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Theoretical Genetics. 4.3, 10.2, and 10.3. Definitions. 1. Define Genotype, Phenotype, Dominant Allele, Recessive Allele, Codominant Allele, Locus, Homozygous, Heterozygous, Carrier, and Test Cross. Definitions. Locus- the particular position on homologous chromosomes of a gene

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Theoretical Genetics

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Theoretical Genetics

4.3, 10.2, and 10.3

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1. Define Genotype, Phenotype, Dominant Allele, Recessive Allele, Codominant Allele, Locus, Homozygous, Heterozygous, Carrier, and Test Cross.

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  • Locus- the particular position on homologous chromosomes of a gene

  • Homozygous-having 2 identical alleles of a gene

  • Heterozygous- having 2 different alleles of a gene

  • Genotype- the alleles (genes) of an organism

  • Phenotype- The physical characteristics of an organism

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  • Dominant Allele- an allele that has the same effect on the phenotype whether it is present in the homozygous or heterozygous state

  • Recessive Allele- an allele that only has an effect on the phenotype when present in the homozygous state

  • Codominant Allele- pairs of alleles that both affect the phenotype when present in a heterozygote

  • Carrier- an individual that has one copy of a recessive allele that causes a genetic disease in individuals that are homozygous for this allele

  • Test Cross- testing a suspect heterozygote by crossing it with a known homozygous recessive

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  • P Generation- Parent Generation

  • F1 Generation- Offspring of the Parent Generation (Filial)

  • F2 Generation- Offspring from when the F1 generation are allowed to self fertilize

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Monohybrid Crosses

2. Determine the genotypes and phenotypes of the offspring of a monohybrid cross using a Punnett Grid.

Flower color gene- R= Redr= white

Cross a Homozygous Red flower with a White flower

Red flower Genotype= RR

Gametes= R or R

White flower Genotype= rr

Gametes= r or r

Phenotypic Ratio:

All (4) Red

Genotypic Ratio:

All (4) Rr

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Monohybrid Crosses

Flower color gene- R= Redr= white

Cross a Heterozygous Red flower with a White flower

Red flower Genotype= Rr

Gametes= R or r

White flower Genotype= rr

Gametes= r or r

Phenotypic Ratio:

2 Red : 2 White

Genotypic Ratio:

2 Rr : 2 rr

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Monohybrid Crosses

Flower color gene- R= Redr= white

Cross 2 Heterozygous Red flowers

Red flower Genotype= Rr

Gametes= R or r

Phenotypic Ratio:

3 Red : 1 White

Genotypic Ratio:

1 RR: 2 Rr : 1 rr

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Multiple Alleles

3. State that some genes have more than two alleles (multiple alleles).

  • Multiple alleles means a gene has 3 or more alleles

  • Example: ABO Blood type- 3 alleles

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Multiple Alleles

4. Describe ABO blood groups as an example of codominance and multiple alleles.

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Multiple Alleles

  • Any two of these alleles are present in an individual

  • A and B are codominant

  • O is recessive

  • Homozygotes- IAIA, IBIB, or ii

  • Heterozygotes- IAi, IBi, or IAIB

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Monohybrid Crosses

Cross a Homozygous A blood type with an AB blood type

A Genotype= IAIA

Gametes= IA

AB Genotype= IAIB

Gametes= IA or IB

Phenotypic Ratio:

2 A blood type and 2 AB

Genotypic Ratio:


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Monohybrid Crosses

Cross a Heterozygous A blood type with a Heterozygous B blood type

A Genotype= IAi

Gametes= IA or i

B Genotype= IBi

Gametes= IB or i

Phenotypic Ratio:

1 A : 1 B: 1 AB : 1 O

Genotypic Ratio:

1 IAi: 1 IBi : 1 IAIB : 1 ii

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Monohybrid Crosses

Cross 2 individuals with O blood

O Genotype= ii

Gametes= i

Phenotypic Ratio:

All (4) O blood type

Genotypic Ratio:

All (4) ii

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Sex Chromosomes

5. Explain how the sex chromosomes control gender by referring to the inheritance of X and Y chromosomes in humans.

  • Two sex chromosomes determine the gender of a child.

  • The X chromosome is relatively large and carries many genes.

  • The Y chromosome is much smaller and carries only a few genes.

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Sex Chromosomes

  • XX  Female

  • XY  Male

  • Females only pass on X in their gametes

  • Males pass on X or Y in their gametes

  • Therefore gender is determined by the Male

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Sex Chromosomes

6. State that some genes are present on the X chromosome and absent from the shorter Y chromosome in humans.

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Sex Chromosomes

7. Define Sex Linkage.

  • Sex linkage is the association of a characteristic with gender, because the gene controlling the characteristic is located on a sex chromosome.

  • Sex linked genes are almost always located on the X chromosome.

  • Females have 2 X chromosomes and therefore have 2 copies of sex linked genes.

  • Males only have 1 X chromosome and therefore only have one copy of sex linked genes.

  • In humans, hemophilia and red-green color blindness are examples of sex-linked characteristics.

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Sex Chromosomes

  • Describe the inheritance of color blindness and hemophilia as examples of sex linkage.

