Chapter 15 (2)

Chapter 15 (2) - Chapter 15 The Chromosomal Basis of...

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Chapter 15 The Chromosomal Basis of Inheritance
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Overview: Locating Genes on Chromosomes Genes Located on chromosomes Can be visualized using certain techniques Figure 15.1
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Concept 15.1: Mendelian inheritance has its physical basis in the behavior of chromosomes The chromosome theory of inheritance states: Genes have specific loci on chromosomes Chromosomes undergo segregation & independent assortment
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The chromosomal basis of Mendel’s laws Figure 15.2 Yellow-round seeds ( YYRR ) Green-wrinkled seeds ( yyrr ) Meiosis Fertilization Gametes All F 1 plants produce yellow-round seeds ( YyRr ) P Generation F 1 Generation Meiosis Two equally probable arrangements of chromosomes at metaphase I LAW OF SEGREGATION LAW OF INDEPENDENT ASSORTMENT Anaphase I Metaphase II Fertilization among the F 1 plants 9 : 3 : 3 : 1 1 4 1 4 1 4 1 4 YR yr yr yR Gametes Y R R Y y r r y R Y y r R y Y r R y Y r R Y r y r R Y y R Y r y R Y Y R R Y r y r y R y r Y r Y r Y r Y R y R y R y r Y F 2 Generation Starting with two true-breeding pea plants, we follow two genes through the F 1 and F 2 generations. The two genes specify seed color (allele Y for yellow and allele y for green) and seed shape (allele R for round and allele r for wrinkled). These two genes are on different chromosomes. (Peas have seven chromosome pairs, but only two pairs are illustrated here.) The R and r alleles segregate at anaphase I, yielding two types of daughter cells for this locus. 1 Each gamete gets one long chromosome with either the R or r allele. 2 Fertilization recombines the R and r alleles at random. 3 Alleles at both loci segregate in anaphase I, yielding four types of daughter cells depending on the chromosome arrangement at metaphase I. Compare the arrangement of the R and r alleles in the cells on the left and right 1 Each gamete gets a long and a short chromosome in one of four allele combinations. 2 Fertilization results in the 9:3:3:1 phenotypic ratio in the F 2 generation. 3
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Morgan’s Experimental Evidence: Scientific Inquiry Morgan Evidence that chromosomes are location of Mendel’s heritable factors
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Morgan’s Choice of Experimental Organism Why fruit flies? Breed at high rate New generation can be bred every 2 weeks Have only 4 pairs of chromosomes
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Wild type (normal, +) phenotypes that were common in fly pops. Traits alternative to wild type Mutant phenotypes Figure 15.3
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Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair Males with white eyes (mutant) with females with red eyes (wild type) F 1 generation all had red eyes F 2 generation showed 3:1 red:white eye ratio, only males had white eyes – What can you deduce from this?
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Figure 15.4 The F 2 generation showed a typical Mendelian 3:1 ratio of red eyes to white eyes. However, no females displayed the white-eye trait; they all had red eyes. Half the males had white eyes, and half had red eyes. Morgan then bred an F
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Chapter 15 (2) - Chapter 15 The Chromosomal Basis of...

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