Transcript Genetics, Heredity, and Biotechnology

Genetics, Heredity, and
Biotechnology
Unit 5
Genetic Variation
• In asexual reproduction, there is zero genetic
variation; offspring are exact genetic copies of the
parent.
• In sexual reproduction, there is much genetic
variation but only before fertilization. The number
of possible chromosome combinations in the
gametes is 2n, where n = the haploid chromosome
number and 2 is the number of chromosomes in a
homologous pair. So, when n = 2, there are 4
possibilities; when n = 3, there are 8. Since
humans have a haploid number of 23 (223),
8,388,608 combinations are possible.
Fertilization
• During fertilization, the two haploid
gametes (sperm and egg) fuse to form a new
diploid cell called a zygote. The zygote
starts as a single cell with a set of 2n
chromosomes, with each parent contributing
one chromosome to each pair. To grow in
size, the zygote begins mitosis and becomes
an embryo. After 8 weeks of development,
the embryo becomes a fetus until birth.
Zygote vs. Embryo
Cell Differentiation
• Stem Cells are the group of cells produced in the
very early stages of embryonic growth; they are
similar to the original zygote.
• When the embryo reaches 20 – 150 cells in size,
this group begins to produce specialized cells that
later become tissues.
• Stem cells can become any type of cell. This
happens because genes within the cell can be
“turned on” or “turned off” at specific times.
stem cells (cont’d)
• Every cell has the same genetic information that
was present in the original zygote. Thus, cell
differentiation occurs by the selective activation or
inactivation of only some of these genes.
• Some cells become skin cells, while others might
become liver cells, but both cells still contain
genes for every other cell type within the
organism.
• In summary, every stem cell has the capacity to
become any type of cell found in that organism.
A Human Stem Cell…
Stem Cell Research
• Stem cells are “pre-cells”. They can become
any type of cell with the proper instructions
from DNA.
• Potential for using stem cells to help cure
many human diseases or injuries is great;
they could help people with nerve damage,
Alzheimer’s, Parkinson’s, or arthritis.
stem cell research (cont’d)…
• Stem cells can be harvested from adult bone
marrow, umbilical cord blood after delivery, or
from human embryos.
• Harvesting from embryos usually kills the
embryo.
• There are many ethical, political, and spiritual
issues related to stem cell research. President Bush
limited federal funds available for such research
while President Obama supports using tax dollars
to fund research.
What do YOU think?
Genetics
From Mendel
to the 21st Century
Principle of Dominance
• Mendel found that some forms of a gene or
trait are dominant over other traits; these
“weaker” traits are called recessive.
• Dominant traits mask, or hide, the presence
of recessive traits.
Principle of Segregation
• Mendel discovered that when forming sex
cells, the paired alleles separate so that each
egg or sperm only carries one form of the
allele.
• The two forms of the allele (one from each
parent) come together again during
fertilization.
Principle of Independent
Assortment
• Mendel noticed this when doing dihybrid
crosses.
• Says that each pair of alleles segregates
independently during the formation of the
egg or sperm.
• Leads to the 9:3:3:1 ratio of phenotypes in
dihybrid crosses.
Modes of Inheritance
1. Sex-Linked Traits
• Sex chromosomes determine the sex of an
organism.
• Males have the genotype XY; females XX.
• If a recessive trait is on the X chromosome
it likely won’t be in the females phenotype.
• Females that have a recessive gene on one
X chromosome are carriers for that trait.
(ex. color blindness, baldness)
2. Incomplete Dominance
• When one trait is not completely dominant
over the other.
• A blending, or mixing, of the two traits.
• If you cross a red flower and a white flower
and get a pink flower, the traits mixed and
neither was dominant.
• Bi-racial children are a classic example of
incomplete dominance.
3. Co-Dominance
• When both traits contribute to the
appearance of the offspring.
• Both alleles are completely expressed.
4. Multiple Alleles and Polygenic
Traits
• Certain traits like blood type, hair color, and eye
color, are determined by two genes from each parent
for every trait (multiple alleles).
• Polygenic Traits are the result of interaction of
multiple genes. Hypertension is genetically linked,
but one gene doesn’t cause it. Weight, ability to
process fats/cholesterol, ability to move salts through
bloodstream (all controlled by genes) combined with
lifestyle (addictive behaviors lie in genes). So…high
blood pressure results from a combination of genes,
or polygenic traits.
Genetic Pedigrees
• A graphical chart used to identify lineage of
individuals.
• Similar to a family tree except it shows inheritance
of genetic disorders within families.
• Used when breeding animals such as dogs or race
horses.
• Males represented with a square and females with
a circle; sufferers of the disorder are shaded.
Pedigree Graph
Mutations
• Mistakes in DNA replication.
• Some are harmful; some are beneficial.
• Play a significant role in creating diversity
of life on Earth today.
• There are two groups of mutations – gene
mutations and chromosomal mutations.
Gene vs. Chromosomal Mutations
Gene Mutations
• Mistakes that affect
individual genes on a
chromosome.
• One base substitutes for
another on a DNA strand
and leads to the wrong
protein being made; this
affects one or more
functions within the
organism.
Chromosomal Mutations
• Mistakes that affect the
whole chromosome.
• There are four types of
chromosomal mutations:
duplication, deletion,
inversion, and translocation.
• ALL MUTATIONS ARE
CAUSED BY
MUTAGENS.
Genetic Diseases
• Most mutations are caught and fixed before
any damage is done to the organism. Many
that aren’t caught have no effect on the
organism. Some, though, have negative
effects on the organism and/or the
organism’s offspring.
• Many diseases that are passed from parent
to offspring are a result of mutations that
were not corrected.
Genetic Disorders and Diseases
• Sickle-cell Anemia. Result of two recessive genes
that causes the red blood cells to take on a sickle
shape; keeps O2 and nutrients from reaching
organs; leads to frequent infections and damage to
major organs.
• Hemophilia. A sex-linked (comes from the
mother) recessive condition involving failure of
blood to clot properly. Mom will pass on the
disease to half her sons, and the trait to half her
daughters.
Genetic Diseases (cont’d)
• Down’s Syndrome. When a person has
inherited an extra copy of chromosome 21,
meaning they have a total of 47.
• Phenylketonuria (PKU). Inherited disease
resulting from a missing enzyme; causes the
amino acid phenylalanine to build up in an
infant’s blood causing brain damage.
Still more genetic diseases…
• Cystic Fibrosis. Causes increased mucus in
the lungs, lung infections, lung disease,
poor growth rate, short life, and infertility.
There is no treatment for CF.
• Tay-Sachs. Caused by a mutation to a gene
found on chromosome 15. Results in a build
up of fatty acids in nerve tissues; symptoms
include blindness, deafness, difficulty
swallowing, and death before age 3.