Development and Inheritance

Development and Inheritance

Muse spring 2440 lecture # 177/15/10

Development

Differentiation Creation of different types of cells required in

development

Occurs through selective changes in genetic activity As development proceeds, some genes are turned off, others

are turned on

Fertilization Also called conception

When development begins

Development

Embryological Development

Occurs during first 2 months after fertilization

Study of these events is called embryology

Fetal Development

Begins at start of ninth week

Continues until birth

Development

Prenatal Development

Embryological and fetal development stages

Postnatal Development

Commences at birth

Continues to maturity when aging process

begins

Fertilization

Fertilization

Fusion of two haploid gametes, each

containing 23 chromosomes

Produces zygote containing 46 chromosomes

Fertilization and the Preparation for Cleavage

Fertilization

Gamete

Provides

Cellular organelles (female)

Inclusions

Nourishment

Genetic programming necessary to support

development of embryo for a week

Fertilization

Fertilization

Occurs in uterine tube within a day after

ovulation

Secondary oocyte travels a few centimeters

Spermatozoa must cover distance between

vagina and ampulla (30 + cm)

Fertilization

Hyaluronidase

Enzyme breaks down bonds between adjacent

follicle cells

Allows spermatozoon to reach oocyte

Acrosin

Is a proteolytic enzyme

Is required to reach oocyte

Fertilization

Acrosomal Caps Release hyaluronidase and acrosin

Penetrate corona radiata, zona pellucida, toward

oocyte surface

Oocyte Activation

Contact and fusion of cell membranes of sperm and

oocyte

Follows fertilization

Oocyte completes meiosis II, becomes mature ovum

Fertilization Polyspermy - would be bad

Fertilization by more than one sperm

Prevented by cortical reaction

Cortical Reaction- initiated upon sperm penetration

Releases enzymes that

Inactivate sperm receptors

Harden zona pellucida

Lift fertilization envelope (vitelline layer)

Fertilization

Female Pronucleus

Nuclear material remaining in ovum after oocyte

activation

Male Pronucleus

Swollen nucleus of spermatozoon

Migrates to center of cell

Fertilization

Amphimixis

Fusion of female pronucleus and male

pronucleus

Moment of conception

Cell becomes a zygote with 46 chromosomes

Fertilization is complete

Fertilization

Cleavage

Series of cell divisions

Produces daughter cells

Differentiation

Involves changes in genetic activity of some cells but

not others

Fertilization

Figure 29–1a Fertilization: An Oocyte and Numerous Sperm at Time of Fertilization.

Fertilization

Figure 29–1b Fertilization and the Preparations for Cleavage.

Fertilization


Fertilization


Fertilization


Gestation

Induction

Cells release chemical substances that affect

differentiation of other embryonic cells

Can control highly complex processes

Gestation

Time spent in prenatal development

Consists of three integrated trimesters, each 3

months long

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

Cleavage

Sequence of cell divisions begins immediately

after fertilization

Zygote becomes a pre-embryo, which

develops into multicellular blastocyst

Ends when blastocyst contacts uterine wall

The First Trimester

Implantation

Begins with attachment of blastocyst to

endometrium of uterus

Sets stage for formation of vital embryonic

structures

Placentation

Occurs as blood vessels form around periphery

of blastocyst and placenta develops

The First Trimester

Placenta

Complex organ permits exchange between maternal and

embryonic circulatory systems

Supports fetus in second and third trimesters

Stops functioning and is ejected from uterus after birth

Embryogenesis

Formation of viable embryo

Establishes foundations for all major organ systems

The First Trimester

Most dangerous period in prenatal life

40% of conceptions produce embryos that

survive past first trimester

The First Trimester

Blastomeres

Identical cells produced by cleavage divisions

Morula

Stage after 3 days of cleavage

Pre-embryo is solid ball of cells resembling

mulberry

Reaches uterus on day 4

The First Trimester

Figure 29–2 Cleavage and Blastocyst Formation.

The First Trimester

Blastocyst

Formed by blastomeres

Hollow ball with an inner cavity

Known as blastocoele

The First Trimester

Trophoblast

Outer layer of cells separate outside world

from blastocoele

Cells responsible for providing nutrients to

developing embryo

The First Trimester

Inner Cell Mass

Clustered at end of blastocyst

Exposed to blastocoele

Insulated from contact with outside

environment by trophoblast

Will later form embryo

The First Trimester

Figure 29–2 Cleavage and Blastocyst Formation.

