Transcript Lecture #26 - Suraj @ LUMS

Lecture 26
Genetically Inherited Diseases
The Monk and His Peas
• An Austrian monk, Gregor
Mendel, developed the
fundamental principles
that would become the
modern science of
genetics.
• Mendel demonstrated that
heritable properties are
parceled out in discrete
units, independently
inherited.
• These eventually were
termed genes.
Mendel’s Experiments
• Mendel's experimental organism was a common garden
pea (Pisum sativum), which has a flower that lends itself to
self-pollination.
• The male parts of the flower are termed the anthers. They
produce pollen, which contains the male gametes (sperm).
• The female parts of the flower are the stigma, style, and
ovary. The egg (female gamete) is produced in the ovary.
• The process of pollination (the transfer of pollen from
anther to stigma) occurs prior to the opening of the pea
flower.
Mendel’s Experiments
• The pollen grain grows a pollen tube which allows the
sperm to travel through the stigma and style, eventually
reaching the ovary.
• The ripened ovary wall becomes the fruit (in this case the
pea pod).
• Mendel tested all 34 varieties of peas available to him
through seed dealers.
• The garden peas were planted and studied for eight years.
• Each character studied had two distinct forms, such as tall
or short plant height, or smooth or wrinkled seeds.
Inherited Characteristics
Definitions
• Genotype – the specific allele composition
of a cell.
• Phenotype – the detectable manifestation of
a specific genotype.
Principle of Segregation -Monohybrid
Cross
Cross female phenotype: Purple X White :male
phenotype
Parent genotype
White (w)
White (w)
Purple (P)
P,w
P,w
Purple (P)
P,w
P,w
Phenotypically all offspring (F1) produce purple flowers, but
each has information for both purple and white pigment
production. Therefore, genotypically the offspring are
heterozygous. In this case the allele for production of the purple
pigment is dominant
Smooth vs Wrinkled Peas
F1 x F1 = F2
Cross female phenotype: Purple X Purple :male phenotype
F1 genotype P
P
w
w
P,P P,w
P,w w,w
In looking at these offspring (F2), one can see that phenotypically
three of four (3/4) produce purple flowers while one of four (1/4)
produce white flowers. We say that such a cross typically produces
a 3:1 ratio of dominate to recessive phenotype expression.
Genotypically, however, one notes that the ratio is 1(homozygous
dominant):2 (heterozygous):1 (homozygous recessive).
Dihybrid Cross
Cross Female = Green pods with Round peas X Yellow
pods with Wrinkled peas = Male
Parent genotype
Green (G),Round
(R)
Green (G), Round
(R)
Yellow (y), Wrinkled
(w)
Gy,Rw
Gy,Rw
Yellow (y),Wrinkled
(w)
Gy,Rw
Gy,Rw
In looking at the phenotypes of the first generation offspring (F1),
we find that all produce green pods and round peas.
Genotypically, however, all F1 individuals have both green and
yellow pod color and round and wrinkled pea appearance
information (heterozygous). Green pod colour and round peas
appearance are, therefore, the dominant alleles
F1 X F1 = F2
Parent genotype
G,R
G,w
y,R
y,w
G,R
GG,RR
G, w
GG,Rw
y,R
Gy,RR
y,w
Gy,Rw
GG,Rw
GG,ww
Gy,Rw
Gy,ww
Gy,RR
Gy,Rw
yy,RR
yy,Rw
Gy,Rw
Gy,ww
yy,Rw
yy,ww
In looking at the phenotypes, we find nine exhibit the dominant
allele for both traits; three that exhibit the dominant green pods
but wrinkled (recessive) peas; three that exhibit yellow (recessive)
pods but the dominant round peas; one that exhibits the recessive
allele for both traits---9:3:3:1. We find a more complex assortment
with regard to genotypes
Expression of Alleles
• Dominance refers to the effects of one allele overriding the
effects of another allele (of the same gene). For example, A
is dominant to a. Dominant traits were defined by Mendel
as those which appeared in the F1 generation in crosses
between true-breeding strains.
• Recessives were those which "skipped" a generation, being
expressed only when the dominant trait is
• Epistasis refers to the effects of one gene overriding the
effects of another gene. For example, gene A (really the aa
genotype) is epistatic to gene B.
Polygenic Inheritance
• Many traits such as height, shape, weight,
color, and metabolic rate are governed by
the cumulative effects of many genes.
• Polygenic traits are not expressed as
absolute or discrete characters, as was the
case with Mendel's pea plant traits.
Multiple Allelic Inheritance
• While many genes have only two possible alleles, and we
are most accustomed to looking for inheritance patterns
involving only two possible alleles, there are genes for
which geneticists have identified more than two possible
alleles.
• These traits are said to be multi-allelic.
• One of the best documented and more interesting multiallelic traits is that of the ABO blood group antigens in
humans.
• The phenotype (A, B or O) on the individual's erythrocytes
is determined by the presence and identity of the terminal
sugar attached to a molecular configuration on the surface
of the erythrocyte membrane.