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Family resemblance can be striking!

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Chapter 14.

Mendel & Genetics

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Gregor Mendel Modern genetics began in mid-1800s in

an abbey garden, where a monk namedGregor Mendel documentedinheritance in peas used experimental method used quantitative analysis

collected data & counted them excellent example of scientific method

He studied at the University of Vienna from 1851 to 1853 where hewas influenced by a physicist who encouraged experimentation andthe application of mathematics to science and a botanist whoaroused Mendel’s interest in the causes of variation in plants. Afterthe university, Mendel taught at the Brunn Modern School and lived inthe local monastery.The monks at this monastery had a long tradition of interest in thebreeding of plants, including peas. Around 1857, Mendel beganbreeding garden peas to study inheritance.

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Mendel’s work Bred pea plants

cross-pollinatedtrue breedingparents (P)

raised seed &then observedtraits (F1)

allowedoffspring tocross-pollinate& observed nextgeneration (F2)

P = parentsF = filial generation

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Mendel collected data for 7 pea traits

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Looking closer at Mendel’s worktrue-breeding

purple-flower peastrue-breeding

white-flower peasX

100%100%

purple-flower peasF1generation(hybrids)

25%white-flower peasF2

generation

75%purple-flower peas 3:1

P

self-pollinate

In a typical breeding experiment, Mendel would cross-pollinate(hybridize) two contrasting, true-breeding pea varieties.The true-breeding parents are the P generation and their hybridoffspring are the F1 generation.Mendel would then allow the F1 hybrids to self-pollinate to produce anF2 generation.

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What did Mendel’s findings mean? Traits come in alternative versions

purple vs. white flower color alleles

different alleles vary in the sequence ofnucleotides at the specific locus of a gene

purple-flower allele & white-flower allele are 2 DNA variations at flower-color locus

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Traits are inherited as discrete units For each character, an organism

inherits 2 alleles, 1 from each parent diploid organism inherits 1 set of

chromosomes from each parent homologous chromosomes diploid = 2 sets of chromosomes like having 2 editions of encyclopedia

Encyclopedia BritannicaEncyclopedia Americana

What’s theadvantage ofbeing diploid?

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What did Mendel’s findings mean? Some traits mask others

purple & white-flower colors areseparate traits that do not blend purple x white ≠ light purple purple masked white

dominant allele fully expressed

recessive allele no noticeable effect non-functional protein

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Genotype vs. phenotype difference between how an organism

“looks” & its genetics description of an organism’s trait =

phenotype description of an organism’s genetic

makeup = genotype

Explain Mendel’s results using…dominant & recessive…phenotype & gentotype

F1

P

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Making crosses using representative letters

flower color alleles = P or p true-breeding purple-flower peas = PP true-breeding white-flower peas = pp

PP x pp

Pp

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Looking closer at Mendel’s worktrue-breeding

purple-flower peastrue-breeding

white-flower peasX

100%100%

purple-flower peasF1generation(hybrids)

25%white-flower peasF2

generation

75%purple-flower peas 3:1

P

PP pp

Pp Pp Pp Ppself-pollinate

????

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Punnett squaresPp x Pp

P pmale / sperm

P

p

fem

ale

/ egg

s PP

Pp pp

Pp

PP

Pp

Pp

pp

75%

25%

3:1

25%

50%

25%

1:2:1

%genotype

%phenotype

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Genotypes Homozygous = same alleles = PP, pp Heterozygous = different alleles = Pp

homozygousdominant

homozygousrecessive

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Phenotype vs. genotype 2 organisms can have the same

phenotype but have different genotypes

PP homozygous dominant

Pp heterozygous

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Mendel chose peas wisely Pea plants are good for genetic research

available in many varieties with distinctheritable features with different variations flower color, seed color, seed shape, etc.

