BIOLOGY CHAPTER 8
Heredity
8.1 OBJECTIVES
Discuss Mendel’s experiments and his results.
 Explain the three principles of genetics Mendel
obtained through his experiments.
 Solve genetics problems using Punnett Squares.

8.1 PRINCIPLES OF GENETICS
Discuss Mendel’s experiments and results.
 Explain the three principles of genetics Mendel
obtained through his experiments.
 Solve genetics problems using Punnett squares.

PRINCIPLES OF GENETICS
The passing of traits from parents to offspring is
termed heredity.
 Can we predict certain traits?
 Can we determine what types of genetic
information parents have by looking at certain
traits of the offspring?
 We can answer these questions through genetics
– the science of heredity.

ORIGIN OF GENETICS
Father of genetics: Gregor Mendel (1882-1884)
 Tended to a garden of pea plants in a monastery.
 Published results from 8 years of research, only
to be ignored until 1900.

MENDEL’S EXPERIMENTS
Peas are grown easily and produce large numbers
of offspring in a short time.
 Chose 7 traits to study:

Seed
Shape
Seed Flowe Flower
Colo r
Position
r
Color
Pod
Color
Pod
Shape
Plant
Heigh
t
Dominant
Trait
Round
Yello
w
Axial (side)
Green
Inflated
Long
Recessive
Trait
Wrinkled
Green White
Terminal
(tips)
Yellow
Constricted
Short
Purple
MENDEL’S PROCESS
Mendel crossed pea plants w/ different traits.
 Offspring of parental cross were called F1
 Offspring of F1 crosses called F2.
 He noted for each trait, there was a dominant
and recessive form.
 In his crosses, recessive trait disappeared in F1
generation and then reappeared in F2.

MENDEL’S HYPOTHESIS
He reasoned that for every trait, there must be a
pair of factors.
 He called them characters, but we now know
them as genes.
 One gene came from each parent.

PUNNETT SQUARES
PUNNETT SQUARES
PUNNETT SQUARES
THE TEST OF SEGREGATION

He predicted if the F1 purple flowered plants
were crossed with white flowered plants, he
would get ratios different from any of his
previously obtained ratios.
TERMINOLOGY
Each alternative form of a gene for a certain trait
are called alleles.
 Combination of genes for a given trait referred to
as genotype. Example: WW and Ww
 The physical appearance of a trait is the
phenotype. Example: Blue and brown eyes.
 Homozygous = two of same alleles: WW or ww
(homozygous dominant and homozygous
recessive)
 Heterozygous = two different alleles: Ww

TWO TRAITS (DIHYBRID CROSSES)
TWO TRAITS (DIHYBRID CROSSES)
TWO TRAITS (DIHYBRID CROSSES)
8.2 SOLVING GENETICS PROBLEMS
Objectives:
 Apply the rules of probability to solve genetics
problems.
 Demonstrate the inheritance of traits resulting
from incomplete dominance, codominance, and
multiple alleles.

PROBABILITY
What is the probability of getting heads or tails
when you toss a coin?
 There is a list of steps to follow on page 208 in
text, but we have already been doing this.
 Remember, we can find either genotypic or
phenotypic ratios.

INCOMPLETE DOMINANCE: 4 O’CLOCK
PLANTS AS AN EXAMPLE
MULTIPLE ALLELES: HUMAN BLOOD
TYPES
8.3 THE CHROMOSOME THEORY OF
HEREDITY
Walter Sutton’s work showed good evidence that
genes reside on chromosomes.
 Some genes appear only on sex chromosomes.
 XX = female and XY = male in humans.
 Different combinations for different species.
 Example: Grasshoppers X = male and YY =
female.
 Sex-linked traits are traits that result from genes
on the sex chromosomes.

SEX-LINKED TRAITS IN HUMANS
Hemophilia – gene is a recessive gene found on
the X chromosome. Mostly, males get the
disease.
 Color-blindness is the same way.

MANY GENES – ONE EFFECT
Height, hair, eye, and skin color all controlled by
multiple alleles.
 Sample genotypes:

AABBCCDD
 AaBbCcDd
 Aabbccdd

Modifier genes can also impact things like eye color.
Brown, blue, hazel, green, mixed etc.