1. Mendel's Law of
Independent Assortment
Definition: The principles that govern
heredity were discovered by a monk named
Gregor Mendel in the 1860's. One of these
principles, now called Mendel’s Law of
Independent Assortment, states that allele
pairs separate independently during the
formation of gametes. This means that
traits are transmitted to offspring
independently of one another.

2. Mendel performed dihybrid crosses, mating of
parent plants that differ in two traits in plants
that were true breeding for two traits.
• For example, a plant that had green pod color and yellow
seed color was cross-pollinated with a plant that had yellow
pod color and green seeds. In this cross, the traits for green
pod color (GG) and yellow seed color (YY) are dominant.
Yellow pod color (gg) and green seed color (yy) are
recessive. The resulting offspring or F1 generation were all
heterozygous for green pod color and yellow seeds (GgYy).

3. Mendel then allowed all
of the F1 plants to
self-pollinate. He
referred to these
offspring as the F2
generation. Mendel
noticed a 9:3:3:1 ratio.
About 9 of the F2
plants had green pods
and yellow seeds, 3 had
green pods and green
seeds, 3 had yellow
pods and yellow seeds
and 1 had a yellow pod
and green seeds.

4. A test cross is a breeding or a mating between an
individual of dominant phenotype, who could be either
homozygous dominant (SS) or heterozygous (Ss), with
an individual that MUST be homozygous recessive
(ss).
• These Punnett squares show the
two different possibilities.
Look them over carefully and
convince yourself that, in a test
cross, a homozygous individual will
produce offspring with only the
dominant phenotype, but a
heterozygous individual will
produce offspring with both
phenotypes (in equal numbers).
Notice that the offspring will
reflect that ratio of the
unknown's gametes because the
other parent contributes only
gametes carrying the recessive
allele.

5. Here’s another example…

7. The rules of probability applied to segregation and
independent assortment can solve complex genetics
problems. For example, Mendel crossed pea varieties that
differed in three traits (trihybrid crosses).
• A trihybrid cross between two organisms with the
genotypes AaBbCc and AaBbCc will result in a 1/64
probability of producing an offspring with the genotype
aabbcc.
Aa x Aa: probability for aa offspring = 1/4
Bb x Bb: probability for bb offspring = 1/4
Cc x Cc: probability for cc offspring = 1/4
Because segregation of each allele pair is an independent event,
the rule of multiplication is used to calculate the overall
probability that the offspring will be aabbcc: 1/4 aa x 1/4 bb x 1/4
cc = 1/64

8. Mendel's two laws explain inheritance in terms of
discrete factors (genes) which as passed from
generation to generation according to simple rules
of chance.
• These principles apply to all sexually
reproducing organisms for simple patterns
of inheritance.
• Experiments with many organisms indicate
that more complicated patterns of
inheritance exist.
• The more complicated patterns of
inheritance include situations where one
allele is not completely dominant over
another allele, there are more than two
alleles for a trait, or the genotype does
not always dictate the phenotype in a
rigid manner.

9. FA MILY PE DIGRE E
A diagram or chart
that s hows the
pattern of inheritance
within a family. A ls o
known as a family
tree. The chart can
inc lude many
generation in the
s ame family. In all
pedigree charts ,
s quares repres ent
males and circles
repres ent females .

10. • The pedigree chart traces a sex-linked trait, the
disease hemophilia, through three generations of
family members. (The chart is also designed to
show the possible combination of genes contributed
from a given set of parents.) The gene for
hemophilia is linked to the X chromosome but is
most likely to be expressed in males. If a boy
receives a copy of the hemophilia-X chromosome
from his mother, he will certainly have the disease.
A girl who receives a copy of the hemophilia-X
chromosome from her mother will not necessarily
have the disease; she will, however, be a carrier. In
that chart, a circle divided in half indicates that the
individual is a carrier for the trait.

11. Many Inherited Human disorders
are controlled by single gene
• Recessive disorders
1. Cystic fibrosis
– most common lethal genetic disorder in US (4% of whites are
carriers)
– due to defective chloride channels causing abnormal
concentration of extracellular chloride leading to mucous
buildup from mucosal epithelium.
2. Tay Sachs disease
– disfunctional enzyme which fails to break down certain lipids
in lysosomes.
3. Sickle-cell anemia
– single amino acid replacement in haemoglobin molecule.

12. Dominant inherited disorders (not as
frequent as recessive disorders)
1. Achondroplasia
-dwarfism
2. Huntingtons disease
-mental deterioration and uncontrollable
movements
3. Alzhiemer’s disease
-mental retardation usually strikes late in life.
4. Hypercholesterolemia
-excess cholesterol in blood; heart disease