Download First Trimester

Development and Inheritance
Muse s12 2440
lecture # 13
7/19/12
Gestation
 First Trimester
 Period of embryological and early fetal development
 Rudiments of all major organ systems appear
 Second Trimester
 Development of organs and organ systems
 Body shape and proportions change
 By end, fetus looks distinctively human
 Third Trimester
 Rapid fetal growth and deposition of adipose tissue
 Most major organ systems are fully functional
The First Trimester
Figure 29–7a The First Trimester.
The First Trimester
Figure 29–7b The First Trimester.
What will I be when I grow up?
What will I be when I grow up?
What will I be when I grow up?
The First Trimester
Figure 29–7c The First Trimester.
The First Trimester
Figure 29–7d The First Trimester.
The Second and Third Trimesters
 Second Trimester
 Fetus grows faster than surrounding placenta
 Third Trimester
 Most of the organ systems become ready
 Growth rate starts to slow
 Largest weight gain
 Fetus and enlarged uterus displace many of mother’s
abdominal organs
The Second and Third Trimesters
Figure 29–8a The Second and Third Trimesters: A Four-Month-Old
Fetus As Seen through a Fiber-Optic Endoscope.
The Second and Third Trimesters
Figure 29–8b The Second and Third Trimesters: Head of a Six-MonthOld Fetus As Seen through Ultrasound.
The Second and Third Trimesters
Figure 29–9c, d Growth of the Uterus and Fetus.
Inheritance
 Nucleated Somatic Cells
 Carry copies of original 46 chromosomes present in
zygote
 Genotype
 Chromosomes and their component genes
 Contain unique instructions that determine anatomical
and physiological characteristics
 Derived from genotypes of parents
 Phenotype
 Physical expression of genotype
 Anatomical and physiological characteristics
Inheritance
 Homologous Chromosomes
 Members of each pair of chromosomes
 23 pairs carried in every somatic cell
 At amphimixis, one member of each pair is
contributed by spermatozoon, other by ovum
Inheritance
 Autosomal Chromosomes
 22 pairs of homologous chromosomes
 Most affect somatic characteristics
 Each chromosome in pair has same structure
and carries genes that affect same traits
Inheritance
 Sex Chromosomes
 Last pair of chromosomes
 Determine whether individual is genetically male or
female
 Karyotype
 Entire set of chromosomes
 Locus
 Gene’s position on chromosome
Inheritance
Figure 29–14 A Human Karyotype.
Inheritance
 Alleles are various forms of given gene
 Alternate forms determine precise effect of gene on
phenotype
 Homozygous
 Both homologous chromosomes carry same allele of
particular gene
 Simple Inheritance
 Phenotype determined by interactions between single
pair of alleles
Inheritance
 Heterozygous
 Homologous chromosomes carry different allele of
particular gene
 Resulting phenotype depends on nature of interaction
between alleles
 Strict Dominance
 Dominant allele expressed in phenotype, regardless
of conflicting instructions carried by other allele
Inheritance
 Recessive Allele
 Expressed in phenotype only if same allele is present
on both chromosomes of homologous pair
 Incomplete Dominance
 Heterozygous alleles produce unique phenotype
 Codominance
 Exhibits both dominant and recessive phenotypes for
traits
Inheritance
 Penetrance
 Percentage of individuals with particular genotype that
show “expected” phenotype
 Expressivity
 Extent to which particular allele is expressed
 Teratogens
 Factors that result in abnormal development
 Punnett Square
 Simple box diagram used to predict characteristics of
offspring
Mutation - change in normal form of gene
Inheritance
Figure 29–15 Predicting Phenotypic Characters by Using Punnett
Squares.
Inheritance
 Polygenic Inheritance
 Involves interactions among alleles on several genes
 Cannot predict phenotypic characteristics using
Punnett square
 Linked to risks of developing several important adult
disorders
 Suppression
 One gene suppresses other
 Second gene has no effect on phenotype
Inheritance
Inheritance
 Complementary Gene Action
 Dominant alleles on two genes interact to produce
phenotype different from that seen when one gene
contains recessive alleles
 Sources of Individual Variation
 During meiosis, maternal and paternal chromosomes
are randomly distributed
 Each gamete has unique combination of maternal
and paternal chromosomes
Inheritance
 Genetic Recombination
 During meiosis, various changes can occur in
chromosome structure, producing gametes with
chromosomes that differ from those of each parent
 Greatly increases range of possible variation among
gametes
 Can complicate tracing of inheritance of genetic
disorders
Inheritance
 Crossing Over
 Parts of chromosomes become rearranged during
synapsis
 When tetrads form, adjacent chromatids may overlap
 Translocation
 Reshuffling process
 Chromatids may break, overlapping segments trade
places
Inheritance
Figure 29–17 Crossing Over and Translocation.