The First Trimester

Implantation

Occurs 7 days after fertilization

Blastocyst adheres to uterine lining

Trophoblast cells divide rapidly, creating

several layers

Stage of Implantation

The First Trimester

Cellular Trophoblast

Cells closest to interior of blastocyst

Syncytial Trophoblast

Outer layer

Erodes path through uterine epithelium by

secreting hyaluronidase

The First Trimester

Figure 29–3 Stages in Implantation.

The First Trimester

Ectopic Pregnancy

Implantation occurs outside of uterus

Does not produce viable embryo

Can be life threatening

Lacunae

Trophoblastic channels carrying maternal blood

The First Trimester

Villi Extend away from trophoblast into endometrium

Increase in size and complexity until day 21

Amniotic Cavity A fluid-filled chamber

Inner cell mass is organized into an oval sheet two

layers thick Superficial layer faces amniotic cavity

Deeper layer is exposed to fluid contents of blastocoele

The First Trimester

Gastrulation

Formation of third layer of cells

Cells in specific areas of surface move

toward central line

Known as primitive streak

Gastrulation

Week 3 - 15 days in

The First Trimester

Primitive Streak Migrating cells leave surface and move between

two layers

Creates three distinct embryonic layers, or germ layers Ectoderm: consists of the superficial cells that did not

migrate into interior of inner cell mass

Endoderm: consists of cells that face blastocoele

Mesoderm: consists of poorly organized layer of migrating cells between ectoderm and endoderm

The First Trimester

Ectoderm makes me nervous

The First Trimester

Mesoderm is myo favorite

The First Trimester

Endoderm gives me endogestion

The First Trimester

Embryonic Disc

Oval, three-layered sheet

Produced by gastrulation

Will form body of embryo

Rest of blastocyst will be involved in forming

extraembryonic membranes

The First Trimester

Figure 29–4 The Inner Cell Mass and Gastrulation.

The First Trimester

Formation of the Extraembryonic

Membranes

Support embryological and fetal development

Yolk sac

Amnion

Allantois

Chorion

The First Trimester

Yolk Sac Begins as layer of cells spread out around outer edges

of blastocoele to form complete pouch

Important site of blood cell formation

Amnion Combination of mesoderm and ectoderm

Ectodermal layer enlarges and cells spread over inner surface of amniotic cavity

Mesodermal cells create outer layer

Continues to enlarge through development

The First Trimester

Amniotic Fluid

Contained in amniotic cavity

Surrounds and cushions developing embryo or

fetus

Allantois

Sac of endoderm and mesoderm

Base later gives rise to urinary bladder

The First Trimester

Chorion

Combination of mesoderm and trophoblast

Blood vessels develop within mesoderm

Rapid-transit system for nutrients that links

embryo with trophoblast

First step in creation of functional placenta

The First Trimester

Chorionic Villi

In contact with maternal tissues

Create intricate network within endometrium carrying

maternal blood

Body Stalk

Connection between embryo and chorion

Contains distal portions of allantois and blood vessels

that carry blood to and from placenta

The First Trimester

Yolk Stalk

Narrow connection between endoderm of embryo

and yolk sac

Decidua Capsularis

Thin portion of endometrium

No longer participates in nutrient exchange and

chorionic villi in region disappear

The First Trimester

Figure 29–5 Extraembryonic Membranes and Placenta Formation.

The First Trimester


The First Trimester


The First Trimester

Umbilical Cord

Connects fetus and placenta

Contains allantois, placental blood vessels, and

yolk stalk

Blood Flow to Placenta

Through paired umbilical arteries

Returns in single umbilical vein

The First Trimester

Figure 29–6 A Three-Dimensional View of Placental Structure.

The First Trimester

The Endocrine Placenta

Synthesized by syncytial trophoblast, released into

maternal bloodstream

Human chorionic gonadotropin (hCG)

Human placental lactogen (hPL)

Placental prolactin

Relaxin

Progesterone

Estrogens

The First Trimester

Human Chorionic Gonadotropin (hCG)

Appears in maternal bloodstream soon after

implantation made by trophoblast

Provides reliable indication of pregnancy

Pregnancy ends if absent

The First Trimester

Human Chorionic Gonadotropin (hCG)

Helps prepare mammary glands for milk

production

Stimulatory effect on other tissues

comparable to growth hormone (GH)

The First Trimester

Placental Prolactin Helps convert mammary glands to active status

Relaxin A peptide hormone secreted by placenta and corpus

luteum during pregnancy

Increases flexibility of pubic symphysis, permitting pelvis to expand during delivery

Causes dilation of cervix

Suppresses release of oxytocin by hypothalamus and delays labor contractions

The First Trimester

Embryogenesis Body of embryo begins to separate from embryonic

disc

Body of embryo and internal organs start to form

Folding, differential growth of embryonic disc produces

bulge that projects into amniotic cavity Projections are head fold and tail fold

Organogenesis Process of organ formation

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?



