Unit 6: Genetics Name ___________________________ Period ______
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Unit 6: Genetics Name ___________________________ Period ______
Name ___________________________ Period ______ Unit 6: Genetics Essential Skills 6-1. Be able to show the expected phenotype and genotype ratios for any single trait Punnett Square cross. (CCS: 2g, 3a) 6-2. Recognize the terms genotype, phenotype, dominant, recessive, heterozygous and homozygous.(CCS: 2g, 3a-b) 6-3. Use a pedigree to determine the probable mode of inheritance of a trait. (CCS: 3c-d) Vocabulary – You should be able to define and use the words in bold. (see website) Study Guide 1. Using the height of plant (Tall (T) & Short (t)). Explain how Mendel obtained his P generation, the results of the F1 generation and the results of the F2 generation. Mendel developed purebred lines of plants (homozygous tall and homozygous short) by continuously self-fertilizing a group of plants. He then crossed the two purebred lines (P generation). The offspring of this generation, called the F1 generation, where all tall. This suggested that the tall trait could coverup the short trait (were dominant). He then took two plants from the F1 generation and crossed them to create the F2 generation. In the F2 generation he observed that ¾ of the offspring were tall and ¼ were short. Because the recessive trait is present in the F2 generation, Mendel predicted that the two alleles from the F1 generation separate during gamete formation. If two recessive alleles are united during fertilization, the offspring will display the recessive trait. 2. How did Mendel’s results with pea plants show that alleles are either dominant or recessive? Use examples to explain. The offspring in the F1 generation were all tall, even though one of their parents was short. This suggests that the tall allele masks the presence of the short allele. Mendel observed a similar phenomena when he crossed purebred purple plants with purebred white plants. All of the offspring were purple and the white trait was covered up. 3. Explain how the following terms relate to each other: a. DNA – Chromosome Chromosomes are made of DNA b. Gene – Chromosome Genes are located on chromosomes c. Gene – Trait A gene codes for a specific trait d. Trait – Allele An allele is an option for a trait 4. What is the Law of Segregation? How is this related to the Law of Independent Assortment? The Law of Segregation says that alleles during gamete formation. The Law of Independent Assortment says that the separation of alleles (on homologous chromosomes) is random. 5. Explain how the process of Meiosis supports Mendel’s Laws. How does crossing-over relate to Mendel’s Laws? Both segregation and independent assortment occur during Meiosis I. Crossing over also contributes to the random separation of alleles and allows alleles located on the same chromosome to become separated. 6. Compare and contrast the following sets of terms: a. Dominant vs. Recessive – Dominant alleles mask recessive alleles b. Genotype vs. Phenotype – Genotype is the genetic make-up of an individual, the phenotype is the physical appearance of the individual c. Homozygous vs. Heterozygous – Homozygous individuals have 2 of the same alleles for a gene where heterozygous individuals have 2 different alleles for a gene. d. Purebred vs. Hybrid – Purebred is homozygous, hybrid is heterozygous 7. How does genotype affect phenotype? Use the terms dominant, recessive, homozygous and heterozygous. The genetic make-up (genotype) of an individual determines what physical trait(s) (phenotype) they will have. Honors Biology 2008-2009 8. Complete the following table that summarizes the 4 basic genetic crosses: (B=brown; b =blue) Word Cross Homozygous dominant x Homozygous recessive Homozygous Dominant x Heterozygous Heterozygous x Heterozygous Homozygous recessive x heterozygous Genotype Cross BB x bb Offspring Phenotype Ratio 100% brown BB x Bb 2 BB: 2 Bb 100% brown Bb x Bb 1 BB: 2 Bb: 1 bb 3 brown: 1 blue bb x Bb 2 Bb x 2 bb 2 Brown : 2 Blue 9. Short tails are dominant to long tails in dogs. a. Offspring Genotype Ratio 100% Bb What are the phenotypic and genotypic ratios of a cross between a dog with a long tail and a dog with a short tail (who had a parent with a long tail)? Alleles: S = short tail, s = long tail Cross: ss x Ss s s S Ss Ss Genotypic ratio: 2 Ss: 2 ss s ss ss Phenotypic ratio: 2 short tail: 2 long tail b. If you had a short tailed dog and did not know the phenotypes of its parents, how could you conclusively determine the genotype of your dog? Explain using examples of a Test Cross. Cross the short tailed dog with a long tailed dog. If any of the offspring have long tails, the dog was heterozygous. If none of the offspring have long tails, the dog was likely to have been homozygous dominant. S ? S s S S s Ss ?s s Ss ss s Ss Ss s Ss ?s s Ss ss s Ss