MIT Department of Biology 7.013: Introductory Biology - Spring 2005 Instructors: Professor Hazel Sive, Professor Tyler Jacks, Dr. Claudette Gardel Solutions to Genetics Section Problems Mice I a) Cross 1: red eyes X white eyes gives all red eyes – a likely model is that eye color is controlled by one gene that has 2 alleles resulting in two phenotypes, red eyes and white eyes. The red eyed phenotype is likely to be dominant. Therefore, choose appropriate symbols (one letter per gene, capital letter for allele with dominant phenotype): R - red allele. Dominant phenotype of red eyes. r - white allele. Recessive phenotype of white eyes. Cross 1 is then: RR X rr ----> all Rr (all red eyes). Cross 2 is then: Rr X Rr ----> 3:1 red:white. The predictions agree with the data. b) Cross 1: long ears X short ears gives some long and some short ears – a likely model is that ear length is controlled by one gene with 2 alleles resulting in two phenotypes, long ears and short ears. From this data, we cannot determine which allele has the dominant phenotype. Cross 2: long ears X long ears gives some long and some short ears – a likely model is that long ears is the dominant phenotype. Therefore, the appropriate symbols are: L - long ears . l - short ears . Cross 1 is then: Ll X ll ----> 1:1 long ears : short ears Cross 2 is then Ll X Ll ----> 3:1 long ears : short ears The predictions agree with the data. Flowers: Cross 1: blue flowers X white flowers gives all pale blue flowers – a plausible model is thatcolor is controlled by one gene with 2 alleles, resulting in blue and white flowers, and thatneither phenotype is dominant. This is an example of Partial or Semi or Incomplete dominance. BB - blue phenotype. B'B' - white phenotype. BB' - pale blue phenotype. therefore: Cross 1 is then: BB X B'B' ----> all BB', pale blue flowers. Cross 2 is then: BB' X BB' ----> 1:2:1 BB, blue flowers : BB', pale blue flowers : B'B',white flowers. The predictions agree with the data.Blood Type Blood type is determined by one gene that has three alleles: IA - allele associated with the A phenotype, co-dominant with the B phenotypeIB - allele associated with the B phenotype, co-dominant with the A phenotypeIO - allele associated with the O phenotype, recessive to both the A and B phenotype a) Cross 1 is then: IAIA X IBIB ----> all type IAIB Cross 2 is then: IAIB X IAIB ----> 1:2:1 types A:AB:B (note skewed ratio with small number of progeny) b) Cross 1 is then: IAIO X IBIO----> 1:1:1:1 types A:B:AB:O (note skewed ratio with small number of progeny) The predictions agree with the data. Mice II: a) Tail-lessness and lethality are controlled by one gene with two alleles:T allele - dominant tail-less phenotype; recessive lethal phenotypet allele - recessive normal-tail phenotype; dominant viable phenotype therefore: TT - tail-less and dead Tt - tail-less and alive tt - normal-tail and alive Cross 1 is then: Tt X tt ----> 1:1, Tt (tail-less) : tt (normal) Cross 2 is then: Tt X Tt ----> 1:2:1 normal:tail-less:dead The predictions agree with the data. b) eye color is determined by one gene with two alleles: B - dominant blue-eye phenotype b - recessive brown eye phenotype tooth length is determined by one gene with two alleles: T - dominant short-tooth phenotype t - recessive long tooth phenotype Cross 1 is then: BBss X bbSS ----> all BbSs Cross 2 is then: BbSs X BbSs ----> 9:3:3:1 (B_S_): (B_ss): (bbS_): (bbss) The predictions agree with the data. Plants Height is determined by one gene with two alleles where the tall phenotype is dominant over the short phenotype. Color is determined by one gene with two alleles where the green phenotype is dominant over the yellow phenotype. Cross 1 is then: TtGG X ttgg ----> 1:1 tall and green (TtGg) : short and yellow ( ttGg)Cross 2 is then: TTGg X ttgg ----> 1:1 (tall and green (TtGg) : tall and yellow
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