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PREDICTING INHERITED TRAITS & PUNNETT SQUARE ANALYSIS Learning Objectives: 1. Students will know and apply the following vocabulary terms: DNA, chromosome, gene, trait, allele, dominant, recessive, genotype, phenotype, homologous chromosomes, homozygous, heterozygous, gamete, zygote, somatic cell, carrier. 2. Students will predict the outcome of genetic crosses involving one characteristic (single trait crosses). 3. Students will predict the outcome of genetic crosses involving two characteristics (2trait crosses). Dog Called Spot Imagine this microscopic drama: a sperm cell from a male dog fertilizes an egg cell from the female dog. Each parent 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 Chromosome From Father L h a T l H a t Two of the puppy’s chromosomes are shown above. It is a homologous pair because each chromosome 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. 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 Hair Length Hair Texture Hair Curliness Coat Pattern DOMINANT Short = L Wiry = T Curly = H Spotted = A RECESSIVE Long = l Silky = t Straight = h Solid = a 2 1. What is the texture of the puppy’s coat? How do you know? ____ ____ 2. What is the texture of the mother’s coat? How do you know? ____ ____ 3. We cannot determine the texture of the father’s coat? Why not? ____ ____ 4. What is the pattern of the puppy’s coat? How do you know? ____ ____ 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 trait for which the puppy is has matching alleles. Give the genotype (letters) and the phenotype (the trait/appearance). HINT: there is only one. When the alleles match the genotype is considered ________________. 8. List the traits for which the puppy’s alleles do not match. In other words, the allele inherited from the mother is different from the allele inherited from the father. (HINT: there are three). Give the genotype (letters) and the phenotype (trait). When the alleles do NOT match the genotype is considered _______________. 3 IMPORTANT BACKGROUND INFORMATION It is important to keep in mind that when gametes (i.e. sperm or egg) are formed the chromosome number is reduced down to one set (compared with the 2 complete sets that are in all other cells). Therefore, when fertilization occurs two sets of chromosomes are again restored. With guidance in class, draw a series of pictures that compare chromosome numbers in a) somatic cells; b) gametes; c) zygotes (fertilized eggs). Predicting Inheritance--Practice with Punnett Squares Example 1: Tay Sachs is a rare and devastating condition (featured in the video clip ‘Cracking the Code’). Tay Sach’s is a recessive trait which means that only people who inherit both copies of the recessive allele will have the disease. What are the chances that a child born to two parents who are both heterozygous will have a child with Tay Sachs? STEP 1: assign codes for the alleles (if not already provided to you). Capital letters represent a dominant trait. For this example we can use: o H = Healthy o h = Tay Sach’s disease Genotype (alleles present) HH Hh hh HINT: when choosing your own codes use letters that look different as capitals and lowercase. Phenotype (trait that is present or ‘expressed’) Healthy (homozygous) Healthy (heterozygous) Tay Sach’s disease (homozygous) 4 STEP 2: determine genotypes of parents. a) Mother: _____ _____ b) Father: _____ _____ STEP 3: determine the genetic possibilities for the gametes (sex cells). Parents pass on their traits to offspring through their sex cells or gametes (sperm or egg). Sex cells (gametes) will have half the genetic material of other cells. Male Gametes (sperm): OR Female Gametes (eggs): OR STEP 4: set-up 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 for the children/offspring (when the two sex-cells unite). Mother: __ __ Egg Egg Sperm Father: __ __ Pollen Step 5: create a chart providing the genotypes and phenotypes possibilities and probabilities for offspring (as a frequency and as a percentage) Genotypes Phenotype (trait) Frequency Percent Step 6: answer the original question in a complete sentence. “For two parents who are healthy but carry a Tay Sach’s allele (both parents are heterozygous), there is a ________ chance their child will be healthy and a __________ chance their child will have Tay Sach’s. 5 Practice Punnett-Square Problems 1. Though earwax tends to get a bad rap, it actually helps to keep your ear canal clean by trapping dirt and dust that gets into your ear and moving it out. Interestingly, earwax actually comes in two types. Jill: ____ ____ Jill has the dry the wax and Jack is heterozygous for Egg Egg sticky earwax. What are the possibilities and probabilities for their children? Sperm Earwax Alleles Wet/Sticky/Yellow = dominant = _____ Jack: ___ ___ Sperm Dry/flaky/grey-yellow = recessive = _____ OFFSPRING POSSIBILITIES and PROBABILITIES Genotype Phenotype (trait) Frequency 2. Percent In certain rabbit breeds, black fur is dominant to white fur. If two heterozygous rabbits mate, what are the possible genotypes and phenotypes for the baby rabbits. What are the chances they will have a baby with white fur? Mother: Egg Egg Sperm Father: Sperm OFFSPRING POSSIBILITIES and PROBABILITIES Genotype Phenotype Frequency Percent There is a ____ % chance that two heterozygous parents will have a baby rabbit with white fur. 6 Practice Problems Continued- Single Trait Crosses Complete each of the following problems in your journal! Be sure to follow all six steps and show your work. Step 1: Assign codes to alleles (unless already provided for you). Step 2: Determine the genotype of the parents. Step 3: Determine the gamete possibilities. Step 4: Set-up and complete the punnett-square. Step 5: Complete genotype & phenotype possibilities & probabilities chart. Step 6: Answer the original question in a complete sentence 3. Ability to taste PTC is dominant over the inability to taste PTC. What are the chances that a child WILL be able to taste PTC, if her mother cannot taste PTC and her father is heterozygous for tasting? Complete all 6 steps in the punnett-square analysis. 