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Unit 3
Chapter 29a
Heredity
 Who we are is guided by the gene-
bearing chromosomes we receive
from our parents in egg and sperm.
 Segments of DNA called genes are
blueprints for proteins, many which
are enzymes, that dictate the
synthesis of all of our body’s
molecules.
Heredity
 Genes are expressed in our hair
color, sex, blood type and so on
 However, these genes are
influenced by other genes and by
environmental influences
Genetics
 Genetics is the study of the
mechanism of heredity
 Genes = give birth to
 Nuclei of all human cells (except
gametes) contain 46 chromosomes
(or 23 pair)
 Sex chromosomes determine the
genetic sex
(XX = female, XY = male)
Genetics
 Karyotype – the diploid chromosomal
complement displayed in homologous
pairs – a picture of our genome
 Genome – genetic (DNA) makeup
represents two sets of genetic
instructions – one maternal and the
other paternal
Alleles
 Alleles - Matched genes at the same
locus on homologous chromosomes
 Homozygous – two alleles controlling
a single trait are the same
 Heterozygous – the two alleles for a
trait are different
Alleles
 Dominant – an allele masks or
suppresses the expression of its
partner – represented by a capital
letter
 Recessive – the allele that is
masked or suppressed –
represented by a lower case letter
Alleles & Genotype
 AA = both alleles dominate –
homozygous dominant
 Aa = one dominant allele and one
recessive allele = heterozygous
 aa = both alleles recessive –
homozygous recessive
Genotype and Phenotype
 Genotype – the genetic makeup
 Phenotype – the way one’s genotype
is expressed
Segregation and Independent
Assortment
 Chromosomes are randomly
distributed to daughter cells
 Members of the allele pair for each
trait are segregated during meiosis
 Alleles on different pairs of
homologous chromosomes are
distributed independently
Segregation and Independent
Assortment
 The number of different types of
gametes can be calculated by this
formula:
2n, where n is the number of
homologous pairs
Segregation and Independent
Assortment
 In a man’s testes, the number of
gamete types that can be produced
based on independent assortment is
223, which equals 8.5 million
possibilities
Independent
Assortment
Figure 29.2
Crossover
 Homologous chromosomes synapse in
meiosis I
 One chromosome segment
exchanges positions with its
homologous counterpart
 Genetic information is exchanged
between homologous chromosomes
 Two recombinant chromosomes are
formed
Crossover
Figure 29.3
Crossover
Figure 29.3
Random Fertilization
 A single egg is fertilized by a single
sperm in a random manner
 Considering independent assortment
and random fertilization, an
offspring represents one out of 72
trillion (8.5 million  8.5 million)
zygote possibilities
Dominant-Recessive Inheritance
 Reflects the interaction of
dominant and recessive alleles
 Punnett square – diagram used to
predict the probability of having a
certain type of offspring with a
particular genotype and phenotype
Dominant-Recessive Inheritance
 Example: probability of different
offspring from mating two
heterozygous parents
T = tongue roller and t = cannot
roll tongue
Figure 29.4
Dominant-Recessive Inheritance
 Examples of dominant disorders:
achondroplasia (type of dwarfism) and
Huntington’s disease
 Examples of recessive conditions:
albinism, cystic fibrosis, and TaySachs disease
 Carriers – heterozygotes who do not
express a trait but can pass it on to
their offspring
Now try some Punnet Square
problems on you own!
Study guide check
Pages 713-718 (6 points)
Unit 3
Chapter 29b
Incomplete Dominance
 Heterozygous individuals have a
phenotype intermediate between
homozygous dominant and
homozygous recessive
Incomplete Dominance
 Sickling gene is a human example
when aberrant hemoglobin (Hb) is
made from the recessive allele (s)
SS = normal Hb is made
Ss = sickle-cell trait (both
aberrant and normal Hb is made)
ss = sickle-cell anemia (only
aberrant Hb is made)
Multiple-Allele Inheritance
 Genes that exhibit more than two
alternate alleles
 ABO blood grouping is an example
 Three alleles (IA, IB, i) determine
the ABO blood type in humans
 IA and IB are codominant (both are
expressed if present), and i is
recessive
ABO Blood Groups
Table 29.2
Sex-Linked Inheritance
 Inherited traits determined by
genes on the sex chromosomes
 X chromosomes bear over 2500
genes; Y chromosomes carry about
15 genes
Sex-Linked Inheritance
 X-linked genes are:
 Found only on the X chromosome
 Typically passed from mothers to
sons
 Never masked or damped in males
since there is no Y counterpart
Polygene Inheritance
 Depends on several different gene
pairs at different loci acting in
tandem
 Results in continuous phenotypic
variation between two extremes
 Examples: skin color, eye color, and
height
Polygenic Inheritance of Skin
Color
 Alleles for dark skin (ABC) are
incompletely dominant over those for
light skin (abc)
 The first generation offspring each
have three “units” of darkness
(intermediate pigmentation)
 The second generation offspring have a
wide variation in possible pigmentations
Polygenic
Inheritance
of Skin Color
Figure 29.5
Environmental Influence on
Gene Expression
 Phenocopies – environmentally
produced phenotypes that mimic
mutations
Environmental Influence on
Gene Expression
 Environmental factors can influence
genetic expression after birth
 Poor nutrition can effect brain
growth, body development, and
height
 Childhood hormonal deficits can
lead to abnormal skeletal growth
Genomic Imprinting
 The same allele can have different
effects depending upon the source
parent
 Deletions in chromosome 15 result in:
 Prader-Willi syndrome if inherited
from the father
 Angelman syndrome if inherited
from the mother
Genomic Imprinting
 During gametogenesis, certain genes
are methylated and tagged as either
maternal or paternal
 Developing embryos “read” these tags
and express one version or the other
Extrachromosomal
(Mitochondrial) Inheritance
 Some genes are in the mitochondria
 All mitochondrial genes are
transmitted by the mother
 Unusual muscle disorders and
neurological problems have been
linked to these genes
Unit 3
Chapter 29c
Genetic Screening, Counseling,
and Therapy
 Newborn infants are screened for a
number of genetic disorders:
congenital hip dysplasia, imperforate
anus, and PKU
Genetic Screening, Counseling,
and Therapy
 Genetic screening alerts new parents
that treatment may be necessary for
the well-being of their infant
 Example: a woman pregnant for the
first time at age 35 may want to
know if her baby has trisomy-21
(Down syndrome)
Carrier Recognition
 Identification of the heterozygote
state for a given trait
 Two major avenues are used to
identify carriers: pedigrees and blood
tests
Carrier Recognition
 Pedigrees trace a particular genetic
trait through several generations;
helps to predict the future
 Blood tests and DNA probes can
detect the presence of unexpressed
recessive genes
 Sickling, Tay-Sachs, and cystic
fibrosis genes can be identified by
such tests
Pedigree
Analysis
Figure 29.6
Fetal Testing
 Is used when there is a known risk of
a genetic disorder
 Amniocentesis – amniotic fluid is
withdrawn after the 14th week and
sloughed fetal cells are examined for
genetic abnormalities
 Chorionic villi sampling (CVS) –
chorionic villi are sampled and
karyotyped for genetic abnormalities
Fetal
Testing
Figure 29.7
Human Gene Therapy
 Genetic engineering has the potential
to replace a defective gene
 Defective cells can be infected with
a genetically engineered virus
containing a functional gene
 The patient’s cells can be directly
injected with “corrected” DNA
Study guide check
Pages 719 – 726 (8 pts)