Download BIOL100 Laboratory Assignment 5: Genetics Name: Part A: Genes

BIOL100 Laboratory Assignment 5: Genetics Name: ____________________________________________ Part A: Genes are regions of chromosomes that code for a specific product, usually a protein. In diploid organisms, each cell contains two copies of each chromosome (except for in the case of the sex chromosomes) and therefore two copies of each gene. Not all genes are the same and types of the same gene that are slightly different are termed alleles. Different alleles are usually written “short‐hand” using a system of lower and upper case letters. For instance, the different alleles of the gene for finger hair might be written as an “H” for the allele that leads to finger hair and as an “h” for the no finger hair allele. Capital letters are usually assigned to an allele that is dominant. Being a dominant allele means that if a person carries that allele then they will display the trait that is coded for by that allele. Alternatively, lower case letters are assigned to traits that are recessive. Recessive traits are only displayed by an organism if that organism does not carry any copies of the dominant allele. Using the example above, we can write out potential allele combinations that a person might carry for the finger hair gene. Remember, a person will carry two copies (alleles) of each gene because we have two copies of each of our chromosomes. Genotype HH Homozygous dominant Hh Heterozygous Hh Homozygous recessive Above, the potential allele combinations for the finger hair gene are written. These are termed genotypes, because they indicate the type of gene alleles that a person could be carrying. If both of the alleles that a person carries are the same, that genotype is referred to as homozygous (i.e. HH and hh are both homozygous genotypes). If the two alleles are different than an individual would be heterozygous for that gene (i.e. Hh). Using the information from above on which trait is dominant and which trait is recessive, we could also assign to each of these genotypes a phenotype. A phenotype is the trait that an organism with a given genotype would display. In the above example we would have the following phenotypes. Note that if a person displays the recessive phenotype, you also know that they are homozygous recessive in their genotype, but that if they display the dominant genotypes you cannot determine whether they are homozygous dominant or heterozygous without getting additional information. Genotype Phenotype HH Finger hair Hh Finger hair Hh No finger hair 1
Go through the following list and determine what your phenotype is for each trait. Do you display the dominant or the recessive phenotype? Your Dominant or Trait Phenotype Recessive? 1. Finger hair—The presence of finger hair on the middle segment of your fingers is determined by a single dominant gene. 2. Tongue rolling—The ability to roll your tongue into a “U” shape is controlled by a single dominant gene. 3. Tasting Phenyl‐thio‐carbamide (PTC)—The ability to taste PTC is controlled by a single dominant gene. There is PTC paper available at your tables. Touch a small piece of the paper to your tongue (it is harmless); if you can taste PTC you will be able to taste a strong bitter taste. If you are unable to taste PTC you will not taste anything. Approximately 70% of the population can taste PTC. 4. Widow’s Peak—This refers to the formation of a point in your hairline at the center of your forehead. Widow’s peak (i.e. having a point) is determined by a single dominant gene. 5. Free Ear Lobe—Free ear lobes are dominant over attached ear lobes. 6. Darwin’s Ear Point—Feel along the upper curved part of your ear. If you can feel a thickening in the cartilage there, then you have Darwin’s ear point (it can be present in just one ear). This is a dominant trait. 7. Cleft Chin—Having a cleft chin (a distinct indentation in the center of the chin) is a dominant trait. 8. Hitchhiker’s thumb—Bend your thumb back as far as you can (without pulling on it or causing any pain). If your thumb can naturally bend backward further than vertical (hyper extensibility), then you have a hitchhiker’s thumb. This is a recessive trait. 9. Interlocking fingers—Shake your hands and then, without over thinking, clasp your hands together interlocking your fingers. Which of your thumbs is on top? Having the left thumb on top when your hands are clasped is controlled by a single dominant gene. Part B: Traits such as those listed above are often used to reconstruct family trees, determine familial relationships, or determine genotypes. In animals and plants, traits such as these can be used to determine genotype so that breeding can be selectively carried out, for instance to create dogs with a desired type of fur or flowers with a specific color. Often, genetics problems can be most easily solved through the use of a Punnett Square. A Punnett Square is a way of graphically depicting the separation of alleles from each parent (remember, only one copy of each chromosome is contained within a sperm or egg and so each parent is only giving one copy to their offspring). By completing the Punnett square, you are depicting the potential genotypes of zygotes and also gaining information about the likelihood of different potential offspring genotypes and phenotypes. 2