  • Both color blindness Xb and hemophilia Xh are recessive sex linked alleles on the X chromosome.

  • The normal alleles are represented by XB and XH

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Sex Chromosomes

  • State that a human female can be homozygous or heterozygous with respect to sex-linked genes.

    10. Explain that female carriers are heterozygous for X-linked recessive alleles.

  • Normal females are homozygotes- XBXB

  • Carrier female carriers are heterozygotes- XBXb the are heterozygotes, but since CB is recessive they see color.

  • CB females are homozygotes- XbXb and do not see color normally

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Sex Chromosomes

11. Predict the genotypic and phenotypic ratios of offspring of monohybrid crosses involving any of the above patterns of inheritance.

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Monohybrid Crosses

Cross a colorblind male and a normal female

Male Genotype= XbY

Gametes= Xb or Y

Female Genotype= XBXB

Gametes= XB

Phenotypic Ratio:

2 carrier females and

2 normal males

Genotypic Ratio:

2 XBXb and 2 XBY

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Monohybrid Crosses

Cross a colorblind male and a carrier female

Male Genotype= XbY

Gametes= Xb or Y

Female Genotype= XBXb

Gametes= XB or Xb

Phenotypic Ratio:

1 carrier ♀, 1 CB ♀ &

1 normal ♂, 1 CB ♂

Genotypic Ratio:

1 XBXb , 1 XbXb ,

1 XBY, and 1 XbY

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  • Deduce the genotypes and phenotypes of individuals in pedigree charts.

  • For dominant and recessive alleles, upper-case and lower-case letters, respectively, should be used.

  • Letters representing alleles should be chosen with care to avoid confusion between upper and lower case.

  • For codominance, the main letter should relate to the gene and the suffix to the allele, both upper case.

  • For example, red and white codominant flower colors should be represented as CR and CW.

  • Sickle cell anemia- HbA is normal and HbS is sickle cell.

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Sex linked-

  • One gender will be affected more than the other

  • If a female has the condition- all of her sons must have it.

    Dominant or Recessive-

  • If both individuals have the condition and they give rise to a child without the condition it is Dominant

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  • Possible genotypes of individuals I, II, and III

  • I- Rh+ Rh+ or Rh+ Rh-

  • II- Rh+ Rh+ or Rh+ Rh-

  • III- Rh+ Rh-

  • However, with respect to I and II, one is Rh+ Rh+ and one is Rh+ Rh-

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  • Deduce, with a reason, whether the allele producing the condition is dominant or recessive.

  • Recessive- in 2nd generation individuals 2 and 3 do not have the condition but they give rise to children that do.

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  • Determine all the possible genotypes of the individual (2nd generation-1) using appropriate symbols.

  • XaY (where a=condition)

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  • Determine all the possible genotypes of the individual (3rd generation-4) using appropriate symbols.

  • XAXa or XAXA where A=normal, a=condition

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Theoretical Genetics


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Dihybrid Crosses

1. Calculate and predict the genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes.

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Dihybrid Crosses

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Dihybrid Crosses

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Dihybrid Crosses

Flower color gene- R= Redr= white

Flower size gene- B= Bigb= small

Cross a Heterozygous Red Big flower with a white small flower

Red Big Genotype= RrBb

Gametes= RB, Rb, rB or rb

White Small Genotype= rrbb

Gametes= rb

Phenotypic Ratio:

4 Red Big : 4 Red Small :

4 White Big : 4 White Small

Genotypic Ratio:

4 RrBb : 4 Rrbb :

4 rrBb : 4 rrbb

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Dihybrid Crosses

Cross a Homozygous Red Heterozygous Big flower with a white Homozygous Big flower

Big Red Genotype= RRBb

Gametes= RB or Rb

Big White Genotype= rrBB

Gametes= rB

Phenotypic Ratio:

16 Red Big

Genotypic Ratio:

8 RrBB : 8 RrBb

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Dihybrid Crosses

Cross two individuals that are heterozygotes for both traits

Big Red Genotype= RrBb

Gametes= RB, Rb, rB, or rb

Phenotypic Ratio:

9 Red Big:3 Red small:

3 white big:1 white small

Genotypic Ratio:


2 RRBb

1 RRbb

2 RrBB

4 RrBb

2 Rrbb

1 rrBB

2 rrBb

1 rrbb

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Sex Chromosomes vs. Autosomes

2. Distinguish between autosomes and sex chromosomes

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Sex Chromosomes vs. Autosomes

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Crossing Over

3. Explain how crossing over between non-sister chromatids of a homologous pair in Prophase I can result in an exchange of alleles.

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Crossing Over

  • Some genes do not follow the law of independent assortment

  • They will deviate from the 9:3:3:1 ratio when 2 heterozygotes are crossed.

  • Combinations of genes tend to be inherited together because of their loci being close together on the same chromosome.

  • This is called Gene Linkage.

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Crossing Over

  • New combinations of alleles can only be produced if crossing over occurs which results in recombination.

  • The chromosomes pair up and form a synapsis.

  • The DNA of one chromosome is cut and a second cut is made at the exact same point on the other chromosome.

