CHAPTER 4GENETIC

INHERITANCE4.2 DEVIATIONS FROM THE

MENDELIAN INHERITANCE

Mendel Inheritance Inheritance deviates from Mendel’s LawInheritance type

Monohybrid Dihybrid Codominant alleles

Incomplete dominant alleles

Multiple alleles Lethal allele

Parents (P) Pure bred typeEg: Purple (PP) x

White (pp)

Pure bred typeEg: Yellow, round

seed (YYRR), Green, wrinkled seed

(yyrr)

MN blood groupMM x NN

Snapdragon flower (Antirrhinum sp)

CRCR (red) x CWCW

(white)

ABO blood groupIAIA x IBIB

Rodent

F1Purple (Pp) Yellow, round seed

(YyRr)MN blood group CRCW (pink) IAIB (AB type) Yy (yellow)

Phenotypic ratio of F2From F1 self cross (F1 x F1)

3 purple : 1 white(3:1)

9 purple, round seed3 purple, wrinkled

seed3 green, round seed

1 green, wrinkled seed

(9:3:3:1)

1 MM : 2 MN : 1NN(1:2:1)

1 CRCR : 2 CRCW : 1 CWCW (1:2:1)

1 IAIA : 2 IAIB : 1 IBIB

(1:2:1)2 Yy (yellow) : 1 yy

(grey)(2:1)

Linked-genes without cross-over

Linked-genes with cross-over

Phenotypic ratio of F2From F1 test cross

1 purple : 1 white(1:1)

1 purple, round seed1 purple, wrinkled

seed1 green, round seed

1 green, wrinkled seed

(1:1:1:1)

Produce only two phenotypes with ratio 1:1

No exact ratio.Produce 4 penotypes:2 phenotypes: larger quantity, parental types2 phenotypes: smaller quantity, recombinant types

Mendel’s Law

Law of Segregation

2 alleles for each gene separate during gamete

formation

Law of Independent Assortment

Each pair of alleles segregates

independently of each other pair of

alleles during gamete formation

4.2 Inheritance that deviates from Mendel’s LawCodominant alleles Incomplete dominant

allelesMultiple alleles Lethal allele

MN blood groupMM x NN

Snapdragon flower (Antirrhinum sp)

CRCR (red) x CWCW (white)

ABO blood groupIAIA x IBIB

Rodent

MN blood group CRCW (pink) IAIB (AB type) Yy (yellow)

1 MM : 2 MN : 1NN(1:2:1)

1 CRCR : 2 CRCW : 1 CWCW (1:2:1)

1 IAIA : 2 IAIB : 1 IBIB

(1:2:1)2 Yy (yellow) : 1 yy (grey)

(2:1)

Linked-genes without cross-over

Linked-genes with cross-over

Produce only two phenotypes with ratio 1:1

No exact ratioProduce 4 penotypes:2 phenotypes: larger quantity, parental types2 phenotypes: smaller quantity, recombinant types

4.2 Deviations from the Mendelian Inheritance Contact Hour = 2 1/2 Hour

Objectives :• Explain codominant alleles (include epistasis) &

calculate genotypic and phenotypic ratios (1:2:1)

• Explain incomplete dominant alleles & calculate genotypic and phenotypic ratios (1:2:1)

• Explain multiple alleles

• Explain polygenes/polygenic inheritance

• Explain recessive lethal alleles & calculate genotypic and phenotypic ratios for lethal allele (2:1)

• Explain linked genes & describe the effects of linked genes with crossing over on the dihybrid test cross ratio• Explain sex-linked genes

EpistasisDEVIATIONS FROM THE MENDELIAN INHERITANCE

What is Epistasis?

• Epistasis come from the Greek – –“epi” means “upon”–“histani” means “to place”

• So it means to place upon or to stand upon

Definition

“The situation in which the alleles at one gene cover up or alter

the expression of alleles at another gene”

(Genetics, Weaver and Hedrick, Wm. C. Brown Publishers,1989)

Or…• Epistasis is a form of gene

interaction in which one gene masks the phenotypic expression of another

• There are no new phenotypes produced by this type of gene interaction

Epistatic versus Hypostatic

• The alleles that are masking the effect are called epistatic alleles

• The alleles whose effect is being masked are called the hypostatic alleles

How is epistasis different from dominance?

Dominance is when an allele suppresses the expression of another allele at the same locus

Pair of Chromosomes

“A” Locus A a

“A” and “a” are alleles to each other. That means they are alternate forms of the gene at the “A” locus.

If “A” suppresses the expression of “a” then “A” is dominant to “a.”

