Chapter 12
12.1 Basic Patterns of Human Inheritance
Recessive Genetic Disorders
 A recessive
trait is
expressed
when the
individual is
homozygous
recessive for
the trait.
Chapter 12
12.1 Basic Patterns of Human Inheritance
Cystic Fibrosis
 Affects the mucus-producing glands,
digestive enzymes, and sweat glands
 Chloride ions are not absorbed into the
cells of a person with cystic fibrosis but
are excreted in the sweat.
 Without sufficient chloride ions in the cells,
a thick mucus is secreted.
Chapter 12
12.1 Basic Patterns of Human Inheritance
Albinism
 Caused by altered genes, resulting in the
absence of the skin pigment melanin in hair
and eyes
 White hair
 Very pale skin
 Pink pupils
Chapter 12
12.1 Basic Patterns of Human Inheritance
Tay-Sachs Disease
 Caused by the absence of the enzymes
responsible for breaking down fatty acids
called gangliosides
 Gangliosides accumulate in the brain,
inflating brain nerve cells and causing
mental deterioration.
12.1 Basic Patterns of Human Inheritance
Galactosemia
 Recessive genetic disorder characterized by
the inability of the body to digest galactose.
Chapter 12
12.1 Basic Patterns of Human Inheritance
Dominant Genetic Disorders
 Huntington’s disease affects the nervous
system.
 Achondroplasia is a genetic condition that
causes small body size and limbs that are
comparatively short.
Chapter 12
12.1 Basic Patterns of Human Inheritance
Chapter 12
12.1 Basic Patterns of Human Inheritance
Pedigrees
 A diagram that traces the inheritance of a
particular trait through several generations
Chapter 12
12.1 Basic Patterns of Human Inheritance
Inferring Genotypes
 Knowing physical traits can determine what
genes an individual is most likely to have.
Predicting Disorders
 Record keeping helps scientists use
pedigree analysis to study inheritance
patterns, determine phenotypes, and
ascertain genotypes.
Chapter 12
12.2 Complex Patterns of Inheritance
Incomplete Dominance
 The heterozygous phenotype is an
intermediate phenotype between the two
homozygous phenotypes. (both alleles are
blended)
Chapter 12
12.2 Complex Patterns of Inheritance
Codominance
 Both alleles are expressed in the
heterozygous condition. (both alleles are seen
at the same time)
Chapter 12
12.2 Complex Patterns of Inheritance
Sickle-cell Disease
Normal red blood cell
 Changes in hemoglobin
cause red blood cells to
change to a sickle shape.
 People who are
heterozygous for the trait
have both normal and
sickle-shaped cells.
Sickle cell
7766x
Chapter 12
12.2 Complex Patterns of Inheritance
Multiple Alleles
 Blood groups in
humans
 ABO blood groups
have three forms
of alleles.
Chapter 12
12.2 Complex Patterns of Inheritance
Coat Color of Rabbits
 Multiple alleles can demonstrate a hierarchy
of dominance.
 In rabbits, four alleles code for coat color:
C, cch, ch, and c.
Chapter 12
12.2 Complex Patterns of Inheritance
Coat Color of Rabbits
Chinchilla
Albino
Light gray
Dark gray
Himalayan
Chapter 12
12.2 Complex Patterns of Inheritance
Epistasis
 Variety is the result of one allele hiding the
effects of another allele.
eebb
eeB_
No dark pigment present in fur
E_bb
E_B_
Dark pigment present in fur
Chapter 12
12.2 Complex Patterns of Inheritance
Sex Determination
 Sex chromosomes
determine an
individual’s gender.
Chapter 12
12.2 Complex Patterns of Inheritance
Sex-Linked Traits
 Genes located on the X chromosome
 Red-green color blindness
 Hemophilia
Chapter 12
12.2 Complex Patterns of Inheritance
Polygenic Traits
 Polygenic traits arise from the interaction of
multiple pairs of genes.
Chapter 12
12.2 Complex Patterns of Inheritance
Environmental Influences
 Environmental factors
 Diet and exercise
 Sunlight and water
 Temperature
Chapter 12
12.2 Complex Patterns of Inheritance
Twin Studies
 Helps scientists separate genetic contributions
from environmental contributions
 Traits that appear frequently in identical twins
are at least partially controlled by heredity.
 Traits expressed differently in identical twins
are strongly influenced by environment.
Chapter 12
12.3 Chromosomes and Human Heredity
Karyotype Studies
 Karyotype—micrograph in which the pairs of
homologous chromosomes are arranged in
decreasing size.
 Images of chromosomes stained during
metaphase
 Chromosomes are arranged in decreasing
size to produce a micrograph.
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