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Chromosomal Abnormalities

Ch. 15 -- Prezi skeleton by David Knuffke
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Ms Schwinge

on 30 October 2017

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Transcript of Chromosomal Abnormalities

Chromosomal Abnormalities
Big Questions:
Make Sure You Can:
How it happens
How can genetic material be changed?

How do changes in the genetic material of an organism lead to changes in the traits of that organism?
Explain how the mutations that are discussed in this presentation arise.

Explain the causes and effects of all of the situations discussed in this presentation

Understand the difference between standard punnett squares and sex linked punnett squares
Morgan’s experiments with
fruit flies
provided convincing evidence that chromosomes are the location of Mendel’s heritable factors
Morgan and His Fruit Flies
Several characteristics make fruit flies a convenient organism for genetic studies:
- They breed at a high rate
- A generation can be bred every two weeks
- They have only four pairs of chromosomes
Morgan noted

wild type
, or normal, phenotypes
that were
common
in the fly populations

Traits alternative to the wild type are called
mutant phenotypes
In one experiment, Morgan mated
male flies with white eyes (mutant) with female flies with red eyes (wild type):

- The
F1
generation
all had red eyes
- The
F2
generation showed the
3:1 red:white
eye ratio,
but only males had white eyes
- Morgan determined that the white-eyed
mutant allele must be located on the
X chromosome
- Morgan’s finding supported the chromosome theory of inheritance
The first solid evidence associating a specific gene with a specific chromosome came from
Thomas Hunt Morgan
, an embryologist
Sex Linked Genes
- In humans and other mammals, there are two varieties of
sex chromosomes: a larger X chromosome and a smaller Y chromosome

- Only the ends of the Y chromosome have regions that are homologous with the X chromosome

- The SRY gene on the Y chromosome codes for the development of testes
- Females are XX, and males are XY
- Each ovum contains an X chromosome, while a sperm may contain either an X or a Y chromosome
- Other animals have different methods of sex determination
- The sex chromosomes have genes for many characters unrelated to sex
- A gene located on either sex chromosome is called a sex-linked gene
- In humans, sex-linked usually refers to a gene on the
larger X chromosome
Sex-linked genes follow specific patterns of inheritance
For a recessive sex-linked trait to be expressed:
- A female needs
two copies
of the allele
- A male needs
only one copy
of the allele
That's why sex-linked recessive disorders are much
more common in males
than in females
Some disorders caused by recessive alleles on the X chromosome in humans:
Color blindness
Duchenne muscular dystrophy
(leads to progressive loss of muscle mass)
Hemophilia
(a blood clotting disorder)
- In mammalian females, one of the two X chromosomes in each cell is
randomly
inactivated during embryonic development

- The inactive X condenses into a
Barr body

- If a female is heterozygous for a particular gene located on the X chromosome, she will be a
mosaic
for that character
Linked Genes
- Each chromosome has hundreds or thousands of genes

- Genes located on the same chromosome that tend to be inherited together are called
linked genes
Linked genes sit close together on a chromosome, making them likely to be inherited together
(left).

Genes on separate chromosomes are never linked
(center).

But not all genes on a chromosome are linked. Genes that are farther away from each other are more likely to be separated during a process called homologous recombination
(right).
Gene 3 is more closely linked to Gene 2 than to Gene 4. Gene 1 and Gene 3 are not linked, but by chance they will still be inherited together 50% of the time, the same as if they were on separate chromosomes.
- Morgan did other experiments with fruit flies to see how linkage affects inheritance of two characters

- Morgan crossed flies that differed in traits of body color and wing size
Parents in
test cross
Most
offspring
- Morgan found that
body color and wing size are usually inherited together in specific combinations (parental phenotypes)

- He noted that
these genes do not assort independently,
and reasoned that
they were on the same chromosome
However, nonparental phenotypes were also produced. Understanding this result involves exploring
genetic recombination
, the production of offspring with combinations of traits differing from either parent.
The genetic findings of Mendel and Morgan relate to the chromosomal basis of recombination
- Mendel observed that combinations of traits in some offspring differ from either parent
- Offspring with a phenotype matching one of the parental phenotypes are called
parental types
- Offspring with non-parental phenotypes (new combinations of traits) are called
recombinant types
, or
recombinants
- A
50% frequency
of recombination is observed for any two genes on different chromosomes
yr
YR
yr
Yr
yR
YyRr
yyrr
Yyrr
yyRr
- Morgan discovered
that genes can be linked, but the linkage was incomplete, as evident from recombinant phenotypes

- He proposed that
some process must sometimes break the physical connection between genes on the same chromosome. That mechanism was the
crossing over
of homologous chromosomes
A
linkage map
is a genetic map of a chromosome based on recombination frequencies

Distances between genes can be expressed as
map units
; one map unit, or centimorgan, represents a 1% recombination frequency

Map units indicate relative distance and order, not precise locations of genes.
The farther apart two genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency
- Genes that are far apart on the same chromosome can have a recombination frequency near 50%

- Such genes are
physically linked
,
but genetically unlinked
, and behave as if found on different chromosomes

-
Using methods like chromosomal banding, geneticists can develop

cytogenetic maps
of chromosomes.
These maps
indicate the positions of genes with respect to chromosomal features
Large-scale chromosomal alterations often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders.