4. A widow’s peak hairline is a dominant trait. What are the chances that two parents who have a widow’s peak (both are heterozygous gentoypes) will have a child without a widow’s peak hairline? Complete all 6 steps in the punnett-square analysis. 5. In mice the ability to run normally is a dominant trait. The recessive trait causes mice to run in circles (geneticists call these ‘waltzing’ mice). Determine the probability of each genotype and each phenotype of the potential offspring when a male heterozygous normal mouse mates with a female homozygous normal mouse. Complete all 6 steps in the punnettsquare analysis. 6. Cystic fibrosis (CF) is an inherited chronic disease that causes the body to produce unusually thick, sticky mucus that clogs the lungs and leads to life-threatening lung infections and obstructs the pancreas. 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. Jennie and Tom are a young married couple planning a family, and decide to have genetic tests prior to having children. The couple received their results and found out that they are both carriers for CF. Carriers are people who have one cystic fibrosis allele and one normal allele, so they do not have the disease but they could pass down the gene to their children. What are the chances that a child born to Tim and Jennifer will have cystic fibrosis? Complete all 6 steps in the punnett-square analysis. 7 7. 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. Huntington’s disease is a dominant trait- which 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? Complete all 6 steps in the punnett-square analysis. Note: use ‘H’ for the Huntington’s allele & ‘h’ = healthy allele. 8. Sickle-cell anemia is a recessive inherited disease. It causes a problem with the protein hemoglobin resulting in blood cells that are distorted or sickle-shaped. This in turn can cause symptoms ranging from pain and fatigue to breathlessness and heart issues. What are all the genotype and phenotype possibilities and probabilities that can result from heterozygous parents? Complete all 6 steps in the punnett-square analysis. 9. In dogs a certain form of deafness is inherited and is a recessive trait. What are the chances that two heterozygous parents will have a deaf puppy? Complete all 6 steps in the punnett-square analysis. 10. What would the offspring phenotype and genotype probabilities be when a pea plant with constricted seed pods crossed with a plant that is heterozygous for inflated seed pods? You will need the pea plant chart on the next page to complete this problem. 8 Gregor Mendel ‘s Work Who was Gregor Mendel? (see page 263 and video clip): Mendel’s principles: Genes are passed from parents to offspring (we now know this occurs through genes on chromosomes). Some alleles are dominant, some are recessive. In sexually reproducing organisms (including humans), somatic cells each have 2 copies of a gene, one gene from each parent. Genes segregate from each other during formation of a gamete (eg. sperm and egg). The alleles for different genes usually segregate independently of each other L l A a Y y R r I i G g T t 9 Two-trait Crosses- predicting two characteristics at one time. 1. What happens when you cross a plant that has green & wrinkled seeds with a plant that is heterozygous for yellow seeds and heterozygous for round seeds? What are the genotype and phenotype possibilities and probabilities for the offspring? STEP 1: Assign alleles (the chart on page 5 provides allele codes). STEP 2: Determine genotype of parents -and- STEP 3: Determine gamete possibilities. a) What is the genotype of a plant that has green and wrinkled seeds (parent 1)? _____ _____ , _____ _____ What are the gamete possibilities for the plant with green and wrinkled seeds? b) What is the genotype of a plant that is heterozygous for yellow seeds and heterozygous for round seeds (parent 2)? _____ _____, _____ _____ What are the gamete possibilities for this plant? STEP 4: Set-up and complete the punnett-square Parent 1: Gamete Gamete Gamete Gamete Gamete Parent 2: Gamete Gamete Gamete 10 STEP 5: Complete the chart—“OFFSPRING POSSIBILITIES and PROBABILITIES” Genotype Phenotype (trait) Frequency Percent 2. 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 2 GENOTYPE: ____ ____, ____ ____ Parent 1 GAMETE Possbilities: Parent 2 GAMETE Possibilities: Parent 2: Gamete Gamete Gamete Gamete Gamete Parent 1: Gamete Gamete Gamete OFFSPRING POSSIBILITIES and PROBABILITIES Genotype Phenotype (trait) Frequency Percent 11 3. Suppose that a gene that determines one’s ability to roll their tongue is on one chromosome and the ability to taste the 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 _______________ R = allele for ________________ t = allele for _______________ r = allele for ________________ DAD’s genotype: ____ ____, ____ ____ MOM’s genotype: ____ ____, ____ ____ DAD’s phenotype: _____________________ MOM’s phenotype: _________________ Mother: Egg Egg Egg Egg Sperm Father: Sperm Sperm Sperm OFFSPRING POSSIBILITIES and PROBABILITIES Genotype Phenotype (trait) Frequency Percent 4. What are the offspring genotype and phenotype possibilities and probabilities, when a mother and father who are both heterozygous for a widow’s peak and both are heterozygous for sticky ear wax? Remember (widow’s peak is dominant and sticky ear wax is dominant) ___ = allele for widow’s peak hairline ____ = allele for sticky earwax ___ = allele for straight hairline ____ = allele for dry earwax DAD’s genotype: ____ ____, ____ ____ MOM’s genotype: ____ ____, ____ ____ DAD’s gamete (sperm) possibilities: MOM’s gamete (egg) possibilities: Set-up complete the Punnett-Square, then complete a chart of the results in your journal. 12