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NOTES: Ch. 14 part 1 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
•
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
•
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 self-fertilize and traits appear in 2 distinct forms (alleles).
– Ex: short or tall…no intermediate height.
•
Mendel studied 7 different traits:
– Seed shape
– Seed color
– Pod color
– Plant height
– Flower color
– Pod shape
– Flower position
•
Mendel crossed pure-breeding or true-breeding plants with one another.
– Homozygous dominant (
) x Homozygous recessive (
)
•
The result was hybrid plants (
)
– ½ “tall” & ½ “short” genetic instructions
•
These hybrid offspring are called the _______________________
–
Punnett Squares show _____________________________________________________
•
After seeing that all the F1 generation was tall, Mendel wondered if the recessive
“short” allele had disappeared
•
He then allowed the ________________________________________ (Tt x Tt)…
– Result = _____________________
(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 ____________. Since ___________________________
__________________________ (tall and short), there must be a tall version of the gene and a
short version of the gene (_______________).
2. Each organism _________________________________________________________________
3. ________________________________________________
4. ___________________________________: _________________________________________
____________________________. The gametes only contain one allele for each trait. At
fertilization, the single alleles are joined to make a pair in the offspring
5. _____________________________________: _______________________________________
______________________________________________________.
•
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.
Extending Mendelian Genetics
INCOMPLETE DOMINANCE:
• inheritance where one allele is not completely dominant over the
other
– Result: ________________________________________
• Ex:
•
Ex:
CODOMINANCE:
• inheritance characterized by full expression of both alleles
in the heterozygote
– Ex:
–
Ex:
“Roan” cattle