For example, in

nondisjunction
pairs of homologous chromosomes do not separate normally during anaphase of meiosis.

As a result, one gamete receives two of the same type of chromosome, and another gamete receives no copy
Aneuploidy
results from the fertilization of gametes in which nondisjunction occurred. Offspring with this condition have an abnormal number of a particular chromosome
A
monosomic zygote
has only one copy of a particular chromosome. Turner Syndrome is when a woman only has one X chromosome.
Polyploidy
is a condition in which an organism has more than two complete sets of chromosomes
-
Triploidy
(3n) is three sets of chromosomes
-
Tetraploidy
(4n) is four sets of chromosomes
- Polyploids are more normal in appearance than aneuploids
- Polyploidy is common in plants, but not animals
A
trisomic zygote
has three copies of a particular chromosome. Down syndrome is an aneuploid condition that results from three copies of chromosome 21
. It affects about one out of every 700 children born in the United States, and the frequency of Down syndrome increases with the age of the mother; a correlation that has not been explained
Breakage of a chromosome can lead to four types of changes in chromosome structure:
-
Deletion
removes a chromosomal segment
-
Duplication
repeats a segment
-
Inversion
reverses a segment within a chromosome
-
Translocation
moves a segment from one chromosome to another
- Alterations of chromosome number and structure are associated with some serious disorders
- But some types of aneuploidy appear to upset the genetic balance less than others, resulting in individuals surviving to birth and beyond
Patau's Syndrome (trisomy 13)
Klinefelter syndrome
is the result of an extra chromosome in a male, producing XXY individuals
Monosomy X, called
Turner syndrome
, produces X0 females, who are sterile; it is the only known viable monosomy in humans
Examples
Edward's Syndrome (trisomy 18)
XYY syndrome is a rare chromosomal disorder that affects males. It is caused by the presence of an extra Y chromosome. Males normally have one X and one Y chromosome. However, individuals with this syndrome have one X and two Y chromosomes. Affected individuals are usually very tall. Many experience severe acne during adolescence. Additional symptoms may include learning disabilities and behavioral problems such as impulsivity
The syndrome cri du chat (“cry of the cat”), results from a specific deletion in chromosome 5

A child born with this syndrome is mentally retarded and has a catlike cry; individuals usually die in infancy or early childhood
Certain cancers, including chronic myelogenous leukemia (CML), are caused by translocations of chromosomes
Exceptions
- For a few mammalian traits, the phenotype depends on which parent passed along the alleles for those traits. This variation in phenotype is called
genomic imprinting

- Genomic imprinting involves the silencing of certain genes that are “stamped” with an imprint during gamete production
It appears that imprinting is the result of the
methylation
(addition of –CH3) of DNA

Genomic imprinting is thought to affect only a small fraction of mammalian genes

Most imprinted genes are critical for embryonic development
Extranuclear genes
(or cytoplasmic genes) are genes found in organelles in the cytoplasm (such as mitochondria or chloroplasts). These genes are inherited
maternally
because the zygote's cytoplasm comes from the egg

The first evidence of extranuclear genes came from studies on the inheritance of yellow or white patches on leaves of an otherwise green plant
Some defects in mitochondrial genes prevent cells from making enough ATP and result in diseases that affect the muscular and nervous systems

For example, mitochondrial myopathy and Leber’s hereditary optic neuropathy
The only tetraploid mammal we've found:
The Red Viscacha Rat
But there's more than one way to be color blind...
Colorblind Test
Colorblindness
The most common type of color blindness is called Deuteranomalia. Around 4.63% of men and 0.36% of women experience this type of color vision deficiency, many of whom don't even know it. People with Deuteranomalia see a more subdued color palette, especially when it comes to colors like green and red.
When somebody has Protanopia, all shades of green and red look rather faded, whereas yellow and blue shades seem largely unaffected. Only around 1% of men experience this type
People with Tritanopia see colors with a greenish/pink tone. It's a very rare form of color blindness and is believed to affect only 0.0001% of men and women.
Total color blindness, or Monochromacy, is the rarest form of color vision deficiency. People who have it can only see in black and white, but it's estimated that only 0.00003% of the world's population are affected by this particular condition.
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