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Genetics Notes Part I The study of genetics examines the inheritance of biological traits. The passing of traits from parents to offspring is called heredity. Biological traits are coded for by the genes on our chromosomes. Half of your chromosomes came from your mother and ½ from your father. It is estimated that more than 8 million combinations are possible from the 23 chromosome pairs you inherited from each of your parents. That means that there are about 64 trillion possibilities of unique combinations that create you. In short, you are uniquely different than any other individual. Historical Development of Genetic Understanding An Austrian monk by the name of Gregor Mendel (1822-1884) conducted some of the first recorded scientific studies on inheritance. He is currently considered the father of modern genetics. The study of inheritance is commonly known as Mendelian genetics. Mendel worked with garden peas. Why? Well, he noticed that peas had very consistent traits. Some produced green peas, others produced yellow peas. Some were tall, others were short. Some produced flowers on the ends of the stem; others produced their flowers on the sides of the stem. Some produced white flowers, others produced purple flowers. By actively pollinating one pure plant (a plant that always produced the same characteristics in the offspring) with a different pure plant, he could see how traits were expressed in the offspring of the cross-pollination. Early hypothesis suggested that if you crossed two different traits, the result would be a trait somewhere between the two. For example, crossing a dark purple flowered plant with a white flowered plant would produce a light purple plant. Crossing a tall plant with a short plant would produce a medium height plant. However, this was NOT found to be the case. It appeared that one trait always dominated the other trait. He found that tall plants produced TALL plants, even if crossed with short plants. Yellow peas produced yellow peas even when crossed with green peas. Although not known at the time, it was the genes that were controlling the genetic traits seen in the offspring. Even though the offspring obtained different possible versions of those same genes (known as alleles) from its two parents, only one of those alleles would be expressed (seen). Your mother may have blue eyes (her chromosomes contain the alleles for blue eye colour) and your dad might have brown eyes (his chromosomes contain the alleles for brown eyes), so you might have blue or brown eyes (unfortunately, with eye colour there are not only 2 possibilities, so it does get a bit more complicated.) In garden peas, the traits that were expressed most often were considered to be dominant traits. Those that were expressed less frequently were known as recessive traits. In the pea experiments, the allele for yellow seeds was dominant over the allele for green seeds. The allele for tall plants was dominant over the allele for short plants. During Mendel’s experiments, he developed a short hand for expressing the traits that were seen. He used a upper case letter to denote a dominant allele and a lower case letter to denote a recessive allele. In his experiment, he began by cross-pollinating a pure round seed plants (identified as RR) with a pure wrinkled seed plants (identified as rr). The offspring were all found to be round in appearance (this is called the phenotype – which trait is expressed/seen), but they all contained the alleles for both round and wrinkled seeds (this is called their genotype – which alleles are present in their DNA). These first generation offspring were called F1 generation which stands for filial 1 (filial is a Latin word that means “son”). All of the F1 offspring contains Rr (one dominant and one recessive gene). This makes them hybrids. Mendel continued his experiment by crosspollinating two of the hybrids (F2). Here is what he found: o Three of the seeds were round and one was wrinkled (phenotypes). ¾ : ¼ o However, looking at their genotypes: 1 was homozygous round (RR), 2 were heterozygous round (Rr) and 1 was homozygous wrinkled (rr). 1:2:1 In order to express the recessive allele, the offspring must be homozygous (possess only recessive alleles) for that trait. If a dominant allele is present it will mask the presence of the recessive allele. This is known as the Principle of Dominance. During the formation of sex cells, the homologous alleles separate and recombine. This is known as the Law of Segregation. Try This Activity Creating a Personal Profile Table #1: Lists of human traits controlled by dominant and recessive alleles. Trait Dominant Eye colour Brown or black or green Hair colour Brown or black Hairline Pointed on forehead Freckles Present Earlobe Suspended Hair texture Curly Eyesight Near or far sighted Eyelashes Long Nose line Convex tip Fingers 6 fingers Rh blood factor Positive Rh factor Ear rim Curled rim Thumb joint Last joint bends out Finger hair Present Folded Hands Left thumb over right Tongue Rolling Can be rolled into U shape Clenched Fist Two wrist cords Chin Dimple Dimple in middle Blood type Type A, B and AB Eyes Astigmatism Recessive Blue or grey Blonde or red Straight across forehead Absent Attached to head Straight Normal vision Short Concave