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Chapter 11 Introduction to Genetics

Did extensive studies on pea plant traits

Documented traits and inheritance patterns
of thousands of plants

Keep in mind that DNA had not been
discovered
 What he called “factors” we know to be genes,
which code for particular traits

Mendel was in charge of the garden
 He really liked pea plants
▪ He had a stock of TRUE BREEDING plants
▪ If allowed to self pollinate they would always produce
offspring identical to themselves
▪ However pea plants can also cross pollinate
▪ Mendel controlled this to produce certain TRAITS
(specific characteristics)

Genetics – the study of heredity

Heredity – the passing of traits from parents to
offspring

Trait – any characteristics that can be passed from
parents to offspring
 examples: dimples & freckles

Genes - a segment of DNA that codes for a specific
trait

Hybrid – cross between parents with different traits

Alleles are slightly different forms of a gene
 The sequence of nitrogen bases may be slightly different
 A person has 2 alleles for each trait

Occur on homologous chromosomes

Of the 2 alleles, you receive one from your mother
and one from your father during fertilization

Dominant Allele – the allele that is
expressed; the stronger allele
 Symbolized by a capital letter

Recessive Allele – the allele that is only
expressed if two of them are present; weaker
allele
 Symbolized by a lower case letter

Example:
 Freckles = Dominant
 No Freckles = Recessive
▪ Dominant Allele = F
▪ Recessive Allele = f
 Dimples = Dominant
 No Dimples = Recessive
▪ Dominant Allele = D
▪ Recessive Allele = d

Genotypes are the genetic make-up of an
organism
 What do the alleles look like:
▪ DD
▪ Dd
▪ dd

Phenotypes are the physical appearance of a
trait
 sometimes appear different due to environmental factors
▪
▪
▪
▪
having dimples
having freckles
no dimples
no freckles
Genotypes
Homozygous
Dominant
Definitions
Abbreviations
Phenotype
2 Dominant
Alleles
FF
Freckles
Genotypes
Definitions
Abbreviations
Phenotype
Homozygous
Dominant
2 Dominant
Alleles
FF
Freckles
Heterozygous
1 Dominant
Allele;
1 Recessive
Allele
Ff
Freckles
Genotypes
Definitions
Abbreviations
Phenotype
Homozygous
Dominant
2 Dominant
Alleles
FF
Freckles
Heterozygous
1 Dominant
Allele;
1 Recessive
Allele
2 Recessive
Alleles
Ff
Freckles
ff
No Freckle
Homozygous
Recessive

Mendel develop 3 principles that hold
true today
 Principle of Dominance and Recessiveness
 Principle of Segregation
 Principle of Independent Assortment

Principle of Dominance and Recessiveness
 One factor in a heterozygous pair may mask the
other factor
 The factor that is expressed is dominant, the one
that is masked (not expressed) is recessive
 In order for a recessive trait to be expressed, the
organism must have a homozygous recessive
genotype

Principle of Segregation
 Gametes (sperm and egg) are formed
during the process of meiosis
 Each gamete receives only one allele for
each trait

Principle of Independent Assortment
 Alleles segregate into gametes randomly and
independently of each other
 Therefore each egg (or sperm) cell should be
different from the another egg (or sperm) cell

http://www.snopes.com/photos/people/mixe
dtwins.asp
Mendel