Chapter 14
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Chapter 14 . Genetics. Introduction. Pre 1800s – blending hypothesis 1850s – Gregor Mendel. Pisum sativum – what makes pea good genetic model? - - - - Pea character (gene) = Pea trait (allele) = . self fertilization Vs. cross fertilization. The monohybrid cross.

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Chapter 14

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Chapter 14

Chapter 14




  • Pre 1800s – blending hypothesis

  • 1850s – Gregor Mendel

Chapter 14

Pisumsativum– what makes pea good genetic model?





  • Pea character (gene) =

  • Pea trait (allele) =

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  • self fertilization

    Vs. cross fertilization

The monohybrid cross

The monohybrid cross

  • Cross true breeding parental plants (P)

  • produces F1 generation

Character gene trait allele dominant phenotype recessive phenotype

-character (gene)-trait (allele)- dominant phenotype- recessive phenotype

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F1 plants all purple

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  • Cross F1 plants

    F2 plants = ~250 white ~750 purple

A monohybrid

Is this quantitative or qualitative data?

Mendel s conclusions

Mendel’s Conclusions



Allele = Alternative version of gene

Dominant allele PRecessive allele p

Each individual inherits 2 alleles/gene


PP homozygous dominant

pp homozygous recessive

Pp heterozygous

Dominant allele masks expression of recessive allele

Dominant allele masks expression of recessive allele

Find the dominant allele

Pick symbols

What is the phenotype of plant with genotype Rr?

What is genotype of a green plant?





  • The 2 alleles segregate in gametes during meiosis

  • Random segregation

Pea chromosome 1

Genotype of plant?

Phenotype of plant?

Human chromosome 11 gene for beta globin

Genotype Hbb/Hbb

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  • Mendel did not know about:

    • DNA/chromosomes/genes

    • Crossing over or the cell cycle

    • Linked alleles

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In mice, black fur is dominant over white fur

Cross heterozygous mouse with homozygous recessive mouse


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In mice, black fur is dominant over white furCross heterozygous mouse with homozygous recessive mouse

  • Make an allele key

  • Show the cross

    3. Write the phenotypes under the genotypes

    4. Determine the parental gametes (random segregation)

    5. Add to Punnett square

    6. Determine offspring

Punnett square

Punnett square

What is the expected ratio of phenotypes?


In a litter of 3, how many are expected to be white?

Which phenotype is the recessive phenotype?

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  • In cats striping is dominant over non-stripes. Mate two heterozygous cats.

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The Scottish fold is a dominant trait

Genotypes FF Ffff


The original Scottish Fold was a white barn cat named Susie, found in Scotland

Test cross

Test cross

Scottish fold cat -> Is she Ff or FF?

If FF, then cross toff kittens?

If Ff, then cross to ff  kittens?

A test cross is always a cross to a homozygous individual.



  • All possible one gene crosses

    B = black fur allele

    b = white fur allele

  • BB X BB  all black

  • BB X Bb 

  • BB X bb 

  • Bb X Bb 

  • Bb X bb 

  • bb X bb  all white

The dihybrid cross 2 genes

The Dihybrid cross (2 genes)

Independent assortment

Independent Assortment

2 genes

Y = yellow pea alleleR = round pea allele

y = green pea alleler = wrinkled

Phenotypes of peas

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True-breeding yellow, round X true-breeding green, wrinkled

Allele key?

P Genotype?

Gametes? (always 4 per parent in 2 gene cross)

R = round allele Y = yellow

r = wrinkled y = green

RY and ry

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Parental generation: RRYY X rryy

Gametes ?

F1 generation?

Gametes of F1 generation?

F2 generation?

F2 generation

F2 generation

Ratio of genotypes?

Ratio of phenotypes?

Extensions of mendel

Extensions of Mendel


Both alleles expressed in phenotype

Ex. Blood type I gene

Alleles IA IB

Phenotype of IA IB ?

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multiple alleles

Ex. blood type

Alleles IA IB i



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polygenic inheritance

multiple genes affect characteristic

Ex. skin – at least 3 genes


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  • environmental effects

    A particular genotype may have a phenotypic range

  • Ex

    • Does blood color have a norm?

    • Skin color?

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Recessively inherited disorders

Recessively inherited disorders

A allele  normal protein

a allele  malfunctioning or no protein


Aa heterozygotes are carriers


disorders range from fetal lethal to mild

Ex albinism a allele does not code for functional melanin protein

Ex. albinism a allele does not code for functional melanin protein

Chromosome 11

Recessive traits

Recessive traits

bb = blond hair

bb = blue eyes

ii = type O blood

Dominantly inherited disorders

Dominantly inherited disorders

A allele  abnormal protein

a allele  normal protein




Achondroplasia d allele causes dwarfism

Achondroplasia D allele causes dwarfism

Multifactorial disorders

Multifactorial disorders

Genetic component + environment (aging, diet, chemicals, radiation, etc.)

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