3. MENDEL’s LAWS
Law of Segregation: member of each pair of alleles are
separated when gametes are formed.
Law of Independent Assortment: pairs of alleles
separate independently of one another during gamete
formation
Law of Dominance: one dominant allele will take
charge over a recessive allele and masked it. The only
way a recessive allele can be seen is when an individual
possess two copies of the recessive allele.
2/6/2020 3
4. DEVIATION FROM MENDELIAN PRINCIPLE
Far beyond dominant and recessive
Some alleles are neither dominant nor recessive, and
many traits are controlled by multiple alleles or
multiple genes.
Codominance
Incomplete dominance
Multiple allele
2/6/2020 4
5. Lethal allele
Polygenic trait
Sex influenced
Sex limited
Sex linked
2/6/2020 5
6. CO DOMINANCE
Both alleles contribute to the phenotype by showing
up simultaneously in heterozygote individual.
Heterozygote for codominant alleles are
phenotypically similar to both parental types.
AB blood type in human
Roan coat in horses
2/6/2020 6
9. INCOMPLETE DOMINANCE
Incomplete dominance is exhibited when the
heterozygote has a phenotype intermediate between
the phenotypes of the two homozygote.
Neither form of the gene is able to mask the other.
Instead, they phenotypically blend together.
curly hair (cc) x straight hair (ss) = wavy hair (cs)
2/6/2020 9
10. Example: A cross between
red (RR) and white (WW)
snapdragons produces pink
flowers (RW).
2/6/2020 10
12. MULTIPLE ALLELES
Genes with more than two allele
No individual have more than two allele for a trait, but
more than two possible allele are seen to control a trait
in a population
Example: human blood type has three different alleles
ABO
2/6/2020 12
14. POLYGENIC TRAIT
Traits controlled by two or more genes
They show a wide range of phenotype which is produced by the
interaction of more than one pair of genes.
Example: human eye, skin colour, hair colour, height, chicken
comb shape, pepper colour etc
2/6/2020 14
15. Skin colour in human is
controlled by more than four
genes
instead of only two alleles
contributing to a trait, there
can be 4, 6 or 8, etc. alleles
contributing to a trait.
Skin color: the more
dominant alleles for each
gene, the darker the skin.
AABbCc = darker skin
aaBbCc = lighter skin
2/6/2020 15
17. LETHAL ALLELE
An allele that has the potential to cause the death of an
organism .
These alleles usually occurs as a result of mutations in
genes that are essential to growth or development.
They can cause death prenatally or after birth.
They may be recessive, dominant or conditional
depending on the gene involved.
2/6/2020 17
18. RECESSIVE LETHAL ALLELES
Expressed only in
homozygous condition
Heterozygous is not affected
Example is seen in Sickle
cell Anemia, Manx cat.
2/6/2020 18
19. DOMINANT LETHAL ALLELES
They are expressed in both homozygous and heterozygous
condition
It requires the present of only one allele in the organism to be
fatal.
It is not usually found in population because, it usually result in
the death of the organism before it can transmit its lethal gene
on to its offspring.
Example : Huntington’s disease.
2/6/2020 19
20. CONDITIONAL LETHAL ALLELE
Allele that will only be fatal in response to
some environmental factors.
Example: Favism
2/6/2020 20
21. SEX LINKED TRAIT
Also known as “X-linked
Red-green colorblindness
is a recessive trait linked to
the X- sex chromosome
XC = normal color vision
Xc = color blindness
Y (doesn’t code for color-
seeing)
2/6/2020 21
22. Hemophilia is a sex-linked recessive trait disorder where individuals
are missing the normal blood clotting protein. Uncontrolled bleeds
from minor cuts or bruises.
2/6/2020 22
23. MUSCULAR DYSTROPHY
X-linked recessive trait
Progressive weakening and
loss of skeletal muscle.
Defective version of gene
that codes for muscle protein
2/6/2020 23
24. SEX LIMITED TRAIT
Sex limited characteristic is encoded by autosomal genes
that are expressed in only one sex.
Both males and females possess sex limited genes and
transmit them to their offspring.
Usually influenced by sex hormones like estrogen,
testosterone
„Breast development is normally limited to females
Beard growth is normally limited to males
2/6/2020 24
25. SEX INFLUENCED TRAIT
Traits where an allele is dominant in one sex but recessive in
the opposite sex.
Example: Pattern baldness in humans
caused by an autosomal gene „Allele (dominant in males, but
recessive in females)
2/6/2020 25
26. PENETRANCE
Penetrance is the proportion of people with a
specific genotype who manifest a particular
clinical characteristic or phenotype.
2/6/2020 26
27. INCOMPLETE PENETRANCE
Occurs when a dominant allele is not always
expressed in the phenotype of the individual.
Example: Polydactyl, the condition of having extra
fingers or toes.
Peavy, 2007
2/6/2020 27
28. EXPRESSIVITY
Expressivity is the series of signs and symptoms that occurs
in different people with the same genetic condition.
The number of digits can vary in polydactyl.
high expressivity: a person with several extra digits.
low expressivity: a person with single extra digit.
Peavy, 2007
2/6/2020 28
29. REFERENCES
Hartl, D. L., & Jones, E. W. (1998). Genetics: Principles and
Analysis 4th ed . Jones and Bartlett Publishers
Lobo, I. (2008). Mendelian ratios and lethal genes. Nature
Education , 1(1), 138.
Miko, I. (2011). Gregor Mendel and the principles of
inheritance. Nature Education (1).
Peavy, T. (2007). Extension to Mendelian Genetics.
Pierce, B. A. (2012). Genetics: A conceptual approach.
Macmillan
Watson J. and Crick F.(2015).Genetics and evolution. Chapter
5;Principle of inheritance and variation; Pages 72-94.
2/6/2020 29
