General Genetic General Genetic

Bio 221 Lab 8Bio 221 Lab 8

Most traits are non-mendelian , meaning they do not follow the rules

of mendelian traits . Almost all traits in humans are non-mendelian.

The trait is determined by :

1- more than one gene .

2- or the environmental effects of the trait .

Allelic or gene interactions

1- The gene in an organism can interact in many different ways .

2- The effect of dominant allele is diluted or modified so that the

phenotypic expression of the concerned trait in a hybrid (Heterozygous

) is distinguishable from both parental types ( Homozygous )

Type of non-mendelian

1- in complete ( partial , semi , intermediate ) dominance :

A type of inheritance in which one allele for a specific trait is not completely

dominant over the other allele. This results in a combined phenotype

 (expressed physical trait). and character is intermediate between the

parents

* Phenotype and genotype ratios in F2 generation are 1:2:1 .

* EX : Red shaded chrysanthemum breed with white shaded chrysanthemum

, the resulting is pink . The mixture of colors shows incomplete dominance

between colors .

2- Co - dominance ( Mosaic dominance ) :

* Both allelic genes of a genetic trait are equally

expressive , there is complete lack of dominance .

When the heterozygote organism shows the

characteristics of two dominant alleles .

* Phenotype and genotype ratios in F2 generation are

1:2:1 .

* EX:

1- White cat and black cat generate offspring that the

dominant alleles share dominance .

2- Red and white coat colors in short-horn cattle

expressions ( Phenotype) , and they are homozygous

dominant and homozygous recessive genotypes .

The hybrids exhibit a roan coat color is intermediate

With incomplete dominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits. 

With codominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype in which both of the parental traits appear together. 

Incomplete VS Codominance

* EX: ( codominace)

• 3- The AB blood group is due

to co-dominance . AB group is

controlled by the genes A and

B . The A and B are equally

dominant . A produces

antigen A and B produces

antigen B .

• 4- Sickle cell anemia is

recessive disease . In

heterozygous which have

abnormal red blood cells under

some conditions .

Incomplete dominance example

Multiple alleles -Which a population has more than two alleles in it . -Multiple alleles increases the number of different phenotypes.-Multiple alleles can be dominant, recessive or co-dominant to each other.

EX: ABO blood groups in humans , in which there are 3 possible alleles A,B or IA and IB ( co dominant ) , and i. type O blood is recessive .

Polygenic inheritanceMore than one gene affecting a trait . Most traits in humans are polygenic , such as weight , height . IQ , personality ,( skin color is environmental determined and determined by 5 genes , each with 2 alleles giving 9 phenotypes . )

Modifier genes A gene that alters the phenotypic expression of another gene.

Mendelian Ratios and Lethal genes

In 1905 , Lucien Cuenot observed unusual patterns when

studying inheritance of a coat color gene in mice .

After mating 2 yellow mice , observed that the offspring never

showed a normal 3:1 phenotype but observed 2:1 , with

2 yellow mice and one non-yellow mice.

This is lethal genes cause the death of the organisms

that carry them .

Sometimes , death is not immediate it may take yeas

depending on the gene .

Kind of Lethal Genes :

1- Recessive Lethal genes . The ratio 3:0 . EX : Cystic fibrosis and Sickle cell anemia

2- Dominant lethal genes . The ratio 2:1 . EX: Huntington disease

3- Semi or Sub Lethal genes . EX : Hemophilia

4- Synthetic Lethal genes.

5- Conditional lethal genes. EX: favism allele

Kind of Lethal Genes 1- Recessive Lethal genes .That occur in dominant or recessive traits but they do not cause death unless an organism carries 2 copies of lethal allel . The ratio 3:0 . EX : Sickle cell anemia .

2- Dominant lethal genes .Are expressed in both homozygotes and heterozygotes . The ratio 2:1 EX: Huntington disease , a neurological disorder in humans ( chromosome 4 ) .

3- Semi or Sub Lethal genes . The allele responsible for hemophilia is carried on the X chromosome , affected mainly in males , and they inherit the allele from their mothers . Hemophilia = is affected in individual bleed as longer period of time until clotting occurs , this means that normally minor wounds can be fatal in a person .

4- Synthetic Lethal genes .Some mutations are only lethal when paired with second mutation

5- Conditional lethal genesEX: favism allele that common among people of Mediterranean , African and Southeast Asian . The disease was named because when affected individuals eat fava beans , they develop hemolytic anemia , a condition in which red blood cells break apart and block blood vessels . But that disease is resistant to malaria because it is difficult for malaria to multiply in cells with deficient a mount of glucose 6 phosphate dehydrogenase .

Epistasis

The effects of one gene are modified by one or several other genes .

Hypo-static : is one whose phenotype is masked by the expression of an allele at a

separate locus, in an epistasis event

Epi-static : The gene whose phenotype is expressed.

