Patterns of Inheritance

1. Patterns of Inheritance Chapter 13

2. Outline • Early Ideas of Heredity • Mendel • Gene Disorders • Multiple Alleles • Pedigrees • Gene Disorders Due to Protein Alteration • Chromosome and Inheritance • Genetic Recombination • Human Chromosomes

3. Early Ideas of Heredity • Classical assumptions • First, heredity occurs within species. • species maintained without significant change since time of creation • Second, traits are transmitted directly and independently. • paradox - all members of same species should eventually have the same appearance • hybrids differ in appearance

4. Early Ideas of Heredity • Early geneticists demonstrated some forms of an inherited character can: • disappear in one generation and reappear, unchanged, in future generations. • segregate among offspring of a cross. • be more likely to be represented than alternative forms.

5. Mendel and the Garden Pea • Advantages of garden pea : • many hybrids previously produced • expect segregation of traits • large number of true-breeding varieties • small and easy to grow • short generation time • sexual organs enclosed in flower • self-fertilization • cross fertilization

6. Mendel and the Garden Pea • Mendel’s experimental design • allowed pea plants to self-fertilize for several generations • assured pure-breeding traits • performed crosses between varieties exhibiting alternative character forms • permitted hybrid offspring to self-fertilize for several generations

7. Mendel’s Experiments

8. What Mendel Found • F1 Generation (first filial) • Offspring of white flower and purple flower cross had flower color resembling one parent (no intermediate color). • All flowers exhibited purple flowers (dominant trait) and none exhibited white flowers (recessive trait).

9. Mendel’s Results • F2 Generation (second filial) • Cross between seeds of F1 generation produced some plants exhibiting white flowers (recessive form reappeared). • dominant : recessive ratio among F2plants was always close to 3:1 • Mendelian Ratio • discovered ¼ of recessives were always true breeding • disguised 1:2:1 ratio

10. Second Filial Generation

11. Mendel’s Model of Heredity • Parents transmit discrete physiological trait information (factors) to offspring. • Each individual receives two factors that may code for same, or alternative, character traits. • Not all copies of a factor are identical. • alleles • homozygous - same alleles • heterozygous - different alleles

12. Mendel’s Model of Heredity • Alleles do not influence each other in any way. • Presence of a particular allele does not ensure its encoded trait will be expressed. • genotype - totality of an individual’s alleles • phenotype - physical appearance

13. Interpretation of Mendel’s Results • Notational convention • P - dominant allele (purple) • p - recessive allele (white) • PP - homozygous dominant • Pp - heterozygous • pp - homozygous recessive

14. Interpretation of Mendel’s Results • F1 generation • PP x pp (parental generation) yielded all Pp offspring • F2 generation • Pp x Pp yielded: (1:2:1) ratio • 1 PP • 2 Pp • 1 pp • Punnett squares

15. Mendel’s Cross

16. Mendelian Inheritance • Mendel’s First Law of Heredity • (Law of Segregation) • Alternative alleles of a character segregate from each other in heterozygous individuals and remain distinct.

17. Testcross • Cross of a plant with an unknown genotype (PP or Pp) with a homozygous recessive individual, will yield one of two possible results: • pp x PP = 100% (Pp) • pp x Pp = 50% (pp) : 50% (Pp)

18. Testcross

19. Mendelian Inheritance • Mendel’s Second Law of Heredity • (Law of Independent Assortment) • Genes that are located on different chromosomes assort independently of one another.

20. Mendelian Inheritance • Phenotype considerations • continuous variation • The greater the number of genes influencing a character, the more continuous the expected distribution of character variation will be. • pleiotropic effects • Individual alleles often have more than one effect on the phenotype.

21. Phenotypic Considerations • Incomplete dominance • Heterozygotes are intermediate in color. • Environmental effects • degree of allele expression may depend on the environment • Epistasis • one gene interferes with the expression of another gene • coat color in Labrador retrievers

22. Epistatic Interactions

23. Gene Disorders • Gene disorder refers to the harmful effect a detrimental allele produces when it occurs at a significant frequency in a population. • Most disorders are rare because affected individuals often die at a relatively young age, or cannot reproduce. • Not all defects are recessive. • Huntington disease

24. Multiple Alleles: ABO Blood Group • Codominance - No single allele is dominant, and each allele has its own effect. • ABO blood groups • human gene that encodes enzyme that adds sugar molecules to lipids on the surface of red blood cells • IB adds galactose • IA adds galactosamine • i adds no sugar

25. ABO Blood Groups

26. Pedigrees • Mutations are accidental changes in genes. • rare, random, and usually result in recessive alleles • pedigrees used to study heredity • hemophilia - inherited condition where blood is slow to clot or does not clot at all • only expressed when individual has no copies of the normal allele • Royal hemophilia - sex-linked

27. Royal Hemophilia Pedigree

28. Gene Disorders Due to Protein Alteration • Sickle-cell anemia is a recessive inherited disorder in which afflicted individuals have defective hemoglobin, and thus are unable to properly transport oxygen to tissues. • Homozygotes have sickle-cell. • Heterozygotes usually appear normal, but are resistant to malaria.

29. Curing Defects with Gene Therapy • Cystic fibrosis • body cells of affected individuals secrete thick mucus that clogs airways of lung • defect in cf gene • Researchers are currently working on transmitting a working copy of cf gene using viruses. • Early attempts using adenovirus vectors produced mixed results.

30. Chromosomes and Mendelian Inheritance • In early 20th century, it was not obvious chromosomes were vehicles of heredity information • chromosomal theory of inheritance first formulated in 1902 • problems quickly arose in trying to track independent assortment

31. Chromosomes and Inheritance • A trait determined by a gene on the sex chromosome is said to be sex-linked. • In Drosophila, sex is determined by the number of copies of the x chromosome. • Mendelian traits assort independently because chromosomes assort independently.

32. Sex Linkage in Drosophila

33. Genetic Recombination • Crossing over • Genes located relatively far apart on a chromosome are more likely to cross over than genes located closer together. • Frequency of crossings can be used to construct a genetic map. • measures distance between genes in terms of recombination frequency

34. Human Chromosomes • Human somatic cells normally have 23 pairs of chromosomes. • divided into seven groups characterized by size and shape • 22 pairs of autosomes • 1 pair of sex chromosomes • XX = Female • XY = Male

35. Human Chromosomes • One x chromosome in females is inactivated early in embryonic development. • Visible as a darkly staining Barr body attached to the nuclear membrane.

36. Alterations in Chromosome Number • Failure of chromosomes to separate correctly during meiosis I or II is called primary nondisjunction. • Down Syndrome caused by trisomy 21 • 1 in 1700 for mothers < 20. • 1 in 1400 for mothers >20<30. • 1 in 750 for mothers >30<35. • 1 in 16 for mothers >45.

37. Nondisjunction in Sex Chromosomes • X Chromosome • XXX or XXY yields Klinefelter syndrome • XO yields Turner syndrome • Y Chromosome • XYY - Jacob syndrome

38. Nondisjunction

39. Genetic Counseling • Genetic counseling identifies parents at risk of producing children with genetic defects and assesses the state of early embryos. • High-risk pregnancies • couples with recessive alleles • mothers older than 35 • amniocentesis • chorionic villi sampling

40. Genetic Counseling • Counselors can look for three things in cell cultures in search of genetic disorders: • aneuploidy or gross alterations • proper enzyme functioning • association with known genetic markers