1. The Basic Principles of Heredity

2. Terminology
Phenotype
Genotype
Locus
Allele
Dominant allele
Recessive allele
Homozygous
Heterozygous

4. Mendel’s principles of inheritance
Segregation
During meiosis, alleles for each locus segregate
Independent assortment
Alleles of different loci distributed randomly into the gamete
Results in recombination
Production of new gene combinations not found in parent

5. Independent assortment
Segregation
Results from homologous chromosomes separating during meiosis
Independent assortment
Orientation of homologous chromosomes on the metaphase plate determines how chromosomes are distributed

6. Chromosomal basis for segregation

7. Seven characters in Mendel’s study of pea plants

8. One of Mendel’s pea crosses

9. Gene loci and their alleles

10. Monohybrid cross Dihybrid cross Test cross
Cross between homozygous parents
Differ at one locus
Dihybrid cross
Differ at two loci
Test cross
Between individual of unknown genotype and homozygous recessive individual

11. Monohybrid cross in guinea pigs

12. Dihybrid cross in guinea pigs

13. Test cross in guinea pigs

14. Genetic ratios can be expressed as probabilities
Product rule predicts combined probability of independent events
Sum rule predicts combined probability of mutually exclusive events

15. Rules of probability

16. Recombination of linked genes
Linkage
Tendency for a group of genes on same chromosome to be inherited together
Recombination of linked genes
Results from crossing-over in meiotic prophase I
By measuring frequency of recombination, can construct linkage map of chromosome

17. Two-point test cross to detect linkage in fruit flies

18. Sex chromosomes
Cells of females of many species contain two X chromosomes
Males have single X chromosome and single, smaller Y chromosome
Y chromosome determines male sex in most species of mammals

19. Sex determination in mammals

20. X-linked red-green colorblindness

21. Incomplete dominance Codominance
Heterozygote is intermediate in phenotype
Codominance
Heterozygote simultaneously expresses the phenotypes of both homozygotes

22. Incomplete dominance in four o’clocks

24. Multiple alleles: Pleiotropy:
more than 2 possible alleles for a gene Ex: human blood types
Pleiotropy:
genes with multiple phenotypic effect. Ex: sickle-cell anemia

25. Multiple alleles in rabbits

27. Polygenic Inheritance:
Epistasis:
a gene at one locus (chromosomal location) affects the phenotypic expression of a gene at a second locus.
Polygenic Inheritance:
an additive effect of two or more genes on a single phenotypic character Ex: human skin pigmentation and height

28. Epistasis in Labrador retrievers

29. Polygenic
inheritance in
human skin
pigmentation

30. Norm of reaction
Range of phenotypic possibilities from a single genotype under different environmental conditions
Example is height in human
Can be modified by factors such as diet
Genes that affect height set norm of reaction
Environment molds phenotype within norm of reaction