  • Crossing over creates a chiasmata which holds the two homologous chromosomes together and the chromosomes switch information (alleles).

  • Resulting in recombination and increased genetic diversity.

  • The new combinations are referred to as recombinants- Ab and aB

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  • Recombination- the reassortment of genes or characteristics into different combinations from those of the parents.

  • Recombination occurs for linked genes by crossing over and for unlinked genes by chromosome assortment.

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Linkage Group

  • Define Linkage Group

  • All of the genes that have their loci on the same chromosome from a Linkage Group.

  • Crossing over allows recombination of linked genes

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Linked Genes

5. Explain an example of a cross between two linked genes.

  • Alleles are shown in vertical pairs when they represent linkage groups.



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Linked Genes

P genotypes- PPLLppll

Phenotypes- purple flowerred flower

long pollenround pollen


F1 Genotype-PpLl

F1 Phenotypepurple flower long pollen

Allow the F1 to self fertilize and product a F2 generation

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Linked Genes

  • Chi Squared Test is used to see if the observed ratios and expected ratios are significantly different.

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Linked Genes

6. Identify which of the offspring are recombinants in a dihybrid cross involving linked genes.

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Linked Genes

  • Two genes A and B are linked together as shown below



  • If the genes are far enough apart such that crossing over between the alleles occurs occasionally, which statement is true of the gametes?

  • All of the gametes will be Ab and aB

  • There will be 25% Ab, 25% aB, 25% ab, & 25% AB

  • There will be approximately equal numbers of Ab and ab gametes.

  • The number of Ab gametes will be greater than the number of ab gametes.

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Recombination in non-linked Genes

Cross tall, white (Ttrr) with short, red (ttRr).

tall, white Genotype= TtrrGametes= Tr or tr

short, red Genotype= ttRrGametes= tR or tr

Phenotypic Ratio:

4 Tall red: 4 Tall white: 4 short red: 4 short white

Genotypic Ratio:

4 TtRr: 4Ttrr: 4ttRr: 4 ttrr

Which are the recombinants?

Tall red- TtRr

Short White- ttrr

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Linked Genes

  • A cross is performed between two organisms with the genotypes AaBb and aabb.

  • What genotypes in the offspring are the result of recombination?

    A. Aabb, AaBb

    B. AaBb, aabb

    C. aabb, Aabb

    D. Aabb, aaBb

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Linked Genes

Explain a cross between 2 linked genes, including the way in which recombinants are produced.

  • Linked genes occur on the same chromosome and tend to be inherited together.

  • Linked genes do not segregate independently therefore they do not follow the Mendelian ratio of 9:3:3:1.

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Linked Genes

  • Purple/White and Starchy/Waxy example from Zea mays

  • Key- C=purple, c=white, W=starchy, w=waxy

  • P generation- CWxcw


  • Phenotypes-purple white


  • Gametes-CWcw

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Linked Genes

  • Gametes-CWxcw

  • F1 Generation-CW


  • Phenotype-purple and starchy

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Theoretical Genetics


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Polygenic Inheritance

  • Define Polygenic inheritance.

  • A single characteristic that is controlled by two or more genes.

  • Each allele of a polygenic character often contributes only a small amount to the over all phenotype.

  • In addition environmental effects smooth out the genotypic variation to give continuous distribution curves.

  • Example- Skin color

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Polygenic Inheritance

How many different possible genotypes are there for a polygeneic character that is controlled by 2 genes each with 2 alleles?

  • AABB

  • AABb

  • AAbb

  • AaBB

  • AaBb

  • Aabb

  • aaBB

  • aaBb

  • aabb


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Polygenic Inheritance

2. Explain the polygenic inheritance can contribute to continuous variation using two examples, one of which must be human skin color.

  • The more genes involved with the characteristic the greater the number of phenotypic classes.

  • Phenotypic variation = genotypic variation + environmental variation.

  • The environmental component smooth the genotypic category differences.

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Polygenic Inheritance

  • The color of human skin depends on the amount of the black pigment Melanin in it.

  • There is a continuous distribution of skin color from very pale to black.

  • At least 4 and possibly more genes are involved.

  • Each gene has allele that promotes melanin production and alleles that do not.

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Polygenic Inheritance

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Polygenic Inheritance

  • The color wheat grains vary from white to dark red, depending on the amount of red pigment they contain.

  • Three genes control grain color.

  • Each gene has 2 alleles, one that causes pigment production and one that not.

  • The figure shows the expected

    distribution of grain color from

    a cross between 2 plants

    heterozygous for all 3 genes

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Polygenic Inheritance

  • Finches are seed eating birds that use their beaks to break open seeds. The depth of beak is under polygenic control of three genes with two alleles each.

  • Allele key:

  • A= add depth (1 unit)

  • a= no depth added

  • B= add depth (1 unit)

  • b= no depth added

  • C= add depth (1 unit)

  • c= no depth added

  • Heterozygous cross: AaBbCc X AaBbCc

  • How many gametes can be produced?

  • 8

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Polygenic Inheritance

Heterozygous cross:

AaBbCc X AaBbCc

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