Chapter 30
Heredity
Genetics
The
study of the mechanism of heredity
Nuclei
of all human cells (except gametes) contain 46 chromosomes
Sex
chromosomes determine the genetic sex (XX = female, XY = male)
Karyotype
the diploid chromosomal complement displayed in homologous pairs
Genome
genetic (DNA) makeup represents two sets of genetic instructions one
maternal and the other paternal
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
Dominant
an allele masks or suppresses the expression of its partner
Recessive
the allele that is masked or suppressed
Genotype and Phenotype
Genotype
the genetic makeup
Phenotype
the way ones 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
The
number of different types of gametes can be calculated by this formula:
2n, where n
is the number of homologous pairs
In
a mans testes, the number of gamete types that can be produced based on
independent assortment is 223, which equals 8.5 million
possibilities
Crossover
Homologous
chromosomes synapse in meiosis I
One
chromosome segment exchanges positions with its homologous maternal counterpart
Crossing
over occurs, forming a chiasma
Genetic
information is exchanged between homologous chromosomes
Two
recombinant chromosomes are formed
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
Example:
probability of different offspring from mating two heterozygous parents
T = tongue roller and t =
cannot roll tongue
Examples
of dominant disorders: achondroplasia (type of dwarfism) and Huntingtons
disease
Examples
of recessive conditions: albinism, cystic fibrosis, and Tay-Sachs disease
Carriers
heterozygotes who do not express a trait but can pass it own to their
offspring
Incomplete Dominance
Heterozygous
individuals have a phenotype intermediate between homozygous dominant and
homozygous recessive
Sickling
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
Sex-Linked Inheritance
Inherited
traits determined by genes on the sex chromosomes
X
chromosomes bear over 2500 genes; Y carries about 15
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
Environmental Influence on Gene Expression
Phenocopies
environmentally produced phenotypes that mimic mutations
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
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
Genetic Screening, Counseling, and Therapy
Newborn
infants are screened for a number of genetic disorders: congenital hip
dysplasia, imperforate anus, and PKU
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
for the heterozygote state for a given trait
Two
major avenues are used to identify carriers: pedigrees and blood tests
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
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
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
patients cells can be directly injected with corrected DNA