Development and Inheritance

1. Development and Inheritance Muse W12 2440 lecture # 13 4/18/12

2. Gestation • First Trimester • Period of embryological and early fetal development • Rudiments of all major organ systems appear • Second Trimester • Development of organs and organ systems • Body shape and proportions change • By end, fetus looks distinctively human • Third Trimester • Rapid fetal growth and deposition of adipose tissue • Most major organ systems are fully functional

3. The First Trimester Figure 29–7a The First Trimester.

4. The First Trimester Figure 29–7b The First Trimester.

5. What will I be when I grow up?

6. What will I be when I grow up?

7. What will I be when I grow up?

8. The First Trimester Figure 29–7c The First Trimester.

9. The First Trimester Figure 29–7d The First Trimester.

10. The Second and Third Trimesters • Second Trimester • Fetus grows faster than surrounding placenta • Third Trimester • Most of the organ systems become ready • Growth rate starts to slow • Largest weight gain • Fetus and enlarged uterus displace many of mother’s abdominal organs

11. The Second and Third Trimesters Figure 29–8a The Second and Third Trimesters: A Four-Month-Old Fetus As Seen through a Fiber-Optic Endoscope.

12. The Second and Third Trimesters Figure 29–8b The Second and Third Trimesters: Head of a Six-Month-Old Fetus As Seen through Ultrasound.

13. The Second and Third Trimesters Figure 29–9c, d Growth of the Uterus and Fetus.

14. Inheritance • Nucleated Somatic Cells • Carry copies of original 46 chromosomes present in zygote • Genotype • Chromosomes and their component genes • Contain unique instructions that determine anatomical and physiological characteristics • Derived from genotypes of parents • Phenotype • Physical expression of genotype • Anatomical and physiological characteristics

15. Inheritance • Homologous Chromosomes • Members of each pair of chromosomes • 23 pairs carried in every somatic cell • At amphimixis, one member of each pair is contributed by spermatozoon, other by ovum

16. Inheritance • Autosomal Chromosomes • 22 pairs of homologous chromosomes • Most affect somatic characteristics • Each chromosome in pair has same structure and carries genes that affect same traits

17. Inheritance • Sex Chromosomes • Last pair of chromosomes • Determine whether individual is genetically male or female • Karyotype • Entire set of chromosomes • Locus • Gene’s position on chromosome

18. Inheritance Figure 29–14 A Human Karyotype.

19. Inheritance • Alleles are various forms of given gene • Alternate forms determine precise effect of gene on phenotype • Homozygous • Both homologous chromosomes carry same allele of particular gene • Simple Inheritance • Phenotype determined by interactions between single pair of alleles

20. Inheritance • Heterozygous • Homologous chromosomes carry different allele of particular gene • Resulting phenotype depends on nature of interaction between alleles • Strict Dominance • Dominant allele expressed in phenotype, regardless of conflicting instructions carried by other allele

21. Inheritance • Recessive Allele • Expressed in phenotype only if same allele is present on both chromosomes of homologous pair • Incomplete Dominance • Heterozygous alleles produce unique phenotype • Codominance • Exhibits both dominant and recessive phenotypes for traits

22. Inheritance • Penetrance • Percentage of individuals with particular genotype that show “expected” phenotype • Expressivity • Extent to which particular allele is expressed • Teratogens • Factors that result in abnormal development • Punnett Square • Simple box diagram used to predict characteristics of offspring Mutation - change in normal form of gene

23. Inheritance Figure 29–15 Predicting Phenotypic Characters by Using Punnett Squares.

24. Inheritance • Polygenic Inheritance • Involves interactions among alleles on several genes • Cannot predict phenotypic characteristics using Punnett square • Linked to risks of developing several important adult disorders • Suppression • One gene suppresses other • Second gene has no effect on phenotype

25. Inheritance

26. Inheritance • Complementary Gene Action • Dominant alleles on two genes interact to produce phenotype different from that seen when one gene contains recessive alleles • Sources of Individual Variation • During meiosis, maternal and paternal chromosomes are randomly distributed • Each gamete has unique combination of maternal and paternal chromosomes

27. Inheritance • Genetic Recombination • During meiosis, various changes can occur in chromosome structure, producing gametes with chromosomes that differ from those of each parent • Greatly increases range of possible variation among gametes • Can complicate tracing of inheritance of genetic disorders

28. Inheritance • Crossing Over • Parts of chromosomes become rearranged during synapsis • When tetrads form, adjacent chromatids may overlap • Translocation • Reshuffling process • Chromatids may break, overlapping segments trade places

29. Inheritance Figure 29–17 Crossing Over and Translocation.

30. Inheritance • Genomic Imprinting • During recombination, portions of chromosomes may break away and be deleted • Effects depend on whether abnormal gamete is produced through oogenesis or spermatogenesis

