Download Ch 14 part 1 (for those on field trip)

Chapter 14:
Mendelian Genetics
GENETICS VOCABULARY
• Trait: some aspect of an organism
that can be described or measured
• Gene: discrete unit of hereditary
information or a specific sequence of
DNA that codes for a protein
• Allele: alternative versions of a gene
– Ex: red vs. white flowers; A, B, or O blood
group instructions
GENETICS VOCABULARY
• Genotype: an organisms actual genetic
make up or set of alleles
– Ex: HbS HbS or Hbs Hbs
• Phenotype: physical or physiological
characteristics of an organism which is
determined by the genotype
– Normal Hb or sickle cell Hb; Blue eyes;
brown feathers; 5 fingers on a hand
GENETICS VOCABULARY
• Homozygous: an individual that inherits 2 identical
alleles for a gene
– Ex: BB or bb
• Heterozygous: an individual that inherits 2 different
alleles for a gene
– Ex: Bb
– Also called “hybrid”
• Dominant: an allele that is fully expressed in the
phenotype of a heterozygote
– Ex: B = Black and b = white; BB or Bb results in black
coloration
– WARNING: Dominant does NOT mean the allele occurs
more frequently!
• Recessive: an allele that is not observed in the
phenotype of a heterozygote
– Ex: only see white coloration when inherit bb
How are traits inherited?
• For a very long time, scientists
assumed that offspring were a result
of blending of traits of their parents.
• Gregor Mendel, an Austrian monk, applied
mathematics to his study of genetics.
• He chose to study the garden pea plant to
investigate how traits were passed from
generation to generation.
• He chose pea plants because they can selffertilize and traits appear in 2 distinct forms
(alleles).
– Ex: short or tall…no intermediate height.
• Mendel studied 7 different traits:
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Seed shape
Seed color
Pod color
Plant height
Flower color
Pod shape
Flower position
• Mendel crossed purebreeding or truebreeding plants with
one another.
– Homozygous dominant (TT)
x Homozygous recessive (tt)
“P” generation
• The result was hybrid plants (Tt)
– ½ “tall” & ½ “short” genetic instructions
• These hybrid offspring are called the F1
generation
– Punnett Squares show the probability of having
offspring with a certain trait
• After seeing that all the F1 generation was
tall, Mendel wondered if the recessive
“short” allele had disappeared
• He then allowed the F1 plants to selfpollinate and reproduce (Tt x Tt)…
– Result = F2 generation
(inside square)
*The short allele was still there!
Mendel’s results led to the following principles:
1.
Each trait (like height) is controlled by a gene. Since
most traits have 2 or more forms (tall and short),
there must be a tall version of the gene and a short
version of the gene (alleles).
2.
Each organism has to inherit 2 alleles, one from each
parent
3. Dominant vs. Recessive
4. Law of Segregation: the 2 alleles must
separate when gametes (sex cells) are
formed. The gametes only contain one
allele for each trait. At fertilization, the
single alleles are joined to make a pair in
the offspring
5. Law of Independent Assortment: how
the chromosomes separate during meiosis
is random and this produces many
different combinations of gametes.
• Traits are inherited independent of
each other.
– For example, in pea plants, the allele for
tallness may be inherited with the allele for
yellow seed color, or the allele for green
seed color. This is because the separation
of the chromosomes during meiosis is
random and produces many combinations of
chromosomes.