chapter14a

chapter14a - Chapter 14: Patterns of Inheritance PART A:...

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BIOL 1020 – CHAPTER 14A LECTURE NOTES Chapter 14: Patterns of Inheritance PART A: INTRODUCTION TO GENETICS I. the basic rules of inheritance were first demonstrated by Mendel A. at the time of Mendel’s work, most thought that parental traits were fluids that “blend” in offspring B. Mendel recognized that this model did not explain what he observed C. Mendel chose a model system and carefully established testing conditions 1. he used pea plants that he could outcross or allow to self-fertilize 2. he chose traits that had two clear possible outcomes (yellow or green seeds, etc.) 3. he established true-breeding or “pure” lines to use for genetic crosses D. terminology for genetic crosses 1. P generation (or P 1 ) = parental generation 2. F 1 generation = first generation offspring (from filial ) 3. F 2 generation = second generation offspring 4. phenotype – appearance or characteristic of an organism 5. genotype – genetic makeup of an organism, determines phenotype 6. gene – unit of heredity; controls a trait that determines a phenotype 7. locus – the location of a particular gene on a chromosome 8. alleles – alternative versions of a gene 9. dominant – allele that dominates over others in determining phenotype 10. recessive – allele whose phenotypic expression is “hidden” when a dominant allele is present 11. hybrid – offspring from a cross between two “pure” lines of different, competing phenotypes II. rules and terminology for examination of genetic inheritance A. Mendel’s law of segregation 1. when Mendel crossed pure lines of different, competing phenotypes, he found that the F 1 generation was uniform and matched one of the parents’ phenotypes example: P 1 yellow seed X green seed à all F 1 yellow seed 2. when F 1 plants were crossed or selfed, the F 2 plants had both P 1 phenotypes in a ratio of roughly 3:1 using offspring from above F 1 X F 1 à F 2 3 yellow seed: 1 green seed 3. thus, contrary to the popular belief of the time, recessive traits are not lost in a mixing of parental phenotypes – they are merely hidden in some “carrier” individuals 4. Mendel explained these ratios with what we now call his law of segregation ; stated in modern terms: individuals normally carry two alleles for each gene, these alleles must segregate in production of sex cells 5. later investigations of cell division revealed the mechanism for segregation: the pairing and subsequent separation of homologous chromosomes during meiosis B. genotype vs. phenotype 1. phenotype is the actual appearance or characteristic, and is determined by genotype; knowing the phenotype will not always directly reveal the genotype (recessive traits can be masked) 2. genotype is the listing of the actual alleles present; if you know the genotype, you should be able to predict the phenotype genotypes are either homozygous or heterozygous homozygous – the homologous chromosomes have the same allele at the locus in question
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chapter14a - Chapter 14: Patterns of Inheritance PART A:...

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