trait for a particular gene, something can be done called a Punnett square. A Punnett square is used to predict the probabilities and possibilities of traits in offspring. Along the top of the Punnett square, the alleles that could come from the father are listed, and the alleles from the mother are listed along the side. In the middle of the Punnett square, the possibilities for offspring are listed. Each square represents a 25% chance of getting that type of offspring. If an offspring has two of the same allele, it is called homozygous. If it has two different alleles, it is called heterozygous.

Questions
Remember
1. What does dominant mean? Recessive?
2. Define, in your own words:
a) Homozygous:
b) Heterozygous:
3. In terms of homozygous, heterozygous, dominant and recessive, label:
a) HH –
b) Mm –
c) bb –
Put it together
4. Give names and percentages for the following Punnett square:
Rr
RRRRr
rRrrr

GenotypeHomozygous or heterozygous AND Dominant or recessive%
Homozygous Dominant
Rr
25%

5. As you did above, complete the Punnett square and percentages for the following:
a) Between homozygous dominant for round peas and heterozygous
b) Between homozygous dominant for unattached earlobes and homozygous recessive for attached earlobes
c) Between homozygous recessive for green eyes and heterozygous
Think about it
6. With a partner, agree on a gene for making a Punnett square. Also, agree on the dominant and recessive alleles. Independently, come up with the genotype of both parents (one of you should be the mother, the other the father), then create a Punnett square for the combination of the two parents. If these two parents have six children, what would be the most likely numbers of dominant and recessive traits in the children?
Review
7. Define biological diversity in your own words.
8. In which cells does mitosis happen?
9. What is the difference between a cell that is haploid and a cell that is diploid?
10. What is a gene?
Alleles
  1. With the genetic model kit, put together a cell that has three pairs of homologous chromosomes. Each chromosome has three shapes on it, in three different colors: red, white and blue. Each one of those shapes represents one allele. When you join a chromosome with its partner, then the matching alleles form genes. Therefore, it takes two alleles (that can be dominant or recessive) to get one gene.
  2. How many alleles does this cell have? How many genes does it have? How many total chromosomes does it have?
  3. Describe the relationship between genes, alleles and chromosomes, in your own words.
  4. Design your own bacteria:
    1. What is it called?
    2. What does it do?
    3. Choose one of the pairs of chromosomes. For each gene, make up a characteristic that this gene represents – this is the dominant trait. For each gene, also specify the recessive trait.
Karyotype Puzzle

Errors can occur during meiosis as an organism creates gametes. Sometimes, extra chromosomes are copied and other times they are deleted. This means that the organism can have too many or too few chromosomes and usually will not make it to birth. Other times, the organism is born with physical or mental differences. In this activity, you will be given the chromosomes of an individual and it is up to you to discover what the genetic disorder is.

  1. Get a packet of chromosomes and a karyotype reference sheet. Order the chromosomes by size, remembering that the Y chromosome (if present) is smaller than the X chromosome.
  2. Once you have a complete karyotype that matches one of the karyotypes in the reference sheet, identify the karyotype and call the teacher over.
  3. What effects do you think that this disorder has on the individual? Make a hypothesis based on what you know about genetics and chromosomes.
  4. Use the reference materials available to research the disorder. What effects does this disorder have on the individual?
Making Genes
  1. Ask a friend or family member for a genetic trait that they are familiar with. Write this trait down and label it the dominant trait.
  2. Figure out what the opposite (recessive) trait would be and write it down as the recessive trait.
  3. Make a random “allele” of DNA that is five codons long. Mark off each codon with a bracket or line. This DNA will represent the dominant trait.
  4. Change one of the codons of DNA from the dominant trait. Mark off each codon as with #4. This DNA will represent the recessive trait.
  5. What is the first letter of the name of the dominant trait (from #1)? This is the letter you will use for step #6.
  6. Using this letter (from #5), combine someone who is homozygous recessive for your chosen trait with someone who is heterozygous. Show the Punnett’s Square.
  7. Show the percentages for each of the genotypes that result from the Punnett’s Square in #6. For each different genotype in #7:
  • Write the genotype, and next to it, the DNA that gets expressed (from either the dominant or recessive allele).
  • Below the DNA, write the corresponding mRNA.
Family Chromosomes

Choose two family members. For these two family members and yourself, you will draw a chromosome map for 5 characteristics. Each chromosome map should show where the characteristic is on the chromosome and what the trait is. Make sure to keep the traits in the same place for each of the three chromosomes.