Ss 10. About 70% of Americans perceive a bitter taste from the chemical phenylthiocarbamide (PTC). The ability to taste this chemical results from a dominant allele (T) and not being able to taste PTC is the result of having two recessive alleles (t). Albinism is also a single locus trait with normal pigment being dominant (A) and the lack of pigment being recessive (a). A normally pigmented woman who cannot taste PTC has a father who is an albino taster. She marries a homozygous, normally pigmented man who is a taster but who has a mother that does not taste PTC. What are the genotypes of the possible children in this dihybrid cross? Alleles: T = taster, t = non-taster & A = normal pigment, a = albino Cross: Aatt x AATt Possible female gametes: At & at Possible male gametes: AT & At At at AT AATt AaTt Genotypic ratio: 1 AATt: 1 AaTt: 1 AAtt: 1 Aatt At AAtt Aatt Phenotypic ratio: 2 normal pigmented tasters : 2 normal pigmented non-tasters 11. What is the key to recognizing a trait that is inherited by incomplete dominance? What is the notation you would use to show the genotypes of a trait with incomplete dominance? In incomplete dominance, the heterozygote will have a phenotype that is an intermediate or blend of the two homozygous phenotypes. You would use a capital letter to show the “more dominant” allele and a lower case letter to show the “more recessive” allele. 12. In northern California there is a creature know as a wildcat. It comes in three colors, black, red, and maroon. This trait is controlled by a single locus gene with incomplete dominance. A homozygous (BB) individual is black, a homozygous (bb) individual is red, and a heterozygous (Bb) individual is maroon. What would be the genotypes and phenotypes of the offspring if a black wildcat were crossed with a red one? Alleles: BB = black, Bb = maroon, bb = red Cross: BB x bb B B B Bb Bb Genotypic ratio: 100% Bb B Bb Bb Phenotypic ratio: 100% maroon wildcats! (Go cats Go!) Name ___________________________ Period ______ 13. What is the key to recognizing a trait that is inherited by Co-dominance? What is the notation you 14. 15. 16. 17. 18. 19. 20. would use to show the genotypes of a trait with Co-dominance? In co-dominance, the heterozygote will have a phenotype that displays both of the alleles. You would use two different capital letters, one for each dominant allele, as the notation. In corn plants, there are two alleles for kernel color, red (R), and yellow (Y). The red and yellow alleles are co-dominant. What would be the genotype and phenotype ratios in the offspring produced by a cross between a plant with red kernels and a plant with both red and yellow kernels? Alleles: RR = red, RY = red & yellow, YY = yellow Cross: RR x RY R R R RR RR Genotypic ratio: ½ RR: ½ RY Y RY RY Phenotypic ratio: ½ red kernels: ½ red & yellow kernels Differentiate between a multiple allele trait and a polygenic trait. A multiple allele trait has more than two options for the trait, but is affected by a single gene. A polygenic trait is affected by two or more genes that interact. Show the expected genotype and phenotype ratios of a cross between a man with type A blood and a woman with type B blood (both the man and woman had at least one parent with type O blood). Alleles: A = type A, B = type B, AB = type AB, OO = type O Cross: AO x BO A O B AB BO Genotypic ratio: 1 AB: 1 BO: 1 AO: 1 OO O AO OO Phenotypic ratio: 1 type AB: 1 type B: 1 type A: 1 type O What is the difference between Autosomal recessive trait, an Autosomal dominant trait? Autosomal recessive traits are caused by a recessive allele on an autosome. Autosomal dominant traits are caused by a dominant allele on an autosome. What is the difference between an autosomal trait and a Sex-Linked trait? What sex chromosomes determine male? Female? Why do sex-linked traits affect males more than females? A sex-linked trait is controlled by a gene located on a sex-chromosome (the X or Y chromosome). A male will have an X & a Y chromosome. A female will have 2 X chromosomes. Sex-linked traits affect males more commonly than females because males only have 1 X chromosome and most sex-linked traits are on the X chromosome. Red-green color blindness is a recessive sex-linked trait. Show the expected genotype and phenotype ratios of a cross between a man who is colorblind and a woman who has normal vision (but her father was colorblind). Alleles: XN = normal vision, Xn = red-green color blind Cross: XnY x XNXn Xn Y N X XN Xn XNY Genotypic ratio: 1 XN Xn: 1 Xn Xn: 1 XNY: 1 XnY n n n n X X X XY Phenotypic ratio: 1 normal female: 1 color-blind female: 1 normal male: 1 colorblind male The following pedigree shows the individuals who have the genetic disorder thunderfanemia. Based on the pedigree, is this disorder caused by a dominant or recessive allele? Fill in the genotype for each individual. The disorder is recessive. Honors Biology 2008-2009