The First Trimester

Figure 29–7c The First Trimester.

The First Trimester

Figure 29–7d The First Trimester.

Summary of changes during embryonic and fetal development

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


Figure 29–8a The Second and Third Trimesters: A Four-Month-Old

Fetus As Seen through a Fiber-Optic Endoscope.


Figure 29–8b The Second and Third Trimesters: Head of a Six-Month-

Old Fetus As Seen through Ultrasound.


Figure 29–9c, d Growth of the Uterus and Fetus.


Pregnancy and Maternal Systems Developing fetus is totally dependent on maternal

organ systems for nourishment, respiration, and waste removal

Maternal adaptations include increases in Respiratory rate and tidal volume

Blood volume

Nutrient and vitamin intake

Glomerular filtration rate

Uterus and mammary glands increase in size


Progesterone Released by placenta

Has inhibitory effect on uterine smooth muscle

Prevents extensive, powerful contractions

Opposition to Progesterone Three major factors

Rising estrogen levels

Rising oxytocin levels

Prostaglandin production


Parturition is forcible expulsion of fetus

Contractions

Begin near top of uterus, sweep in wave toward cervix

Strong, occur at regular intervals, increase in force

and frequency

Change position of fetus, move it toward cervical

canal

Labor

Dilation Stage

Begins with onset of true labor

Cervix dilates

Fetus begins to shift toward cervical canal

Highly variable in length, but typically lasts over 8 hours

Frequency of contractions steadily increases

Amniochorionic membrane ruptures (water breaks)

Labor

Figure 29–11 The Stages of Labor.

Labor

Expulsion Stage

Begins as cervix completes dilation

Contractions reach maximum intensity

Continues until fetus has emerged from vagina

Typically less than 2 hours

Delivery

Arrival of newborn infant into outside world

Labor


Labor

Placental Stage

Muscle tension builds in walls of partially empty

uterus

Tears connections between endometrium and

placenta

Ends within an hour of delivery with ejection of

placenta, or afterbirth

Accompanied by a loss of blood

Labor


Labor

Immature Delivery Refers to fetuses born at 25–27 weeks of gestation

Most die despite intensive neonatal care

Survivors have high risk of developmental

abnormalities

Premature Delivery Refers to birth at 28–36 weeks

Newborns have a good chance of surviving and

developing normally

Labor

Forceps Delivery

Needed when fetus faces mother’s pubis

instead of sacrum

Risks to infant and mother are reduced if

forceps are used

Forceps resemble large, curved salad tongs

Used to grasp head of fetus

Labor

Breech Birth

Legs or buttocks of fetus enter vaginal canal first

instead of head

Umbilical cord can become constricted, cutting off

placental blood flow

Cervix may not dilate enough to pass head

Prolongs delivery

Subjects fetus to severe distress and potential injury

Labor

Dizygotic Twins

Also called fraternal twins

Develop when two separate oocytes were

ovulated and subsequently fertilized

Genetic makeup not identical

70% of twins

Labor

Monozygotic Twins

Identical twins

Result either from

Separation of blastomeres early in cleavage

Splitting of inner cell mass before gastrulation

Genetic makeup is identical because both

formed from same pair of gametes

Labor

Rates of Multiple Births

Twins in 1 of every 89 births

Triplets in 1 of every 892 (7921) births

Quadruplets in 1 of every 893 (704,969) births

Octuplets = ridiculous

Postnatal Life

Figure 29–13 Growth and Changes in Body Form and Proportion.

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

Red-green colorblindness

Most common type of color

blindness

Red and green are seen as

same color

Males have only one X

– They express whatever they

inherit from their motherColor blind maleXcY

Normal maleXCY

Color blind femaleXcXc

Normal female

(carrier)XCXc

Normal femaleXCXC

PhenotypeGenotype

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

Multiple-allele inheritance

Some genes have more

than 2 alleles

ABO blood group

IA produces A antigen

IB produces B antigen

i produces neither

A and B are codominant

– Both genes expressed

equally in heterozygote

OIi

ABIA IB

BIB IB or IB i

AIA IA or IA i

Phenotype

(blood type)Genotype

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

Neural tube deficits are more common if the mother lacks

adequate folic acid in the diet – environmental effect

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

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Development and Inheritance