208 Understanding Heredity one example We ve learned that DNA affects how our bodies work, and we have learned how DNA is passed from generation to generation. Now we ll see how small DNA differences, even as small as ONE BASE of your three billion base DNA sequence, can cause big problems as well. In this case, we ll see what happens when one particular base on Chromosome 11 differs. The result of the difference can be a medical condition called Sickle Cell Disease. See if you can figure out how genetic traits get inherited from the following scenario about a family with a history of Sickle Cell Disease. After you read the scenario, draw a family tree, called a pedigree, to try to understand the pattern of inheritance. A pedigree uses the following basic symbols: Sharon and George are healthy adults with no history of Sickle Cell Disease. Their daughter Tracy is healthy. Their son Robert is healthy. Their third child, Michael, has SCD. George thinks this has something to do with him, because his sister, Monique, suffers from SCD. George and Monique s parents, Linda and Ron, do not have SCD. Sharon has no history of SCD in her family. Sharon s parents Ruth and Franklin do not have SCD. Ruth has never heard of anyone in her family having SCD. Franklin has heard stories of a great-aunt of his who died from SCD many years ago. So what are the rules for inheritance of Sickle Cell Disease? See if you can figure out why certain people in this family have it and certain people don t. Begin by drawing the pedigree. If you need help, look on the back for some ideas to help you figure this out:
209 1. There are two competing versions of the hemoglobin gene in this family: Normal hemoglobin and Sickle Cell Hemoglobin. 2. Remember that everyone carries TWO copies of the hemoglobin gene. One copy is on the Chromosome 11 inherited from their mother, and one copy is on the Chromosome 11 inherited from their father. a. If you think someone has two normal hemoglobin genes, label them NN. b. If you think someone has one normal hemoglobin gene and one sickle cell gene, label them NS. c. If you think someone has two sickle cell genes, label them SS. 3. When someone has a child, only ONE copy of their hemoglobin gene goes into each sperm cell or egg cell they produce. a. If your two copies of the hemoglobin gene are the same (both normal or both sickle cell) then obviously you pass along one of those same copies to each child. Since you have two of the same copy of that gene, you are called homozygous for that gene. b. If your two copies of the hemoglobin gene are different (you inherited one normal copy and one sickle cell copy from your parents) then you have a 50/50 chance of passing along either copy to each child. Since you have two different copies of that gene, you are called heterozygous for that gene. So...who in this family is NN? Who is NS? Who is SS? Below, we ll draw something called a Punnett Square to examine the kinds of kids Sharon and George can expect to have:
210 Understanding Heredity more examples We ll look at a few more examples of how genes can get inherited. Albinism is a group of conditions that affect coloring (pigmentation) of the skin, hair, and eyes. Affected individuals typically have very fair skin and white or light-colored hair. They have an increased risk of skin damage and skin cancers, including melanoma, with sun exposure. The most common cause of albinism is a large deletion in one gene on Chromosome 15. See if you can figure out how albinism is inherited from this family scenario. Joshua and Bella have a son named Luke. Luke has been diagnosed with albinism. Joshua and Bella have normal skin pigment. Bella s parents, Sam and Susan, both have normal skin pigment. Joshua s parents, David and Elizabeth, both have normal skin pigment. Joshua s sister, Sara, has albinism. 1. Begin by drawing the pedigree for this family. 2. Remember that for this particular situation, the particular gene on Chromosome 15 can either be normal (N) or a. 3. Everyone has two copies of this gene. a. If someone has two normal genes (NN), they have normal skin pigment. b. If someone has two a genes (aa), they have albinism. c. If someone has one normal and one a gene (Na), they also have normal skin pigment because normal is dominant and a is recessive. 4. Finally, draw a Punnett Square to show the kinds of kids Joshua and Bella can expect to have.
Lest you think all of genetics is about diseases and disorders, here are some examples of inheritance that have nothing to do with disorders or disease. 1. Tongue Rolling Can you roll your tongue (curl your tongue into a tube shape)? If you can, you are a tongueroller. The tongue rolling gene is dominant. Let s call it R. The non-tongue-rolling gene is recessive. Let s call it n Draw a Punnett Square to show how two rollers could have a child who can not roll her tongue. 211 The tongue rolling gene has not yet been mapped to a particular chromosome. Why not, do you think? By the way the rules for tongue-rolling inheritance are full of exceptions unfortunately, true inheritance is often much more complex than we would like! 2. Ear lobes Your earlobes can be free or attached. Free earlobes are dominant. Let s call this gene F. Attached earlobes are recessive. Let s call this gene a. What happens if two parents, both heterozygous for earlobe genes, have children? What might they expect?
212 3. Blood types Your blood type is controlled by genes. Other examples have shown that there are two versions of a gene. For example, in the case of hemoglobin, we saw there was a normal version and a sickle cell version. In the case of blood types, there are three versions of the gene. Different versions of a gene are called alleles of a gene. One version makes A proteins on your red blood cells. Let s call this gene A. One version makes B proteins on your red blood cells. Let s call this gene B. One version makes no proteins on your red blood cells. Let s call this gene O. A is dominant to O. So, if a person gets an A gene and an O gene, what blood type will they have? B is dominant to O. So, if a person gets a B gene and an O gene, what blood type will they have? A and B are co-dominant. So, if a person gets an A gene and a B gene, what blood type will they have? How can a person have type O blood? What genes would they need to inherit? Draw a Punnett Square that shows how two parents might have four different children all with different blood types.
213 4. Curly and straight hair There is a gene that controls hair straightness. Curly hair is encoded by a dominant allele, C. Straight hair is encoded by a recessive allele, s. Brad, who has curly hair, and Sandi, who has straight hair, have a child. If Brad's father had straight hair, what are the odds that Brad and Sandi's child will have curly hair? 5. Puppies!! In Labrador retrievers, fur color is determined by a gene. The allele for dark fur (D) is dominant over the allele for yellow fur (y). Construct a Punnett square for a cross between two Labs that are both heterozygous for dark fur (Dy). If a yellow Lab were crossed with a heterozygous dark Lab (Dy), how many yellow labs would you expect to find if they had a litter of eight puppies?
214 6. Just for fun a bad hair day The genes that control hair color are not as straightforward as some other genes. But here s a scenario for fun that will test your understanding of inheritance. The earth has passed through a radiation cloud, producing a variety of mutations. One of these results in a new set of hair colors: the allele for purple hair (P) is dominant over that for green hair (G), which in turn is dominant over that for orange hair (o). Bandar, a man who has purple hair, marries Clarinda, a woman who has green hair. Clarinda's mother's hair was a bright shade of orange. Bandar s father s hair was also orange. What can be predicted about the hair color of Bandar and Clarinda s child?
215 7. Challenge Question - Achondroplasia Shown below is a pedigree chart for the inheritance of achondroplasia (ay-kondruh-play-zhuh), a form of dwarfism. One form of achondroplasia can be caused by a mutation of a gene on Chromosome 4. Examine the pedigree chart, and answer the following questions. Is the gene that causes this form of dwarfism a recessive or dominant trait? How do you know? Using (D) to represent the dominant allele and (d) to represent the recessive allele, write the genotypes of the indicated individuals http://serendip.brynmawr.edu/sci_edu/waldron/genetics.html