FUNDAMENTALS OF GENETICS
Chapter 9
The velociraptor said that they were
going to play a game of chicken, but in
the end they played a game of horse...
Genetics is the filed of biology devoted to
understanding how characteristics are
transmitted from parents to offspring. Genetics
was founded with the work of Gregor Johann
Mendel. This section describes Mendel’s
experiments and the principles of genetics that
resulted from them.
9.1 MENDEL’S LEGACY
Mendel’s Study of Traits
Mendel= “father of GENETICS”: STUDY OF HEREDITY
Your traits resemble your PARENTS
• Hair, color/eye shape, height, weight, etc…
This is due to HEREDITY: Passage of traits from parents to
offspring.
TRAIT: GENETICALLY DETERMINED VARIANT OF A
CHARACTERISTIC.
• Ex. Yellow, white, pink, etc… flower color.
Mendel’s Experiments: Peas
Chose peas because:
• Have two distinct forms.
• Flowers are both male
and female.
• Grows easily and quickly.
Mendel studied 7 traits:
• Flower color
• Seed color
• Seed texture
• Pod color
• Pod appearance
• Flower position along
stem
• Plant height
Mendel’s Experiments
Mendel initially studied each characteristic and contrasting
traits individually.
• Done using plants that were TRUE-BREEDING:
BREEDING FOR A TRAIT THAT IS ALWAYS PRODUCED
WHEN OFFSPRING ARE SELF-POLLINATED.
Mendel initially did MONOHYBRID CROSSES: CROSS
INVOLVING ONE PAIR OF CONTRASTING TRAITS.
• Ex. Crossing purple flowered plants with white flowered
plants
Mendel’s Experiments
Mendel's experiments followed 3 steps:
1. Forming the P (PARENT) Generation
2. F₁ (FIRST FILIAL) Generation
3. F₂ (SECOND FILIAL) Generation
Step 1: Forming P generation
Peas SELF-POLLINATED for several generations to form the P
GENERATION: PARENT GENERATION FOR START OF A
BREEDING EXPERIMENT.
All plants in the P generation are TRUE-BRED, so that they
only produce offspring like themselves.
• Ex. Purple flowers should only produce purple flowers.
Step 2: F₁ Generation
CROSS-POLLINATION of 2 P generation plants with
contrasting traits.
• Ex. Purple and white flowers
The generation that follows is called the F₁ GENERATION:
FIRST FILIAL GENERATION.
Step 3: F₂ Generation
Self-pollination of F₁ GENERATION plants to form the F₂
GENERATION: SECOND FILIAL GENERATION.
Mendel’s Results:
All F₁ plants had only 1 form of the trait (ex. either all purple
or all white flowers), but when self-pollinated, the missing
trait reappeared in the F₂ generation.
• Figure 9.3 pg. 175; Table 9-1 pg. 176
Also, found consistent mathematical RATIOS applicable to the
traits shown in offspring.
Recessive and Dominant Traits
Mendel found that crossing of two P generation plants always
forms a 3:1 ratio in the F₂ generation:
• 3 dominant trait plants
• 1 recessive trait plants
DOMINANT TRAIT: Trait or allele that is fully expressed;
masks the recessive allele.
RECESSIVE TRAIT: Trait or allele that is expressed only when
2 recessive alleles for the same characteristic are inherited.
LAW OF SEGREGATION:
A pair of alleles is separated when gametes are formed.
• One chromatid in each of 4 gametes formed.
LAW OF INDEPENDENT ASSORTMENT:
Alleles separate independently when gametes are formed.
• Traits are not inherited based on other characteristics.
Modern Genetics
Mendel’s findings were the start of modern genetics.
MOLECULAR GENETICS: STUDY OF THE STRUCTURE AND
FUNCTION OF CHROMOSOMES AND GENES.
ALLELE: Two or more alternative forms of a gene.
• Represented by letters:
• Capital letters = DOMINANT ALLELE
• Ex. Purple flowers are dominant; allele for purple is “P”
• Lower-case letters = RECESSIVE ALLELE
• Ex. White flowers are recessive; allele for white is “p”
Today, geneticists rely on Mendel’s work to predict the
likely outcome of genetic crosses. In this section, you
will learn how to predict the probable genetic makeup
and appearance of offspring resulting from specified
crosses.
9.2 Genetic
Crosses
Genotype and Phenotype Basics
GENOTYPE: Organism’s genetic makeup inherited from each
parent.
• Ex. Purple-flowered plant, PP or Pp
PHENOTYPE: Organisms appearance based on presence of
dominant and/or recessive genes.
• Ex. A: PP or Pp; Plants show purple flowers.
• Ex. B: pp; Plants show white flowers.
Genotype and Phenotype Basics
HOMOZYGOUS: When both paired alleles are the same.
• HOMOZYGOUS DOMINANT: Both alleles are the
dominant trait; PP.
• HOMOZYGOUS RECESSIVE: Both alleles are the recessive
trait; pp.
HETEROZYGOUS: When paired alleles are different from each
other.
• Ex. Pp
Genotype and Phenotype Basics
GENOTYPIC RATIO: Ratio of genotypes that appear in
offspring of a monohybrid cross.
