Patterns of Inheritance
CHAPTER 7
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Mendel’s rules of inheritance apply to autosomal
genetic disorders.
– A heterozygote for a recessive disorder is a carrier.
– Carriers do not have the disorder but carry the gene.
– Disorders caused by dominant alleles are uncommon.
(dominant)
MALES AND FEMALES CAN DIFFER IN SEX-LINKED TRAITS.
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Genes on sex chromosomes are called sex-linked genes.
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Male mammals have an XY genotype.
Y chromosome genes in mammals are responsible for male
reproductive characteristics.
Female mammals have an XX genotype.
Y
X
FEMALES CARRY SEX-LINKED GENETIC DISORDERS.
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Males (XY) express all of their sex linked genes.
Expression of the disorder depends on which parent
carries the allele and the sex of the child.
Sex-linked (X-linked) Inheritance
•Example: Colorblindness
•Females carry this allele on X chromosome. If son inherits affected X chromosome
from mother, then the son will have colorblindness because the Y chromosome
cannot mask/dominate the X chromosome. It is more rare for females to be colorblind
because they have to inherit both copies of the allele from each parent. If female has
one copy of the allele, then she is a carrier and holds a 50% of passing this allele onto
offspring.
B = Normal allele
b = Colorblind allele
What #
do you
see
inside
the
circle?
Sex-linked (X-linked) Inheritance
•Example: Hemophilia
•Hemophilia is a bleeding disorder in which it takes a long time for the blood to clot.
•Affected individual could bleed to death.
•
Female mammals have an XX genotype.
– All body cells of females have 2 X chromosomes. The cell only needs one
to be activated; therefore the other is randomly “turned off” by inactivation.
– This is random and not all cells will have the same activated X
chromosome. This is why female cats can have more than two coat colors.
INCOMPLETE DOMINANCE INHERITANCE
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In incomplete dominance, neither allele is completely dominant nor
completely recessive.
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Heterozygous phenotype is intermediate between the two homozygous
phenotypes
Example: Beta Fish
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Green offspring results from mix of yellow and blue parents.
Purple offspring results from mix of red and blue parents.
Primary colored offspring (Yellow, Blue, and Red) are purebreds.
Incomplete Dominance
•Example: Snapdragons (flowers)
•Red parent crossed with white
parent = 100% pink offspring in
F1 generation.
•F1 pink offspring crossed with
F1 pink offspring = F2 generation
of 25% red, 50% pink, and 25%
white
Codominant Inheritance

Codominant alleles will both be completely expressed.
– Codominant alleles
are neither dominant
nor recessive.
– The ABO blood
types result from
codominant alleles.
Type O blood is universal donor; Type AB blood is universal acceptor.
Why can’t person with blood Type A receive blood Type B transfusion?
MANY GENES MAY INTERACT TO PRODUCE ONE TRAIT.
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Polygenic traits are produced by two or
more genes.
Example: Eye color and skin color.
These are complex patterns that are
controlled by several genes located on
different chromosomes.
This explains why there are different
shades of light and dark skin, as well as
eyes.
Order of dominance:
brown > green > blue.
•
•
An epistatic gene can interfere with other genes to prevent their expression. They
“block” other genes.
Example: Albinism= the lack of pigment in skin, hair, and eyes.
PHENOTYPE IS ALSO INFLUENCED BY ENVIRONMENT.
• The sex of sea turtles depends on
temperature of the environment in
which the egg is incubated. Eggs in
warm nest = female. Eggs in cold
nest = male. Could an increase in
female sea turtles indicate global
warming?
• Height is an example of a phenotype
strongly affected by the environment.
If person is malnourished, he or she
may not grow as tall as their genes
intended.
GENE LINKAGE
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Thomas Hunt Morgan found that linked traits are on
the same chromosome; called gene linkage.
Chromosomes, not genes, assort independently
during meiosis.
He studied fruit flies because they have variations
easy to observe and have a short life cycle, which
means you can get generations of data within weeks.
Wild type
Mutant

Linked genes are not inherited together every time.
•
Chromosomes exchange homologous genes during Prophase I of Meiosis I.
LINKAGE MAPS ESTIMATE DISTANCES BETWEEN GENES.
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Genes can be mapped to specific locations on chromosomes.
The closer together two genes are, the more likely they will be inherited
together.
The farther apart two genes are, the least likely they will be inherited
together.
Cross-over frequencies are related to distances between genes.
Linkage maps show the relative locations of genes.
•
Cross-over frequencies can be converted into map units.
– gene A and gene B cross over 6.0 percent of the time
– gene B and gene C cross over 12.5
percent of the time
– gene A and gene C cross over 18.5 percent of the time
A PEDIGREE IS A CHART FOR TRACING GENES IN A FAMILY.
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Phenotypes are used to infer genotypes on a pedigree.
Circles = females Squares = males
Non shaded shapes = Normal
Shaded shapes = Disorder
½ shaded shapes = Carrier
•
If the phenotype is more common in males, the gene is likely sex-linked.
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A karyotype is a picture of all chromosomes in a cell.
Karyotypes can show changes in chromosomes.
XY
Karyotypes can show different types of chromosomal mutations:
Deletion, Duplication, Inversion and Translocation.