Epistasis involves two gene pairs

Epistasis is when an allele at one locus masks (covers up) or alters the expression of an allele at a different locus

Pair of Chromosomes

“A” Locus A

“B” Locus B

a

B

If the “A” allele at the A locus alters or masks the expression of the “B” allele at the B locus, then A is epistatic to B.

(The B locus could be on the same chromosome as the A locus, or it could be on a different chromosome.)

Recessive or Dominant?

• Epistasis can be described as either recessive epistasis or dominant epistasis

• Let’s look at an example of recessive epistasis….

Labrador Retrievers• Fur color in Labrador Retrievers is controlled by two

separate genes– Fur color is a polygenic trait!

Gene 1 : Represented by B : Controls color

Gene 2 : Represented by E : Controls expression of B

Labrador Retrievers• If a Labrador retriever

has a dominant B allele, they will have black fur

• If they have two recessive alleles (bb) they will have brown fur

Labrador Retrievers

• If a retriever receives at least one dominant “E” allele, they will remain the color that the “B” allele coded for– Either black of brown

• However, if a dog receives a pair of homozygous recessive “e” alleles, they will be golden regardless of their “B” alleles!

Labrador Retrievers

• BBEE and BbEe --> Black retrievers• bbEE and bbEe --> Brown retrievers• BBee, Bbee, or bbee --> Golden retrievers

Try this cross…

• You have decided to cross your golden retriever (bbee) with the neighbor’s chocolate retriever (bbEe). What color pups will they have?

bbee x bbEe

• Gamete/FOIL : be• Gamete/FOIL : bE or be

• Genotypes of F1 generation: bbEe and bbee

• Pups phenotypes:Brown and golden

Dominant Epistasis

• Let’s have a look at dominant epistasis…

• Squash fruit color is controlled by two genes

• Gene 1 is represented by a W• Gene 2 is represented by a G

Squash Fruit Color

• Genotypes and Phenotypes:

• W-G- white

• W-gg white

• wwG- green

• wwgg yellow

Squash Fruit Color

• Which allele is epistatic in squash color?

• How do you know?

The dominant W allele is epistasis

Because every time a dominant W allele shows up in a squash genotype, the squash fruit color is white

Try this cross….

• Cross a green squash (wwGg) with a white squash (Wwgg).

• What color are the offspring?

Wwgg x wwGg

• FOIL: Wg or wg• FOIL: wG or wg

• F1 generation genotypes:

• Phenotypes:

DEVIATION FROM THE MENDELIAN INHERITANCE

4.2.1 Codominant alleles

4.2.1 Codominant allelesObjectives :a) Explain codominant alleles (Include epistasis)b) Calculate genotypic and phenotypic ratios (1:2:1)

Codominant Alleles

• The situation in which the both alleles of a pair are fully expressed in the phenotype of the heterozygous (F1)

• The heterozygote offspring has the characteristics of both the homozygous parents

• Phenotypic ratio 1:2:1 instead 3:1

Genotypes Phenotypes

MM Blood group M( Homozygous for

allele M )

NN Blood group N( Homozygous for

allele N )

MN Blood group MN( Heterozygous )

Eg : human MN blood groups system in humans are due to the presence of two specific molecules on the surface of red blood cells

P : Blood group M Blood group N MM X NN

G : M N

F1 : MN X MN

G : M N M N

F2 : MM MN MN NN

Genotypic ratio: 1 MM : 2 MN : 1 NNPhenotypic ratio: 1 blood group M : 2 blood group MN : 1 blood

group N

DEVIATION FROM THE MENDELIAN INHERITANCE

4.2.2 Incomplete dominant alleles

4.2.2 Incomplete dominant allelesObjectives :a) Explain incomplete dominant

allelesb) Calculate genotypic and phenotypic ratios (1:2:1)

Incomplete Dominant Alleles• The situation in which the phenotype of

heterozygotes (F1) is intermediate between the phenotypes of individuals homozygous for either allele (dominant or recessive) (P generation)

• The offspring shows partial expression of both alleles

• Phenotypic ratio 1:2:1 instead of 3:1

• Eg :Color of snapdragon flower (Antirrhinum sp.)

• A cross between individuals that are homozygous with red flowers (RR) with individuals that are homozygous with white flowers (WW)

• will produced, Heterozygote (offspring) with pink flowers which intermediate between red and white

• The phenotypic ratio becomes 1 red:2 pink: 1 white instead of 3:1

Color of Snapdragon flower Antirrhinum sp.

