I delivered this presentation to fellow postgraduate students. It's on the various traits, normal and pathological, that are transmitted by single genes.

1. General Overview Of Patterns
Of Transmission Of Single
Gene Traits
ADEPOJU, Paul Olusegun
Cell Biology & Genetics Unit
Dept of Zoology,
University of Ibadan
Ibadan, Nigeria

2. Introduction
• We are all humans, but we don’t look alike –
not even identical twins
• The basis for the similarity and the reasons
for the diversity that coexist in all species
have puzzled and intrigued people for
thousands of years. It actually birthed
genetics
• Yoruba myths, traits and transmission
genetics – Babatunde, Iyabo et al

3. Historical Contributions
• Mendel and his peas (Pisum sativum)
• Bateson and Punnett – epistasis (influence of
gene interaction on phenotypes)
• Harris – Pearson’s goodness of fit test
• Morgan et al – Drosophila melanogaster (heritable
mutation in fruit flies)
• Sutton – marriage of cell biology and genetics to
suggest genes might actually be on chromosomes
• Timofeeff-Ressovsky – penetrance and
expressivity

4. Gregor Mendel and His Peas
• The way in which traits are passed from one generation to the next,
sometimes skipping some generations, was first explained by GM
(1885).
• Why Peas?
• Good model system – he could easily control fertilization by
transferring pollen with a small paintbrush – self or cross fertilization
• Traits – height (tall or short), pod shape (inflated or constricted), seed
shape (smooth or wrinkled), pea color (green or yellow) etc
• Quality control measures – tall plants had tall children, grandchildren
etc. i.e. pure breeders (homozygous)
• Unlike preceding blend theory, GM’s crosses yielded offspring that
resembled one of the parent plants, not a blend (examples)
DOMINANT & RECESSIVE TRAITS

5. Law Of Segregation
“Every individual possesses a pair of alleles
(assuming diploidy) for any particular trait and that
each parent passes a randomly selected copy
(allele) of only one of these to its offspring. The
offspring then receives its own pair of alleles for
that trait. Whichever of the two alleles in the
offspring is dominant determines how the
offspring expresses that trait (e.g. the color and
height of a plant, or the color of an animal's fur).”

6. Law Of Independent Assortment
• Also known as "Inheritance Law", states that separate genes for
separate traits are passed independently of one another from
parents to offspring.
• IOW, alleles of different genes assort independently of one another
during gamete formation.
• While Mendel's experiments with mixing one trait always resulted in
a 3:1 ratio between dominant and recessive phenotypes, his
experiments with mixing two traits (dihybrid cross) showed 9:3:3:1
ratios. But the 9:3:3:1 table shows that each of the two genes is
independently inherited with a 3:1 phenotypic ratio.
• Mendel concluded that different traits are inherited independently of
each other, so that there is no relation, for example, between a cat's
color and tail length.
• This is actually only true for genes that are not linked to each other.

7. Applications of Patterns of
Transmission
• predicting the clinical status of individuals
possessing mutations
• critical for assessing risk to the family
members of a patient affected with a genetic
disorder.
• to rule out certain genetic disorders in a
differential diagnosis when there is a family
history of disease.

8. Autosomal
Sex-linked Dominant – AA or Aa
Single Gene Traits Recessive – AA or aa
Mitochondrial
• Definition from etymology
• They are controlled by a single gene with two
alleles; each allele producing a distinct
phenotype.
• Alleles are different expressions of the same
gene.
• All can be used to demonstrate Mendel's
Law of Segregation.

9. External Influences
• Anticipation
• Mosaicism
• Uniparental disomy
• Genomic imprinting – e.g. Igf2 gene; Prader–
Willi and Angelman syndromes

11. • Dominant Inheritance
a single copy of a mutation will result in disease (a
variation on this principle is dominant disorders with
reduced penetrance)
• Recessive Inheritance
an individual will not be affected if he/she has at least one
normal allele. Individuals with one normal allele and
one mutated allele are called carriers. If a patient
possesses no normal allele, only genes with recessive
mutations, they will be affected with the disorder.
• Mitochondrial Inheritance
the clinical status of a patient is correlated to the
proportion of mitochondria with mutations versus
mitochondria with normal gene copies.

12. ADI
• Mutation Location: Autosomal Chromosome
• Genetic transmission: Individuals possessing one copy of a mutation
will be affected.
• Examples: Huntington's Disease
Charcot-Marie-Tooth Disease Type 1
Spinocerebellar Ataxia
Myotonic Dystrophy
• Characteristics:
– The child of an affected parent has a 50% chance of inheriting the parent's
mutated allele and thus being affected with the disorder.
– A mutation can be transmitted by either the mother or the father. All children,
regardless of gender, have an equal chance of inheriting the mutation.
 Some autosomal dominant disorders may be characterized by reduced penetrance, i.e.,
an individual may inherit a mutation and not manifest clinical symptoms. However, these
individuals may transmit the mutation and have affected offspring.

