Heritable variation among individuals

1. Heritable variation among individuals • Read Chapter 5 of your text

2. Heritable variation among individuals • Variation provides the raw material of evolution. • Without variation there could be no selection because there would be no differences to select for or against.

3. Discovery of genes • Heredity was a big problem for Darwin because he didn’t know how it worked. • Darwin knew offspring resembled their parents, but it was widely believed that heritability was a sort of blending process akin to the way different paints can be mixed to produce a new shade. • The problem with blending inheritance is that a new trait would be diluted in a large population and disappear.

4. Discovery of genes – inheritance is particulate • Gregor Mendel (1822-1884) proved that inheritance is not a blending process. • Instead he showed that discrete particles (we now call them genes) which remain intact through many generations carry the hereditary information. • An individual allele may sometimes be hidden in a generation (e.g. a recessive allele as a heterozygote), but later reappear intact in a later generation when present as a homozygote. • Demonstrated this with his famous experiments using pea plants (see box 5.2 pages 142-143 of the text or any introductory biology text for a description of Mendel’s work)

5. Gene-centered thinking • Different versions of genes, which we call alleles, are the ultimate target of natural selection because they can last for generations passing from body to body. • Changes in population allele frequencies result in evolution. • Important to remember that individual bodies built by genes are temporary assemblages of sets of genes.

6. Gene-centered thinking • Individual organisms live and die. Each body (“survival machine in Dawkin’s term from his book the Selfish Gene”) is built by a temporary collection of genes working together. • Alleles that work well with others and help to build well adapted bodies will become more common and those that don’t will be disappear.

7. Gene-centered thinking • To illustrate the idea of selection judging individual genes from the products they build, imagine trying to select the best crew of rowers for an 8-man boat from a large pool of potential rowers. • By randomly making crews and racing boats against each other and repeating the practice many time you would eventually realize that certain rowers tended to be found more often in winning boats and others in losing boats. • Even though strong rowers would sometimes be in losing boats, on average, they would win more often than weaker rowers. Using the information on wins you could then build a very strong crew. • Similarly, genes that tend to build more successful bodies on average would be favored by selection and spread.

8. Genes • Mendel did not know what genes were, but we know today that they are made of DNA and that they work by coding the structure of proteins. • Proteins are made of chains of amino acids joined together and DNA dictates the identity and order in which amino acids are joined together.

9. Structure of DNA • DNA made up of sequence of nucleotides. Each nucleotide includes a sugar, phosphate and one of four possible nitrogenous bases (adenine and guanine [both purines], and thymine and cytosine [both pyrimidines]).

10. 4.1a

11. 4. 4.1b + 4.1d

12. Structure of DNA • The opposite strands of the DNA molecule are complementary because the strands are held together by bonds between the opposing bases and adenine bonds only with thymine and cytosine only with guanine. • Thus, knowing the sequence on one strand enables one to construct the sequence on the other strand.

13. 4.2

14. Structure of DNA • The sequence of nucleotides in a gene codes for the protein structure as each three nucleotide sequence codes for one amino acid in the protein chain.

16. 4.3a

17. Transcription and translation • To make a protein the DNA must first be transcribed into an RNA copy (called mRNA for messenger RNA) and that mRNA translated into a protein or polypeptide.

18. Production of protein from DNA requires transcription and translation Gene expression: process by which information from a gene is transformed into product

19. Ribosomes translate mRNA into protein

20. One gene one protein • The expression “one gene one protein” is widely used, but most genes actually code for multiple proteins because they join different “exons” the executable or coding portions of a gene together to make different proteins. This process is called alternative splicing.

21. RNA splicing can create multiple proteins from a single gene

22. Mutations: creating variation • A change in the structure of DNA, which may perhaps result in a change in the protein coded for, is called a mutation. • Mutations are the ultimate source of all genetic variation. • A change to a gene can result in a new allele (version of a gene) being produced.

23. Where do new alleles come from? • When DNA is synthesized, an enzyme called DNA polymerase reads one strand of the DNA molecule and constructs a complementary strand. • If DNA polymerase makes a mistake and it is not repaired, a mutation has occurred.

24. Mutation and genetic variation • Mutations are raw material of evolution. • No variation means no evolution and mutations are the ultimate source of variation.

25. Types of mutations • A mistake that changes one base on a DNA molecule is called a pointmutation.

26. Examples of point mutations

27. Type of mutations • A point mutation in a gene coding for the structure of one of the protein chains in a hemoglobin molecule is responsible for the condition sickle cell anemia.

28. Types of mutations • Not all mutations cause a change in amino acid coded for. These are called silent mutations. • Mutations that do cause a change in amino acid are called replacementmutations.

30. Types of mutations • Another type of mutation occurs when bases are inserted or deleted from the DNA molecule. • This causes a change in how the whole DNA strand is read (a frame shift mutation) and produces a non-functional protein.

32. Types of mutations • There are multiple other forms of mutations that involve larger quantities of DNA. • Genes may be duplicated as may entire chromosomes or even entire genomes. • Genes may also be inverted.

34. Where do new genes come from? • Mutation can produce new alleles, but new genes are also produced and gene duplication appears to be most important source of new genes.

35. Gene duplication • Duplication results from unequal crossing over when chromosomes align incorrectly during meiosis. • Result is a chromosome with an extra section of DNA that contains duplicated genes

36. 4.7

37. Gene duplication • Extra sections of DNA are duplicates and can accumulate mutations without being selected against because the other copies of the gene produce normal proteins. • Gene may completely change over time so gene duplication creates new possibilities for gene function.

38. Globin genes • Human globin genes are examples of products of gene duplication. • Globin gene family contains two major gene clusters (alpha and beta) that code for the protein subunits of hemoglobin.

39. Globin genes • Hemoglobin (the oxygen-carrying molecule in red corpuscles) consists of an iron-binding heme group and four surrounding protein chains (two coded for by genes in the Alpha cluster and two in the Beta cluster).

40. Globin genes • Ancestral globin gene duplicated and diverged into alpha and beta ancestral genes about 450-500 mya. • Later transposed to different chromosomes and followed by further subsequent duplications and mutations.

41. From Campbell and Reese Biology 7th ed.

42. Globin genes • Lengths and positions of exons and introns in the globin genes are very similar. Very unlikely such similarities could be due to chance.

43. Exons (blue), introns (white), number in box is number of nucleotides. 4.9

44. Globin genes • Different genes in alpha and beta families are expressed at different times in development. • For example, in a very young human fetus, zeta (from alpha cluster) and epsilon (from beta cluster) chains are present initially then replaced. Similarly G-gamma and A-gamma chains present in older fetuses are replaced by beta chains after birth.

45. 4.8 Gestation (weeks) Post-birth(weeks) Fetal hemoglobin has a higher affinity for oxygen than adult hemoglobin. Enhances oxygen transfer from mother to offspring.

46. Chromosomal alterations • Two major forms important in evolution: inversions and polyploidy.

47. Inversions • A chromosome inversion occurs when a section of chromosome is broken at both ends, detaches, and flips. • Inversion alters the ordering of genes along the chromosome.

48. 4.10

49. Inversions • Inversion affects linkage (linkage is the likelihood that genes on a chromosome are inherited together i.e., not split up during meiosis). • Inverted sections cannot align properly with another chromosome during meiosis and crossing-over within inversion produces non-functional gametes. • Genes contained within inversion are inherited as a set of genes also called a “supergene”

50. Inversions • Inversions are common in Drosophila (fruit flies) • Frequency of inversions shows clinal pattern and increases with latitude. • Inversions are believed to contain combinations of genes that work well in particular climatic conditions.