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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

Chapter 14



  • Pre 1800s – blending hypothesis

  • 1850s – Gregor Mendel

Pisumsativum– what makes pea good genetic model?





  • Pea character (gene) =

  • Pea trait (allele) =

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

A monohybrid

Is this quantitative or qualitative data?


Allele = Alternative version of gene

Dominant allele P Recessive allele p

Each individual inherits 2 alleles/gene

Genotype Phenotype?

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

In mice, black fur is dominant over white fur

Cross heterozygous mouse with homozygous recessive mouse


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?

The Scottish fold is a dominant trait heterozygous cats.

Genotypes FF Ffff


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

Test cross
Test cross heterozygous cats.

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.

Review heterozygous cats.

  • 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 heterozygous cats. Dihybrid cross (2 genes)

Independent assortment
Independent Assortment heterozygous cats.

2 genes

Y = yellow pea allele R = round pea allele

y = green pea allele r = wrinkled

Phenotypes of peas

X heterozygous cats.



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

Parental generation: RRYY X heterozygous cats. rryy

Gametes ?

F1 generation?

Gametes of F1 generation?

F2 generation?

F2 generation
F2 generation heterozygous cats.

Ratio of genotypes?

Ratio of phenotypes?

Extensions of mendel
Extensions of Mendel heterozygous cats.


Both alleles expressed in phenotype

Ex. Blood type I gene

Alleles IA IB

Phenotype of IA IB ?

multiple alleles heterozygous cats.

Ex. blood type

Alleles IA IB i


IAiIBi ii

polygenic inheritance heterozygous cats.

multiple genes affect characteristic

Ex. skin – at least 3 genes


  • environmental effects heterozygous cats.

    A particular genotype may have a phenotypic range

  • Ex

    • Does blood color have a norm?

    • Skin color?

? heterozygous cats.

Recessively inherited disorders
Recessively inherited disorders heterozygous cats.

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 heterozygous cats. a allele does not code for functional melanin protein

Chromosome 11

Recessive traits
Recessive traits heterozygous cats.

bb = blond hair

bb = blue eyes

ii = type O blood

Dominantly inherited disorders
Dominantly inherited disorders heterozygous cats.

A allele  abnormal protein

a allele  normal protein




Multifactorial disorders
Multifactorial heterozygous cats. disorders

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