Lesson 11

Day 0 of embryonic development - zygote

formed from sperm and egg nuclei fusion (fertilization) - divides without growth to form small cells called blastomeres

Day 3 of embryonic development - morula

when zygotic division reaches 32 cell

Day 4-6 of embryonic development - blastocyst

hollowed out morula - has inner cell mass (forms the embryonic body) and outer trophoblast layer (forms the chorion and fetal part of placenta)

Implantation

blastocyst attaches to uterine wall

Day 7 of embryonic development

blastocyst implants with inner cell mass facing uterine wall - trophoblast secrete enzyme to erode uterine mucosa to expose vascular network in submucosa - chorionic villi from trophoblast and uterine tissue will form placenta - as implanted blastocyte grows, 3 embryonic layers form (amnion, yolk sac, allantois)

Amnion

encases embryo in fluid-filled chamber - protects against trauma, temp and adhesion from rapid embryonic growth

Yolk sac

forms embryo’s first blood cells and primordial germ cells here migrate to embryo body to form germ cells in gonadal tissue

Allantois

helps form umbilical cord

Day 14 of embryonic development

uterine mucosa envelops embryo (decidua capsularis) - uterine wall beneath inner cell mass (decidua basalis) will form part of placenta - embryo in gastrula stage - 3 primary germ layers present and begin to differentiate for the next 6 weeks (endoderm, ectoderm, mesoderm)

Endoderm

forms mucosa of digestive and respiratory system and their associated glands

Ectoderm

forms skin and nervous system

Mesoderm

forms everything in between (skeleton, skeletal muscles, walls of digestive organs, urinary and circulatory system, etc.)

Week 9 of embryonic development

embryo is referred to as a fetus - major focus of development at this point is for fetal growth and tissue/organ specialization/growth

Genetics

all somatic (body) cells contain 46 chromosomes (diploid, 2n) - egg and sperm contain 23 chromosomes (haploid, n) - their fusion yields 23 pairs of homologous chromosomes (46)

Gene

unit/area on DNA (or sometimes RNA) that contains hereditary information

Alleles

genetic information coding for the same traits found on each pair of homologous chromosomes

Homozygous trait

both alleles express the same information

Example for genetics

a gene for eye color can have 3 different alleles (blue, brown, green) - a person that is homozygous for blue eyes has 2 blue alleles, one on each chromosome

Heterozygous trait

one allele is dominant, while the other allele is recessive (is masked by the expression of the dominant allele)

Dominant gene

requires only 1 copy of the gene for expression

Recessive gene

requires both alleles to contain the gene for expression

Genotype

an individual’s genetic makeup (whether he/she is homozygous/heterozygous for various alleles)

Phenotype

the physical appearance of the genes

Example for heterozygous trait

in reference to eye color, brown is dominant to blue - if a person has one blue allele and one brown allele, they have a heterozygous genotype - however, their phenotype is brown eyes

4 main types of inheritance

dominant-recessive inheritance, incomplete dominance, sex-linked inheritance, and co-dominance

Most traits are…

quantitative - alleles for that trait will be co-expressed and have an additive effect

Some traits and many medical disorders can be associated with…

only one gene

Dominant-recessive inheritance

Mendelian genetics - dominant alleles are expressed, recessive alleles are expressed only when they are homozygous, genotype and phenotype determined by using Punnett square

Albinism

inability to produce melanin (recessive trait)

Skin, hair, eyes with melanin

dominant traits

Achondroplasia

dwarfism (dominant traits)

Average height and limb proportions

recessive traits

Heterochromia

dominant trait

Both eyes the same color

recessive trait

Dominant alleles of a gene do not mean that that allele is…

the most common one found in humans - achondroplasia and heterochromia are very rare in humans, despite the fact they are the dominant trait

Incomplete dominance

heterozygous alleles yield phenotype different than from homozygous dominant phenotype

Many genetic medical disorders that appear to be dominant-recessive are…

actually better described as being incomplete dominance, such as the disease sickle cell trait

A person with recessive disorder sickle cell anemia has…

two of the recessive alleles which results in irregularly shaped red blood cells that are not as efficient at carrying oxygen and also clump together blocking small blood vessels

A person with one recessive allele for the sickle cell trait makes…

exactly half normal cells and half sickle cells, indicating both the dominant (normal) alleles and the recessive (abnormal) allele are begin expressed at the same time

A person heterozygous for the sickle cell alleles…

does not have sickle cell anemia, but has sickle cell trait

People who are heterozygous for sickle cell trait usually…

do not have any symptoms of the disease and live perfectly healthy lives - they are immune to malaria, which is why this allele is common in places where malaria is prevalent

Sex-linked inheritance

inheritance of genes from the X (23rd) chromosome - the sex chromosome contain genes that determine color vision, blood clotting, baldness, etc.

Autosomes

the other 22 pairs of chromosomes

X chromosome is unique in that it is…

the only chromosome of which you only require one copy - females are expressed as XX, males as XY

Y chromosome has…

very few genes, most of which involve the production of testosterone during fetal development and adolescent maturity (puberty) in males

Like other chromosomal disorders, people can end up with…

multiple or fewer copies of sex chromosomes - everyone must have at least one X chromosome

-Y in an embryo results in a…

failed pregnancy (miscarriage)

Having extra copies of X in females and extra Ys in males does…

not appear to cause any physiological problems for people

Turners syndrome

X-

Klinefelters syndrome

XXY

Miscarriage/failed pregnancy

-Y