Epistasis can be contrasted with dominance which is an interaction between alleles at the

same gene locus . The interaction between two or more genes to control a single

phenotype and identify and recognize the 9:3:3:1 that results of crossing 2 dihybrids

produced a modified mendelian .

Kind of Epistasis :

1- Dominant Epistasis : (12:3:1 ) from ( 9: 3 :3 :1 ) EX: Fruit color in squash

2- Dominant duplicate Epistasis : (15:1 ) from ( 9: 3:3 :1 ) EX: Kernel Color in wheat

3- Recessive Epistasis: (9:4:3 ) from ( 9: 3: 3 :1 ) EX: mice coat color

4- Recessive duplicate Epistasis : (9:7) from ( 9: 3:3 :1 ) EX: Flower Color in sweet pea

Kind of Epistasis :

1- Dominant Epistasis .

* Definition :Complete dominance at both gene pairs ; when one gene is dominant , it

hides the effects of other gene . And the ratio is 12:3: 1

* EX: Fruit color in squash .

Genotype Phenotype Enzymatic Activites

9 W_G_ White Dominant white allele negates effect of G allele 12

3 W_gg White Dominant white allele negates effect of G allele

3 wwG_ yellow Recessive color allel allows yellow allele expression 3

1 wwgg Green Recessive color allel allows green allele expression 1

1- Dominant Epistasis : (12:3:1 ) from ( 9: 3 :3 :1 ) EX: Fruit color in squashwhen one gene is dominant gene 1= ( white dominant (WW) , colored (ww) ) . gene 2= ( yellow is dominant (YY) , green ( yy ) ) .

♂/♀ WY Wy wY wy

WY WWYY WWYy WwYY WwYy

Wy WWYy WWyy WwYy Wwyy

wY WwYY WwYy wwYY wwYy

wy WwYy Wwyy wwYy wwyy

The effect of dominant gene ’Y’ is masked by the dominant gene ’W’ (epistatic gene)

P WWYY X wwyy

(white) ↓ (green)

F1 WwYy

(white) (selfed)

F2 White:Yellow:Green

12 : 3 : 1

2- Dominant duplicate Epistasis .

* Definition :Complete dominance at both gene pairs ; when either gene is dominant ,

it hides the effects of other gene . And the ratio is 15 : 1

* EX: Kernel Color in wheat .

Genotype Phenotype Enzymatic Activites

9 A_B_ Colored Kernels Functional enzymes from both genes 15

3 A_bb Colored Kernels Functional enzymes from A genes

3 aaB_ Colored Kernels Functional enzymes from B genes

1 aabb White Kernels Non Functional enzymes from both genes

1

Dominant duplicate Epistasis : (15:1 ) from ( 9: 3:3 :1 ) EX: Kernel Color in wheat when either gene is dominantGene A = Enzyme AA Gene B = Enzyme BB

AABB aabb

AaBb

AaBb AaBb

AABB 9يعمالن الجينين

AAbb3

A يعمل

aaBB3

B يعمل

aabb1

اليعمل

15 1

3- Recessive Epistasis .

*Definition: Complete dominance at both gene pairs ; when one gene is homozygous

recessive , it hides the effects of other gene . And the ratio is 9:4:3

* EX: mice coat color .

in some casses recessive allele c masks the effect of dominant allele (B) .

When black mouse crossed to albino mouse , only black progeny were produced , but in F2 , 9

black , 3 cream and 4 albino (9:3:4) were produced .

Coat color in mouse is controlled by 2 dominant genes B and C

When B is dominant , mouse color will be black .

When both recessive genes b and c are homozygous recessive , albino mice are produced .

In this case when c is homozygous it masked the effect of the B gene.

Black mice ( BBCC) X Albino (bbcc) mice

F1 BbCc( black)

Recessive Epistasis: (9:4:3 ) from ( 9: 3: 3 :1 ) EX: mice coat color when one gene is homozygous recessive

4- Recessive duplicate Epistasis .

* Definition : Complete dominance at both gene pairs ; when either gene is

homozygous recessive , it hides the effects of other gene . And the ratio is 9: 7

* EX: Flower Color in sweet pea .

. The following table explanation for the ratio 9:7 because either the genes can provide

the wild type phenotype .

Genotype Phenotype Enzymatic Activites

9 C_P_ Colored : anthocyanin produced Functional enzymes from both genes 9

3 C_pp Flowes white : no anthocyanin produced p enzyme non-functional 7

3 ccP_ Flowes white : no anthocyanin produced c enzyme non-functional

1 ccpp Flowes white : no anthocyanin produced C and p enzymes non-functional

Recessive duplicate Epistasis : (9:7) from ( 9: 3:3 :1 ) EX: Flower Color in sweet pea; when either gene is homozygous recessiveGene1 =CGene2=P

CCPPColored

ccppwhite

CcPp

CcPp CcPp

CCPPالجينين كال عمل9

CCppانزيم P اليعمل

3

ccPPانزيم C اليعمل

3

ccpp اليعمل

1

9Colored

7White