Mendelian Genetics and Inheritance Problems

Transcription

1 Biology 211 Mendelian Genetics and Inheritance Problems Mendel discovered and described many of the basic rules of genetics after studying the pattern of how inheritable traits were passed from generation to generation in pea plants. His approach was mathematical. He made various crosses, counted the number of offspring with the different traits and described several Laws/Principles that would account for the ratios he saw in his results. 1) When Mendel crossed true breeding, purple-flowered pea plants with true-breeding, white flowered plants the offspring (F 1 ) generation all had purple flowers. When he let this generation self itself and counted the flowers in the next (F 2 ) generation, he found 705 plants had purple flowers and 224 had white flowers (approximately a 3:1 ratio). Use a Punnet square to show why one would expect a 3:1 ratio if crossing purple and white flowers. Which color is dominant, purple or white? 2) You cross a heterozygous purple-flowered pea plant with a white-flowered plant and counted 800 seeds. Approximately how many of those 800 would you expect to grow into white-flowered plants? 3) What were the genotypes (represent the dominant allele with an A and the recessive allele with an a) of the parents in a cross that produced: a) ½ dominant phenotype and ½ recessive phenotype progeny b) all heterozygous progeny c) ¾ dominant phenotype and ¼ recessive phenotype progeny 1

2 4) Cystic fibrosis is a genetic disease that causes bronchial mucus to be very thick. People with cystic fibrosis usually die of pneumonia or other respiratory tract infection by the their mid-20 s. Two parents, neither of whom have cystic fibrosis, give birth to a son with cystic fibrosis. Is cystic fibrosis dominant or recessive? What are the genotypes of the parents? (Cystic fibrosis is not sex-linked.) 5) Huntington's Disease, a rare, lethal, degenerative disease of the nervous system is caused by a dominant allele. This disease does not show symptoms until a person is about 45 to 55 years old. A 50-year-old man has just been diagnosed with Huntinton's disease. He wants to know what are the chances that any of his children have inherited the disease. As his doctor, what do you tell him? Mendel worked with many characteristics (genes) in his crosses. This led to his Law of Independent Assortment. 6) List all the possible gametes that a purple flowered, yellow, round seeded pea plant can produce if it is heterozygous for those characteristics. (These characteristics assort independently.) 7) Assuming complete dominance, what would be the ratio of the phenotypes of the offspring of selfing a plant that was heterozygous for tall plants (tall is dominant) with purple flowers (purple is dominant over white). 2

3 Not all traits can be explained by classic Mendelian patterns of inheritance. For example not all alleles are dominant or recessive, but instead show incomplete or codominance. In these instances a cross between two true-breeding plants will result in a phenotype that is either intermediate between the two original phenotypes (incomplete dominance) or both phenotypes will be visible in the offspring (co-dominance). 8) In snap-dragons red is not dominant to white in flower color but instead heterozygotes are pink. What would be the result of crossing a pure-breeding (homozygous) snapdragon with red flowers with a pure-breeding white-flowered plant? Show the genotypes and then describe what the plant would look like. Also all the characteristics that Mendel examined had only two alleles. His flowers were either purple or white, his seeds were either wrinkled or round, plants tall or short, etc. In many instances there are more than two alleles in the population, still any person can only have two versions since they only have two sets of chromosomes. 9) The ABO blood types are a classic case where there are three alleles of the gene, I A, I B and i. A person with A blood has either I A I A or I A i, a person with type B blood has I B I B or I B i, and a person with type O blood has two copies of the i i allele. Blood typing is often used in cases where one wants to determine whether an individual could be the parent of another individual. What are the possible blood types of children of a mother with AB blood and a father with O blood? Which two alleles of this blood type gene are codominant and which allele is recessive? 10) Color pattern in a species of duck is determined by one gene with three alleles. Alleles H and I are codominant, and allele i is recessive to both. How many phenotypes are possible in a population of ducks (ducks are diploid) that contains all the possible combinations of these alleles? 3

4 Mendel did not examine sex-linked traits. These are traits that are found on the X or Y chromosomes. Sex-linked traits show different patterns of inheritance than normal traits because males have one X and one Y chromosome while females have two X chromosomes. Of course, females do not show traits that are only coded for on the Y chromsome. 11) Hemophilia is inherited as an X-linked recessive in humans. A normal woman and a normal man have a male child who has hemophilia. What are the genotypes of the parents? What is the chance any subsequent male children will have hemophilia? What percentage of their daughters will be carriers? Even though Mendel s dihybrid crosses showed Independent Assortment of his traits (genes), many traits do not. If a dihybrid cross is performed some traits will give ratios that show dependent assortment (i.e. the traits are linked, close to each other on the same chromosome) while other traits will give ratios that can not be explained by either dependent or independent assortment (due to crossing-over). 12) In hephalumps, the gene for number of lumps is completely linked to the gene for webbed toes. Lumps (L) is dominant to having no lumps and webbed toes (T) is dominant to non-webbed toes. If a homozygous dominant hephalump mated with a homozygous recessive hephalump what would be the genotype and phenotype of their offspring? Please write the possible gametes that this individual, the F 1 offspring hephalump can produce. If those genes were NOT linked, i.e., showed independent assortment, what types of gametes could the F 1 individual produce? 4

5 14) In barflies, the gene for glassy eyes is completely linked to the gene for red nose. Each of these traits is recessive to the normal alleles. If a true-breeding, glassy-eyed, normal-nosed female is crossed with a true-breeding, normal-eyed, red-nosed male, and their F 1 progeny are mated with each other, what is the expected phenotypic ratio of the F 2 progeny? Look at the following family pedigrees and determine the most likely mode of inheritance for each one (i.e., dominant, recessive, X-linked dominant, X-linked recessive). Where possible fill in the genotypes of family members. If you can only determine one of the alleles just write that one allele down and leave a question mark for the second allele. Squares = male, circle= female. HINTS for doing pedigree analysis problems: 1) Check to see if the trait is equally distributed between males and females. If there are more males affected you should look to see if the trait is sex-linked. If it is passing from father to sons it must be Y-linked. More commonly it is passing from the mother to sons (and daughters) and is X-linked. 2) Check to see if the trait shows up in every generation. If the trait is skipping generations it may be recessive. 3) If the trait shows up every generation, it can be dominant or it can be recessive. Often several options work. If they do, check to see what pattern of inheritance is best supported by the ratio of the offspring. If additional information has been given in the question, use that information. For example if you are told the trait is rare, it is unlikely anyone has two alleles for that trait. 5

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