Observing
Patterns
in
Inherited
Traits
Chapter 7
Skin Color
Terms Used in Modern Genetics

Heredity- characteristics passing from

Genes
parents to offspring through genes
Each gene has a specific locus on a
chromosome
 Heterozygous (Aa or Bb)
 Homomzygous (AA or BB)

Gregor Mendel
Genetics

An allele is dominant if its effect masks
the effect of a recessive allele paired
with it
Mendel’s Law of
Segregation
 Only
one of the 2
alleles gets put
into each gamete
Segregation of Alleles
at
a
Gene
homozygous
homozygous
dominant parent
recessive parent
Locus
(chromosomes
duplicated before
meiosis)
meiosis I
meiosis II
(gametes)
(gametes)
fertilization
produces
heterozygous
offspring Fig. 11-5, p. 172
genotype
phenotype
Construction of a Punnett Square
Punnett square
Dihybrid Cross

2 loci
AABB x aabb
 F2 phenotype ratio is 9:3:3:1

Mendel’s Law
of Independent Assortment

Mendel’s law of independent
assortment
One trait does not influence the
inheritance of the other
 Exception: linked genes

parent plant homozygous
parent plant
for purple
homozygous
flowers
for white flowers
and long
and short stems
stems
aabb
AABB
AB
x
ab
Fig. 11-9a, p. 175
AaBb
AaBb
AaBb
What gametes will
they form?
F1
generation
Fig. 11-9b, p. 175
AB
Ab
aB
ab
AB
AABB
AABb
AaBB
AaBb
Ab
AABb
AAbb
AaBb
Aabb
aB
AaBB
AaBb
aaBB
aaBb
ab
AaBb
Aabb
aaBb
aabb
D Out of 16 possible genetic outcomes of this dihybrid cross, 9 will result in plants that
are purple-flowered and tall; 3, purple-flowered and short; 3, white-flowered and tall;
and 1, white-flowered and short. The ratio of phenotypes of this dihybrid cross is 9:3:3:1.
Fig. 11-9c, p. 175
Codominance


Two nonidentical alleles of a gene are both
fully expressed in heterozygotes, so
neither is dominant or recessive
Multiple allele systems
Incomplete Dominance

One allele is not fully dominant over its
partner
Epistasis

Epistasis
Two or more gene products influence a
trait
 Typically, one gene product suppresses the
effect of another

Epistasis in Coat Colors
Pleiotropy

Pleiotropy

One gene product
influences two or
more traits
Example: Marfan
syndrome
 Or sry

Sex-linked traits
Genes and the Environment

Expression of some genes is affected
by environmental factors such as
temperature, altitude, or chemical
exposure