As an example: Two individuals, a female with finger hair and a male without finger hair are trying to have a child. What is the likelihood that they have a child with finger hair? Note, the female had a father who did not have finger hair. The first step is to determine the genotypes of the male and female: Because figure hair is dominant and the male doesn’t have finger hair, he must be homozygous recessive with a genotype of “ff.” The female has finger hair, so she must have at least one “F” but her father was homozygous recessive (ff) and she had to get one of her alleles from her father so she must also have an “f.” This means that she is heterozygous for finger hair (Ff). The second step is to set up a Punnett Square: Female potential gamete alleles: F f f Male potential gamete alleles: f Complete the Punnett Square by filling in the alleles from the female and male down and across, respectively, to indicate the potential zygote genotypes. Female potential gamete alleles: F f f Ff ff Male potential gamete alleles: Ff ff f The final step is to determine the ratios of these different genotypes and their respective phenotypes. This couple has a 50% chance of having a child with an Ff genotypes and 50% chance of having a child with an ff genotype. The phenotype of Ff is finger hair and the phenotype of ff is no finger hair. Therefore, the couple has a 50% chance of having a child with finger hair. 3
Complete the following genetics problems. These problems will help you develop your skills in the process of solving these types of problems. Some new terms will also be introduced as you work through several of the problems below. Make sure that you show the work that you do to solve each problem. This will usually involve making a Punnett Square. 1. In corn, the gene for high sugar production (s) is recessive to starch production (S). What are the phenotypes for the following individuals: a) ss b) Ss c) SS 2. If a high sugar corn plant is crossed with a heterozygous starchy plant, what is the likelihood that the seeds produced with be for a high sugar plant? 3. In dogs, long fur (f) is recessive to short fur (F). If a long furred female is crossed with a homozygous short furred male, a) what are the potential genotypes of their puppies? b) what are the potential phenotypes of their puppies? 4. In cats, having six toes is controlled by a single dominant gene. A six‐toed heterozygous cat is crossed with a normal toed cat. What is the likelihood that their kittens will be born with 6 toes? 4
5. Sickle cell anemia is a recessive trait. If two individuals who carry the allele for sickle cell anemia have children, a) what is the likelihood that the child will have sickle cell anemia? b) what is the likelihood that their child will be a carrier for sickle cell anemia? 6. In some flowers, pigments show incomplete dominance. In a trait that has incomplete dominance, the homozygous dominant of a trait will show a strong phenotype (red flowers), the homozygous recessive will have a weak trait phenotype (white flowers), and the heterozygote will display an trait phenotype that is intermediate (pink flowers). a) If you cross two pink flowers (Rr x Rr), what are the likely genotypic ratios of the offspring? b) If you cross two pink flowers, what are the likely phenotypic ratios of the offspring? c) What are the likely ratios in phenotypes produced when a red flower and a white flowered individual are crossed? 5
7. Some alleles are co‐dominant. In co‐dominant alleles, the phenotype of both alleles are expressed. A common example of co‐dominance is blood type. The alleles for blood type are as follows: IA, IB, and i. An individual with type A blood (a phenotype) could have a genotype of either IA IA or IAi. An individual with type B blood could have a genotype of either IB IB or IBi. An individual with type AB blood has the genotype IAIB and an individual with type O blood has the genotype ii. a) If a male with type AB blood marries a woman with type A blood (and her father had type O blood), what are the possible blood types of their children? b) If two people, both with type AB blood have children, what are the possible blood types of their children? c) If two people, both with type AB blood have children, what is the possibility that they have a child with type O blood? 8. Some alleles are sex‐linked. Sex‐linked alleles occur on the X chromosome only, so human males (XY) only have one copy of these alleles. This means that they will display the dominant phenotype if their one copy of the allele is the dominant allele and they will display the recessive trait if their one copy of the allele is the recessive allele. Hemophilia is a sex‐linked recessive phenotype. If a woman who is a carrier for hemophilia (this means that she "carries" a copy of the allele but that she doesn't display the phenotype) marries a man who does not have hemophilia, a) what are the chances that, if they have a male child, that the child will have hemophilia? 6
b) what is that likelihood that a child of theirs of any sex will have hemophilia? 9. All of the Punnett Square crosses that you have completed above have been monohybrid crosses (only dealing with a single gene). Of course, with sexual reproduction there are many many genes that are "crossing" with any breeding event. The following example will involve two genes and is called a dihybrid cross. Remember, the chromosomes independently sort in Metaphase 1, so with two genes you will have four possible allele combinations for each parent (a 4 x 4 Punnett Square). In rabbits, there is a gene for short hair (H) that is dominant over the gene for long hair (h). There is also a gene for a Brown coat color (B) that is dominant over black coat color (b). a) Complete a dihybrid Punnett Square for the crossing of two heterozygous short‐haired, brown coated rabbits. b) What is the likelihood that the cross will result in a long haired, black coated rabbit? c) What is the likelihood that a rabbit will be born that has the same genotype as the parents? 7