CHAPTER 8
INTRO TO GENETICS
Gregor Mendel -FATHER OF
GENETICS
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GENETICS-STUDY OF HEREDITY
HEREDITY-PASSING OF
CHARACTERISTICS FROM PARENT TO
OFFSPRING
TRAIT-INHERITED CHARACTERISTIC
TRUE-BREEDING-PURE FOR A TRAIT
Monohybrid Cross
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Crossed tall and short pea plants that
were true-breeding for those traits
Monohybrid cross -produced offspring
whose parents differed by one trait
(height)
Offspring were called hybrids
P1 Tall plants X short
plants
F1 Tall Pea Plants
Monohybrid
Cross
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P1 Tall plants X short
plants
F1 Tall Pea Plants
F2 F1 x F1
3 Tall; 1 short
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EACH PERSON RECIEVES TWO
COPIES OF A GENE -ONE FROM
EACH PARENT -WHEN
REPRODUCTION TAKES PLACE.
ALLELES
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THERE ARE ALTERNATE FORMS OF A
GENE CALLED ALLELES
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YOUR TRAITS ARE DETERMINED BY
THE ALLELES YOU RECEIVE FROM
YOUR PARENTS.
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WHEN TWO ALLELES OCCUR TOGETHER,
ONE OF THEM MAY BE EXPRESSED
(PHYSICALLY SEEN) WHILE THE OTHER
MAY NOT BE OBSERVED IN THE
ORGANISM’S APPEARANCE (HIDDEN)
The expressed gene is called the dominant
gene. (represent by capital letters)
The hidden gene is called the recessive
gene. (represented by lower case letters)
Law of segregation
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When gametes are formed during
meiosis, the alleles separate so that
each gamete recieves only one allele for
each trait.
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If the two alleles of a gene are the same
the individual is said to be homozygous.
TT -homozygous dominant
tt -homozygous receive
If the two alleles of a gene are different
the individual is said to be heterozygous
Tt
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The set of alleles an organism recieves
from their parents is calle the.
genotype
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genotype =type of gene
The physical appearance of an organism
(which is determined by the genotype) is
called the phenotype.
phenotype=physical
Example
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Let's say that for the redthroated booby bird, red
throat is the dominant trait and
white throat is recessive. Since
the "red-throat code" and the"
white-throat code" are alleles
(two forms of the same gene),
we abbreviate them with two
forms of the same letter. So
we use "R" for the dominant
allele/trait (red throat) and "r"
for the recessive allele/trait
(white throat).
Our possible genotypes &
phenotypes would be like so:
Symbol
Genotype
name
Phenotyp
e
RR
Homozygou
s pure
dominant
Red
throat
Rr
Heterozygou Red
s
throat
rr
Homozygou
s pure
recessive
White
throat
Punnet square
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Diagram that predicts the possible
genotypes and phenotypes of offspring
Use these parental genotypes: Tt x tt.
Dihybrid crosses (DI=2)
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Cross that involves parents who differ by
two traits
Law of independent assortment-the
inheritance of one trait had no effect on
the inheritance of another trait.
Seed color has no effect on seed shape.
Dihybrid cross
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Cross involving parents that differ by two
traits
Law of independent assortment-genes
for different traits are inherited
independently of each other
RRYY x rryy = RrYy
RrYy x RrYy
• R and r will separate; Y and y will separate
• Recombine to produce 4 different gametes
Example of a Dihybrid Cross
Diagram of a Dihybrid Cross
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Images and examples are taken from
various websites.