PREDICTING INHERITED TRAITS & PUNNETT SQUARE ANALYSIS

Transcription

1 PREDICTING INHERITED TRAITS & PUNNETT SQUARE ANALYSIS

2 GENETICS TERMS AND VOCABULARY DNA- Deoxyribonucleic acid. It is the molecule that codes for our traits. CHROMOSOME - A structure found in the nucleus of a cell. It consists of DNA and proteins. A chromosome contains smaller segments called GENES. GENE- A segment of a chromosome that determines a particular trait of an organism by coding for specific proteins. GAMETE- and sperm cells (sex cells). They have half the chromosomes compared do other cells in the body (23 individual chromosomes for humans). SOMATIC CELLS- Cells in the body other than egg or sperm. They have a full set of DNA. For humans this is 23 pairs of chromosomes for a total of 46 chromosomes. ZYGOTE- A fertilized egg. When a sperm and egg unite a zygote is formed that eventually develops into a mature organism. Zygotes will have a two sets of chromosomes (in humans this is 23 pairs or 46 total), one set from the mother and one from the father. ALLELES- Different versions of the same gene. We have two alleles for each gene, one is inherited from the mother and the other is inherited from the father. PHENOTYPE- a person or organism s characteristics or traits. A person s phenotype can be influenced by their genetic make-up (genotype) as well as the environment. GENOTYPE- this is a person s or organism s genetic make-up. The genotype is indicated by the set of alleles. Knowing what versions of the gene one has or carries will help you determine their traits (phenotype). HETEROZYGOUS- having two different alleles for a given trait (the allele inherited from the mother is different from the allele inherited from the father). Note: in the above example Joe is heterozygous for tongue rolling (Rr), because the versions of the gene (alleles) are NOT identical) HOMOZYGOUS- having identical alleles for a trait (the allele inherited from the mother is identical to the allele inherited from the father). DOMINANT TRAIT- only one allele for the trait needs to be present for the trait to show up (be expressed) RECESSIVE TRAIT- both alleles for the trait must be present for the trait to show up (be expressed) 2

3 A Dog Called Spot Imagine this microscopic drama: a sperm cell from a male dog fertilizes an egg cell from the female dog. Each dog s gamete contain 39 chromosomes and the zygote (which will eventually develop into a puppy) will have a total of 78 chromosomes, one set from the mother and one from the father. Chromosome From Mother L h a T Chromosome From Father l H a t Two of the puppy s chromosomes are shown above. It is a homologous pair because each chromosomes contains alleles (versions of a gene) that code for the same traits. One of the chromosomes in the pair came from the mother and one came from the father. The only difference between the two is that one may have a dominant allele (capital letter) and the other a recessive allele (lower case) for a given trait. To have a dominant trait the puppy only needs to have one copy of the dominant allele. However, to have a recessive trait, they puppy must have both copies of the recessive allele. Using the chromosomes above, the chart below, and your vocabulary list to answer the following questions. TRAIT DOMINANT RECESSIVE Hair Length Short = L Long = l Hair Texture Wiry = T Silky = t Hair Curliness Curly = H Straight = h Coat Pattern Spotted = A Solid = a 3

4 1. What is the texture of the puppy s coat? How do you know? Explain. 2. What is the texture of the father s coat? How do you know? Explain. 3. What is the texture of the mother s coat? How do you know? Explain. 4. What is the pattern of the puppy s coat? How do you know? Explain. 5. Is the pattern of the puppy s coat the same as pattern of the parent s coat? How do you know? Explain. 6. Does either parent have curly hair? Which one(s)? How do you know? Explain. 7. List the traits for which the puppy is homozygous. Tell the genotype (letters) and the phenotype (the trait/appearance). 8. List the trait for which the puppy is heterozygous. Tell the genotype (letters) and the phenotype (trait). 4

5 9. Based on the information provided in this scenario, why can t you completely describe the puppy s parents even though you can accurately describe the puppy? 10. What are the chances that a puppy will have a spotted coat, if his mother is solid and his father is heterozygous for a spotted coat? Mother s Genotype: Father s Genotype: s Offspring/Puppy Possibilities: Genotype Probability (fraction) Probability (%) Phenotype 5

