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Modern Genetics
Gene Linkage: If the genes of two different traits
(non-allelic) are located on the same chromosome
pair (homologous chromosomes) they are said to
be linked, and are therefore inherited together.
Crossing Over: In Meiosis 1(in Prophase) the
chromatids of a pair of homologous
chromosomes often twist around each other.
They can then break, exchange segments, and
rejoin. This is called Crossing Over and it
produces important variation in offspring, during
sexual reproduction.
Mrs. Degl
Mrs. Degl
Sex Determination
•In human body cells there are 22 pairs of autosomes
(a chromosome that is not a sex chromosome) and 1
pair of sex chromosomes
•The sex chromosomes are the X and Y chromosomes
•Females have 2 X chromosomes (XX)
•Males have 1 X and 1 Y chromosome (XY)
•During Meiosis the sex chromosomes (like all of the
rest of the chromosomes) separate so each gamete is
either X or Y
•All females gametes (eggs) will be X
•Male gametes (sperm) can be either X or Y
•At fertilization the sex of the baby is determined when
either the X or Y sperm enters the egg (which is X),
resulting in either a girl (XX) or boy (XY)
Mrs. Degl
Sex-linked traits
•T.H. Morgan’s work with Drosophila (fruit flies) found that
some rare recessive traits appear more often in males than
in females.
•He concluded that these traits must be on the X
chromosome. These are called sex-linked genes.
•Recessive traits that are sex-linked occur more frequently
in males because in females there is usually a normal
dominant allele on the other X chromosome to overpower
the recessive X chromosome, labeled (X’).
•In males there is not another X chromosome to overpower
the recessive one (males are XY), so the recessive
phenotype takes over, producing X’Y.
Mrs. Degl
Human Sex-linked Disorders
Hemophilia- disease in which the blood does
not clot properly
Colorblindness- inability to see certain colors,
most commonly red and green
• Both of these disorders are more common in males
than in females because a female will not show the
disorder as long as she has one normal gene.
Females who are heterozygous for a sex-linked
trait are said to be carriers for that trait.
• Females with one normal dominant and one
recessive gene are called carriers and have no
symptoms, but can pass it along to offspring.
Mrs. Degl
A punnett square
for the X-linked
characteristic
color blindness in
humans.
Female carrier XCXc
crossed with normal
male XCY:
Mrs. Degl
Genetic Mutations
• Changes in genetic material are called mutations
• If a mutation occurs in the sex cell, it may be
transmitted to the offspring
• Mutations occurring in body cells may be passed
on to new cells of the individual due to mitosis,
but will not be transmitted to the offspring by
sexual reproduction
There are two groups of mutations:
1. Chromosomal Alterations
2. Gene Mutations
Mrs. Degl
1. Chromosomal Alterations
These involve changes in the structure or
number of chromosomes.
One example is Nondisjunction
• One or more pairs of
homologous chromosomes
fail to separate.
• This results in gametes with
more (or less) than their
normal haploid
chromosome number.
Mrs. Degl
Down Syndrome is caused by the presence of an
extra chromosome (# 21).
•It is characterized by mental retardation, an abnormal
pattern of palm creases, a flat face, sparse, straight hair, and
short stature.
•People with Down syndrome have a high risk of having
cardiac anomalies, leukemia, cataracts, and digestive
blockages.
•Life expectancy of Down syndrome individuals is in the
middle teens but some live much longer.
•The gene responsible for Alzheimer’s is on chromosome 21.
Down’s are at increased risk for developing Alzheimer’s.
•Down Syndrome is associated with maternal age.
Mrs. Degl
Polyploidy- sometimes all 22 chromosomal pairs
fail to separate. The resulting 2n gamete fuses with
the normal n gamete, producing a 3n zygote. This is
common in plants but rare in humans
Polyploidy in Plants
Polyploidy is a major evolutionary mechanism in plants.
Approximately 47% of all flowering plants are polyploid.
Some examples of polyploid plant species are corn, wheat,
cotton, sugarcane, apples, bananas, watermelons, and
many flowers
Polyploidy in Humans
Polyploids have defects in nearly all organs.
Most die as embryos or fetuses. Occasionally an infant
survives for a few days.
Mrs. Degl
Changes in Chromosome Structure
Translocation: when a segment of one chromosome
breaks off and reattaches to a non-homologous
chromosome.
Addition: when a segment breaks off one chromosome
and reattaches to the homologous chromosome.
Inversion: when a segment breaks off and reattaches in
reverse on the same chromosome (it is upside-down).
Deletion: when a segment breaks off and does not
reattach to any other chromosome.
Mrs. Degl
2. Gene Mutations
•A random change in the chemical makeup of the DNA is a
gene mutation. The effects of some mutations, such as
albinism, are noticeable, but others are not.
•Inheritable mutations are usually harmful.
•Fortunately, most mutations are recessive, and therefore
the dominant allele prevails.
•If both parents carry the recessive mutation, than the child
will possess the mutation (Sickle Cell Anemia, Tay-Sachs).
•Exposure to chemicals and radiation can increase the rate
of mutations. X-rays, UV Rays, Cosmic Rays,
formaldehyde, and asbestos are examples of Mutagenic
Agents.
Mrs. Degl
Heredity and the Environment
The development of inherited
traits can be influenced by
environmental factors such
as temperature, sunlight, and
nutrients.
•Normally, Himalayan Rabbits are white with black ears,
nose, tail and feet. The black hair helps the colder areas
absorb heat.
•When some fur is shaved off the rabbit and that area is kept
covered with ice, the new hair grows in black. The artificial
temperature change causes the hair to grow in black, so that
it can absorb heat to help the rabbit.
Mrs. Degl
Selective Breeding
Breeders of animals and plants in today's world are
looking to produce organisms that will possess
desirable characteristics, such as high crop yields,
resistance to disease, high growth rate and many
other phenotypical characteristics that will benefit
the organism and species in the long term.
There are 3 methods of selective breeding:
•Artificial Selection
•Inbreeding
•Hybridization
Mrs. Degl
Artificial Selection: Individuals with the most desirable
traits are crossed or allowed to mate in the hopes that
their offspring will show the desired traits. Examples are
crossing two tall plants or having two sheep with thick
wool mate to get tall plants or sheep with very thick wool.
Inbreeding: Two offspring from the same parents can be
crossed or allowed to mate to produce desired
characteristics. However, the danger of inbreeding is that
it is more likely that two recessive traits can be inherited.
Hybridization: Crossing two different species to get
desired traits. This is more common in plants but can be
done with animals as well. Can these species reproduce?
Mrs. Degl
Zorse
Liger
Hybridization Breeding
Mrs. Degl