or straight 5 fingers Negative Rh factor Not curled rim Last joint straight Absent Right thumb over left Cannot be rolled Three wrist cords No dimple Type O No astigmatism Table #2: Use information from Table 1 to complete your personal profile. Which additional traits can you include? Trait (use the letter Appearance or physical Possible genetic makeup Dominant or recessive indicated) condition (phenotype) (genotype) Eye colour E/e Hairline L/l Earlobe T/t Ear rim R/r Freckles F/f Thumb joint J/j Finger hair P/p Tongue rolling Y/y Folded hands D/d Nose line N/n Hair colour H/h Chin dimple G/g Clenched fist K/k Single-Trait Inheritance Define each of the following: 1. Genotype: ___________________________________________ _____________________________________________________ 2. Phenotype:___________________________________________ _____________________________________________________ 3. Homozygous: _______________________________________ _____________________________________________________ 4. Heterozygous: _______________________________________ _____________________________________________________ The cross between a TT tall pea plant and a tt short pea plant is called a monohybrid cross. Crossing a Tt tall pea plant with another Tt tall pea plant is also a monohybrid cross. A monohybrid cross is a cross that involves one allele pair of contrasting traits. A special chart, called a Punnett square, helps us organize the results of a cross between the sex cells of two individuals. Problem #1 Consider a cross between a pea plant that is heterozygous for round seeds and a pea plant that has wrinkled seeds. The allele for round seeds is dominant over that for wrinkled seeds. R can be used to indicate the round dominant allele and r can be used to represent the wrinkled recessive allele. A) Determine the genotypes of the offspring. B) Determine the phenotypes of the offspring. Wrinkled Parent (rr) Punnett Square r R Round parent (Rr) r r Problem #2 Consider a cross between two pea plants that are heterozygous for round seeds. The allele for round seeds is dominant over that for wrinkled seeds. R can be used to indicate the round dominant allele and r can be used to represent the wrinkled recessive allele. A) Determine the genotypes of the offspring. B) Determine the phenotypes of the offspring. Round Parent (Rr) Punnett Square R r R Round parent (Rr) r Problem #3 Consider a cross where only the offspring are observable. It is still possible to determine the genotypes of the parents in many cases even if the parents are unknown. Offspring phenotype Round seed peas Wrinkled-seed peas A) Numbers 5472 1850 Determine the ratio of round plants to wrinkled plants. You may need to round the ratio. round _________ _____ wrinkled B) Now list the possible genotypes for each phenotype: Offspring phenotype Round seed peas Wrinkled-seed peas C) Genotype Possibilities RR or Rr rr Based on your results from the last two questions, what phenotype did the parents of these results have? Problem #4 A plant that is homozygous for purple flowers is crossed with a plant that has white flowers. If purple is dominant over the white condition, what are the genotypes and phenotypes of the F1 generation? White Parent (pp) Punnett Square Purple Parent (PP) Problem #5 Table #1: Results of Mendel’s Experiments Trait Plant height Flower colour Flower position on stem Pod colour Pod shape Seed colour Seed shape Alleles DOMINANT recessive Possible genotypes TALL dwarf PURPLE white AXIAL (at branches) top GREEN yellow INFLATED constricted YELLOW green ROUND wrinkled TT or Tt tt PP or Pp Pp AA or Aa Aa GG or Gg Gg II or Ii Ii YY or Yy Yy RR or Rr rr For each of the crosses listed below, create a Punnett square and determine the following information: parent phenotypes, parent genotypes, parent gametes, F1 genotypes and F1 phenotypes. a) Two heterozygous tall parents are crossed. Parent phenotypes Punnett Square Parent genotypes Parent gametes F1 genotypes F1 phenotypes b) A heterozygous tall plant is crossed with a dwarf plant. Punnett Square Parent phenotypes Parent genotypes Parent gametes F1 genotypes F1 phenotypes c) Two plants that are heterozygous for purple flowers are crossed. Punnett Square Parent phenotypes Parent genotypes Parent gametes F1 genotypes F1 phenotypes d) A plant that is homozygous for green pods is crossed with a plant that has yellow pods. Punnett Square Parent phenotypes Parent genotypes Parent gametes F1 genotypes F1 phenotypes e) A plant that is homozygous for round seeds is crossed with a plant that is heterozygous for round seeds. Punnett Square Parent phenotypes Parent genotypes Parent gametes F1 genotypes F1 phenotypes Problem #6 In guinea pigs, the allele for a black coat is dominant over the allele for a white coat. A black guinea pig was crossed with a white guinea pig. All F1 offspring have black coats. A) Describe how you can determine whether or not the black parent is homozygous or heterozygous for the black condition. Indicate the letter you will use to represent an allele Punnett Square b b B BB Bb B BB Bb b b B BB Bb b Bb bb Punnett Square B) If 10 offspring were produced, indicate how many you would expect to have black coat colour, if the black parent were heterozygous. Punnett Square b b B Bb Bb b bb bb Test Cross Sometimes in agriculture a particular