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Genetics
*transmission of traits – heredity
*variation
*genetics
Two main hypotheses on how traits were transmitted:
*blending inheritance
*particulate inheritance
The father of transmission genetics:
Gregor Johann Mendel
1822-1884
Mendel tracked heritable characters for three generations
*P - parental generation
*F1 – first filial generation
*F2 – second filial generation
-Example:
F2
P
X
Tall
Dwarf
F1 – all Tall
Tall
*genes and alleles
Mendel’s hypotheses (to explain his results)
1. Alternative versions
of genes (alleles)
account for variation
in inherited characters
2. For each character,
an organism inherits two
alleles, one from each
parent
3. If two alleles differ, one is
dominant, the other recessive
4. The two alleles for each character
segregate (separate) during gamete
production.
P:
X
Tall
Dwarf
DD
dd
F1 – all Tall
Tall
Dd
Mendel’s Law of Segregation
Punnett Square predicts the results of a genetic cross between
individuals of known genotype
Tall
P:
Gamete
formation:
DD
D
Dwarf
X
D
dd
d
d
*genotype
*Homozygous
*phenotype
*Heterozygous
Mendel’s Law of Independent Assortment
*What happened when he looked at two characters?
If they segregate together:
If they segregate independently:
Dihybrid cross- A genetic cross between two individuals involving two
characters
Punnett square and the law of
Example:
independent assortment:
P1
GW
GW
GW
GW
X
gw GgWw GgWw GgWw GgWw
yellow, round
green, wrinkled
GGWW
ggww
gw GgWw GgWw GgWw GgWw
gw GgWw GgWw GgWw GgWw
F1
All
yellow, round
GgWw
gw GgWw GgWw GgWw GgWw
Punnett square and the law of
independent assortment:
GW
F1
F1
X
All
yellow, round
GgWw
GgWw
F2
9/16 yellow, round
gw
GGWw GGww
GgWw Ggww
gW GgWW GgWw
ggWW ggWw
gw
ggWw ggww
GgWw
Ggww
9:3:3:1 Phenotypic ratio; Genotypic ratio as follows:
1/16 GGWW, 2/16 GGWw, 2/16 GgWW, 4/16 GgWw
3/16 yellow, wrinkled
1/16 GGww, 2/16 Ggww
3/16 green, round
1/16 ggWw, 2/16 ggWw
1/16 green, wrinkled
gW
GW GGWW GGWw GgWW GgWw
Gw
All
yellow, round
Gw
1/16 ggww
Mendelian inheritance is based on probability
Example- coin toss
*1/2 chance landing heads
*Each toss is an independent event
*Coin toss, just like the distribution
of alleles into gametes
*The rule of multiplication – determines
the chance that two or more independent
events will occur together
½ x ½ = ¼
Sample problem
Albinism in humans is inherited as a simple recessive trait. Determine the genotypes
of the parents and offspring for the following families. When two alternative
genotypes are possible, list both. (A) Two non albino (normal) parents have five
children, four normal and one albino. (B) A normal male and an albino female have six
children, all normal.
1) establish gene symbols:
2) Establish: genotype
AA
Aa
aa
A=normal (not albino)
a=albino
Move on to part (A): Parents are
both
phenotypically
normal,
genotypes could be EITHER AA
or Aa, an albino phenotype could
only result from an aa genotype.
*One a had to come from the
mother and one a had to come
from the father, so, the parents
must be genotypically Aa.
A
phenotype
normal
normal
albino
a
Answer to (A):
A
AA
Aa
Genotype of the
parents = Aa
a
Aa
aa
Genotype of normal
children = AA or Aa
Genotype of albino
child = aa
2. (B) A normal male and an albino female have six children, all normal.
1) The female is phenotypically albino; genotype can only be aa
2) The male is phenotypically normal; genotype can be AA or Aa
3) Since all children are
4) BUT male COULD also be Aa !
normal one might assume
*IF the father was
the male to be AA
genotypically Aa, then
what is the likelihood
A
A
A
a
(chance or probability)
of this couple having 6
a
a
Aa
aa
Aa
Aa
normal children?
a
Aa
Aa
a
Aa
aa
F1:
F1:
Genotype: all Aa
Genotype: ½ Aa, ½ aa
or a 1:1 ratio
Phenotype: all normal
Phenotype: ½ normal, ½ albino
or a 1:1 ratio
Recall the product law!
½x½x½x½x½x½
= 1/64