Mendel had strict control over whichplants mated with which each pea plant has male & female structures pea plants can self-fertilize Mendel could also cross-pollinate plants:

moving pollen from one plant to another

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Mendel chose peas luckily Pea plants are good for genetic research

relatively simple genetically most characters are controlled by a single

gene each gene has only 2 alleles, one of which is

completely dominant to the other

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Mendel’s laws of heredity (#1) Law of segregation

when gametes (eggs & sperm) areproduced during meiosis, homologouschromosomes separate from each other

each allele for a trait segregates (ispackaged) into a separate gamete

PPP

Ppp

p

pPp

P

p

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Law of Segregation What meiotic

event createsthe law ofsegregation

and Mendeldidn’t evenknow DNA orgenes existed!

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Dominant phenotypes It is not possible to predict the genotype

of an organism with a dominantphenotype

PP?

Pp?So how do youfigure out thephenotype?

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Test cross Cross-breed the dominant phenotype

— unknown genotype — with ahomozygous recessive (pp) todetermine the identity of the unknownallele

x

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x

PP pp

Test cross

p p

P

P

Pp

Pp Pp

Pp

p p

P

p

Pp

pp pp

Pp

x

Pp pp

100% 1:1

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Monohybrid cross Some of Mendel’s experiments

followed the inheritance of singlecharacters flower color seed color monohybrid crosses

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Dihybrid cross Some of Mendel’s experiments

followed the inheritance of 2 differentcharacters seed color & seed shape dihybrid crosses

Helped him understand other geneticmechanisms (“rules”)

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Inheritance of seed shape Seed shape

wrinkled peas smooth peas

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Dihybrid crosstrue-breeding

yellow, round peastrue-breeding

green, wrinkled peasxYYRR yyrr

P

YyRr100%F1

generation(hybrids)

yellow, round peas

self-pollinate

F2generation

9/16yellowroundpeas

9:3:3:13/16

greenroundpeas

3/16yellow

wrinkledpeas

1/16green

wrinkledpeas

Wrinkled seeds in pea plants with two copies of the recessive alleleare due to the accumulation of monosaccharides and excess waterin seeds because of the lack of a key enzyme. The seeds wrinklewhen they dry.Both homozygous dominants and heterozygotes produce enoughenzyme to convert all the monosaccharides into starch and formsmooth seeds when they dry.

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Making gametes How are the alleles handed out?

together or separately?

YyRr

YR yr

YyRr

Yr yRYR yr

Which system explains the data?

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Dihybrid crossYyRr YyRr

YR Yr yR yr

YR

Yr

yR

yr

YYRR YYRr YyRR YyRr

YYRr YYrr YyRr Yyrr

YyRR YyRr yyRR yyRr

YyRr Yyrr yyRr yyrr

x9/16

yellowround

3/16greenround

3/16yellow

wrinkled

1/16green

wrinkled

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Mendel’s laws of heredity (#2) Law of independent assortment

each pair of alleles — for each trait —segregates into gametes independently= independent assortment 4 classes of gametes — YR, Yr, yR, yr —

are produced in equal amounts

YyRr

YR Yr yR yr

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Law of Independent Assortment What meiotic event

creates the law ofindependentassortment?

and Mendeldidn’t evenknow DNA orgenes existed!

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Review: Mendel’s laws of heredity Law of segregation

monohybrid cross = single trait each allele for a trait segregates (is

packaged) into separate gametes established by Meiosis 1

Law of independent assortment dihybrid (or more) cross

2 or more traits each pair of alleles for each trait

segregates into gametes independently established by Meiosis 1

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Some interesting historical facts While Mendel was acknowledged by his

contemporaries as an outstanding plantbreeder, his revolutionary genetics workwas overlooked for 34 years.

Mendel published “Experiments on PlantHybrids” in 1865. In 1900, 16 years afterMendel’s death, a number of scientistsindependently rediscovered his work.

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Some interesting historical facts Charles Darwin, a contemporary of Mendel, proposed

that evolution by natural selection was dependent onvariation in the population.

But Darwin was unable to propose a mechanism forhow this variation was transmitted.

The key was Mendel’s work, and nearly a centuryafter Mendel published his findings, historians founda copy of Mendel’s paper in Darwin’s study. Hepresumably never read it.