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Patterns of Gene Inheritance
Chapter 23
Genetics is the study of genes
What is a gene?
A. A factor that controls a heritable characteristic
B. Something on a chromosome
C. Information stored in a segment of DNA
D. Something that encodes a protein
Gregor Mendel
LOTS O’ DEFINITIONS
•
Gene---A piece of DNA that encodes a particular
trait. EX a gene for eye-color
•
Allele—an alternate form of a gene. EX—allele
for blue eyes and allele for brown eyes
•
Phenotype—the physical expression of a gene
or allele. Ex. Blue eyes
•
Genotype—the genetic composition of an
individual.
MORE DEFINITIONS
•
Locus—the location of a gene on a
chromosome. Plural=loci
•
Dominant allele—expressed whether
alone or in pairs. Symbolized by a capital
letter. EX Brown eye allele=B
•
Recessive allele—expressed only in the
absence of a dominant allele. Symbolized
by lowercase. EX blue eye allele=b
•
What is the maximum # of alleles a diploid
individual can have at any given locus?
STILL MORE DEFINITIONS!!!
•
Genotype—the genetic composition of an
individual
–
Homozygous----containing a pair of the same
alleles. Can be
•
Homozygous recessive—two recessive
alleles EX. bb, or
•
Homozygous dominant– two dominant
alleles EX BB
–
Heterozygous—containing two different
alleles. EX Bb
Law of Segregation
•
Each individual has two factors (called
genes today) for each trait.
•
Factors segregate during gametogenesis.
Ploidy level of gametes?
•
Fertilization gives each new individual two
factors again.
•
Ploidy level after fertilization?
Gene locus
Defs
Example--Widow’s Peak
Gametogenesis
•
Homologous pairs separate during
meiosis. When?
∴ a gamete has only one allele from each pair
of alleles.
∴ If the primary spermatocyte is genotype
Ww, the sperm cell would contain either a W
or a w, but not both.
•
Ww represents the genotype of an
individual.
•
Gametes are represented by W or w.
One-Trait Crosses
GENOTYPE?
GENOTYPE?
GENOTYPE?
Punnett Square
EXAMPLE 1—A Monohybrid Cross
Monohybrid cross
The One-Trait Testcross
QUESTION: How can we determine the
QUESTION: How can we determine the
genotype of a dominant phenotype?
genotype of a dominant phenotype?
Testcross: cross the dominant phenotype (unknown genotype) with
the recessive phenotype (known genotype).
The Law of Independent
Assortment
•
Law of Segregation involves 1 character. What
about 2 (or more) characters?
•
Each pair of alleles segregates independently of
the other pairs
•
All possible combinations of alleles can occur in
the gametes
•
EX: two pairs of alleles segregate independently
of each other.
•
When will different alleles not sort
independently?
E
n
e
N
F
a
t
h
e
r
M
o
t
h
e
r
N
e
E
n
e
n
E
N
e
n
E
N
e
N
E
n
Telophase II
Replication
Independent Assortment
N
E
n
e e
n
E
N
OR
Alignment of
Homologs at
Metaphase I
Segregation and Independent
Assortment
MEIOSIS & GENETIC VARIATION
Independent
Assortment
•
Diploid organisms can
produce 2
n
diff. gametes
•
Ea. homologous pair can
orient in two different ways
•
Humans: 2
23
= 8,388,608
(est. 8.4 million)
Each homologous pair can
orient in either of two ways
