Chapter 12 – Patterns of Inheritance

1. Chapter 12 – 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. 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

23. Test cross: Determine unknown genotype by mating it with a known genotype