Extensions of mendelian inheritance

1. EXTENSIONS OF MENDELIAN INHERITANCE

2. CONTENTS
 What Is Inheritance?
 What Is Mendelian Inheritance?
 Extensions Of Mendelian Inheritance
 Factors Affecting Expected Mendelian Ratios
 Conclusion
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3. INHERITANCE
 The reception of
genetic qualities by
transmission from
parent to offspring.
 Condition, or trait from
past generations
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5. MENDELIAN INHERITANCE
Mendel’s experiments with pea plants suggested that:
1) Two types of “units” or alleles exist for every gene;
2) Alleles maintain their integrity in each generation (no blending).
3) In the presence of the dominant allele, the recessive allele is hidden, with no
contribution to the phenotype.
Therefore, recessive alleles can be “carried” and not expressed by individuals.
Such heterozygous individuals are sometimes referred to as “carriers.” Since
then, complexity exists, but that the fundamental principles of Mendelian
genetics still hold true. In the sections to follow, we consider some of the
extensions of Mendelism.
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6. EXTENSIONS OF MENDELIAN
INHERITANCE
 Mendel studied traits with only one
mode of inheritance in pea plants.
 The inheritance of the traits he
studied all followed the relatively
simple pattern of dominant and
recessive alleles for a single
characteristic.
 There are several important modes of
inheritance, discovered after Mendel’s
work, that do not follow the dominant
and recessive, single-gene model.
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7. FACTORS AFFECTING EXPECTED
MENDELIAN RATIOS
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• Multiple alleles
• Codominance
• Incomplete dominance
• Polygenic TRAIT
• Genetic Interactions Between non-allelic genes on different loci
• Environmental effects
• Lethal alleles
• Linkage

8. MULTIPLE ALLELES
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• MORE THAN TWO ALTERNATIVE FORMS OF A GENE
• Example:
ABO blood groups result from a series of three alleles, IA, IB and i that
combine to produce four phenotypes (A, B, AB and O).

9. CODOMINANCE
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 A variation on incomplete dominance is codominance, in which both
alleles for the same characteristic are simultaneously expressed in the
heterozygote.
 An example of codominance occurs in the ABO blood groups of
humans. The A and B alleles are expressed in the form of A or B
molecules present on the surface of red blood cells. Homozygotes
(IAIA and IBIB) express either the A or the B phenotype, and
heterozygotes (IAIB) express both phenotypes equally.
 The IAIB individual has blood type AB. In a selfcross between
heterozygotes expressing a codominant trait, the three possible
offspring genotypes are phenotypically distinct. However, the 1:2:1
genotypic ratio characteristic of a Mendelian monohybrid cross still
applies.

10. INCOMPLETE DOMINANCE
 Incomplete dominance, meaning that one of the alleles appears in the
phenotype in the heterozygote, but not to the exclusion of the other,
which can also be seen.
 The allele for red flowers is incompletely dominant over the allele for
white flowers.
 In the snapdragon, Antirrhinum majus, a cross between a homozygous
parent with white flowers (CWCW) and a homozygous parent with red
flowers (CRCR) will produce offspring with pink flowers (CRCW).
 In this case, the genotypic ratio would be 1 CRCR:2 CRCW:1 CWCW,
and the phenotypic ratio would be 1:2:1 for red: pink: white.
 The basis for intermediate color in the heterozygote is a pigment
produced by the red allele is diluted in the heterozygote and therefore
appears pink because of the white background of the flower petals.

11. POLYGENIC TRAIT
 A polygenic trait is one
whose phenotype is
influenced by more than one
gene.
 Traits that display a
continuous distribution, such
as height or skin color,
are polygenic.
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12. EPISTASIS
 Epistasis is a process in
which the
effect of one gene is
dependent on the
presence of one or more
'modifier genes’.
 It can also be defined as
the interaction
of two genes on different
loci.
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13. ENVIRONMENTAL EFFECTS
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• Internal and external environmental factors, like gender and
temperature, influence gene expression. Similarly, drugs, chemicals,
temperature, and light are among the external environmental factors
that can determine which genes are turned on and off, thereby
influencing the way an organism develops and functions.

14. LETHAL ALLELES
 Some genes are required for life
(essential genes) and mutations in them
may result in death (lethal alleles).
 Roughly one-third of all genes are
essential.
 Dominant lethal alleles result in the
death of both homozygotes and
heterozygotes.
 Recessive lethal alleles cause death
only when homozygous.
 An example of lethality is the yellow
body color gene in mice
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