6 Practice with Punnett Squares Punnett squares will help you predict the chances or probability that a child (or offspring of any organism) will be born with a particular trait IF you know the genetic make-up of the parents. Example Punnet Square: What are the chances that the children of Joe (who is heterozygous for a widow s peak) and Sally (who does not have a widow s peak) will have a widow s peak hairline? 1. The first step in a punnett-square analysis is to determine what traits are associated with what genes. For this example you will look at the presence of a widow s peak hairline which is dominant over a straight hairline. You will need to assign allele codes for the alleles (if not already provided to you). Capital letters represent a dominant trait. For this example we can use: o W = allele for widow s peak hairline (dominant - only one allele needs to be present for trait to be expressed) o w = allele for straight hairline (recessive - both alleles must be present for trait to be expressed) Genotype (alleles present) WW The mother has a straight hairline so what would her genotype be: The father has a widow s peak and is heterozygous for the trait, therefore the father s genotype would be: Phenotype (trait that is present or expressed ) Widow s Peak Hairline (homozygous) Ww Widow s Peak Hairline (heterozygous) ww Straight Hairline (homozygous) 2. Parents pass on their traits to offspring (children) through their sex cells or gametes (sperm or egg). Because sex cells (gametes) will have half the genetic material of other cells, your next step will be to determine the genetic possibilities for sex cells. Joe s sperm: OR Sally s s: OR 6

7 3. Next you would create and complete the punnett-square. The top row represents the gamete (sex cell) possibilities for one parent, while the first column represents the gamete (sex cell) possibilities for the other parent. The interior of the punnett-squares show the genotype possibilities when the two sex cells unite and offspring or children are produced. Mother: ww Father: Ww ww ww Ww Ww 4. Once the punnett-square is completed you will need to determine the frequency of each trait (phenotype) based on the genotypes presented in the square. Create a table to do this. Offspring Genotype Phenotype (trait) Frequency Percent Ww widow s peak hairline 2/4 50% ww straight hairline 2/4 50% 5. Finish each problem by answering the original question. For any child that Joe and Sally have together, there is a 50% chance that the child will have a widow s peak and a 50% chance that the child will have a straight hairline 7

8 Practice Punnett-Square Problems 1. Cystic fibrosis (CF) is an inherited chronic disease that affects the lungs and digestive systems of about 30,000 children and adults in the United States. A defective gene causes the body to produce unusually thick, sticky mucus that clogs the lungs and leads to life-threatening lung infections and obstructs the pancreas. 50 years ago few children with cystic fibrosis lived to attend elementary school. Today many people with CF live into their 30 s and 40 s and beyond due to advances in treating the disease. Unfortunately however there is no cure for the disease. Cystic fibrosis caused by a single gene and is a recessive trait, therefore both copies of the gene must be present for the person to be affected. Jennifer and Tim are a young married couple planning a family. Tim s younger sister had CF and died before she finished high school. It is possible Tim carries the gene for CF. They have decided to have genetic tests before trying to conceive in order to determine whether they could have a child affected by the diseases. The couple received their test results and was devastated to find out that they are both carriers. Carriers are people who have one cystic fibrosis allele and one normal allele. They are not affected by the disease. What are the chances that a child born to Tim and Jennifer will have cystic fibrosis? Note: Use C for the healthy allele since it is dominant over c the cystic fibrosis allele. Tim: Jennifer: Offspring Genotype Phenotype (trait) Frequency Percent 8

9 2. Huntington's Disease is a devastating, degenerative brain disorder for which there is, at present, no effective treatment or cure. Huntington s slowly diminishes the affected individual's ability to walk, think, talk and reason. Eventually, the person with the disease becomes totally dependent upon others for his or her care. Signs of the disease don t usually show up until age 30 or 40. However, Huntington s disease is a dominant traitwhich means you only need to inherit one copy of the allele to have the disease. Sheila does not show any signs of having Huntington s disease right now. Sheila s father does have the disease (and is heterozygous ). Sheila s mother is healthy and does not carry the gene for Huntington s. What is Sheila s probability of having the gene Huntington s and therefore developing the disease later in life? Note: use H for the Huntington s allele & h = healthy allele. Mother: Father: Complete the chart below and finish with a one sentence written response to the original question. 9

