Organismic level of Organization of Genetic information. Gene Interaction

1. LECTURE 4
ORGANISMIC LEVEL OF
ORGANIZATION OF GENETIC
INFORMATION.
GENE INTERACTION.

2. PLAN
• Fundumentals of cenetics.
• Mendel’s Laws
• Mexanizm of gene interaction.
• - interaction of allelic gene
- interaction of non-allelic gene
- Genotype –environment interaction

3. A gene is the basic physical and functional unit of heredity.
Genes, which are made up of DNA,
act as instructions to make molecules called proteins.
In humans, genes vary in size from
a few hundred DNA bases to more than 2 million bases.
The Human Genome Project has estimated
that humans have between 20,000 and 25,000 genes.

4. Gregor Mendel
(1822-1884)
•Augustinian monk who cross-bred pea plants with
different characteristics
•Observations led to laws regarding the transmission of
hereditary characteristics from generation to generation
•Many of the concepts from his observations still hold
true today

5. Sponsored
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6. • Dominant = only one allele of a gene necessary to
express the trait (A)
• Recessive = both alleles of a gene must be identical to
express the trait (a)
• Heterozygous = alleles of a particular gene are non-
identical (Aa)
• Homozygous = alleles of a particular gene are identical
(AA, aa)
• Genotype = the genetic constitution of an organism. The
genotype determines the hereditary potentials and
limitations of an individual from embryonic formation
through adulthood.
• Phenotype= all the observable characteristics of an
organism, such as shape, size, colour, and behaviour,
that result from the interaction of its genotype (total
genetic inheritance) with the environment.

7. Mendel's Laws are as follows:
1.the Law of Dominance
2. the Law of Segregation
3. the Law of Independent
Assortment

8. Dominance - the ability of one allele to express its
phenotype at the expense of an alternate allele; the
major form of interaction between alleles; generally
the dominant allele will make a gene product that
the recessive can not; therefore the dominant allele
will express itself whenever it is present

9. • Monohybrid cross - a cross between
parents that differ at a single gene pair
(usually AA x aa)
• Monohybrid - the offspring of two parents
that are homozygous for alternate alleles
of a gene pair

10. the Law of Segregation
Two members of a gene pair segregate
from each other in the formation of
gametes; half the gametes carry one
allele, and the other half carry the other
allele

11. Principle of Independent
Assortment
Genes for different traits assort
independently of one another in gamete
production

13. •Therefore, the independent segregation means
that among each of the possible genotypes
formed by one pair of alleles, the ratio of
homozygous dominant to heterozygous to
homozygous recessive is 1 : 2 : 1 for the other
independent pair of alleles.
•The principle of independent segregation of two
pairs of alleles in different chromosomes (or
located sufficiently far apart in the same
chromosome) has come to be known as the
principle of independent assortment.
(1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1 ratio of genotypes;
9:3:3:1 phenotypes)

14. Gene interaction
Interaction of allelic genes:
• Complete dominance
• Incomplete dominance
• Codominance
• Superdomonance (overdominance)

15. Complete dominance
In all of Mendel’s experiments, he worked with
traits where a single gene controlled the trait and
where one allele was always dominant to the
other.
Although the rules that Mendel derived from his
experiments explain many inheritance patterns,
the rules do not explain them all. There are in
fact exceptions to Mendel’s rules, and these
exceptions usually have something to do with
the dominant allele

16. Incomplete dominance
• The kind of inheritance of allelic genes where a
cross between organisms with two different
phenotypes (AAxaa) produces offspring with
third phenotype that is blending (Aa) of the
parental traits. Incomplete is a condition when
neither allele is dominant over the other, when
the interaction enzymes are slightly different in
their activity

18. Codominance
All humans and many other primates can be
typed for the ABO blood group. There are four
principal types: A, B, AB, and O. There are two
antigens and two antibodies that are mostly
responsible for the ABO types. The specific
combination of these four components
determines an individual's type in most cases.
The table below shows the possible
permutations of antigens and antibodies with the
corresponding ABO type

21. Overdominance
• The kind of gene interaction in which the
phenotypic expression of the
heterozygous condition exceeds the
phenotype of homozygous dominant
condition.

22. Interaction of non-allelic genes
• Complementation
• Epistasis
• Polygenic inheritance

23. Complementation
Gene interaction where the manifestation
of a character is determined by presence
of two dominant genes of different
allelomorphic pairs simultaneously (A_B_)
(ratio 9:3:4, 9:6:1, 9:7)

25. Epistasis
Epistasis is the term applied when one gene interferes
with the expression of another.
A good example of epistasis is the genetic interactions that produce
coat color in horses and other mammals. In horses, brown coat
color (B) is dominant over tan (b). Gene expression is dependent on
a second gene that controls the deposition of pigment in hair. The
dominant gene (C) codes for the presence of pigment in hair,
whereas the recessive gene (c) codes for the absence of pigment. If
a horse is homozygous recessive for the second gene (cc), it will
have a white coat regardless of the genetically programmed coat
color (B gene) because pigment is not deposited in the hair. The
figure above demonstrates this scenario. Several of the white
horses have genotypes for brown or tan coat color in the first gene,
but are completely white because they are homozygous recessive
for the gene controlling pigment deposition.

27. Polygenic inheritance
• Polygenic inheritance is a pattern responsible for
many features that seem simple on the surface.
Polygenic traits are not expressed as absolute or
discrete characters, as was the case with
Mendel's pea plant traits. Instead, polygenic
traits are recognizable by their expression as a
gradation of small differences (a continuous
variation). The results form a bell shaped curve,
with a mean value and extremes in either
direction.

28. Human polygenic traits include:
• Height
• Weight
• Eye Color
• Intelligence
• Skin Color
• Many forms of behavior

30. Pleiotropy
• So far we have only considered genes that affect a
single phenotypic character. This actually is a rare
situation because it is more common that one gene can
have multiple effects

31. Pleiotropy
• Primary (Marfan`s syndrome, Hartnup
diseases)
• Secondary (sickle-cell anaemia,
phenylketonuria)

32. Marfan`s syndrome

33. Hartnup diseases

37. Variation in Gene Expression
Not all traits are expressed 100% of the time
even though the allele is present.
Penetrance - the frequency of expression of an
allele when it is present in the genotype of the
organism (if 9/10 of individuals carrying an allele
express the trait, the trait is said to be 90%
penetrant)
Expressivity - variation in allelic expression
when the allele is penetrant

38. For example the dominant allele P produces polydactyly in humans, a
trait that is characterized by extra toes and/or fingers. Two normal
appearing adults have been known to mate and produce offspring
that express polydactyly. Thus one parent must carry at least one
dominant allele (P allele) and its genotype is probably Pp. This
parent with the Pp genotype exhibits reduced penetrance for
the P allele.
Not all phenotypes that are expressed are manifested to the same
degree. For polydactyly, an extra digit may occur on one or more
appendages, and the digit can be full size or just a stub. Therefore,
when the P allele is present it expresses variable expressivity.
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