31. Inheritance • Chromosomal Abnormalities • Damaged, broken, missing, or extra copies of chromosomes • Few survive to full term • Produce variety of serious clinical conditions • Humans are poorly tolerant of changes in gene copy number (to few or too many = lethal or bad news) • Mutation • Changes in nucleotide sequence of allele

32. Inheritance • Spontaneous Mutations • Result of random errors in DNA replication • Errors relatively common, but in most cases error is detected and repaired by enzymes in nucleus • Errors that go undetected and unrepaired have potential to change phenotype • Can produce gametes that contain abnormal alleles

33. Inheritance • Carriers • Individuals who are heterozygous for abnormal allele but do not show effects of mutation

34. Inheritance • Sex Chromosomes • X Chromosome • Considerably larger • Have more genes than do Y chromosomes • Carried by all oocytes • Y Chromosome • Includes dominant alleles specifying that the individual will be male • Not present in females

35. Autosomes, sex chromosomes and sex determination Karyotype shows 46 chromosomes arranged in pairs by size and centromere position 22 pairs are autosomes – same appearance in males and females 23rd pair are sex chromosomes XX = female XY = male

36. Inheritance • Sperm • Carry either X or Y chromosome • Because males have one of each, can pass along either 50% chance of each

37. Inheritance • X-Linked • Genes that affect somatic structures • Carried by X chromosome • Inheritance does not follow pattern of alleles on autosomal chromosomes

38. Sex determination Males produce sperm carrying an X or Y Females only produce eggs carrying an X Individual’s sex determined by father’s sperm carrying X or Y Male and female embryos develop identically until about 7 weeks Y initiates male pattern of development SRY on Y chromosome Absence of Y determines female pattern of development

39. Inheritance Figure 29–18 Inheritance of an X-Linked Trait

40. Inheritance of red-green color blindness

41. Sex-linked inheritance Genotype Phenotype XCXC Normal female XCXc Normal female (carrier) XcXc Color blind female XCY Normal male XcY Color blind male • Genes for these traits on the X but not the Y • Red-green colorblindness • Most common type of color blindness • Red and green are seen as same color • Males have only one X • They express whatever they inherit from their mother

42. Inheritance • Human Genome Project • Goal was to transcribe entire human genome • Has mapped thousands of human genes • Genome • Full complement of genetic material

43. Inheritance Figure 29–19 A Map of Human Chromosomes.

44. Inheritance • Passage of hereditary traits from one generation to the next • Genotype and phenotype • Nuclei of all human cells except gametes contain 23 pairs of chromosomes – diploid or 2n • One chromosome from each pair came from father, other member from mother • Each chromosome contains homologous genes for same traits • Allele – alternative forms of a gene that code for the same trait • Mutation – permanent heritable change in allele that produces a different variant

45. Inheritance

46. Phenylketonuria or PKU example • Unable to manufacture enzyme phenylalanine hydroxylase • Allele for function enzyme = P • Allele that fails to produce functional enzyme = p • Punnet square show possible combinations of alleles between 2 parents • Genotype – different combinations of genes • Phenotype – expression of genetic makeup • PP – homozygous dominant – normal phenotype • Pp – heterozygous – normal phenotype • 1 dominant allele codes for enough enzyme • Can pass recessive allele on to offspring – carrier • pp - homozygous recessive – PKU • 2 recessive alleles make no functional enzyme

47. Inheritance • Alleles that code for normal traits are not always dominant • Huntington disease caused by dominant allele • Both homozygous dominant and heterozygous individuals get HD • Nondisjunction • Error in cell division resulting in abnormal number of chromosomes • Aneuploid – chromosomes added or missing • Monosomic cell missing 1 chromosome (2n-1) • Trisomic cell has additional chromosome (2n +1) • Down Syndrome – trisomy 21 – 3 21st chromosomes

48. Variations of Dominant-recessive inheritance • Simple dominance-recessive • Just described where dominant allele covers effect of recessive allele • Incomplete dominance • Neither allele dominant over other • Heterozygote has intermediate phenotype • Sickle-cell disease

49. Sickle-cell disease Sickle-cell disease HbAHbA – normal hemoglobin HbSHbS – sickle-cell disease HbAHbS – ½ normal and ½ abnormal hemoglobin Minor problems, are carriers for disease

50. Incomplete Dominance • Heterozygous individuals have an intermediate phenotype • Example: Sickling gene • SS = normal Hb is made • Ss = sickle-cell trait (both aberrant and normal Hb are made); can suffer a sickle-cell crisis under prolonged reduction in blood O2) • ss = sickle-cell anemia (only aberrant Hb is made; more susceptible to sickle-cell crisis)