Download Lab. 8 Deviation of Mendel`s first law “Monohybrid” part 1

Lab. 8
Deviation of Mendel’s first
law “Monohybrid” part 1
Main topics:


Deviation of Dominance
a. Co-dominance
b. Incomplete Dominance
Deviation of lethal alleles
a. Dominance lethal alleles
b. Recessive lethal alleles
Deviation from Mendel’s law
Sometimes during the crosses the ratio obtained does not obey Mendelian’s law.
Deviation from Mendel’s
law I:
1. Dominance:
a. Co-dominance
b. Incomplete dominance
2. Lethal alleles
c. Dominance lethal alleles
d. Recessive lethal alleles
3. Mutation:
a. Multiple alleles
4. Sex :
a. Sex- linkage
b. Sex- limited
c. Sex- influences
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1. Deviation of Dominance
a. Co-dominance:
Co-dominance: is basically means existing together. This means when there are two
alleles that share a co-dominant relationship, progeny will express both alleles.
For example:
(1) In roan horses, the white horse
(2) In roan cows, these cows have possibilities
below is the father of the foal. The red
of several colors of hairs that aren't a blend of
horse on the right is the mother and
the
the roan foal (baby horse) is on the
because the traits for each phenotype are
far right.
dominant. RR = red cow hairs, Rr = red and
dominant
and
recessive
phenotypes
white cow hairs, rr = white cow hairs
Ex. Color of Shorthorn cattle
Genetics Analysis:
P:
Gp:
F1:
Gf1:
F2:
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b. Incomplete Dominance
It is the expression of alleles that are
neither dominant nor recessive. These
alleles mix together and result in expression
of a physical trait that is a mixture of the two
alleles.
For example:
In snapdragon flower, the phenotypic ratio
and genotypic ratio for F2 generation is the
same which is 1:2:1. This is due to
incomplete dominance where recombinant progeny is produced. A red flower and white
flower cross would produce 1 red flower, 2 pink flowers and 1 white flower. This is due
to the intensity of the pigment cannot express itself in heterozygous state and produce a
combination of the pigment red and white which is pink.
Ex. Color of Snapdragon flower
Genetics Analysis:
P:
Gp:
F1:
Gf1:
F2:
2. Lethal Alleles
In 1905, Lucien Cue not 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.
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Essential genes are those that are absolutely required for survival and the absence
of their protein product leads to a lethal phenotype
• It is estimated that about 1/3 of all genes are essential for survival
Nonessential genes are those not absolutely required for survival
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 :
a. Postnatal lethal alleles cause abnormalities
in the progeny that cause them to die early on
in development. An example is parrot jaw,
which is observed in cattle. The phenotype
results in a deformed mouth so the calf is
unable to eat correctly and eventually starves
to death.
b. Embryonic lethal cause the death of the fetus,
and fertility studies are often required in order to
positively determine that an embryonic lethal
exists. An example of an embryonic lethal is the AY
allele in mice (seen in the diagram on the right).
This allele causes a yellow phenotype and is
dominant to the a+ allele, which causes the
grayish agouti color seen in many wild mice. However, embryos that are
homozygous for the AY allele die before they are born. Thus, the live progeny
from an AYa+ x AYa+ mating show a 2 yellow: 1 agouti phenotypic ratio.
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1. Dominant Lethal Alleles
•
The ratio 2:1. EX: Huntington disease
, coat color gene in mice and Manx cat
(Modifies 3:1 ratio to 2:1)
•
Organisms die before they can produce
the progeny.
•
So, the mutant dominant lethal allele is
removed from the population in the same generation it
arose
Ex. Color fur mice
Genetics Analysis:
P:
Gp:
F1:
Gf1:
F2:
Other example: Manx cat
Example of Manx Inheritance patterns
Mendel's ratio has now changed to
2:1.
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2. Recessive Lethal Allele
The recessive lethal allele kills the carrier
individual only in homozygous condition. They
maybe of two kinds
(i) One which has no obvious phenotypic effect
in heterozygotes.
(ii) One which exhibits a distinctive phenotype
when in heterozygous condition.
•
In many cases, lethal genes become
operative at the onset of sexual maturity
•
Conditional lethal alleles may kill an organism only when certain environmental
conditions prevail. For example, temperature-sensitive (ts) lethal A developing
Drosophila larva may be killed at 30 C But it will survive if grown at 22 C.
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Ex. Maize seedlings
Genetics Analysis:
P:
Gp:
F1:
Gf1:
F2:
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Exercise8
1- A red carnation is crossed with a white carnation and the resulting offspring are all
pink (indicating incomplete dominance). What would be the result of a cross between a
pinkand a white carnation?
Pink carnation genotype: RW
White carnation genotype: WW
2- A curly haired person and a straight haired person mate and all their offspring have
wavy hair (note that wavy hair is a phenotype in between that of the curly and straight
haired individuals). What would be the result of a cross between two wavy haired
individuals? (Give phenotype percent).
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3- In shorthorn cattle, when a red bull (RR) is crossed with a white cow (WW), all the
offspring are roan—a spotted, red and white or milky red color.
1. What offspring are expected from mating a roan bull and a roan cow?
2. What phenotypes would you expect from a cross between a red bull and a white
cow?
4- What is the F1 genotypic and phenotypic ratio from crossing two yellow individuals?
Yy
produces yellow fur
YY lethal
yy
Agouti
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