10 3. Hypocholesterolemia is a disorder that causes excess cholesterol in the blood and heart disease. This disorder is caused by a dominant allele. If each of your parents were heterozygous for this condition, what are your chances of being healthy (NOT having hypocholesterolemia)? Mother: Father: 4. Ability to taste PTC is dominant over the inability to taste PTC. If your mother cannot taste PTC and your father is heterozygous for tasting. What are the chances that you WILL be able to taste PTC? Mother: Father: 10

11 Gregor Mendel was an Austrian monk who worked on inheritance during the 1800 s. Although at the time nothing was known of DNA or genes, Mendel s work on garden peas laid the foundation for understanding patterns of inheritance. In fact the type of genetics we are studying in this packet is called Mendelian Genetics. Please read chapter 11-1 pages before and answer the following questions. 1. True-breeding plants were the basis of Mendel s experiments. (a) Please define truebreeding plants? (b) What are hybrid plants? 2. Why weren t the hybrid plants simply a blend of the characteristics of the parent plants? 3. Please understand the concepts of: P-generation plants, F1 generation plants, and F2 generation plants. 4. When a true bred tall plant was crossed with a true bred short plant the offspring (F1) were all tall. Explain why these offspring were all tall rather than a blend? 5. (a) When the F1 plants (all tall) were crossed with each other what were the results in the F2 generation? (b) Please explain these results in terms of segregation of alleles and gamete formation? 11

12 Read pages (11-2) and the material presented earlier in this packet to answer each of the following questions (use the chart below and construct a punnett-square for each question) 1. What would the offspring phenotype and genotype probabilities be when a plant with constricted seed pods crossed with a plant that was heterozygous for inflated seed pods? Parent 1: Parent 2: 2. What would the offspring genotype and phenotype probabilities be when a white flowered plant crossed with a plant that was homozygous for purple flowers. Parent 1: A P p a Y y Parent 2: R r I i G g T t 12

13 Two-trait Crosses Predicting traits can get more complicated when you are examining more than one trait at a time. Please read pages and answer the following questions in preparation to perform punnett-square analysis for more than one trait at a time. 1. Explain what happened when Mendel crossed true-bred round & yellow seeded plants (genotype: RRYY) with true-bred wrinkled & green seeded plants (genotype: rryy)? 2. What did the F2 generation look like when Mendel crossed the F1 generation (genotype: RrYy) with one another? 3. Explain the principle of independent assortment. Punnett-Squares: practice with two-trait crosses 4. What happens when you cross a plant that has green & wrinkled seeds with a plant that is heterozygous for yellow seeds and homozygous for round seeds? What are the genotype and phenotype possibilities for the offspring? a) What is the genotype of a plant that has green and wrinkled seeds? b) What is the genotype of a plant that is heterozygous for yellow seeds and homozygous for round seeds? c) For the plant described in (a), what are the game possibilities for that plant? d) For the plant described in (b), what are the gamete possibilities? 13

14 #4 continued Complete the punnett-square. Parent 1: Parent 2: What are the offspring genotype and phenotype probabilities? (Construct a chart below). 5. Describe the offspring genotype and phenotype possibilities when you cross a plant that has white flowers and is heterozygous for a tall stem (parent 1) with a heterozygous purple plant that is heterozygous for a tall stem (parent 2). Parent 1 GENOTYPE: Parent 1 GAMETE Possbilities: Parent 2 GENOTYPE: Parent 2 GAMETE Possibilities: Parent 1: Parent 2: 14

15 A new example. 6. Suppose that the gene that determines one s ability to roll thier tongue is on one chromosome nd the ability to taste a certain bitter chemical called PTC is on another chromosome. Both tasting and rolling are dominant. Determine the probability of the different genotypes and phenotypes for the offspring when the mom is Ttrr and the dad is ttrr? T = allele for t = allele for R = allele for r = allele for MOM s genotype: DAD s genotype: MOM s phenotype: DAD s phenotype: MOM s gamete (egg) possiblities: DAD s gamete (sperm) possibilities: Mother: Father: 15