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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

Theoretical Genetics

4.3, 10.2, and 10.3


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

  • 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
  • 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
  • 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
monohybrid crosses
Monohybrid Crosses

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

Flower color gene- R= Red r= 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

monohybrid crosses7
Monohybrid Crosses

Flower color gene- R= Red r= 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

monohybrid crosses8
Monohybrid Crosses

Flower color gene- R= Red r= 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

multiple alleles
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
multiple alleles10
Multiple Alleles

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

multiple alleles11
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
monohybrid crosses12
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:


monohybrid crosses13
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

monohybrid crosses14
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

sex chromosomes
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.
sex chromosomes16
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
sex chromosomes17
Sex Chromosomes

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

sex chromosomes18
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.
sex chromosomes19
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
sex chromosomes20
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
sex chromosomes21
Sex Chromosomes

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

monohybrid crosses22
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

monohybrid crosses23
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

  • 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.

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
  • 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-
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.
Determine all the possible genotypes of the individual (2nd generation-1) using appropriate symbols.
  • XaY (where a=condition)
Determine all the possible genotypes of the individual (3rd generation-4) using appropriate symbols.
  • XAXa or XAXA where A=normal, a=condition
dihybrid crosses
Dihybrid Crosses

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

dihybrid crosses35
Dihybrid Crosses

Flower color gene- R= Red r= white

Flower size gene- B= Big b= 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

dihybrid crosses36
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

dihybrid crosses37
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

sex chromosomes vs autosomes
Sex Chromosomes vs. Autosomes

2. Distinguish between autosomes and sex chromosomes

crossing over
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.

crossing over41
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.
crossing over42
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
  • 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.
linkage group
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
linked genes
Linked Genes

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

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



linked genes46
Linked Genes

P genotypes- PPLL ppll

Phenotypes- purple flower red flower

long pollen round pollen

Gametes- PL pl

F1 Genotype- PpLl

F1 Phenotype purple flower long pollen

Allow the F1 to self fertilize and product a F2 generation

linked genes47
Linked Genes
  • Chi Squared Test is used to see if the observed ratios and expected ratios are significantly different.
linked genes48
Linked Genes

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

linked genes49
Linked Genes
  • Two genes A and B are linked together as shown below

A b

a B

  • 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.
recombination in non linked genes
Recombination in non-linked Genes

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

tall, white Genotype= Ttrr Gametes= Tr or tr

short, red Genotype= ttRr Gametes= 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

linked genes51
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

linked genes52
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.
linked genes54
Linked Genes
  • Purple/White and Starchy/Waxy example from Zea mays
  • Key- C=purple, c=white, W=starchy, w=waxy
  • P generation- CW x cw

CW x cw

  • Phenotypes- purple white

starchy waxy

  • Gametes- CW cw
linked genes55
Linked Genes
  • Gametes- CW x cw
  • F1 Generation- CW


  • Phenotype- purple and starchy
polygenic inheritance
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
polygenic inheritance58
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


polygenic inheritance59
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.
polygenic inheritance60
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.
polygenic inheritance62
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

polygenic inheritance63
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
polygenic inheritance64
Polygenic Inheritance

Heterozygous cross:

AaBbCc X AaBbCc