Inheritance
 Genomic Imprinting
 During recombination, portions of
chromosomes may break away and be
deleted
 Effects depend on whether abnormal gamete
is produced through oogenesis or
spermatogenesis
Inheritance
 Chromosomal Abnormalities
 Damaged, broken, missing, or extra copies of
chromosomes
 Few survive to full term
 Produce variety of serious clinical conditions
 Humans are poorly tolerant of changes in gene copy
number (to few or too many = lethal or bad news)
 Mutation
 Changes in nucleotide sequence of allele
Inheritance
 Spontaneous Mutations
 Result of random errors in DNA replication
 Errors relatively common, but in most cases error is
detected and repaired by enzymes in nucleus
 Errors that go undetected and unrepaired have
potential to change phenotype
 Can produce gametes that contain abnormal alleles
Inheritance
 Carriers
 Individuals who are heterozygous for
abnormal allele but do not show effects of
mutation
Inheritance
 Sex Chromosomes
 X Chromosome
 Considerably larger
 Have more genes than do Y chromosomes
 Carried by all oocytes
 Y Chromosome
 Includes dominant alleles specifying that the individual will be
male
 Not present in females
Autosomes, sex chromosomes and sex
determination
 Karyotype shows 46
chromosomes arranged in
pairs by size and centromere
position
 22 pairs are autosomes –
same appearance in males
and females
 23rd pair are sex
chromosomes
 XX = female
 XY = male
Inheritance
 Sperm
 Carry either X or Y chromosome
 Because males have one of each, can pass
along either
50% chance of each
Inheritance
 X-Linked
 Genes that affect somatic structures
 Carried by X chromosome
 Inheritance does not follow pattern of alleles on
autosomal chromosomes
Sex determination
 Males produce sperm
carrying an X or Y
 Females only produce
eggs carrying an X
 Individual’s sex
determined by father’s
sperm carrying X or Y
 Male and female embryos
develop identically until
about 7 weeks
 Y initiates male pattern
of development
 SRY on Y chromosome
 Absence of Y
determines female
pattern of development
Inheritance
Figure 29–18 Inheritance of an X-Linked Trait
Inheritance of red-green color blindness
Sex-linked inheritance
 Genes for these traits
on the X but not the Y
Genotype
XCXC
 Red-green
colorblindness
 Most common type of
XCXc
XcXc
color blindness
 Red and green are
seen as same color
 Males have only one X
– They express
XCY
XcY
Phenotype
Normal
Normal
female
female
Color
blind
(carrier)
female
Normal male
Color blind
male
Inheritance
 Human Genome Project
 Goal was to transcribe entire human genome
 Has mapped thousands of human genes
 Genome
 Full complement of genetic material
Inheritance
Figure 29–19 A Map of Human Chromosomes.
Inheritance
 Passage of hereditary traits from one generation
to the next
 Genotype and phenotype
 Nuclei of all human cells except gametes contain 23
pairs of chromosomes – diploid or 2n
 One chromosome from each pair came from father,
other member from mother
 Each chromosome contains homologous genes for
same traits
 Allele – alternative forms of a gene that code for the
same trait
 Mutation – permanent heritable change in allele that
produces a different variant
Inheritance
Phenylketonuria or PKU example
 Unable to manufacture enzyme phenylalanine
hydroxylase
 Allele for function enzyme = P
 Allele that fails to produce functional enzyme = p
 Punnet square show possible combinations of alleles
between 2 parents
 Genotype – different combinations of genes
 Phenotype – expression of genetic makeup
 PP – homozygous dominant – normal phenotype
 Pp – heterozygous – normal phenotype
– 1 dominant allele codes for enough enzyme
– Can pass recessive allele on to offspring – carrier
 pp - homozygous recessive – PKU
– 2 recessive alleles make no functional enzyme
Inheritance
 Alleles that code for normal traits are not always
dominant
 Huntington disease caused by dominant allele
 Both homozygous dominant and heterozygous
individuals get HD
 Nondisjunction
 Error in cell division resulting in abnormal number of
chromosomes
 Aneuploid – chromosomes added or missing
 Monosomic cell missing 1 chromosome (2n-1)
 Trisomic cell has additional chromosome (2n +1)
– Down Syndrome – trisomy 21 – 3 21st chromosomes
Variations of Dominant-recessive
inheritance
 Simple dominance-recessive
 Just described where dominant allele covers effect
of recessive allele
 Incomplete dominance
 Neither allele dominant over other
 Heterozygote has intermediate phenotype
 Sickle-cell disease
Sickle-cell disease
 Sickle-cell disease
 HbAHbA – normal
hemoglobin
 HbSHbS – sickle-cell
disease
 HbAHbS – ½ normal and
½ abnormal
hemoglobin
 Minor problems, are
carriers for disease
Incomplete Dominance
 Heterozygous individuals have an
intermediate phenotype
 Example: Sickling gene
 SS = normal Hb is made
 Ss = sickle-cell trait (both aberrant and normal
Hb are made); can suffer a sickle-cell crisis
under prolonged reduction in blood O2)
 ss = sickle-cell anemia (only aberrant Hb is
made; more susceptible to sickle-cell crisis)
(b) Sickled erythrocyte results from
a single amino acid change in the
beta chain of hemoglobin.
