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Genetics and
Human
Development
Human
Karotype
.
GENETIC FOUNDATIONS OF
DEVELOPMENT
• Each of us carries a “genetic code” that
we inherited from our parents
• A fertilized human egg carries this human
code
• fertilized human egg cannot grow into
any other species
Human Genetics
• Each of us began life as a single cell
weighing about one twenty-millionth of
an ounce
• This cell contained our entire genetic
code
Chromosomes
• Nucleus of each human cell contains
chromosomes
• thread-like structures made up of
deoxyribonucleic acid, or DNA.
• DNA is a complex molecule that has a
double helix shape, like a spiral staircase,
and contains genetic information
DNA Molecule
http://videos.howstuffworks.com/hsw/21318-human-genome-the-dna-molecule-and-chromosomes-video.htm
Genes
• Genes - units of hereditary information
– short segments of DNA
• Genes direct cells to reproduce
themselves and to assemble proteins
• Proteins
– the building blocks of cells
– the regulators that direct the body’s
processes
(Mader, 2008)
Human Genome Project
• Preliminary map of the human genome
• the complete set of developmental
instructions for creating proteins that
initiate the making of a human organism
• Each human has 20,000 – 30,000 genes
Enduring Nature of Genes
• Three processes by which cells split
and are formed
–Mitosis
–Meiosis
–Fertilization.
Mitosis
• All body cells, except the sperm and egg, have
46 chromosomes arranged in 23 pairs
• These cells reproduce by a process called
mitosis
• During mitosis, the cell’s nucleus—including
the chromosomes—duplicates itself and the
cell divides
• Two new cells are formed, each containing the
same DNA as the original cell.
Meiosis
• Sex cell division is different type of cell division
• Eggs and sperm (or gametes) are formed in
meiosis
• A cell of the testes (in men) or ovaries (in women)
duplicates its chromosomes but then divides
twice, thus forming four cells
• Each of these has only half of the genetic material
of the parent cell.
• By the end of meiosis, each egg or sperm has 23
unpaired chromosomes.
•
http://videos.howstuffworks.com/hsw/6015-genetics-fertilization-and-meiosis-video.htm
Fertilization
• During fertilization, an egg and a sperm
fuse to create a single cell, called a zygote
• In the zygote, the 23 unpaired
chromosomes from the egg and sperm
combine to form one set of 23 paired
chromosomes
• One chromosome of each pair is from the
mother’s egg and the other from the
father’s sperm.
• Each parent contributes half of the
offspring’s genetic material.
•
http://videos.howstuffworks.com/hsw/11519-the-reproductive-system-process-of-conception-video.htm
The 23rd Pair of Chromosomes
• Ordinarily, in females the 23rd pair
consists of two chromosomes called X
chromosomes
• In males the 23rd pair consists of an X
and a Y chromosome
• The presence of a Y chromosome is what
makes an individual male
Sources of Variability
• Combining genes of two parents
increases genetic variability in the
population
• chromosomes in the zygote are not exact
copies of the parents
• Another source of variability comes from
DNA
– a mutated gene, which is a permanently
altered segment of DNA
Genotype and Phenotype
• Genotype -- all of a person’s genetic
material
• Phenotype -- observable characteristics
• Phenotypes include
– physical characteristics (such as height,
weight, and hair color)
– psychological characteristics (such as
personality and intelligence)
Genetic Principles
•
•
•
•
Dominant-recessive genes
Sex-linked genes
Genetic imprinting
Polygenically determined characteristics
Dominant-Recessive Genes Principle
• Dominant -- one gene of a pair always
exerts its effects
• Recessive gene – exerts its influence only
if the two genes of a pair are both
recessive
– may be overridden by a dominant gene
– May be carried from generation to
generation but not expressed in phenotype
Sex-Linked Genes
• X-linked inheritance -- mutated gene is carried
on the X chromosome
– very different implications for males than females
(Turner, 2006)
– males have no “backup” copy to counter the
harmful gene
– females have a second X chromosome, which is
likely to be unchanged
• Females who have one changed copy of the X gene are
known as “carriers”
Polygenic Inheritance
• Genetic transmission is usually quite
complex
– Few characteristics reflect the influence of
only a single gene or pair of genes
• polygenically determined characteristics - determined by the interaction of many
different genes
Chromosome Abnormalities
• Down syndrome -- caused by the
presence of an extra copy of
chromosome 21
– once in every 700 live births
– Women aged 16 - 34 are less likely to give
birth to a child with Down syndrome
• Sex-Linked Chromosome Abnormalities
Sex-Linked Chromosome Abnormalities
• Klinefelter syndrome – extra X
chromosome
• Fragile X syndrome -- results from an
abnormality in the X chromosome
– X chromosome becomes constricted and
often breaks
• Turner syndrome -- females have either
an X chromosome missing or part of one
X chromosome is deleted
• XYY syndrome – male has extra Y
Gene-Linked Abnormalities
• Phenylketonuria (PKU) – individual
cannot properly metabolize
phenylalanine, an amino acid
– once in every 10,000 to 20,000 live births
– left untreated, phenylalanine builds up in
the child, producing mental retardation and
hyperactivity
• Sickle-cell anemia -- genetic disorder that
impairs the body’s red blood cells
– most often in African Americans
– red blood cell becomes a hook-shaped
“sickle” that cannot carry oxygen properly
– 1 in 400 African American babies is affected
• 1 in 10 African Americans is a carrier
• 1 in 20 Latin Americans is a carrier
• Other diseases that result from genetic
abnormalities
– cystic fibrosis
– diabetes
– hemophilia
– Huntington disease
– spina bifida
– Tay-Sachs disease
Twins
• Monozygotic Twins – identical
• Dizygotic Twins - no more alike than any other
siblings – they simply gestated together
• http://videos.howstuffworks.com/health/frate
rnal-twins-videos-playlist.htm#video-8197
• Conjoined twins
The Hensel Twins
• http://www.youtube.com/watch?v=7ZUzi0RhBpM