Patterns of Gene Inheritance

1. Patterns of Gene Inheritance

2. Genetics is the study of genes What is a gene? A. A factor that controls a heritable characteristic B. Something on a chromosome C. Information stored in a segment of DNA D. Something that encodes a protein

3. Gregor Mendelthe “father” of genetics

4. LOTS O’ DEFINITIONS • Gene---A piece of DNA that encodes a particular trait. EX a gene for eye-color • Allele—an alternate form of a gene. EX—allele for blue eyes and allele for brown eyes • Phenotype—the physical expression of a gene or allele. Ex. Blue eyes • Genotype—the genetic composition of an individual.

5. MORE DEFINITIONS • Locus—the location of a gene on a chromosome. Plural=loci • Dominant allele—expressed whether alone or in pairs. Symbolized by a capital letter. EX Brown eye allele=B • Recessive allele—expressed only in the absence of a dominant allele. Symbolized by lowercase. EX blue eye allele=b • What is the maximum # of alleles a diploid individual can have at any given locus?

6. STILL MORE DEFINITIONS!!! • Genotype—the genetic composition of an individual • Homozygous----containing a pair of the same alleles. Can be • Homozygous recessive—two recessive alleles EX. bb, or • Homozygous dominant– two dominant alleles EX BB • Heterozygous—containing two different alleles. EX Bb

7. Law of Segregation • Each individual has two factors (called genes today) for each trait. • Factors segregate during gametogenesis. Ploidy level of gametes? • Fertilization gives each new individual two factors again. • Ploidy level after fertilization?

8. Gene locus Defs

9. Example--Widow’s Peak

10. Gametogenesis • Homologous pairs separate during meiosis. When? •  a gamete has only one allele from each pair of alleles. •  If the primary spermatocyte is genotype Ww, the sperm cell would contain either a W or a w, but not both. • Ww represents the genotype of an individual. • Gametes are represented by W or w.

11. One-Trait Crosses GENOTYPE? GENOTYPE? GENOTYPE?

12. Punnett SquareEXAMPLE 1—A Monohybrid Cross

13. Monohybrid cross

14. QUESTION: How can we determine the genotype of a dominant phenotype? The One-Trait Testcross Testcross: cross the dominant phenotype (unknown genotype) with the recessive phenotype (known genotype).

15. The Law of Independent Assortment • Law of Segregation involves 1 character. What about 2 (or more) characters? • Each pair of alleles segregates independently of the other pairs • All possible combinations of alleles can occur in the gametes • EX: two pairs of alleles segregate independently of each other. • When will different alleles not sort independently?

16. Segregation and Independent Assortment

17. MEIOSIS & GENETIC VARIATION Independent Assortment • Diploid organisms can produce 2n diff. gametes • Ea. homologous pair can orient in two different ways • Humans: 223 = 8,388,608 (est. 8.4 million) Each homologous pair can orient in either of two ways

18. HYPOTHETICAL EXAMPLE OF INDEPENDENT ASSORTMENT Eye color Hair color Gene for browneyes Gene for blueeyes r Gene for black hair Gene for red hair

19. INDEPENDENT ASSORTMENT During meiosis I, tetrads can line up 2n different ways. OR Meiosis I & II Brown eyesBlack hair Blue eyesRed hair Blue eyesBlack hair Brown eyesRed hair

20. Dihybrid cross What are the possible genotypes of this individuals gametes? Probabilities

21. Genetic Disorders • Pedigree charts- show patterns of inheritance

22. Autosomal Dominant Disorders • Autosomes---Non-sex chromosomes • Dominant allele • when under what genotype(s)will an individual show the disorder?

23. Examples: Autosomal Dominant Disorder • Neurofibromatosis • Small benign tumors, • Gene on chromosome 17. • Huntington Disease • Progressive degeneration of the nervous system • Normal until middle age • No treatment • Gene has been isolated (chromosome 4), genetic testing can diagnose

24. Huntington disease

25. Autosomal recessive pedigree chart • Autosomes---Non-sex chromosomes • Recessive allele • when under what genotype(s)will an individual show the disorder?

26. Autosomal Recessive Disorders • Tay-Sachs Disease • Common among United States Jews of central and eastern European descent. • Death by the age of three or four. • Defective enzyme in lysosomes. • Cystic Fibrosis • Most common lethal genetic disorder among Caucasians • Defective chloride ion transport protein • Osmotic imbalance results in thick mucous in lungs and pancreatic ducts

27. Cystic fibrosis therapy

28. Phenylketonuria (PKU) • allele on chromosome 12. • lack an enzyme needed for metabolism of phenylalanine (an amino acid) • Urine test diagnostic. • Brain damage unless controlled by diet.

29. Polygenic Inheritance • Polygenic (Quantitative) Traits • Governed by more than one gene pair. • Several genes determine the phenotype. • Produce bell-shaped curve. • EX: Skin color

30. EX 2 Polygenic Inheritance :

31. Polygenic Disorders • Examples: cancer, schizophrenia, hypertension, diabetes, etc • several genes involved • also environmental influences.

32. Multiple Allelic Traits • Often more than two alleles exist for a particular gene locus. • Maximum # of alleles at any given locus? • Each individual inherits only two alleles for these genes!!! • EX: Human Blood type

33. Inheritance of blood type Details

34. Incompletely Dominant Traits • Codominance--both alleles equally expressed in a heterozygote. EX ABO Blood type • Incomplete dominance--heterozygote shows intermediate phenotype, representing a blending of traits. EX Wavy hair in Caucasians • Phenotypic ratio ? • 1 : 2 : 1.

35. Incomplete dominance

36. Example 2-Incomplete Dominance

37. Sickle-Cell Disease • Caused by incompletely dominant alleles. • Single nucleotide mutation causes abnormal hemoglobin. • HbA = normal hemoglobin • HbS = sickled condition. • Genotype of normal? • Genotype of sickled homozygote? • Genotype of intermediate phenotype?

40. Everything’s Relative

41. WHAT’S HAPPENING??

42. REMEMBER!!! • Relate genetics to meiosis • For genetics problems—first try to figure the genotype(s) of the gametes