Welcome to IB 201!

Welcome to IB 201!

Genetics and Evolution

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A phenotype ratio of 9:3:3:1 in the offspring of a mating between two individuals that are heterozygous for two traits occurs when:A. the genes reside on the same chromosomeB. each gene contains two mutationsC. the gene pairs assort independently during meiosisD. only recessive traits are scoredE. none of the above

Strategies for Learning Genetics

Strategy

Take notes during lecture and review before exam

Summarize lecture in your own words after class

Self-questioning during lecture

Exam Grades

Worst

Intermediate

Best

Why Genetics and Evolution?

Evolution is unifying principle of biology.

“Nothing in biology makes sense except in the light of evolution”

Theodosius Dobzhansky (geneticist)

Darwin’s Postulates

Individuals within species are variable. Some of these variations are passed on to

offspring. In every generation, more offspring are

produced than can survive. Survival & reproduction of individuals are not

random. Those that survive and reproduce are those with the most favorable variations. They are naturally selected.

Darwin’s Success

He compiled massive amounts of evidence that all these postulates are true.

Showed conclusively that individuals within species are variable, and that some of these variations are passed on to the offspring.

Darwin’s ProblemsBecause mechanisms of inheritance were not

known, two objections were raised:1. Inheritance acts like pigment in paint

(“blending inheritance”). Any new variant would mix with existing traits and be swamped by them. Thus, even if new variation could be created, it would not persist.

2. Variation within species is limited. Once existing variation is exhausted, evolution by natural selection will grind to a halt.

Discoveries in genetics solved both these problems.

Mendel solves one of Darwin’s Problems

When Origin was published, an unknown monk was cultivating peas in central Europe. Gregor Mendel would demonstrate that objection 1 is invalid by showing that inheritance is particulate, not blending.

Darwin had Mendel’s paper but did not read it,; he also did crosses but did not go beyond the F1!

Morgan solves the other In 1900 DeVries, Von Tschermak, and Correns

published experiments similar to Mendel’s, and attributed priority of discover to him.

Within a few years, T.H. Morgan & colleagues showed that new hereditary variation (mutation) occurs in every generation.

By beginning of the 20th century, both objections to Darwin’s principles had been conclusively eliminated by discoveries in genetics.

“Modern Synthesis”

Evolution by natural selection was not fully accepted until the hereditary mechanism was understood.

The “Modern Synthesis” is the synthesis of Mendelism and Darwinism that occurred in the early 20th century.

The Modern Synthesis is the foundation of modern evolutionary biology.

Modern genetics is also based on evolutionary biology and the Modern

Synthesis Medical population genetics: patterns of genetic

disease and disease resistance Genetic epidemiology:identification and spread of

new disease vectors: Hanta virus, AIDS, SARS Genome biology: Sequencing projects for non-

human organisms can tell us where did human genes come from, which genes are conserved and which have diverged,and how did eukaryotic genomes come to be organized the way they are (introns, exons, repetitive DNA).

Course OutlineDeviations from Mendelism: Epistasis; Unusual Modes of InheritanceGenetic Data Analysis:

Probability & StatisticsChromosomal Inheritance:

Chromosomal Abnormalities; Sex DeterminationMapping:

Gene and Genome MappingTraits Affected by Genes & Environment

Quantitative TraitsGenes in Populations

Genetic Mechanisms of Evolution; Population Genetics of Disease and Disease resistance

Genomes and Genome Evolution

Additional Deviations from Mendelism

Lethal AllelesEpistasis

Unusual sex linkageSex influenced inheritance

Genetic Anticipation

Manx Cats

Lethal alleles

F1: Mm x Mm

F1: 1 MM 2 Mm 1 mm

F2: 1 Lethal: 2 Manx: 1 Normal

F2 phenotypic ratio: 2:1 instead of 3:1

Other lethal mutations

Achondroplasia (humans)

Yellow body color (domestic mice) Curly wings (Drosophila)

Agouti: wild type

F2 Phen. ratio: 9 agouti : 3 black : 4 albino

agouti

F1: agouti

P: agouti albino

F2: 9/16 B- C- 3/16 bb C- 3/16 B- cc 1/16 bb cc

Simple dominant phenotype?

novel phenotype

BB CC x bb cc

black albino albino

Bb Cc

Epistasis

Locus 1 Locus 2BB Bb bb CC Cc ccagouti agouti black no effect no effect albino

Biochemical model

CC or Cc: tyrosinase is produced (involved in production of melanin)

BB or Bb: controls distribution of the pigment

C enzyme present? B enzyme present?