Human Blood Types

Antigens are molecules that can trigger an immune reaction in the body. Antigens can be found on the surface of red blood cells. Human blood type A has the A antigen. Blood type B has the B antigen. Type AB has both antigens. Human blood type O carries neither antigen. The gene for blood type antigens has three alleles—A, B, and O.

Sometimes a patient is given blood during surgery or after a serious injury. The donated blood must not introduce a new antigen into the body. Otherwise, the patient’s body will respond to the donated blood just as it would to a pathogen. So a person with type A blood can be given type O, but a person with type O blood cannot be given type A.

Human red blood cells may also carry another antigen, known as Rh factor. Rh+ individuals carry this antigen. Rh– patients do not. Blood with a positive Rh factor can only be given to patients with a positive Rh factor. Rh– can be given to patients with a positive or negative Rh factor. The circle graph below shows the percentage of each blood type in the U.S. population.

bloodtypes

Analyze and Conclude

  1. People with blood type O− are sometimes called universal donors. Why?
  2. What blood type would a universal acceptor—a person who can receive any blood type—have?
  3. Study the circle graph. Which blood type is most common in the U.S. population? What percentage of the population has that blood type?
  4. Find all of the parts of the circle graph that represent a positive Rh factor. What percentage of the U.S. population has a positive Rh factor?
  5. What percentage of the U.S. population has a negative Rh factor?
  6. Which blood type can be donated to the largest percentage of people? Which type can be donated to the smallest percentage of people? Explain your answer.

Build Science Skills

Blood types A and B are codominant. O is recessive. Rh factor is determined by a different gene. The allele for a positive Rh factor is dominant over the allele for a negative Rh factor. Based on this information, could a person with O+ blood have two parents with O− blood?

Calculating Haploid and Diploid Numbers

One characteristic shared among members of a species is the number of chromosomes in its body cells. Because chromosomes in body cells exist as pairs, this number is referred to as the diploid number. It is represented by the term 2N, which means “two times N.” The haploid number, N, represents the chromosomes that contain a full, single set of genes.

The number of chromosomes varies widely among organisms. The record for the smallest chromosome number goes to a subspecies of the ant Myrmecia pilosula. The females have just a single pair of chromosomes (2N = 2, N = 1). The male ants have only a single chromosome. The record for the largest chromosome number goes to the fern Ophioglossum reticulatum. It has 630 pairs of chromosomes or 1260 chromosomes per body cell.

If you know either the haploid or diploid number, you can calculate the other. For example, if the haploid number (N) is 3, the diploid number (2N) is 2 × 3, or 6. If the diploid number (2N) is 12, the haploid number (N) is 12/2, or 6. The table shows haploid or diploid numbers for several types of organisms. Complete the table. Then use it to answer the Analyze and Conclude questions.

Trait Survey

Organism

Haploid Number

Diploid Number

Amoeba

N = 25

Chimpanzee

N = 24

Earthworm

N = 18

Fern

2N = 1010

Hamster

N = 22

Human

2N = 46

Onion

2N = 16

Analyze and Conclude

  1. Which organism in the table has the most chromosomes? Which has the fewest?
  2. Which cells in a body will have the haploid number of chromosomes?
  3. How many chromosomes are in a chimpanzee’s body cells? How many chromosomes are in a chimpanzee’s gametes?
  4. Why is a diploid number always even?
  5. If you were asked to organize the data from this table into a graph, which kind of graph would you choose? Explain your choice.

Build Science Skills

Compare the number of chromosomes for the different types of organisms in the table. What can you conclude about the complexity of an organism and the number of chromosomes it has? Hint: Are there any single-celled organisms in the table?

Genetically Modified Crops in the United States

Crops with genetically modified (GM) traits were first introduced in 1996. Some varieties of soybean, cotton, and corn have been modified to survive the use of herbicides. Some varieties of cotton and corn have been modified to be more resistant to insect pests. The graph shows data on the use of different modified crops in the United States between 1996 and 2007. In the legend, “HT” stands for herbicide tolerance, and “Bt” stands for insect resistance.

gmcrops

Analyze and Conclude

  1. Which two GM crops were widely and rapidly adopted by U.S. farmers? Which crop had only a slight increase?
  2. Compare the percentage of soybeans that had the HT trait in 1996 to the percentage in 2007.
  3. Compare the percentage of corn that had the HT trait in 1996 to the percentage in 2007.
  4. Add up the percentages of acres of GM cotton crops in 2007. Explain your result.
  5. Do you think that the percentages of HT soybeans and HT corn will continue to rise over the next several years? Use the graph to support your prediction.

Build Science Skills

Why is it important for farmers to have plants that are modified to contain an HT trait, a Bt trait, or both traits? How do these traits help crop yields?