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Chapter 14 – Patterns of Inheritance

Chapter 14 – Patterns of Inheritance . Gregor Mendel, documented mechanisms of inheritance decades before chromosomes and genes were observed Characters Traits. What genetic principles account for the passing of traits from parents to offspring? The “ blending ” hypothesis

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Chapter 14 – Patterns of Inheritance

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  1. Chapter 14 – Patterns of Inheritance

  2. Gregor Mendel, documented mechanisms of inheritance • decades before chromosomes and genes were observed • Characters • Traits

  3. What genetic principles account for the passing of traits from parents to offspring? • The “blending” hypothesis • The “particulate” hypothesis

  4. Planned breeding experiments • Organisms used should: • show variation in characteristics • produce large number of offspring • allow controlled matings to be conducted • have short life cycle • be convenient to handle

  5. Mendel brought an experimental approach to genetics • Mendel chose to work with garden peas because: • Many true breeding varieties • Variation in particular traits • Could control mating

  6. Mendel studied inheritance of 7 characters existing in 2 forms -- either/or

  7. Parental generation (P) true-breeding First filial generation (F1) hybrids

  8. Fig. 14-3-3 EXPERIMENT P Generation (true-breeding parents)  Purple flowers White flowers F1 Generation (hybrids) All plants had purple flowers F2 Generation No blending of traits! 224 white-flowered plants 705 purple-flowered plants

  9. When plants of contrasting traits (that bred true) were crossed: purple flowers x white flowers The offspring (F1)all exhibited one of the traits The traits did not blend Traits that appear in all F1 = dominant Traits that are not expressed = recessive

  10. All traits behaved the same • P(parental) dominant x recessive • F1: • all dominant • F2: • 3/4 dominant : 1/4 recessive

  11. The Law of Segregation • Mendel developed a hypothesis to explain the inheritance pattern observed in F2 offspring • Five concept model • concepts can be related to what we now know about genes and chromosomes

  12. First concept • Parents don’t transmit traits directly to offspring but as “heritable factors” • Mendel’s “heritable factors” are genes

  13. Second concept • Alternative versions of genes account for variations in inherited characters • alleles • locus

  14. Third concept • For each character an organism inherits two alleles, one from each parent • Mendel made this deduction without knowing about chromosomes • The two alleles at a locus on a chromosome may be identical, or two alleles at a locus may differ • Homozygous (dominant or recessive) • Heterozygous

  15. pair of homologous chromosomes Both chromosomes carry the same allele of this gene so the organism is homozygous at this locus This gene represents another homozygous locus The chromosomes carry different alleles of this gene so the organism is heterozygous at this locus

  16. Fourth concept • If alleles at a locus differ: • dominant allele determines appearance while recessive allele has no noticeable effect • Genotype • Phenotype

  17. Fifth concept • The two alleles for a heritable character separate (segregate) during gamete formation • egg or sperm (haploid) gets only one of the two alleles present in somatic cells (diploid) • corresponds to distribution of homologous chromosomes during meiosis

  18. True-breeding parents: homozygous for trait F1 generation: all heterozygous dominant genotype and phenotype F2 generation: heterozygous and homozygous genotypes variety of phenotypes

  19. Monohybrid Cross Conclusions • Pea possesses 2 genes (alleles) for each character • Genes do not blend • one is dominant and masks the other • When gametes form, the 2 alleles separate so that each gamete gets only 1 • Law of segregation • alleles of a gene separate from each other during gamete formation

  20. Punnett square • diagram for predicting results of a genetic cross • capital letter for dominant allele, and lowercase letter for recessive allele

  21. Widow’s peak is coded for by a single gene and has 2 only alleles • W codes for peak, w codes for straight hair line

  22. If a man homozygous for a straight hair (ww) line has a child with a woman homozygous for a Widow’s peak (WW) what type of hair line will that child have? A) all homozygous recessive with straight hairline B) all homozygous dominant with Widow’s peak C) all heterozygous with Widow’s peak

  23. Hitch-hiker’s thumb is coded for by a single gene and has 2 only alleles H codes for a curve, h codes for a straight thumb

  24. If a man heterozygous for a curved thumb has a child with a woman also heterozygous for a curved thumb what’s the chance their child will have straight thumbs? A) 25 % - homozygous recessive with straight thumb B) 0% - all homozygous dominant with curved thumb C) 0% - all heterozygous with curved thumb D) 50% - heterozygous with straight thumb

  25. Both you and your sister have attached earlobes but your parents both have unattached earlobes. Unattached earlobes are dominant (E) over attached (e). How is this possible? A) both parents are homozygous dominant B) both parents are homozygous recessive C) both parents are heterozygous for the trait

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