Genetics & Inheritance (chpt. 29 pgs. 60 -68)
Genes or DNA segments contain the blueprints for proteins and many of these are enzymes that dictate the
synthesis of virtually all of the body's molecules.
Chromosomes contain DNA
Genes are functional segments of DNA
human genome aprx. 38,000 genes
Genetics:
Gregor Mendel:
Vocabulary of Genetics:
nuclei of all human cells (except
) contain the diploid # of chromosomes
diploid genome or genetic DNA makeup represents 2 sets of instructions
2 of this set are sex chromosomes (X and Y)
autosomes
karyotype (pg. 60 figure 29-14) typically shown as homologous chromosomes
homologous chromosomes:
locus:
Gene pairs (alleles)
-chromosomes. are paired the genes
-alleles (genotype) may code for the same or for alternative forms of a trait
genotype vs phenotype:
-dominant
-recessive
-homozygous
-heterozygous
Interactions between alleles
• most human traits determined by multiple alleles or interaction of several gene
1. Strict Dominance or Dominant-Recessive Inheritance
Dominant allele always expressed
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2. Incomplete Dominance (Intermediate Inheritance)
The heterozygous genotype has a different phenotype
3. Codominance
the heterozygous genotype exhibits both dominant and recessive phenotypes
Other types of Inheritance:
4. Multiple Allele Inheritance: more than 2 alleles code for the trait
5. Sex-Linked Inheritance: allele is inherited on the X chromosome – not the Y
6. Polygene Inheritance: involves many alleles and chromosomes
Predicting Inheritance: Punnett square
Calculates percentage of offspring from each fertilization
Draw box – Put mom’s gametes (alleles) on one side and dad’s alleles on the other
Combine all possible fertilizations
1. Dominant-Recessive: interaction of dom/rec. alleles (figure 29-15 p. 62)
Punnett square
Tongue rolling is dominant – T
If mom can roll her tongue and dad can’t:
t
t
T
t
so probability of having homozygous dom. =
the prob. of having heterozygous =
the prob. of having recessive =
% offspring that can roll their tongue:
% that cannot:
• dominant traits:
• genetic disorders caused by dominant genes are fairly uncommon
• there are some dominant disorders where the person is less
impaired or survives long enough to reproduce.
• Huntington's disease: (see applications manual)
What if an individual with Huntington’s disease mated with someone without it?
• recessive traits: many recessive inheritance cause no problems
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and some are more desirable
-i.e. normal vision
-most genetic diseases are inherited as simple recessive traits
See applications manual:
-i.e. albinism: lack skin pigmentation
-cystic fibrosis:
-Tay-Sachs disease:
carrier:
2. Incomplete Dominance
•the heterozygote has a phenotype intermediate between homozygous dominant and
homozygous recessive
•very common in plants i.e. snapdragons
R = red r= white so RR =
R
rr =
Rr=
R
r
r
In humans:
• sickling gene (s): when hypoxia occurs hemoglobin changes shape and causes RBC to
assume abnormal sickle shape.
SS:
Ss:
S
ss:
s
S
s
3. Codominance & Multiple Allele Inheritance: ABO blood type: based on presence/absence
antigens
3 alleles:
IA codes for antigen A
IB
i
• each receives 2 of these IA and IB are codominant and i is recessive
IB IB or IB i =
IA IB =
ii =
IA IA or IA i =
IA
I
i
i
B
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4. Sex-Linked Inheritance
•inherited traits determined by genes on sex chromosome are
sex linked
XX XY =
•Y chromosome contains the genes that determine maleness and only 1/3 as
large as X
•a gene found only on the X chromosome is said to be sex linked
•for colorblindness which is a recessive x linked disorder:
XCXC
XC Y
XCXc
Xc Y
c c
XX
•male inherits a bad x-linked recess. allele
•the female must have 2 bad X's to express the disorder
XC
Y
C
X
Xc
•X-linked traits passed from mothers to sons
•mother can pass it to daughter
5. Polygene Inheritance
•polygene inherit. results in continuous variations between 2 extremes & explains
many human characteristics.
•skin color
existing in 2 allele forms: Aa, Bb, Cc, Dd
A B C D:
a b c d:
-individual with AABBCCDD genotype would
vs.
aabbccdd
amount of brown pigment in the iris (determines eye color) is also regulated by
polygenes as are intelligence and height.
Sexual Sources of Genetic Variation:
1. Segregation of Individual Chromosomes
2. Crossover of Homologous Chromosomes & Gene Recombination
3. Random Fertilization
1. Segregation of Individual Chromosomes
A. homologous chromosomes: synapses to form tetrads during
spermatogenesis and oogenesis.
• chance determines the alignment & orientation of the tetrads
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• the number of diff. gamete types resulting from independent assortment of the
homologous chromosomes is 2n
-a cell with 6 prs of homologous would produce 26
-in male testes the # of gamete types produced by independent
assortment is 223 = 8.5 million
2. Crossover of Homologues & Gene Recombination
• additional variation results from the crossing over & exchange
• only 2 of the 4 appear to take part in crossing over and recombination but these
2 make many crossovers during synapsis.
• each crossover causes recombination of many genes not just 2
• crossovers occur at random points:
• humans have 23 tetrads
3. Random Fertilization
• single egg fertilized by single sperm on a haphazard basis
• the variation occurring from ind. assortment & random fertilization
Influence of Environmental Factors on Gene Expression
our genotype is very
our phenotype is more like
sometimes maternal factors
Fetal Alcohol Syndrome:
environ. factors after birth
Genetic Screening/Counseling
•newborn infants routinely screened
Fetal Testing:
•amniocentesis:
•chorionic villi sampling (CVS) newer procedure
Human Genome Project (page 66)
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