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Biology CP
Genetics Guided Notes
Name: _______________________
Per: _____ Date: _______________
Genetics:
 Study of heredity
 How certain traits are passed from parents to offspring
Chromosomes:
 Transmit the traits
 Contain units of hereditary information
Genes:
 Chromosome part (unit) that determines a trait of a living
thing
 1000’s of different genes – thus many different traits
 Each chromosome is made up of different genes
 Are arranged one next to another
 Genes are paired (remember – chromosomes in body cells
are paired – one from your mother, one from your father)
 Each trait has a pair of genes that code for it
 Example:
Chromosome pair
Genes
How genes are passed to offspring:
1. each parent has two genes for each trait
2. use letter to represent genes (use two letters – one from
each parent – represents a pair of genes)
3. gametes – have only one gene for each trait (aren’t paired)
4. Example:
d = dwarf, D = Normal
Body cells
Dwarf Father
dd
Normal Mother
DD
Gametes
d
d
D
D
Dominant Genes:
 Genes that prevent other genes from showing
 Use a capital letter(s)
Recessive Genes:
 Genes which do not show if there is a dominant gene, even
though they are present
 Only way the recessive trait shows is if there are two of the
same recessive genes.
 Use a lowercase letter(s)
Dwarf is recessive to Normal body shape
Dwarf Body Shape:
dd
Normal Body Shape: DD
Homozygous Dominant:
 Two dominant genes for the same trait
 Both genes are the same
 Example: DD
OR
Pure or Purebred:
 gene pairs are identical
Homozygous Recessive:
 Two recessive genes for a trait
 will show the recessive trait
 Example: dd
OR
Pure Recessive
Heterozygous or Hybrid
 Two different genes are present for a trait (a dominant and a
recessive gene)
 Shows the dominant trait; the recessive gene does not show
 Example: Dd
Gregor Mendel
 Father of genetics
 Discovered basic principles of heredity through his plant
breeding research
 Reasons for such success – used garden pea plants for his
experiments
 Garden Pea – good choice:
1. seeds are readily available
2. small and easy to grow (cultivate)
3. produce large numbers of offspring
4. mature quickly
5. may use: self-pollination or cross-pollination
6. many varieties available – display one of two contrasting
forms
** Used logical experimental methods and careful recordkeeping!
Self-pollination: fertilization is carried on in a single plant
Cross-pollination: involves the sex cells of two plants
Characteristics in Garden Pea Plant:
Pea Trait:
Dominant
Round
Seed Shape
Yellow
Seed Color
Colored
Seed Coat Color
Inflated
Pod Shape
Green
Pod Color
Axial
Flower Position
Tall
Stem Length
Recessive
Wrinkled
Green
White
Constricted
Yellow
Terminal
Short
Important Vocabulary:
Probability:
The likelihood that an event will occur
Parental generation (or P generation)
First filial generation (or F1 generation): Results of parental
cross appearing in the first generation offspring
Second filial generation (or F2 generation):
Results when the
F1 generation self-pollinate (2nd generation)
Principle of Dominance:
States that one gene in a pair may
prevent the other gene in the pair of being expressed
 Example: Rr
R=Round seed, r=wrinkled seed
Principle of Segregation:
States that the members of each pair
of genes separate, or segregate, when gametes are formed
 Example:
Parents RR
rr
Gametes
X
R R
r
r
Allele: Refers to either member of a pair of genes that
determines a single trait
 Example:
o dominant allele for seed color in peas (Y) produces
yellow seeds
o the recessive allele (y) produces green seeds
Genotype:
The pairs of alleles in the cells of an organism
 Example: RR, Rr, rr (genetic makeup)
Phenotype: A trait that is actually expressed in an organism
 Example: RR=Round seeds (physical makeup/appearance)
Punnett Square: A chart that shows all the possible gene
combination for a cross.
 Male and female gametes go on top and side of chart
 Inside chart, all the possible gamete combination are found
 Example:
o In humans, free earlobes (E) are dominant over attached
earlobes (e). Use a Punnett square to show the results of
a cross between a male with pure free earlobes and a
female with pure attached earlobes.