trait is desired. For example, white sheep are preferred over black sheep because the wool of black sheep is more brittle and harder to dye. Therefore, farmers strive to selectively breed their flock to produce only white sheep. How do they ensure the offspring of a mating will be white? Let’s look at the genetics behind the alleles that code for white or black wool. White wool is the dominant condition and black wool is the recessive condition. To ensure the offspring are all white, ranchers will use a homozygous white ram (male sheep). How do they know the ram is homozygous? Ranchers perform a test cross. The test cross is always performed between the unknown genotype and a homozygous recessive genotype. In this case, the recessive individual would be a black ewe (female sheep). In a cross between a white ram whose genotype is unknown (either WW or Ww) and a black ewe (ww), if any of the offspring show the recessive trait, then the ram must be heterozygous. If all the offspring show the dominant trait, then the ram must be homozygous. Punnett Square White Ram Unknown genotype Homozygous? W W Black ewe (recessive) w w Punnett Square Ww Ww (white) (white) White Ram Ww Ww (white) (white) Unknown genotype Heterozygous? W w Black ewe (recessive) w w Ww Ww (white) (white) ww ww (black) (black) Problem #7 For Labrador Retrievers, black fur (F) colour is dominant over yellow (f). A) What would be the genotype of a homozygous black dog? ________________________________ B) Could a heterozygous black dog have the same genotype as a dog with yellow fur? Explain. Problem #8 A heterozygous pea plant with round seeds (R) is cross-pollinated with a pea plant that has wrinkled seeds (r). Remember that the round condition is dominant over the wrinkled seed. For the cross, indicate each of the following: A) What would be the genotype of the parents if the round-seed plant is heterozygous?____________ B) What are the genotypes and phenotypes of the F1 generation? Punnett Square F1 Wrinkled r r R Rr Rr r rr rr Round C) What are the genotypes and phenotypes of the F2 generation, if two round-seed plants from F1 are cross-pollinated? Punnett Square F2 R r R RR Rr r Rr rr Problem #9 For Dalmatian dogs, the spotted condition (S) is dominant over non-spotted (s). A) Using a Punnett square, show the results of a cross between two heterozygous parents. Non-Spotted Punnett Square F1 Spotted B) A spotted female Dalmatian mates with an unknown father. From the appearance of the pups, the owner concludes that the male was a Dalmatian. The owner notes that the female had six pups, three spotted and three non-spotted. What are the genotype and phenotype of the unknown male? Male's genotype: Spotted Female Punnett Square F1 S s _______________ s Ss ss Male's phenotype: s Ss ss _______________ Unknown Male? Problem #10 A human neurological disorder referred to as Huntington's chorea is caused by a dominant allele. Because the allele doesn't express itself until a person reaches 50 years of age, early detection has been difficult. In one family, a woman begins to show symptoms. Her father had Huntington's chorea, but her mother never developed the disorder. The woman's husband shows no symptoms, nor does anyone in his immediate family. A) What is the genotype of the woman who has developed Huntington's chorea? B) What is the probable genotype of the woman's husband? C) If the man and woman have 6 children together, how many are likely to develop Huntington's chorea? Woman Punnett Square F1 H h h Hh hh h Hh hh Man Selective Breeding People have been using selective breeding for a long time as a way of improving varieties of plants and animals. Selective breeding involves the crossing of desired traits from plants or animals to produce offspring that have one or several of the favoured characteristics. For example, farmers might selectively cross-pollinate a cold-hardy wheat with one that grows quicker in order to create a new variety that can survive in Canada's colder, shorter growing season. Purebred pets and thoroughbred horses have been selectively bred over many generations using inbreeding. Inbreeding is the mating of closely related individuals for the purpose of maintaining or perpetuating certain characteristics. New varieties of plants and animals can be developed by hybridization. This process is the opposite of inbreeding. In hybridization, the goal is to combine different traits from different parents to have all of these favorable traits present in the offspring. Hybrids tend to be more vigorous than either parent. Standard poodle Labrador retriever Labradoodle Pedigree Charts A pedigree chart can be used to solve genetic puzzles when traits have been recorded over many generations. A pedigree shows the passing of a trait through the generations. Pedigree charts use many symbols to represent gender, relationships between individuals, and whether or not the individual expresses a trait or carries the allele as part of a heterozygous genotype. hjg Problem #11 A) Indicate whether each family member is homozygous or heterozygous for shortsigthedness or homozygous for normal vision. B) If couple II - 7 & 8 (in row II) had another child, what genotype might the child have? (Hint: What genotype is possible but not shown in the chart? Would the child have normal vision or be shortsighted?