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Patterns of Gene Inheritance
(Chapters 10 & 11)
Gene: unit of heredity
 Individual genes, or DNA segments, contain the genetic blue-print which is ultimately expressed
in our hair color, blood type, etc.
 DNA is packaged in chromosomes
Homologous chromosomes: chromosomes of the same size and shape that contain genes for the same trait
 Each somatic cell contains 23 pairs of homologous chromosomes (diploid number)
o 1 pair of the chromosomes are sex chromosomes (XY or XX) that determine gender
o other 22 pairs of chromosomes called autosomal chromosomes guide the expression of
every other trait
Mendel’s Law of Segregation
 each individual has to factors for each trait
 factors separate during meiosis
 each gamete contains one factor for each trait
Terminology of genetics
Alleles: genes coding for the same traits on each pair of homologous chromosomes
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Alleles may be identical or different in their influence
For example, member of a gene pair, or alleles, coding for hairline shape on your forehead may
specify straight across or widow’s peak
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When both alleles in a homologous chromosome pair have the same expression, the individual is
said to be homozygous for that trait
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Homozygous: when two alleles controlling a single trait are identical
When alleles differ in their expression, the individual is heterozygous for that trait and typically
one of the alleles, called the dominant allele will exert its effects
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Heterozygous: when two alleles controlling a single trait differ in their expression for the
trait
Dominant allele: gene that is always expressed if present
Recessive allele: gene that is not expressed in the presence of a dominant allele
Genotype: refers to an individual’s genetic makeup (i.e., whether homozygous or heterozygous for the
various alleles)
 two letters are used to designate each trait since homologous chromosomes each contain
genes for a particular trait
Trait: hairline shape
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W = Widow’s peak (dominant allele)
w = Straight hairline (recessive allele)
Genotype written as:
Phenotype
Homozygous dominant WW
Heterozygous
Ww
Homozygous recessive ww
Widow’s peak
Widow’s peak
straight hairline
Phenotype: characteristics of the individual based on expression of the genes
Punnett Square:
 Method used to calculate probable results of a genetic cross  i.e., predict genotypes and
phenotypes of offspring
Example:
Trait  earlobe shape
D = detached earlobes (dominant allele)
d = attached earlobes (recessive allele)
Mother = homozygous dominant
Father = homozygous recessive
What type of earlobe shape will their children have?
 Heterozygous  detached earlobes
If these children (Dd) reproduce with individuals of the same genotype, what type of
earlobe shape will their children have?
Other Forms of Inheritance:
 Certain traits follow the rules of simple Mendelian inheritance (i.e., dominant-recessive type of
inheritance), but other inheritance patterns exist for many traits
Multiple Alleles: more than two alleles control a particular trait
For example: blood type
 Blood types: A, B, AB, and O are phenotypes caused by three different alleles
 Each person has only two of the three alleles
 A and B are dominant and will be fully expressed in the presence of the other  called
codominance
 Type O is recessive
Phenotype
A
B
AB
O
Genotype
AA, Ao
BB, Bo
AB
oo
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Blood typing can sometimes aid paternity suits, but they can only suggest that an individual is the
supposed father. In most cases they can only exclude possible paternity.
♀ Ao x ♂ Bo
♀ Bo x ♂AB
Incomplete dominance
 the heterozygote has a phenotype intermediate between the homozygous dominant and the
homozygous recessive
 Example: Sickle-Cell Anemia
Genetic Disorders
 Sickle-cell gene demonstrates incomplete dominance
 Autosomal dominant disorders
o Genotype AA or Aa (heterozygous) will have disorder
o Example: Huntington’s Disease
 Neurological disorder resulting in degeneration of brain cells
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Autosomal recessive disorders
o Genotype aa will have disorder, but Aa is a carrier
o Example: cystic fibrosis
 Most common lethal genetic disorder in US
 1 in 20 caucasians are carriers
o Carrier: individual who has the abnormal recessive gene  not
expressed since they are heterozygous
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Nondisjunction:
o failure of homologous chromosomes or sister chromatids to separate during meiosis in
the process of oogenesis or spermatogenesis
Result: abnormal number of chromosomes inherited by gametes
 offspring inherit an extra chromosome or are missing a chromosome
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procuces syndrome: a group of symptoms that appear together and indicate the
presence of a particular disorder
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Autosomal Nondisjunction:
 Down Syndrome  usually have three copies of chromosome 21
because the egg had two copies instead of one
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Sex Chromosome Nondisjunction
 Turner syndrome (XO)
 Triplo-X (XXX)
Sex-linked Inheritance (or X-linked inheritance)
XX  female
XY  male
The gender of a newborn is determined by the father:
o If a Y containing sperm fertilizes the egg, then the XY combination results in a male
o If an X-containing sperm fertilizes the egg, the XX combination results in a female
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X-linked inheritance  Body traits that are inherited from the X-chromosome
A male always receives sex-linked condition from his mother
The Y chromosome from the father does not carry an allele for the trait
Usually the trait is recessive  the female must receive two alleles before she has the
condition
Common X-linked recessive disorders:
o Color blindness
o Hemophilia: blood clotting disorder
o Muscular dystrophy: degeneration of muscle tissue
Example: red-green color-blindness
XB = normal color vision
Xb = color blind
♀ XBXb (carrier) ♂ XBY (normal)