13. ARI
• Mutation Location: Autosomal Chromosome
• Genetic transmission: Individuals possessing two copies of a
mutation will be affected.
• Examples: Spinal Muscular Atrophy (SMA)
Friedreich's Ataxia
• Xtics:
– An individual will be a "carrier" if they posses one mutated allele
and one normal gene copy.
– There is a 50% chance that a carrier will transmit a mutated gene
to a child.
– All children of an affected individual will be carriers of the disorder.
– A mutation can be transmitted by either the mother or the father.
– All children, regardless of gender, have an equal chance of
inheriting mutations.

14. If two carrier parents have a child…
• 25% chance that both will transmit the mutated gene; in this case, the
child will inherit only mutated copies of the gene from both the mother
and the father and thus will be affected with the disorder.
• 50% chance that one carrier parent will transmit the mutated gene and
the other will transmit the normal gene; in this case, the child will have
one mutated gene and one normal gene and will be a carrier of the
disorder.
• 25% chance that both carrier parents will transmit the normal gene; in
this case the child will have only normal genes and will not be affected
and will not be a carrier.

15. XLD – pro male
• Mutation Location: X-Chromosome
• Genetic transmission: Individuals possessing one copy
of a mutation will be affected.
• Examples: Charcot-Marie-Tooth Disease Type X1
• Xtics
– A male or female child of an affected mother has a 50% chance
of inheriting the mutation and thus being affected with the
disorder.
– All female children of an affected father will be affected
(daughters possess their fathers' X-chromosome).
– No male children of an affected father will be affected (sons do
not inherit their fathers' X-chromosome).

16. XLR – pro female
• Mutation Location: X-Chromosome
• Genetic transmission: Individuals possessing no normal gene copies
will be affected; typically, only males are affected.
• Examples: Duchenne/Becker Muscular Dystrophy; Norrie Disease;
Spinal and Bulbar Muscular Atrophy (Kennedy's Disease)
• Xtics:
– Females possessing one X-linked recessive mutation are considered
carriers and will generally not manifest clinical symptoms of the disorder.
– All males possessing an X-linked recessive mutation will be affected (males
have a single X-chromosome and therefore have only one copy of X-linked
genes).
– All offspring of a carrier female have a 50% chance of inheriting the
mutation.
– All female children of an affected father will be carriers (daughters posses
their fathers' X-chromosome).
– No male child of an affected father will be affected (sons do not inherit their
fathers' X-chromosome).

17. M.I.
• Mutation Location: Mitochondrial DNA (brief recap)
• Genetic Transmission: Dependent on proportion of normal and
mutated mitochondrial DNA (mtDNA).
• Examples: Kearns-Sayre Syndrome, MELAS - Mitochondrial
Myopathy Encepholopathy, Lactic Acidosis, and Stroke Like
Episodes, MERRF - Myoclonus with Epilepsy Ragged Red Fibers
• Also implicated in DM, deafness, heart disease, Alzheimer’s
disease, Parkinson disease, and Leber’s hereditary optic
neuropathy

18. Xtics of MRI
• All children of a mother with a mtDNA mutation are at risk to be
either affected with the disorder or asymptomatic carriers of the
disorder.
• An individual will be affected with a mitochondrial disorder if the
percentage of mitochondria possessing mutated mtDNA reaches a
threshold value beyond which the normal mtDNA does not
compensate for the mutated mtDNA.
• The mixture of mitochondria possessing mutated mtDNA and
mitochondria with normal DNA is referred to as heteroplasmy.

19. Non Pathological Single Gene Traits

20. Tongue Rolling
AUTOSOMAL DOMINANT

21. Widow’s Peak
AUTOSOMAL DOMINANT

22. HAND CLASPING (L/R INTERLOCKING FINGER)
AUTOSOMAL DOMINANT

23. ATTACHED EARLOBES
AUTOSOMAL DOMINANT

24. HITCHHIKER THUMB
AUTOSOMAL RECESSIVE

25. MID-DIGITAL HAIR

26. OTHERS
CONDITION DETAILS PATTERN
Wet ear wax D
PTC Tasting D
Darwin tubercle little bump on the inside of the ear D
S-methylthioester detection you smell asparagus odor in urine? R
Pigmented iris any color but blue D
Polydactyl more than 5 fingers and/or toes D
Short big toe The big toe is shorter than your second D
Long eyelashes >1cm D
Wooly hair R
Dimples D
Freckles D

27. Cleft Chin
• Impaired fusion of left and right sides of lower
jaw
• Dimple Appearance
• Congenital
• Low incidence
• Genetic (AD), but could be acquired due to
facial asymmetry resulting from one jaw being
slightly long.
• Also known as superhero chin

28. Cleft Chin
Kirk Douglas Hulk Hogan

29. Variation In Degree of Prominence
Michael DOUGLAS Kirk DOUGLAS

30. Thank You
ADEPOJU, Paul Olusegun
(November, 2012)