Gregor Mendel
• Largely responsible for the development
of the concept of the gene.
• High school substitute
teacher and monk who lived in a monastery in what is now the Czech Republic.
• Studied heredity of the
garden pea (1856-1863).
• Published his findings
in a local natural history journal in 1866.
• Few people paid any attention to the article until
1900 when three biologists independently cited it.
Mendel’s
Experiments
• He chose an ideal model
system to study heretibility, the garden pea.
-small
-easily cultured
-short life span
-exhibits great variability
-true-breeding strains
• He carefully recorded
his data.
• He formed testable
hypotheses.
• He tested his hypotheses
using numerical analysis.
• He examined seven traits
in the garden pea.
Monohybrid cross
•
A cross involving two contrasting traits of one character.
-(Monohybrid Cross Diagram)
-Explanation for results of Monohybrid cross
(diagram)
Test Cross
•
To
distinguish the genotype of the individuals possessing the dominant phenotype
in the F2 generation.
•
Cross
the individual with the dominant phenotype with a homozygous recessive
individual.
• Expected results- DD X dd = Dd (Tall)
- Dd X dd =1/2 Dd (Tall) ,1/2 dd (Dwarf)
Ø
Phenotypic
ratio =1:1
Mendel’s First Three Postulates
•
Unit Factors in Pairs-Genetic traits are controlled by unit factors (genes) which exist in pairs in individual organisms.
•
Dominance/ Recessiveness-
When unit factors are contrasting, one is dominant to the other which is recessive.
•
Segregation- Unit
factors segregate randomly during
gametogenesis.
Dihybrid Cross
•
A genetic cross
involving two characters.
1. seed color: yellow and green
2. seed shape : smooth and wrinkled
•
P1: yellow, smooth X green, wrinkled
F1: yellow, smooth X yellow, smooth
F2: (9/16) yellow, smooth (3/16)green, smooth (3/16) yellow,
wrinkled (1/16) green, wrinkled
If, P1 : yellow, wrinkled
X green, smooth
F1 : yellow, smooth
F2: 9:3:3:1
Ø
Results remain
the same
Punnett
Square (diagram)
Explanation
for results of the Dihybrid cross
•
1 Dihybrid cross, = 2 monohybrid crosses
•
Product law- “When
two independent events occur simultaneously, the combined probability of the
two outcomes is = to the product of their individual probabilities of
occurrence.”
•
E.g. yellow (3/4), smooth (3/4) = 9/16 yellow, smooth
yellow (3/4), wrinkled (1/4) = 3/16 yellow, wrinkled
green (1/4), smooth (3/4) = 3/16 green, smooth
green (1/4), wrinkled (1/4) =
1/16 green, wrinkled
Law of Independent Assortment
Mendel’s fourth postulate-derived from the
results of dihybrid crosses:
• During gamete formation, segregating pairs of unit
factors assort independently of each other.
-
All possible
combinations of gametes will be
formed with
equal frequency.
Test Cross
• To distinguish the genotype of individuals possessing
the dominant phenotype.
• Dihybrid test cross ratio = 1:1:1:1
Trihybrid cross
•
Three- factor cross
•
The processes of
segregation and independent assortment applies as in monohybrid and dihybrid crosses.
•
P1 AABBCC X aabbcc
•
F1 AaBbCc X AaBbCc
•
F2 27:9:9:9:3:3:3:1 ratio
Explanation of Trihybrid cross results
•
Forked-line method or Branched
diagram --“product law” is applied (fig.3.10).
•
Crosses involving 2 or
more gene pairs
-Determine
the # of heterozygous gene pairs =(n).
-2n = # of different gametes
formed.
-3n=
# of different genotypes formed.
-2n= # of different
phenotypes formed.
A note on
probability
• Probability = no. times
event is expected / no. times event is possible.
• Product Law of
Probability: “The
probability of two independent events occuring sumultaneously is equal to the
product of their separate probabilities.” ––Applies to “and”
statements.
• Sum Law of
Probability:“The
probability of either one or the other of two mutually exclusive events is the
sum of their individual probabilities.” –– applies to “or” statements.
·
Chi-square Analysis
·
Pedigree Analysis In Humans