Probability
=
The likelihood that a specific event will occur
It may be expressed as a . . .
1. decimal
2. percentage
3. fraction
A few examples . . .
1.
Coin toss
Probability of heads: _______
Probability of tails: _______
2.
Roll a die (dice)
Probability of getting a “1”: _______
Probability of getting a “5”: _______
3.
Genetics (think Punnett Square)
“If I cross two hybrids for the happy gene (H), what is the
probability that they will have happy offspring?”
HH Hh
Hh hh
¾ Happy = 75% = .75
¼ Sad = _____ = _____
In guinea pigs, both BB and Bb result in a black coat.
PROBLEM: How can you determine whether a black guinea pig
is homozygous (BB) or heterozygous (Bb)?
SOLUTION:
Perform a testcross:
An individual of unknown
genotype is crossed with a homozygous
recessive individual.
 The testcross can determine the genotype of any individual
whose phenotype is dominant.
SAMPLE PROBLEM:
BB or Bb?
X
Bb – known genotype
B = Black Fur
b = White fur
IF
BB x bb
Bb
Bb
IF
Bb
Bb
ALL OFFSPRING Bb
Bb x bb
Bb
bb
Bb
bb
½ Bb and ½ bb
Complete Dominance: One trait is completely dominant over
another
 Example: PP and Pp both have purple flowers, while pp has
white flowers
Incomplete Dominance:
Occurs when two or more alleles for a
trait influence the phenotype. Produces an intermediate
phenotype between the dominant and recessive phenotype.
 Example: R = Red, r = white
Cross RR x rr
Rr
Rr
Rr
Rr
Cross Rr x Rr
All Rr offspring
RR Rr
Rr rr
Rr = PINK
1 RR = Red
2 Rr = Pink
1 rr = White
PHENOTYPC RATIO IS 1 RED : 2 PINK : 1 WHITE
Codominance:
Both alleles for a trait are expressed in a
heterozygous offspring. Neither allele is dominant or recessive,
nor do the alleles blend in the phenotype.
 Example:
In cats,
o LL = Long tail
o NN = No tail
o NL = Short tail
If we cross:
P:
NN
G:
N
F1:
x
N
NL
NL
LL
L
NL
NL
L
ALL NL = Short Tail
Mendel’s Next question:
“Does the gene that controls one trait have anything to do
with the gene that controls a different trait?”
How did Mendel find his answer?
He did another cross. This time, a two-factor cross.
Dihybrid Cross = A cross between individuals that involved two
pairs of contrasting traits.
Example:
P:
Mendel crossed two purebred plants that produced:
1. Round, yellow seeds
2. Wrinkled, green seeds
RRYY
Gametes:
F1:
RY
RY
RY
RY
X rryy
RY
RY
ry
ry
RY
RY
ry
ry
ry
ry
ry
ry
RrYy RrYy RrYy RrYy
“
“
“
“
“
“
“
“
“
“
“
“
All Seeds:
Genotype:
Phenotype:
RrYy = 16/16
Round, yellow = 16/16
Mendel then crossed two of these offspring, for an F2 generation:
P:
RrYy
Gametes:
F2:
RY
Ry
rY
ry
X RrYy
RY
Ry
RY
Ry
rY
ry
rY
ry
RY
RRYY
RRYy
RrYY
RrYy
Ry
RRYy
RRyy
RrYy
Rryy
rY
RrYY
RrYy
rrYY
rrYy
ry
RrYy
Rryy
rrYy
rryy
If genes for seed shape and seed color are inherited
independently then the seeds produced should be:
9/16
3/16
3/16
1/16
RATIO:
yellow, round seeds
yellow, wrinkled seeds
green, round seeds
green, wrinkled seeds
9
:
3
:
3
:
1
PRINCIPLE OF INDEPENDENT ASSORTMENT:
 Process by which two or more pairs of genes segregate
independently of one another during the formation of
gametes.