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Chapter 11 Mendel & The Gene Idea

Chapter 11 Mendel & The Gene Idea. Mendelian genetics. Character heritable feature, i.e., fur color Trait variant for a character, i.e., brown True-bred all offspring of same variety Hybridization crossing of 2 different true-breds P generation parents

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Chapter 11 Mendel & The Gene Idea

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  1. Chapter 11 Mendel & The Gene Idea

  2. Mendelian genetics • Character • heritable feature, i.e., fur color • Trait • variant for a character, i.e., brown • True-bred • all offspring of same variety • Hybridization • crossing of 2 different true-breds • P generation • parents • F1 generation • first filial generation • F2 generation • second filial generation

  3. The Law of Dominance • In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. • Offspring that are hybrid for a trait will have only the dominant trait in the phenotype. • When an organism has two different alleles for a trait, the allele that is expressed, overshadowing the expression of the other allele, is said to be dominant. The gene whose expression is overshadowed is said to be recessive. • Mendel’s Law of Dominance

  4. Leading to the Law of Segregation • Alternative versions of genes (alleles) account for variations in inherited characteristics • For each character, an organism inherits 2 alleles, one from each parent • If the two alleles differ, then one, the dominant allele, is fully expressed in the organism’s appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance • The alleles for each character segregate (separate) during gamete production (meiosis). • Mendel’s Law of Segregation

  5. The Law of Independent Assortment • Law of Segregation involves 1 character. What about 2 (or more) characters? • Monohybrid cross vs. dihybrid cross • The two pairs of alleles segregate independently of each other. • Mendel’s Law of Independent Assortment

  6. Genetic vocabulary……. • Punnett square: predicts the results of a genetic cross between individuals of known genotype • Homozygous: pair of identical alleles for a character • Heterozygous: two different alleles for a gene • Phenotype: an organism’s traits • Genotype: an organism’s genetic makeup • Testcross: breeding of a recessive homozygote X dominate phenotype (but unknown genotype)

  7. Alternative Inheritance Patterns • Incomplete dominance: appearance between the phenotypes of the 2 parents. Ex: snapdragons • Codominance: two alleles affect the phenotype in separate, distinguishable ways. Ex: Tay-Sachs disease • Multiple alleles: more than 2 possible alleles for a gene. Ex: human blood types

  8. Alternative Inheritance Patterns • Pleiotropy: genes with multiple phenotypic effect. Ex: sickle-cell anemia • Epistasis: a gene at one locus (chromosomal location) affects the phenotypic expression of a gene at a second locus. Ex: mice coat color • Polygenic Inheritance: an additive effect of two or more genes on a single phenotypic character Ex: human skin pigmentation and height

  9. Human disorders • The family pedigree • Recessive disorders: • Cystic fibrosis • Tay-Sachs • Sickle-cell • Dominant disorders: • Huntington’s • Testing: • Amniocentesis • chorionic villus sampling (CVS)

  10. Chapter 12 The Chromosomal Basis of Inheritance

  11. Chromosomal Theory of Inheritance • Genes have specific loci on chromosomes and chromosomes undergo segregation and independent assortment

  12. Chromosomal Linkage • Morgan • Drosophilia melanogaster • XX (female) vs. XY (male) • Sex-linkage: genes located on a sex chromosome • Linked genes: genes located on the same chromosome that tend to be inherited together

  13. Genetic Recombination • Crossing over • Genes that DO NOT assort independently of each other • Genetic maps • The further apart 2 genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency • Linkage maps • Genetic map based on recombination frequencies

  14. Human Sex-Linkage • SRY gene: gene on Y chromosome that triggers the development of testes • Fathers= pass X-linked alleles to all daughters only (but not to sons) • Mothers= pass X-linked alleles to both sons & daughters • Sex-Linked Disorders: Color-blindness; Duchenne muscular dystropy (MD); hemophilia • X-inactivation: 2nd X chromosome in females condenses into a Barr body (e.g., tortoiseshell gene gene in cats)

  15. Chromosomal Errors • Nondisjunction: • members of a pair of homologous chromosomes do not separate properly during meiosis I or sister chromatids fail to separate during meiosis II • Aneuploidy: • chromosome number is abnormal • Monosomy – missing chromosome • Trisomy - extra chromosome (ex. Trisomy 21 Down Syndrome) • Polyploidy - extra sets of chromosomes

  16. Chromosomal Errors • Alterations of chromosomal structure: • Deletion: removal of a chromosomal segment • Duplication: repeats a chromosomal segment • Inversion: segment reversal in a chromosome • Translocation: movement of a chromosomal segment to another

  17. Genomic Imprinting • A parental effect on gene expression • Identical alleles may have different effects on offspring, depending on whether they arrive in the zygote via the ovum or via the sperm. • Fragile X syndrome: higher prevalence of disorder and mental disability in males

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