Parent : Red x WhiteRR WW

Gamete : R W

F1 genotype: RWF1 X F1 : RW x RW

Gamete: R W R W

F2 generation : RR RW WWPhenotypic ratio: 1 red : 2 pink : 1 white

DEVIATION FROM THE MENDELIAN INHERITANCE

4.2.3 Multiple alleles

4.2.3 Multiple allelesObjective :a) Explain multiple alleles

Multiple Alleles• A condition when more than two alleles occupying the

same gene locus on a pair of homologous chromosomes

• However, only 2 alleles can be present in a single organism

• Eg : ABO blood group in humans

• There are 4 blood types : A, B, AB and O

• The ABO locus has three common alleles : IA , IB , IO

ABO Blood Group

IOIO / iiO

IAIBAB

IBIB , IBIOB

IAIA , IAIOA

GenotypePhenotype (Blood group)

i /IO is a recessive allele IA & IB are both dominant (codominant) alelles

Multiple alleles control the ABO blood groups.

DEVIATION FROM THE MENDELIAN INHERITANCE

4.2.4 Polygenes/polygenic inheritance

4.2.4 Polygenes/polygenic inheritanceObjective :a) Explain polygenes / polygenic

inheritance

Polygenes / polygenic inheritance• An additive effect of two or more gene loci on a single

phenotypic character (a character is controlled by the cumulative effect of more than one gene)

• Eg: height in human and human skin color

• These are called quantitative characters

• For example, height in human; skin pigmentation in humans is controlled by at least three (probably more) genes

• Height in human and other animal show a continuous variation

• Traits show continuous variation are not determined by a single gene, but a large number of genes at different loci so this is called polygenic inheritance

• The overall expression of an polygenic traits depends upon the sum of the influences of all the genes involved

Height in Humans• Range of phenotypes resulting from polygenic

trait

• Let’s consider three genes controlling skin colour:- A, B, C = dark-skin - a ,b, c = fair skin

• An AABBCC person would be very dark, while an aabbcc individual would be very light

• An AaBbCc person would have skin of an intermediate shade

• Because the alleles have a cumulative effect, the genotypes AaBbCc and AABbcc would make the same genetic contribution (3 dominant alleles & 3 recessive alleles) to skin darkness

• This polygenic inheritance resulted a bell-shaped curve, called a normal distribution, with a mean value and extremes in either direction

• Environmental factors, such as exposure to the sun, also affect the skin-color phenotype

Polygenic InheritanceIndividuals based on degrees of skin darkness

PPHHVery darkXXXX

PpHH; PPHhDarkXXX

PPhh; ppHH; PpHh

Quite darkXX

ppHh ; PphhFairX

pphhVery fair-

GenotypePhenotype0 darkness

Differences between multiple alleles and multiple genes

Multiple alleles• Multiple alleles are the presence of

more than two alleles for a trait within a gene pool,

• Alleles refer to different versions of the same gene. So a single gene can have multiple alleles.

• For example in fruit flies there is a single gene that controls eye color, and the eye color of the fly depends on the alleles they have for that gene (since they have two copies of every gene, being diploid).

Polygenetic traits• while polygenetic traits are controlled

by multiple genes.

• A polygenic trait refers to any inheritable trait that is controlled by multiple genes, and each of these genes can have multiple alleles.

• For example, eye color in humans is a polygenic trait. There are at least three different genes, each with multiple alleles, that determine eye color in humans. Polygenic traits don't follow patterns of mendelian inheritance.

DEVIATION FROM THE MENDELIAN INHERITANCE

4.2.5 Lethal genes

4.2.5 Lethal geneObjectives :a) Explain recessive lethal alleleb) Calculate genotypic and phenotypic ratios for lethal allele (2:1)

Lethal Alleles

• A gene that leads to the death of an individual

• The alleles can be dominant or recessive

• Most lethal alleles occur in homozygous recessive forms

•There are also homozygous dominant forms

•Some lethal alleles cause death during fetal stage; the offspring never appears

•Other cause death of the organism later in life

Example: Lethal gene in coat color of rodents (mice)

• Wild mice have grey-colored fur (agouti yy) while mutants have yellow fur (Yy)

• A self cross between mice with yellow fur produces offspring in the phenotypic ratio of 2 yellow to 1 agouti

• These results suggest that the allele for yellow (Y) is dominant to the allele for agouti (y)

• So, although the Y allele is dominant for fur color but it is recessive for lethal, if exists in homozygous form, it becomes lethal