Colorless precursor ‡ Yes: CC or Cc some melaninproduced

Yes: BB or Bb agouti

No: cc no melanin produced No: bb black

EpistasisNormal dihybrid ratio is altered from 9:3:3:1 to 9:3:4

C and B gene have an epistatic interaction

Yellow Brown

X

F2 generation

F1 generation

Parentalgeneration

Red9/16

Yellow3/16

Brown3/16

Green1/16

RedSelf-fertilization

Figure 10.18bCrosses between pure lines produce novel colors.

R-Y- rrY- R-yy rryy

R-Y-

rrYY RRyy

Codominance?

Genotype Color Explanation of color

R-Y-

rrY-

R-yy

Red

Yellow

Brown

Red pigment + no chlorophyll

Yellow pigment + no chlorophyll

Red pigment + chlorophyll

rryy Green Yellow pigment + chlorophyll

Gene 1 Gene 2

R = Red

r = Yellow

(-) = R or r

Y = Absence of green (no chlorophyll)

y = Presence of green (+ chlorophyll)

(-) = Y or y

Figure 10.18c

Model to explain 9 : 3 : 3 : 1 pattern observed above: Two genes interact to produce pepper color.

Practice Problem

In Labrador retrievers, coat color is controlled by two loci each with two alleles B,b and E,e respectively. When pure breeding Black labs with genotype BB EE are crossed with pure breeding yellow labs of genotype bb ee the resulting F1 offspring are black. F1 offspring are crossed (Bb Ee x Bb Ee). Puppies appear in the ratio:

9/16 black; 9/16 black; 3/16 chocolate3/16 chocolate;; 4/16=1/4 yellow.

What genotypes correspond to these three phenotypes?

9/16 B- E- 3/16 B- ee 3/16 bb E- 1/16 eebbB- E- B- ee bb E- and bb ee

Other kinds of epistasis

1/16 aabb9/16 A-B- 3/16 A-bb 3/16 aaB-

Hint: usually given numbers, not fractions 27 agouti; 12 albino; 9 black 28 agoute; 11 albino; 4 black

Practice ProblemIn the summer squash (Cucurbita pepo) fruit shape is determined

by two genes. Two different true-breeding spherical types were crossed. The F1's were all disk, and the F2's segregated 35 disk, 25 spherical and 4 long. Explain these results.

What’s the first step? Notice novel phenotype: disk, long.

What’s the next step? Notice there are three F2 phenotypes. What kind of inheritance will give three F2 phenotypes?

Incomplete, codominanceEpistasis

Expected F2 ratio?1:2:1Variation on 9:3:3:1

Practice Problem, cont.In the summer squash (Cucurbita pepo) spherical fruit is

recessive to disk, True-breeding spherical types from different geographic regions were crossed. The F1's were disk, and the F2's segregated 35 disk, 25 spherical and 4 long. Explain these results.

Are the phenotypic ratios closer 1:2:1 or to a variant of 9:3:3:1 ?

If phenotypic ratios closer to a variant of 9:3:3:1, then what variant is it?Total # of individuals = 35 + 25 + 4 = 6464/16 = 49*4 = 366*4 = 241*4 = 4Phenotypic ratio close to 9:6:1

Practice Problem, cont.In the summer squash (Cucurbita pepo) spherical fruit is

recessive to disk, True-breeding spherical types from different geographic regions were crossed. The F1's were disk, and the F2's segregated 35 disk, 25 spherical and 4 long. Explain these results.

If phenotypic ratios are close to 9:6:1, then what are the genotypes associated with each phenotype?

35 disk 25 spherical 4 long9/16 A- B- 3/16 A- bb + 3/16 aa B- 1/16 aa bb

What were the genotypes of the original spherical parents?

AA bb aaBB