This document discusses several concepts related to genetics: 1. It describes incomplete dominance and codominance, using the example of flower color in petunias. 2. It explains lethal alleles and how they can cause death in homozygous or heterozygous individuals. Examples given include coat color in mice and genes in cattle and humans. 3. Multiple alleles are discussed, which have varying levels of dominance and control traits like eye color in fruit flies and coat color in rabbits.

1. Example for trait
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2. Extension to Mendel’s Law
1. Alleles’ interaction
2. Pleiotropism
3. Epistasis interaction
4. Linkage and crossing over
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3. a) Incomplete dominance
b) Lethal allele
c) Co dominant allele
d) Multiple allele
e) Penetrance and expressivity
Alleles’ interaction
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4.  Lack of dominant and recessive relationship
 Allele - same degree of expression in heterozygous condition
a) Incomplete dominance
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7. Q1) The 4O’ clock plant has one variety with red flowers and other
variety with white flowers. From a pure crossing of these two
varieties all the F1 has pink flowers. The F2 offspring produce 142
pink, 68 white and 73 red flowers. How these flowers are inheriting?
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8. Q2) Give the genotypes and phenotypes of progeny from the
following crosses in the 4O’ clock plant.
i. White X White
ii. White X Pink
iii. Red X Pink
iv. Red X Red
v. Pink X Pink
vi. Red X White
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b) Lethal allele
 Some genes - cause death during early stage of development
(before sexual maturation)
 A fully dominant lethal allele - arise by mutation from a wild
type allele
 Dominant lethal genes - lost from the heterozygous
organism called as gene erosion
 Most of all lethal genes are recessive (death only
homozygous)
 There are two types of lethal alleles

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1. One that has no obvious phenotypic effect in heterozygous
a. Mice coat colour
Dominant homozygous causes death during embryonic stage
b. Aurea gene in Antirrhinum majus
It produces yellow leaf due to lack of chlorophyll in heterozygous
c. Dexter gene in cattle
d. Achondroplastic dwarfness in man

11. Coat colour in mice

12. Dexter gene in cattle

13. Antirrhinum majus

14. Achondroplastic dwarfness

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2. One that exhibit a distinctive phenotype when heterozygous
The amount of chlorophyll in snapdragons is controlled by a pair of
alleles C1 and C2
One of which C2 exhibits lethal effect when homozygous and a
distinctive colour phenotype when heterozygous
Thus with regard to colour these alleles are incomplete dominant
However, with regard to viability the C2 allele is fully recessive that
the C2 allele only causes death when C1 is absent

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Snapdragon plant

17. Alleles that lack of dominant and recessive relationship and are both
observed phenotypically to some degree are co-dominant
This means that the phenotypic effect of each allele is observable in
heterozygous condition
Hence the heterozygous genotypes give rise to a phenotype
distinctly different from either of the homozygous genotype but
possess characteristics of each other
For co-dominant alleles all upper case base symbol with different
superscripts are used.
c. Co - dominant allele

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Qu) Coat colours of short horned breed of cattle represent a classical
example of co-dominant allele. Red is governed by the genotype
CRCR, roan (mixture of red and white) by CRCW and white is CWCW.
a. When roan short horns are crossed among themselves what
genotypic and phenotypic ratios are expected among their progeny?
b. If red short horns are crossed with roans and the F1 progeny are
crossed among themselves to produce the F2. What percentage of F2
will probably be roan?

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More than two alleles - at a gene locus in a population
The series called multiple allele series
These alleles may have arisen as a result of mutation of the
dominant (wild) allele in a gene pair
d. Multiple allele

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Common characters of multiple allele
1. They occupy same locus within the homologous chromosome. In a diploid cell
any two alleles of such allele series are present in a pair of homologous
chromosome. The gamete of an organism contains only one allele of such
series.
2. Multiple allele controls the particular character but with varying degree of
efficiency.
3. The normal gene of the series act as dominant, overall such series alleles may
also behave as dominant, recessive and co-dominant among them.
4. The capital letter is commonly used to designate allele that is dominant to all
others in the series. The corresponding lower case letter designates the allele
that is recessive to all others in the series.
5. Other alleles, intermediate in their degree of dominance between these two
extremes are usually assigned the upper case letter with some suitable
superscript.

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Examples:
i. Eye colour of Drosophila
ii. Coat colour in rabbit
iii. Blood group in man
iv. Self sterility genes in plants. Eg: Red clove and Tobacco

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Eye colour of Drosophila

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Differences in environmental condition/ in genetic backgrounds
may cause individuals that are genetically identical at a particular
locus to exhibit different phenotypes
The percentage of individuals with a particular gene combination
that exhibit the corresponding character to any degree represents
the penetrance of the trait.
e. Penetrance and expressivity

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The polydactyl (extra fingers and or toes) in human can be produced
by a dominant gene P. The wild type condition with five digits on
each limb is produced by the recessive gene p. However, some
heterozygous individuals (Pp) are not polydactylous. If 20% of Pp
individuals do not show polydactyl (that are wild type) the gene has
a penetrance of 80%. A trait although penetrant may be quite
variable in its level of expression. The degree of effect produce by a
penetrant genotype is term expressivity.

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A gene that has multiple phenotypic effects
It is a special interest because it helps in understanding the
relationships between different organisms of the same individuals
Examples:
I. Drosophila bar eyed individuals may be significantly altered by
the wing nature
II. Seed coat colour gene of sweet pea controls flower colour and
also red spot in leaf axils
III. Sickle cell anemia in human
2. Pleiotropism

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