• The YY genotype is lethal and the mice die before born

• So, the phenotypic ratio is 2:1

Example of recessive allele for coat color of rodents

P : Yellow X Yellow Yy

Yy

G : Y y Y y

F1:

YY Yy Yy yy Yellow Yellow Yellow

GreyPhenotypic ratio : Lethal 2 : 1

Other ExamplesGenetic disease

Expression of gene

Main symptoms

Defect

Cystic fibrosis Recessive autosomal

Unusually thick mucus clogs lungs, liver and pancreases

Failure of chloride ion transport mechanism in cell surface membranes of epithelial cells

Huntington’s chorea (disease)

Dominant autosomal

Uncontrolled movement

Brain cell metabolism is inhibited

DEVIATION FROM THE MENDELIAN INHERITANCE

4.2.6 Linked genes

4.2.6 Linked genesObjectives :a) Explain linked genesb) Describe the effects of linked genes with crossing over on the dihybrid test cross ratio

Linked Genes

Genes located on the same chromosome

The genes tend to inherited together - since the chromosome is passed along as one unit.

Do not obey Mendel’s laws because it doesn’t follow the law of independent assortment

They are inherited together unless separated by crossing over during prophase 1 of meiosis

Linked genes without cross-over

Self-cross on heterozygous (for both characters) F1 produces F2 with phenotypic ratio 3:1

Test cross on heterozygous (for both characters) individual produces offspring with phenotypic ratio 1:1

Test cross

Dihybrid-obey Mendel`s Law

Dihybrid of linked genes (no crossing

over)F1 :

G:

F2 :

Normal wings, grey

bodyVvEe

Vestigial wing, black

bodyvveeve

Normal wings, grey

bodyVE/ve

Vestigial wing, black

bodyve/veveVE Ve vE ve

1 normal wings,grey body : 1 normal,black :

1 short, grey : 1 short, black

VE ve

VE/ve ve/ve

1 VE/ve : 1 ve/ve

VvEe Vvee vvEe vvee

1 : 1 : 1 : 11 normal wings, grey body :

1 short, black

1 VvEe : 1 Vvee :1 vvEe : vvee

1 : 1

V

E

V

E

v

e e

v

xP :

V

E e

vG :

V

E e

vF1 :

Wild typeNormal wings, grey body

Wild type/Normal wings, grey body

Mutant/Vestigial wing, black body

Dihybrid of linked genes (without Crossing Over)

V

E

v

e

v

e e

v

xF1x ve/ve :(test cross)

V

E e

vG :

V

E e

vF2 :

Wild type1 Normal wings, grey body

Wild type/Normal wings, grey body

Mutant/Vestigial wing, black body

Continue…..Dihybrid of linked genes (without Crossing Over)

v

e

e

vv

emutant:1 vestigial wings, black body

F2 Phenotypic Ratio:

Test cross

Dihybrid-obey Mendel`s Law

Dihybrid of linked genes (with crossing

over)F1 :

G:

F2 :

Normal wings, grey

bodyVvEe

Vestigial wing, black

bodyvveeve

Normal wings, grey

bodyVE/ve

Vestigial wing, black

bodyve/ve

veVE Ve vE ve

1 normal wings,grey body : 1 normal,black : 1 short, grey : 1 short, black

ve Ve

VE/ve ve/veVvEe Vvee vvEe vvee

1 : 1 : 1 : 1

1 VvEe : 1 Vvee :1 vvEe : vvee

Phenotypic ratio is not

1 : 1: 1:1

VE vE

Ve/ve vE/ve normal wings,grey body :

normal,black : short, grey : short, black

If test cross of dihybrid inheritance for linked genes,

phenotype ratio is not 1: 1:1:1

Linked genes with cross-over

V

E

V

E

v

e e

v

xP :

V

E e

vG :

V

E e

vF1 :

Wild typeNormal wings, grey body

Wild type/Normal wings, grey body

Mutant/Vestigial wing, black body

Dihybrid of linked genes (with Crossing Over)

V

E

v

e

v

e e

v

xF1x ve/ve :(test cross)

v

e E e

vG :

V

E e

vF2 :

Wild type1 Normal wings, grey body

Wild type/Normal wings, grey body

Mutant/Vestigial wing, black body

Continue…..Dihybrid of linked genes (with Crossing Over)

V

e

e

vv

Emutant:1 vestigial wings, black body

F2 Phenotypic Ratio:

