Transcript Heredity Part 2 - Pima Community College

Heredity
Part 2
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 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
• 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 29.3.1
Crossover and Genetic
Recombination
Figure 29.3.2
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
Dominant-Recessive Inheritance
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 Tay-Sachs disease
• Carriers – heterozygotes who do not express a
trait but can pass it on to their offspring
Incomplete Dominance
• Heterozygous individuals have a phenotype
intermediate between homozygous dominant and
homozygous recessive
• 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)
Incomplete dominance in snapdragon
color
Incomplete dominance in human hypercholesterolemia
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(codominant)
Table 29.2
Multiple alleles for the ABO blood groups
Sex-Linked Inheritance
• Inherited traits determined by genes on the
sex chromosomes
• X chromosomes bear over 2500 genes; Y
chromosomes carry about 15 genes
• 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,
metabolic rate,inteligence.
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
A model for polygenic inheritance of skin color
Sickle cell disease, multiple effects of a single human gene
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
Thank you
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 wellbeing 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
• 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
• What makes a dominant gene dominant?
• Usually, the masking effect is done by virtue of
the fact that the recessive gene has a loss of
some function that the dominant gene has. For
example, in the case of ABO blood types, the O
type is recessive because it does not produce any
antigens or antibodies, whereas A and B types
(which are co-dominant) do. Or, in the case of
eye color, there is a complete loss of pigment in
blue-eyed people, therefore to express the
phenotype, both copies of the gene (after all,
humans are diploid) must have that same loss of
• What makes an allele recessive (the opposite
of dominant) is if it DOESN'T code for
something, or if it codes for an ALTERED
something. In the case of blue eyes, there is
no gene that says "make melanin!", so your
eyes don't get dark, they stay light blue or
gray. In the case of green eyes you have a
gene that says "make some really weird
melanin!" and so your eyes get a green
pigment instead of a normal dark pigment.