PHENOTYPIC RATIO: Ratio of phenotypes that appear in
offspring of a monohybrid cross.
Ratios are written in simplest form with colons between each,
and follow a specific order:
1. HOMOZYGOUS DOMINANT
2. HETEROZYGOUS
3. HOMOZYGOUS RECESSIVE
Probability in Genetics
PROBABILITY: Likelihood that a specific event will occur.
Probability = NUMBER OF TIMES AN EVENT IS EXPECTED TO HAPPEN
NUMBER OF TIMES AN EVENT COULD HAPPEN
Used to predict what genotypes and phenotypes will be
produced from a cross.
• Ex. Mendel produced a total of 8,023 plants. 6,022 plants
showed yellow seed color. 2,001 plants showed green
seed color.
• Probability of producing yellow seed color =
6,022/8,023 = 0.75 = 75%
• Probability of producing green seed color =
2,001/8,023 = 0.25 = 25%
Predicting Results of Monohybrid Crosses
MONOHYBRID CROSS: Cross in which one trait is tracked.
• Ex. Crossing a true-breeding purple pea plant with a truebreeding white pea plant.
MALE
Biologists use a PUNNETT SQUARE
to determine the products of
monohybrid cross.
FEMALE
Examples of Monohybrid Crosses:
HOMOZYGOUS Do. x HOMOZYGOUS Re.
PARENT #1: Pea plant homozygous dominant for purple flower: PP
PARENT #2: Pea plant homozygous recessive for white flower: pp
WHITE
PURPLE
p
p
P
Pp Pp
P
Pp Pp
** Predicted genotype(s):
4:0 ratio; Pp
** Predicted phenotype(s):
4:0 ratio; All purple flowers
Examples of Monohybrid Crosses:
HOMOZYGOUS Do. x HETEROZYGOUS
PARENT #1: Guinea pig homozygous dominant for black fur: BB
PARENT #2: Guinea pig heterozygous for black dominant and
brown fur recessive: Bb
BLACK D./BROWN R.
BLACK
B
b
B
BB Bb
B
BB Bb
** Predicted genotype(s):
1:1 ratio; BB and Bb
** Predicted phenotype(s):
4:0 ratio; All black fur
Examples of Monohybrid Crosses:
HETEROZYGOUS x HETEROZYGOUS
PARENT #1: Rabbit heterozygous for black fur dominant and
brown fur recessive: Bb
PARENT #2: Rabbit heterozygous for black fur dominant and
brown fur recessive: Bb
BLACK D./ BROWN R.
BLACK D./
BROWN R.
B
b
B
BB Bb
b
Bb bb
** Predicted genotype(s):
1:2:1 ratio; BB, Bb and bb
** Predicted phenotype(s):
3:1 ratio; 75% Black fur
and 25% brown fur.
Testcross
TESTCROSS: An individual of a unknown dominant genotype
(DD or Dd) is crossed with a homozygous recessive (dd)
individual.
OFFSPRING RATIOS determine the unknown genotype.
• Ex. DD and dd would produce only dominant traits; Dd and
dd would produce a 1:1 ratio dominant traits and
recessive traits.
Intermediate Characteristics
INCOMPLETE DOMINANCE: OFFSPRING PHENOTYPE THAT
IS INTERMEDIATE OF THE PARENT’S PHENOTYPES.
• Ex 1. Snapdragon with red flowers is crossed with a
snapdragon that has white flowers and produces a
snapdragon with PINK flowers; neither the red allele (R)
or the white allele (r) is completely dominant over the
other.
• Ex 2. Child of a straight haired parent and a curly-haired
parent has wavy hair.
Codominance
CODOMINANCE: Both alleles for a gene are expressed at the
same time in a heterozygous offspring.
• Neither allele is dominant or recessive, nor do the blend.
• Ex. AB blood groups; both A and B are dominant, but a
person can have blood type AB.
Inheritance of Traits
PEDIGREE: Family history showing how a trait is inherited
over several generations.
Used to determine if a person is a carrier for a GENETIC
DISORDER and/or to determine if their children will be
affected by the disorder.
RECESSIVE OR
DOMMINANT
SYMPTOM
DEFECT
FREQUENCY
Recessive
Poor blood
circulation
Abnormal
Hemoglobin
molecules
1:500
(African Americans)
Dominant
Excessive
cholesteral levels
in the blood
Abnormal form of
cell surface receptor
for cholesterol
1:500
Recessive in
early childhood
Deterioration of
central nervous
system; death in early
childhood
Defective form
of a brain enzyme
1:3,500
(Ashkenazi Jews)
Cystic Fibrosis
Recessive
Mucus clogs
organs
Defective
chloride-ion
transport protein
1:2,500
(Caucasians)
Hemophelia A
Sex-linked
Recessive
Failure to
clot blood
Defective form
of a blood-clotting
factor
1:10,000
(males)
Dominant
Gradual
deterioration of
Brain tissue/ CNS
Inhibitor of brain
Cell metabolism is
made
1:10,000
DISORDER
Sickle Cell
Anemia
Hypercholesterolemia
Tay-Sachs
Disease
Huntington's
Disease