Inheritance

1. Inheritance Chapter 29

2. Gregor Mendel 1822 - 1884 “Father of Genetics”

3. What Mendel did • He bred peas in the monastery garden at Brno, Czech Republic (then part of the AustroHungarian Empire). • Observed occasional variations in the appearance of these plants. • Selectively bred plants to consistently produce “characteristics” that were unusual. • Saw a pattern in the way that the unusual characteristics showed up. • Was the first to propose that these characteristics were passed from one generation to another by the gametes.

4. The Abby where Mendel worked

5. What Mendel did not do • He didn’t use the word “gene” to refer to subject of his work. • He didn’t see chromosomes. • He never used a Punnett square. • He never achieved fame in his lifetime for his work.

6. Charles Darwin 1809 - 1882 • Proposed the “Theory of Evolution”. • Actually, talked about “descent with modification from a common ancestor”. He didn’t use the word “evolution” very often. • Voyage of the Beagle 1831 – 1836. • Presented paper with Alfred Russell Wallace in 1858. • Published first edition of “Origin of Species” in 1859.

7. Some Vocabulary • Genetics – study of inheritance. • Autosomes – the 22 pairs of chromosomes that do not determine genetic sex. • Sex chromosomes – the 23rd pair, the X and the Y. • Karyotype – the diploid chromosomes displayed in their condensed form and paired as homologs

8. A typical karyotype

9. More Vocabulary • Alleles - a matched pair of two genes, coding for the same or alternative forms of a particular trait. Found at the same location (locus) on homologous chromosomes. • Homozygous – having the same alleles for a trait • Heterozygous – having different alleles for the same trait.

10. More words • Dominant – an allele that expresses itself and masks its partner. Example: brown hair is dominant over blond. • Recessive – the reverse of the above. The allele that is masked • Allele pairs are expressed as a pair of letters representing the trait. Example: Mendal’s peas came in tall and short. Tall is the dominant allele for height in peas. Therefore it is written as a capital “T”. • A heterozyote for height would be Tt, with the lowercase t representing the recessive.

11. Genotype vs. Phenotype • Genotype – the actual alleles an organism has is it’s genotype. In our heterozygote pea plant that would be Tt. • Phenotype – that which is expressed. Our pea plant maybe genotypically heterozygotic but phenotypically it is tall. • Homozygote dominant = TT phenotype = tall • Homozygote recessive = tt phenotype = short • Heterozygote = Tt phenotype = tall

12. Mendel’s Laws • Mendal discovered that if you bred plants that had two alleles for each trait that you would get the same ratios of phenotypes & genotypes whenever you crossed heterozygotes. It was like clockwork! • This was because of independent assortment and segregation, which became known as “Mendal’s Laws”

13. It works like this… Phenotypic ratio = 3:1 or 3 tall : 1 short Genotypic ratio = 1:2:1 or 1 homozygote dominant 2 heterozygotes 1 homozygote recessive

14. Example: PKU

15. Violation of Mendel’s Laws • Mendal’s laws only hold if: • there is random fertilization • the alleles are located on separate chromosomes • the alleles have a simple dominant/recessive relationship • there are only two alleles for that trait • they are not lethal to the zygote

16. Penetrance • Percentage of individuals with particular genotype that shows “expected” phenotype Expressivity • Extent to which particular allele is expressed

17. Teratogens • Factors that result in abnormal development

18. Sources of variation: segregation & independent assortment Assortment leads to many possibilities as far as gamete formation goes. For any genome it can be calculated as 2n, where n=the number of chromosome pairs.

19. So for a human with 23 chromosome pairs, the possible combinations of gametes = 223or8,388,608!(and that’s with out recombination)

20. Suppression • 1 gene suppresses other • Second gene has no effect on phenotype

21. Complementary Gene Action • Dominant alleles on 2 genes interact to produce phenotype different from when 1 gene contains recessive alleles

22. Sources of Individual Variation • During meiosis, maternal and paternal chromosomes are randomly distributed • Each gamete has unique combination of maternal and paternal chromosomes

23. Crossing Over and Translocation Figure 29–17

24. 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

25. Crossing Over • Parts of chromosomes become rearranged during synapsis • When tetrads form, adjacent chromatids may overlap

26. Translocation • Reshuffling process • Chromatids may break, overlapping segments trade places

27. 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

28. Chromosomal Abnormalities • Damaged, broken, missing, or extra copies of chromosomes • Few survive to full term • Produce variety of serious clinical conditions

29. Mutation • Changes in nucleotide sequence of allele

30. 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

31. A Map of Human Chromosomes

32. Human Genome Project • Goal is to transcribe entire human genome • Has mapped more than 38,000 human genes

33. Karyotyping • Determination of individual’s complete chromosomal complement

34. Types of inheritanceAside from simple dominant/recessive • Incomplete dominance – a dominant allele does not completely mask the recessive (red flower + white flower = pink flower). • Codominance – both traits are expressed together (red flower + white flower = stripes). • Multiple alleles – More than one allele for a trait. ABO blood group is an example. • Polygene – several alleles interact to produce a trait. Results are a continuous or quantitative phenotype, as in skin color.

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

36. Incomplete dominance:Sickle Cell

37. Codominance of multiple alleles

38. Polygenic inheritance

39. Sex-linked inheritance • Males only have one X chromosome. Therefore, if a trait is found only on the X it will be expressed in a male regardless of whether it is dominant or recessive. • X – inactivation occurs in females. Every normal woman has two Xs but they only need one. Therefore, one X chromosome turns off, forming a Barr body. • Because X – inactivation is random in most cases, it leads to a fine mosaic of cells in females.

40. Sex determination in humans

41. Color-blindness:a sex-linked trait

42. Environmental influences • Phenocopy – Developmental influences impact genetic expression in ways that appear to be genetic but are not inheritable. • Temperature, nutrition, non-genetic pathologies can have impacts that are expressed in ways that appear genetic.

43. Genetic defects • Aneuploidy – a defective set of genes. • Triploidy – an extra set of chromosomes • Trisomy – an extra single chromosome • Monosomy – a missing homolog • Trisomy of the 23rd chromosome – XXX = “super female” XXY = Klinefelter’s syndrome • Trisomy of the 21st chromosome leads to Down’s Syndrome.

44. Down syndrome

45. Klinefelter’s -a type trisomy affecting the sex chromosomes

46. Turner Syndrome: monosomy of the 23rd chromosome, X_

47. Monosomy of the 23rd chromosome Name that condition!

48. A Peek into the Future:Screening for genetic disorders

49. Thanks for a great term!