1
2
3
4
5
6
7
146
Figure 17.8b
Multiple-allele inheritance
Phenotype
 Some genes have
more than 2 alleles
 ABO blood group
 IA produces A antigen
 IB produces B antigen
Genotype
(blood
IA IA or IA i
type)
A
IB IB or IB i
B
IA IB
AB
Ii
O
 i produces neither
 A and B are codominant
– Both genes
expressed equally in
heterozygote
Blood type inheritance
Complex inheritance
 Polygenic inheritance – most inherited traits
not controlled by one gene
 Complex inheritance – combined effects of
many genes and environmental factors
 Skin color, hair color, height, metabolism rate, body
build
 Even if a person inherits several genes for tallness,
full height can only be reached with adequate
nutrition
Skin color is a complex trait
 Depends on
environmental
conditions like sun
exposure and nutrition
and several genes
 Additive effects of 3
genes plus
environmental affect
produces actual skin
color
Polygene 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
Hair color genes
Eye color genes
Homologous chromosomes synapse during
prophase of meiosis I. Each chromosome consists
of two sister chromatids.
H Allele for brown hair
h Allele for blond hair
E Allele for brown eyes
e Allele for blue eyes
Paternal chromosome
Homologous pair
Maternal chromosome
Figure 29.3 (1 of 4)
Chiasma
One chromatid segment exchanges positions
with a homologous chromatid segment—in other
words, crossing over occurs, forming a chiasma.
H Allele for brown hair
E
Allele for brown eyes
h
e
Allele for blue eyes
Allele for blond hair
Paternal chromosome
Homologous pair
Maternal chromosome
Figure 29.3 (2 of 4)
The chromatids forming the chiasma break, and the
broken-off ends join their corresponding homologues.
H Allele for brown hair
E
Allele for brown eyes
h
e
Allele for blue eyes
Allele for blond hair
Paternal chromosome
Homologous pair
Maternal chromosome
Figure 29.3 (3 of 4)
Random Fertilization
 A single egg is fertilized by a single sperm
in a random manner
 Because of independent assortment and
random fertilization, an offspring
represents one out of 72 trillion (8.5 million
 8.5 million) zygote possibilities
Gamete 1
Gamete 2
Gamete 3
Gamete 4
At the conclusion of meiosis, each haploid gamete
has one of the four chromosomes shown. Two of the
chromosomes are recombinant (they carry new
combinations of genes).
H Allele for brown hair
E
Allele for brown eyes
h
e
Allele for blue eyes
Allele for blond hair
Paternal chromosome
Homologous pair
Maternal chromosome
Figure 29.3 (4 of 4)
Environmental Factors in Gene
Expression
 Phenocopies: environmentally produced
phenotypes that mimic conditions caused
by genetic mutations
 Environmental factors can influence
genetic expression after birth
 Poor nutrition can affect brain growth, body
development, and height
 Childhood hormonal deficits can lead to
abnormal skeletal growth and proportions
Nontraditional Inheritance
 Influences due to
 Genes of small RNAs
 Epigenetic marks (chemical groups attached
to DNA)
 Extranuclear (mitochondrial) inheritance
Small RNAs
 MicroRNAs (miRNAs) and short interfering
RNAs (siRNAs)
 Act directly on DNA, other RNAs, or proteins
 Inactivate transposons, genes that tend to
replicate themselves and disable or hyperactivate
other genes
 Control timing of apoptosis during development
 In future, RNA-interfering drugs may treat
Epigenetic Marks
 Genomic imprinting tags genes as
maternal or paternal and is essential for
normal development
 Allows the embryo to express only the
mother’s gene or the father’s gene
Epigenetic Marks
 Information stored in the proteins and
chemical groups bound to DNA
 Determine whether DNA is available for
transcription or silenced
 May predispose a cell to cancer or other
devastating illness
Epigenetic Marks
 The same allele can have different effects
depending on which parent it comes from
 For example, deletions in chromosome 15
result in
 Prader-Willi syndrome if inherited from the
father
 Angelman syndrome if inherited from the
Extranuclear (Mitochondrial)
Inheritance
 Some genes (37) are in the mitochondrial
DNA (mtDNA)
 Transmitted by the mother in the
cytoplasm of the egg
 Errors in mtDNA are linked to rare
disorders: muscle disorders and
neurological problems, possibly
Sins of the father? Epigenetics at work
Scientists at Australia’s University of New South Wales fed healthy,
svelte, male rats a high-fat diet (43 percent of calories from fat—a
typical American diet). Not surprisingly, the rats put on weight and fat,
and developed insulin resistance and glucose intolerance—basically,
type 2 diabetes, the scientists reported last month in Nature. None of
that was surprising. What made the scientists take notice was the
daughters these rats sired: although their mothers were of normal
weight and ate a healthy diet while pregnant, daughters of the highfat-diet dads developed insulin resistance and glucose resistance as
adults—even though they never ate a high-fat diet themselves.
Mothers’ diet while pregnant affects their children’s health as adults
because of how nutrients and toxic compounds pass through the
placenta. But fathers have no contact with their daughters except
through the sperm that created them. These rat fathers were not
genetically diabetic. The conclusion is therefore inescapable: the
fathers’ high-fat diet altered their sperm in a way that induced adultonset disease in their daughters.