Introduction to Genetics
I. Gregor Mendel- Austrian monk who tended the monastery garden. He had several varieties of true
breeding peas (when self pollinate they make offspring identical to them selves). To test inherited traits
he cross-pollinated the pea plants. Two different types of pea plants produce offspring together which
are called hybrids.
II. Genes and Dominance
a. Mendel discovered that inheritance is determined by factors that are passed from one generation
to the next, called genes.
i. Different forms of genes are called alleles. Each organism will have two alleles for each
trait.
b. Mendel also discovered the principal of dominance. Which states: some alleles are dominant
and some are recessive.
i. Dominant alleles for a trait will always express that form of the trait.
ii. Recessive alleles for a particular form of a trait will only be expressed when the
dominant form is not present.
iii. Heterozygous organisms have two different alleles.
iv. Homozygous organisms have two identical alleles.
v. Genotype genetic makeup
vi. Phenotype physical characteristics
III. Segregation
a. When Mendel crossed two different pure breed plants he noticed that one trait, the recessive, was
not expressed in the offspring. The F1 cross- to find out if the gene would be expressed. He
crossed two of the offspring. The trait did come back. He called this segregation, or separation.
b. When each F1 plant flowers and produces gametes (sex cells), the two alleles segregate from
each other so that each gamete carries only a single copy of each gene. Therefore, each F1 plant
produces two types of gametes.
Punnett Squares- used to predict and compare the genetic variations that will result from a cross.
IV. Independent Assortment- genes for different traits can segregate independently during the formation
of gametes. Independent assortment helps account for the many genetic variations observed in plants,
animals, and other organisms
V. Multiple Alleles- more than two forms of the trait, so there are more than two alleles.
VI. Polygenetic Traits- traits controlled by two or more genes
VII. Codominance
Codominance is a situation in which both alleles contribute to the phenotype. This type of combination
does not result in blending, but usually a spotting effect. An example of this is roan horses. (see TV)
Genotype
Phenotype
Red Hair
Roan
White Hair
Cross a red horse and a white horse:
Cross two roan horses:
What is the probability of producing a white horse from this match?
Cross a roan and a white horse:
What is the probability of producing a white horse from this match?
VIII. Incomplete Dominance
Incomplete dominance is a situation in which both alleles contribute to the phenotype. In this cross, the
parents alleles blend together and the offspring have a phenotype somewhere between their parents. (see
TV)
Genotype
Phenotype
No sickle cell anemia, malaria prone
No symptoms of sickle cell, but resistant to
malaria
Sickle cell anemia expressed
Cross two parents that are heterozygous for sickle cell anemia:
What is the likelihood that this couple will produce offspring that are resistant to malaria?
Cross a parent with sickle cell and a parent with malaria:
IX. Multiple Alleles
Multiple alleles occur when a gene has more than two possibilities. This does not mean that an
individual can have more than two alleles. Only that more than two possible choices exist. One of the
best known examples is found in rabbits, another is human blood type. (TV)
Blood Type
(Phenotype)
O
AB
A
B
Genotype
Can donate blood to:
Can receive blood from:
A,B,AB and O
O, AB
AB, A
AB,B
O
A,B,AB and O
O,A
O,B
Cross a female with type O and a male with O:
Cross a female with type A and a male with B:
(Hint their daughter has type O blood!)
A man with AB blood marries a woman who says she is carrying his child. The child is born with type
O blood. Can this be his child?
A child has type O blood. His mother is type B, his father is type A, and his brother is AB. How is this
possible?
X. Sex-linked Genes
Sex-linked genes are found on the X chromosome. More than 100 disorders have been linked to the X
chromosome. The Y chromosome is very small and contains only a few genes, including the sex
determining factor. Males have just one X chromosome , so all X-linked alleles are expressed even if
they are recessive.
Examples of sex-linked genes are:
Colorblindness (recessive)
Hemophilia (recessive)
Hearing loss (dominant)
XI. Dihybrid Cross
Dihybrid crosses are punnett squares involving two traits at the same time.