General Genetic
Bio 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
Incomplete VS Codominance
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.
* 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 .
Incomplete dominance example
• 4- Sickle cell anemia is
recessive disease . In
heterozygous which have
abnormal red blood cells under
some conditions .
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 codominant 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 inheritance
More 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 genes
EX: 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
3 W_gg
White
Dominant white allele negates effect of G allele
3 wwG_
yellow
Recessive color allel allows yellow allele expression
1 wwgg
Green
Recessive color allel allows green allele expression
12
3
1
1- Dominant Epistasis : (12:3:1 ) from ( 9: 3 :3 :1 ) EX: Fruit color in squash
when one gene is dominant
gene 1= ( white dominant (WW) , colored (ww) ) .
gene 2= ( yellow is dominant (YY) , green ( yy ) ) .
 The effect of
dominant gene ’Y’
is masked by the
dominant gene
’W’ (epistatic
gene)
 P WWYY X wwyy
♂/♀
WY
WY
Wy
wY
wy
WWYY WWYy
WwYY
WwY
y
Wy
WWYy
WWyy
WwYy
Wwy
y
wY
WwYY
WwYy
wwYY
wwY
y
wy
WwYy
Wwyy
wwYy
wwyy
(white) ↓ (green)
 F1
WwYy
(white) (selfed)
 F2
White:Yellow:Gree
n

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
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
15
1
Dominant duplicate Epistasis : (15:1 ) from ( 9: 3:3 :1 ) EX: Kernel Color in
wheat
when either gene is dominant
Gene A = Enzyme AA
Gene B = Enzyme BB
AABB
aabb
AaBb
AaBb
AABB
9
‫الجينين يعمالن‬
AAbb
3
‫يعمل‬A
15
AaBb
aaBB
3
‫يعمل‬B
aabb
1
‫اليعمل‬
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 9:7because either the genes can provide
the wild type phenotype .
Genotype
9 C_P_
Phenotype
Colored : anthocyanin produced
Enzymatic Activites
Functional enzymes from both genes
9
3 C_pp
Flowes white : no anthocyanin produced
p enzyme non-functional
3 ccP_
Flowes white : no anthocyanin produced
c enzyme non-functional
1 ccpp
Flowes white : no anthocyanin produced
C and p enzymes non-functional
7
Recessive duplicate Epistasis : (9:7) from ( 9: 3:3 :1 ) EX: Flower Color in
sweet pea
; when either gene is homozygous recessive
Gene1 =C
Gene2=P
CCPP
Colored
ccpp
white
CcPp
CcPp
CCPP
‫عمل كال الجينين‬
9
9
Colored
CCpp
‫اليعمل انزيم‬P
3
CcPp
ccPP
‫اليعمل انزيم‬C
3
7
White
ccpp
‫اليعمل‬
1