MENDEL AND PATTERNS
OF INHERITANCE
CHAPTER 9- FUNDAMENTALS OF
INHERITANCE
A ROYAL TRAGEDY:
A PEDIGREE OF RUSSIAN ROYAL FAMILY
MENDEL’S PEAS:
HIS EXPERIMENTS WITH PATTERNS OF INHERITANCE AN
USE OF PUNNETT SQUARES
VOCABULARY OF INHERITANCE
1. Gregor Mendel- Generally known as the ‘Father of
genetics. He conducted experiments with pea plant to
discover patterns of inheritance of traits. His scientific
laws include the law of segregation and law of
independent assortment.
2. Inheritance- The passing down of genetic material from
one generation to the next through reproduction.
3. Probability- The mathematical tool that enables
predictions to be made.
4. Expected Results- Results that are predicted based on
mathematical probability.
5. Observed Results- Actual results that are a result of
observed events and due to chance.
6. Sample Size- The number of trials that comprise an
experiment. The more the better.
7. Trait- A characteristic that is passed down as the result
of genetic inheritance.
8. Gene- A unit of hereditary information located on
chromosomes. A particular piece of genetic
information.
9. Phenotype- Observable traits when the information in
an organism’s genetic plan is expressed.
10. Genotype- An organism’s genetic plan. All of the
genetic information in an organism.
11. DNA- (deoxyribonucleic acid) The molecule that
stores genetic information.
12. Chromosome- Large DNA molecules organized
around structural proteins. Each chromosome contains
the DNA for just a small part of the total genetic
makeup of an organism.
PATTERNS OF INHERITANCE
13. Allele- Different versions of the same gene (such as
blood type- A, B, O, etc.). A particular combination of
genes determines an organism’s phenotype for that gene.
14. Heterozygous- A genotype that contains two different
alleles for a certain trait or gene (blood type AB).
15. Homozygous- A genotype that contains two identical
alleles for a certain trait or gene (blood type AA).
16. Dominant Trait- A trait that is expressed in either the
heterozygous or homozygous form. Only one allele
needs to be present for it to be expressed.
17. Recessive Trait- A trait that can only be expressed if
both alleles are present for that trait. The genotype must
be in the homozygous form.
PATTERNS OF INHERITANCE
18. Law of Independent Assortment- The process
demonstrating that alleles for two different traits
can be passed down separately from each other.
The allele assortment for one gene does not depend
on the allele assortment for another gene (eye color
does not depend on hair color).
19. Law of Segreation- Every organism has 2
alleles of each gene. When gametes are
made, each gamete receives only one of
these alleles. During fertilization, the offspring
will receive one allele for each gene from
each parent.
MENDEL’S LAWS OF HEREDITY
• Mendel’s law of segregation
• Every organism has 2 alleles
of each gene
• When gametes are made,
each gamete receives only
one of these alleles
• During fertilization, the
offspring will receive one
allele for each gene from
each parent
DOMINANT
HOMOZYGOUS
RECESSIVE
ALLELES
TRAIT
GENE
GENOTYPE
HETEROZYGOUS
PHENOTYPE
FINDING THE GENOTYPES OF THE PARENTS’
GAMETES
• For a monohybrid cross (a cross involving one
trait), there can be up to two different types of
gametes for each parent.
♂
♀
Bb x Bb
FINDING THE GENOTYPES OF THE PARENTS’
GAMETES
• For a dihybrid cross (a cross involving two traits), there can be up to
FOUR different gametes.
♂
R r Y y
x
RY
Ry
rY
ry
♀
R r Y y
RY
Ry
rY
ry
DIHYBRID CROSSES
• And we end up with something like this!!!
INCOMPLETE DOMINANCE
• A pattern of inheritance where
the heterozygous genotype
produces a phenotype that
falls between the homozygous
dominant and homozygous
recessive phenotypes
MENDEL’S LAWS OF HEREDITY
• Mendel’s law of independent assortment
• Genes for different traits are inherited
independently of each other (hair color does not
depend on freckles).
• If You have brown hair (Bb) and freckles (Ff)
– You can pass on to your children one of the
following combinations: B + F,
B + f,
b+
F,
b+f
MECHANISMS OF INHERITANCE:
MEIOSIS
• MEIOSIS: a special type of cell division during
which sex cells called GAMETES are produced.
• GAMETES: (sperm & egg) cells containing half
the genetic material.
– These cell are joined during fertilization
– One male cell and one female cell come together
– The genetic material in the gametes determine the
patterns of heredity in sexually reproducing
organisms
HOW GENOTYPES OF GAMETES ARE
DETERMINED
• Types of gametes are determined from the
genotypes of the parents
• The genotypes of the parents’ gametes must be
determined in order to predict the possible
genotypes of the offspring
– When determining patterns of heredity, the first
generation of the organisms being tested is called the
PARENTAL (P) generation
– The first generation of offspring is called the FIRST
FILIAL (F1) and the second generation is called the
SECOND FILIAL (F2) and so on.
• Once the genotypes of the parents’ gametes are
determined, the Punnett square can be set up!
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.
• Short Answer: Most Traits are passed down
separately from one another.
– Example: Hair Color does not depend eye
color
LINKED TRAITS
• Genetic linkage occurs when particular genetic loci
or alleles for genes are inherited jointly. Genetic loci
on the same chromosome are physically connected
and tend to stay together during meiosis, and are
thus genetically linked. This is called autosomal
linkage. Alleles for genes on different chromosomes
are usually not linked, due to independent
assortment of chromosomes during meiosis.
• Short Answer: Some traits are located very close to
and are inherited together
– Example: In drosophila (fruit flies) body color and wing
shape are always inherited together
X-LINKED (SEX LINKED) TRAITS
• X-linked traits are traits that are passed on from
parents to offspring on the X chromosome (the
chromosome related to gender).
• A common X-linked trait is colorblindness. The genes
needed to distinguish red from green are on the Xchromosome.
– A female with one defective and one normal Xchromosome has normal vision.
– However, a male with a defective color vision gene on his
X-chromosome, is colorblind.
– There are no genes for normal color vision on the Ychromosome to cover for the defective X-chromosome.
– One way of tracing a trait through generations is a pedigree
chart. Here is a colorblindness pedigree chart.
X-LINKED PEDIGREE