1. Genes – pieces of DNA that code for a particular character or trait
MENDEL’S THEORY
Genes – pieces of DNA that
code for a particular
character or trait

2. Mendel’s Theory of Heredity:
1. For each inherited trait, an individual has two copies
of the gene – one from each parent.
2. There are alternative versions of these genes – the different versions of the gene are called alleles.
3. When two different alleles occur together, one of
them may be completely expressed, while the other
may have no observable effect on the organisms
appearance.
dominant – the expressed form of a trait
recessive – the trait not expressed, unless two are
present

3. 4. When gametes are formed, the alleles for each gene in an individual separate independently of one another. Thus, gametes carry only one allele for each inherited trait. When gametes unite during fertilization, each gamete contributes one allele.

4. The Laws of Heredity:
1. Law of Segregation – states that the two alleles for a character or trait segregate (separate) when gametes are formed
2. Law of Independent Assortment - states genes
for different traits are sorted separately from one
another so that the inheritance of one trait is not
dependent on the inheritance of another
3. Law of Dominance – states an organism with
alternate forms of a gene will express the form
that is dominant.

5. Vocabulary:
dominant traits – are expressed by writing the first letter of the trait as an uppercase letter
ex: purple flower - P
recessive traits – are expressed by writing the first letter of the dominant trait but as a lowercase letter
ex: white flower - p

6. Selecting Your Alleles:
Height: tall is dominant; short is recessive
Eye color: brown is dominant; green is recessive
Hair: curly is recessive; straight is dominant T t B b s S

7. More Vocabulary:
homozygous – the two alleles of a particular gene are the same ex: PP or pp
heterozygous – the two alleles of a particular gene are different ex: Pp
genotype – the set of alleles (genes) for a trait
ex: PP, Pp, pp
phenotype - the physical appearance of a trait
ex: purple flower, white flower

8. Genotype or Phenotype? PP Red hair Yellow seeds Rr Dimples Tt Yy ss
Short PP
Yellow seeds
Dimples Yy
Set of alleles P G G P G P G G P G

9. Practice:
For each genotype below, indicate whether it is heterozygous (He), homozygous (HO D) or (HO r)
Rr ______ HH _______
ii ______ Ww _______
For each genotype below determine what phenotypes would be possible.
Curly hair is dominant to straight hair.
Cc ________ CC _________
cc ________ He
HO D
HO r He
Curly
Curly
Straight

10. Practice: For each phenotype below, list the genotypes.
Remember to use the letter of the dominant
trait.
Green eyes are dominant to blue eyes.
______ green eyes _______ blue eyes
______ green eyes GG gg Gg