Codominance – mixture of white and red
hairs = roan coats in horses & cattle
Incomplete Dominance – blending of
red and white to form the pink
phenotype of the four o’clock flower
Chapter 14
Human Genetics
Sec. 14-1 Human Heredity
I. Human Traits
A. Pedigree: chart that shows the genetic
relationships within a family & how traits
are passed from one generation to another
1. Circles represent Females
2. Squares represent Males
3. Circles/squares not shaded = doesn’t have
trait or is a carrier (carriers may be ½ shaded)
4. Circles/squares shaded = has trait
Pedigree Examples
Half-filled in = Heterozygous/carrier of
recessive trait/disorder
II. Back to Human Blood Types
A. ABO Blood Groups
II. Back to Human Blood Types
A. ABO Blood Groups
B. Rh Blood Group
1. Determined by a single gene with 2 alleles,
positive & negative.
2. Was discovered in the rhesus monkey, & is
why its called the Rh group.
3. Positive (Rh+) allele is dominant
4. Rh+/Rh+ and Rh+/Rh- = positive
5. Rh-/Rh- = negative
III. Autosomal Disorders in Humans
A. Recessive Disorders = caused by recessive
alleles
1. To have these disorders, individual must be
homozygous recessive
2. The presence of just one dominant allele
prevents the individual from having the
disorder.
3. Heterozygotes are carriers of the disorder
Recessive Disorders
Disorder
Albinism
Major Symptoms
lack of pigment in skin, hair, & eyes
excess mucus in lungs, digestive tract, liver;
Cystic Fibrosis increased susceptibility to infections; death in
childhood unless treated
Galactosemia Accumulation of galactose in tissues; mental
retardation; eye & liver damage
Phenylketonuria Accumulation of phenylaline in tissues; lack of
(PKU)
normal skin pigment, mental retardation
Tay-Sach's
Lipid accumulation in brain cells; mental deficiency;
disease
blindness; death in early childhood
4. Cystic Fibrosis – caused by a deletion of 3
DNA nucleotides, which leads to a
dysfunctional chloride transport protein
B. Dominant Disorders – caused by a dominant
allele
1. It is only necessary to have one dominant
allele to have these types of disorders
(homozygous dominant or heterozygous)
2. Only individuals with 2 recessive alleles
(homozygous recessive) are normal.
Dominant Disorders
Disorder
Major Symptoms
Achondroplasia
dwarfism (one form)
Huntington's
mental deterioration and uncontrollable movements;
disease
appears in middle age
Hypercholesterolemia excess cholesterol in blood; heart disease
Disorders & Pedigrees
Does this pedigree show a recessive or
dominant disorder?
Disorders & Pedigrees
Does this pedigree show a recessive or
dominant disorder?
C. Codominant Disorders – controlled by a
codominant allele
1. Sickle Cell Disease – the sickle cell allele
changes just 1 DNA nucleotide in the gene
for the protein hemoglobin
•
•
•
This single change leads to the amino acid
valine being substituted for glutamic acid
The hemoglobin protein cannot fold properly
and changes shape of the blood cell
Sickle-shaped blood cells carry less oxygen
Sickle-Cell Disease
Chapter 11, Sect. 5 - Linkage
I. Gene Linkage – genes located on the same
chromosome are inherited together = linked
A. Locus – specific location of a gene on a
chromosome
B. Thomas Hunt Morgan – bred fruit flies and
realized certain traits were always inherited
together, a.k.a. linked
C. Linkage Maps (also called gene maps)
1. Shows relative locations (loci) of genes
found on the same chromosome
2. If two genes are linked, are
they always inherited
together?
3. What event in meiosis
might allow linked genes
to be inherited separately?
Chapter 14, Sect. 2 – Human
Chromosomes
I. Sex-Linked Genes – genes
found on the X or Y chromosome
A. Colorblindness = recessive
1. Males (XY) – if the X has the
allele, male will be colorblind
2. Females (XX) – both X’s must
have the allele to be colorblind
Colorblindness Example
Colorblindness Pedigree
Other Sex-linked Genes
B. Hemophilia – recessive, sex-linked blood
disorder
1. Recessive alleles lead to a protein necessary for
blood clotting to be missing.
C. Duchenne Muscular Dystrophy – recessive,
sex-linked muscle disorder
1. Defective version of the gene that codes for a
muscle protein leads to weakening and loss of
muscle tissue
II. X Chromosome Inactivation
A. Barr Bodies
1. If 1 X chromosome is enough in males, 1 is
enough for females too (even though
females have 2 X’s)
2. In females, one of the 2 X chromosomes is
randomly switched off & forms a smaller,
dense barr body.
3. Each cell of the female randomly turns off
an X, but not all cells turn off the same X!
III. Chromosomal Disorders
A. Nondisjunction – an error in meiosis where
homologous chromosomes are not seperated.
1. Leads to abnormal numbers of
chromosomes in gametes
2. If an abnormal gamete is fertilized, the
resulting individual will have an abnormal
number of chromosomes
Nondisjunction in Meiosis
B. Down Syndrome
1. Trisomy – have 3 copies of a chromosome
2. Trisomy 21 – 3 copies of chromosome 21 &
leads to Down Syndrome
C. Sex Chromosome Disorders
1. Turner’s Syndrome – female only has 1 X
chromosome; (XO)
•
sterile as they don’t fully develop at puberty
2. Triple X Syndrome – Female has an extra X
chromosome (XXX)
•
normal appearance; may be infertile; may
have intellectual impairments
3. Klinefelter’s Syndrome – male with an
extra X (XXY)
•
Sterile, may display female characteristics