Merill

A

B

A

C

Mendel’s law of segregation

law of dominance

• Crossing true-breeding (homozygous) round (RR) with wrinkled (rr) peas, Mendel first obtained a population of exclusively round (Rr) individuals

Punett square

random segregation of gametes (RR:Rr:rr = 1:2:1)

Mendel’s law of independent assortment

• even alleles for separate traits are passed on independently from parents to offspring

• each individual contains two alleles determining the value of a discrete phenotypic trait

• Allele pairs separate (segregate) during game formation, then randomly unite at fertilization

Alleles at two (or more) different loci are sorted into gametes independently of one another

In case of traits being encoded by genes on different chromosomes, Mendel’s ratios were consistent with chromosome theory.

In case of linkage on the same chromosome, however, no recombinant genotypes would be expected:

Q10 C)

Q11 D)

Genes did not always segregate independently (1:1:1:1 ratio in test crosses) nor did they show full linkage (1:1:0:0).

• Thomas Hunt Morgan introduced the fruit fly as a model for genetics to test for patterns of inheritance using a variety of easy scorable phenotypes such as eye colour, wing shape

• For example, in a testcross between a female wild type fly homozygous for normal colouration (+) and long wings (+) with a homozygous male mutant of black body colouration (b) and vestigial (rudimentary) wings (vg) he obtained ratios that were neither predicted by Mendelian laws nor by chromosome theory:

—> clear evidence for crossing-over in the female germline

c)

111+106 / 465+586+111+106 = 217/1268 = 0.171, hence 17.1% recombinant genotypes

order and linear distances between genes using "three-factor crosses". Distance was arbitrarily expressed in units of recombinant frequencies as multiple of 0.01 (or 1%), a unit later denoted as one centimorgan (cM = 1 recombinant in 100 offspring).

A) 21

is species and sex-specific

1) 10

=10

LD can be thought of as a deviation from a null model of Linkage equilibrium (|D|)

Linkage equilibrium assumes: no selection, random mating, free recombination, large pops

LD will break down over time

• 
  

• a non-random association of alleles at different loci within a population

• depends on multiple factors like local recombination rate, non-random mating, mutation rate, genetic drift and population structure

• linkage disequilibrium is not the same as physical linkage

  • but physical linkage can result in LD

• LD can even occur between loci on different chromosomes.

Pleiotropy – single allele -> many traits

Epistasis – many loci interact -> one trait

4 possible ‘haplotypes’ allele combinations in gametes

‘haplotypes’ => Alleles inherited together from a single parent (i.e. together in gametes)

Prob of A and prob of B = prob A x prob B

Total = 1

Genotypic frequencies with LD

(deviation D)

a) i.e. D ≠ 0, linkage disequilibrium

DAB = PAB-PAPB =0–(0.5x0.5)=-0.25

b) DAB = 0

c) DAB = 0.25

red: only LD

• Quantitative genetics is the study of the inheritance of quantitative/continuous phenotypic traits, like human height and body size, grain colour in winter wheat or beak depth in birds

• These phenotypes are more commonly found in nature than Mendelian traits.

• influenced by both (many) loci (i.e. they are oligogenic or polygenic) and environment

  • • P (phenotypic value of an individual) = G (genotypic value) + E (environmental deviation)

• alleles at all loci contributing to the phenotypic variation act predominantly additively, leading to a normal distribution of phenotypic values in a population.

• can normally measure only P

• make use of the resemblance between relatives to estimate how much of the variation in a phenotypic trait has a genetic basis.

• ◦ phenotypic variance VP

  ◦ genotypic variance VG

  ◦ environmental variance VE

  ◦ VG and VE are artificial constructs, we cannot measured them directly

  instead estimate them using statistical models

  ◦ VP = VG +VE and V P = VA +VD +VI +VE

◦ fraction of the phenotypic variance in a population that can be attributed to variance in genotypes

The evolution of new species – speciation –

Species are groups of interbreeding populations, which are (normally) reproductively isolated from other such groups.

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