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
QUESTION: How can we determine the
genotype of a dominant phenotype?
The One-Trait Testcross
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?
Independent Assortment
E
E
e
E e
e
n N
N
N
n
N
e
E
e
n
e
OR
n
n
E
Alignment of
Homologs at
Metaphase I
Replication
E
e
N
E
N
N
n
Telophase II
n
Segregation and Independent
Assortment
MEIOSIS & GENETIC VARIATION
Independent
Assortment
•
Diploid organisms can
produce 2n diff.
gametes
•
Each homologous pair can
orient in either of two ways
•
Ea. homologous pair can
orient in two different ways
Humans: 223 = 8,388,608
(est. 8.4 million)
HYPOTHETICAL EXAMPLE OF
INDEPENDENT ASSORTMENT
Eye color
Hair color
Gene
for
brown
eyes
Gene
for
blue
eyes
r
Gene
for
black
hair
Gene
for red
hair
INDEPENDENT
During meiosisASSORTMENT
I, tetrads can line up 2 different ways.
n
OR
Meiosis I & II
Brown eyes
Black hair
Blue eyes
Red hair
Brown eyes
Red hair
Blue eyes
Black hair
Linked Genes and Independent Assortment
e
L
l
Replication
E
L
E e
L
l
E
e
l
Alignment of
Homologs at
Metaphase I
E
L
E
L
l
e
l
N
n
e
OR
n
n N
N
n
n
N
N
Dihybrid cross
What are the possible
genotypes of this
individuals gametes?
Probabilities
Two-trait Testcross-How?
Genetic Disorders
• Pedigree charts- show patterns of inheritance
Autosomal Dominant Disorders
• Autosomes---Non-sex chromosomes
• Dominant allele
• when under what genotype(s)will an
individual show the disorder?
Examples: Autosomal Dominant
Disorder
• Neurofibromatosis
– Small benign tumors,
– Gene on chromosome 17.
• Huntington Disease
– Progressive degeneration of the nervous
system
– Normal until middle age
– No treatment
– Gene has been isolated (chromosome 4),
genetic testing can diagnose
Huntington disease
Autosomal recessive pedigree
chart
• Autosomes---Non-sex chromosomes
• Recessive allele
• when under what genotype(s)will an
individual show the disorder?
Autosomal Recessive Disorders
• Tay-Sachs Disease
– Common among United States Jews of central and
eastern European descent.
– Death by the age of three or four.
– Defective enzyme in lysosomes.
• Cystic Fibrosis
– Most common lethal genetic disorder among
Caucasians
– Defective chloride ion transport protein
– Osmotic imbalance results in thick mucous in lungs
and pancreatic ducts
Cystic fibrosis therapy
Phenylketonuria (PKU)
• allele on chromosome 12.
• lack an enzyme needed for metabolism
of phenylalanine (an amino acid)
• Urine test diagnostic.
• Brain damage unless controlled by diet.
Polygenic Inheritance
•
•
•
•
•
Polygenic (Quantitative) Traits
Governed by more than one gene pair.
Several genes determine the phenotype.
Produce bell-shaped curve.
EX: Skin color
EX 2 Polygenic Inheritance :
Polygenic Disorders
• Examples: cancer, schizophrenia,
hypertension, diabetes, etc
– several genes involved
– also environmental influences.
Multiple Allelic Traits
• Often more than two alleles exist for a
particular gene locus.
• Maximum # of alleles at any given locus?
• Each individual inherits only two
alleles for these genes!!!
• EX: Human Blood type
Inheritance of blood type
Details
Incompletely Dominant Traits
• Codominance--both alleles equally
expressed in a heterozygote. EX ABO
Blood type
• Incomplete dominance--heterozygote
shows intermediate phenotype,
representing a blending of traits. EX Wavy
hair in Caucasians
• Phenotypic ratio ?
• 1 : 2 : 1.
Incomplete dominance
Example 2Incomplete
Dominance
Sickle-Cell Disease
• Caused by incompletely dominant alleles.
• Single nucleotide mutation causes abnormal
hemoglobin.
• HbA = normal hemoglobin
• HbS = sickled condition.
• Genotype of normal?
• Genotype of sickled homozygote?
• Genotype of intermediate phenotype?
Sickle-Cell Disease
Let’s
Get
Small!!!
• Simple Dominance
Molecular Explanation-Dominance
– One-half the amount of gene product
sufficient for phenotype
• Incomplete Dominance
– Recessive allele not expressed in
heterozygote
– Homozygous dominant –2 doses, full
phenotype
– Heterozygote—1 dose, reduced phenotype
• Codominant
– Both alleles expressed, combined
phenotype
Everything’s Relative
WHAT’S HAPPENING??
SUMMARY
•
•
•
•
•
•
•
•
•
•
Physical Basis of Genes
Lots o’ Definitions
Law of Segregation
Law of Independent Assortment
One-Trait Crosses
Law of Independent Assortment
Two-Trait Crosses
Genetic disorders
Polygenic Traits
Incomplete Dominance/Codominance
REMEMBER!!!
• Relate genetics to meiosis
• For genetics problems—first try to
figure the genotype(s) of the
gametes