V

E

v

e

v

e

vv

e

V

erekombinant:1 vestigial wings, grey body

rekombinant:1 normal wings, black body

Linked genes with cross-over• If crossing over occurs in Linked genes :The test cross results on F2 generation is not in

the ratio of 1:1:1:14 types of offspring produced: - 2 types with characters . Show the parental phenotypes . Larger in number - 2 types with characters: . Show the recombinant phenotypes . Smaller in number with no specific ratio (not in equal numbers)

The formation of a small proportion of recombinant gametes in which alleles of these genes, change places on the chromosomes during crossing over

it enables linked alleles to separate and to recombine, forming new linkages (recombinants)

crossing over be most common in pairs of loci of end chromosome length

VE Ve vE ve

P :Normal

wings, grey bodyVE/VE

Vestigial wing, black body

ve/ve

VE veVE/ve

All normal wings, grey bodyVE/ve ve/ve

G :F1 :

F1 undergoes test cross :

Gametes formedafter cross over ve

VE/Ve Ve/ve vE/ve ve/veF2 genotype :F2 phenotype : Normal

wing, grey body

Normal wing, black

body

short wing, black body

short wing, black body

Parental/Non-recombinant

250

Crossing over produces

recombinant50 + 52

Parental/Non-recombinant

248

V v

E e

V

E

v

E

V

e

v

e

Linked genes with crossing over on the dihybrid test cross in body colour and wing size of Drosophila

b+ = gray body vg+ = normal wings b = black body vg = vestigial wings

Results shows that the ratio of parental-type offspring to recombinant (non-parental type) type offspring is not 1:1:1:1

SEX DETERMINATION IN HUMANS

• Every human cell contains 23 pairs of chromosomes (46 chromosomes)

• 1 - 22 pairs are identical in male and female and known as autosomes / autosomal chromosomes

• Human sex is determined by a pair of sex chromosomes (the 23rd pair) called X and Y

• Females have two large X chromosomes (XX)• males have one X chromosome and one Y

chromosome (XY)• During meiosis, the sex chromosomes pair up and

segregate into the daughter cells• Males are heterogametic sex because they produce

different sperm: approximately 50% contain an X chromosome and 50% have a Y chromosome

• Females produce homogametic sex because all of their egg contain an X chromosome

• Sex chromosomes (especially X) also have genes for many other characters

DEVIATION FROM THE MENDELIAN INHERITANCE

4.2.7 Sex-linked genes

4.2.7 Sex-linked genesObjective :a) Explain sex-linked genes

Sex Linked Genes

• Genes which is located on the sex chromosomes

• Human females have XX chromosomes , meaning they have two sex-linked alleles

• In males, they are XY but the Y chromosome is smaller & cannot mirror all the genes found on the X chromosome, so males have only one sex-linked allele

• There are no known Y-linked traits, because the Y chromosome carries so few genes

• Males will suffer from the effects of X-linked genetic diseases more often than females

XcXc

Color blind X XCY

Normal

Xc Xc XC Y

XCXc XCXc XcY XcY

P:

G:

F1:

100% carrier 100% color blind

Example of colour blindness in human for sex-linked gene

XHXh

Female,normal (carrier)X XHY

Male,normal

XH Xh XH Y

XHXH XHXh XHY XhY

P:

G:

F1:normal female

Example of haemophilia in human for sex-linked gene

Haemophiliac malenormal male normal female(carrier)

Example of Drosophila eye colour for sex-linked gene

Transmission of sex-linked genes

Past Year Questions • Session 2000/2001 A tall tomato plant is controlled by a dominant allele where as a short tomato plant is controlled by a recessive allele. A hairy stem is controlled by a dominant allele where as a smooth stem is controlled by a recessive allele. Both genes are not linked. By using appropriate symbols for the allele, i) Draw a genetic diagram showing a test cross on a tomato plant which is heterozygote for both genes. ii)State the genotypic and phenotypic ratios from the test cross.

[10 marks]

Past Year Questions • Session 2000/2001 According to the Mendel’s first law, the F2 phenotypic

ratio is 3:1. Researches by geneticist later show there are monohybrid crossings that deviate from the ratio. Using a suitable example, design a crossing to show how the F2 ratio deviates from the ratio of 3:1.

[12 marks]• Session 2000/2001 Show how haemophilia can happen to sons whose

mother is a carrier and the father is a normal person. [8

marks]

References

• Campbell & Reece, Biology 8th edition• Solomon, Berg & Martin, Biology 8th edition• www.urbandaleschools.com/uploads/users/p

etersg/epistasis_1.ppt• http://www.tutornext.com/deviations-mendel

ian-laws/8810

Next Subtopic…

Subtopic 3.3: Genetic Mapping

“ STUDY SMART”