1. 4.3 Alterations In Chromosome Structure and Number
Big Ideas
Errors in meiosis can lead to errors in chromosome number in gametes. Embryos that develop from these gametes will have missing or extra chromosomes
Duplication, deletion, inversion and translocation are types of damage that can occur to chromosomes during meiosis
Genetic testing can be an important part of diagnosing treating or preventing particular genetic conditions

2. Atypical Chromosome Numbers
In most cases, a human embryo with an atypical number of chromosomes does not survive and the mother has a miscarriage
Some embryos with atypical chromosome numbers do survive depending on which chromosomes are affected
A developmental disability is a lifelong disability due to mental or physical impairment. Developmental disabilities can be caused by social, environmental or physical factors. Some of the physical factors include abnormalities in genes or chromosome number.

3. Trisomy: condition in which an individual has three copies of a particular chromosome
Down Syndrome (Trisomy 21): individual carries an extra copy of chromosome 21
Characterized by mild to severe developmental delays, certain facial features, below average height
Other traits typical of individuals with Down Syndrome include heart defects and an impaired immune system. Although people with Down Syndrome have life expectancies that are shorter than average, they can still have productive and happy lives. This can also occur in other animal species.

4. Meiosis occurs repeatedly in a persons lifetime as the testes produce sperm or the ovaries produce eggs
Almost always the meiotic spindle distributes chromosomes to the daughter cells without error
Occasionally mistakes can occur during meiosis that alter chromosome number
Non-disjunction: occurs when homologous chromosomes do not separate during Anaphase I or Anaphase II – page 118
Anaphase I: separates homologous chromosomes Anaphase II: separates sister chromatids

5. This results in gametes with either missing or extra chromosomes
If an abnormal gamete produced by a non-disjunction event unites with a normal gamete in fertilization, the result is a zygote that contains an atypical number of chromosomes
Mitosis will then carry this number of chromosomes to all of the embryonic cells

6. Other conditions caused by non-disjunction
Trisomy 13 (Patau Syndrome): results in severe intellectual disability and physical abnormalities (heart, brain, spinal cord, eyes, muscle tone, extra digits, cleft lip); affects 1 in births, 5-10% live past year 1
Trisomy 18 (Edwards Syndrome): results in severe intellectual disability and low birth weight (small cranium, heart defects); affects 1 in 5000 births, 5-10% live past year 1
XXY (Klinefelter Syndrome): affects male sexual development and results in testes that do not produce enough testosterone (reduces development of secondary sex characteristics); affects 1 in births, testosterone injections help treat this condition
Why does Klinefelter syndrome only affect males? Only males carry a Y chromosome

7. Damage To Chromosome Structure
Mutation: any change to the DNA of a cell
Some mutations change only one or a few base pairs in a DNA sequence, others occur at the level of chromosomes
Large-scale mutations that damage chromosome structure can occur when something unusual happens during crossing over

8. Ex: Translocation Down Syndrome
Translocation: a fragment of one chromosome attaches to a non-homologous chromosome
Ex: Translocation Down Syndrome
translocation between chromosomes 21 and 14
inheritance of two normal copies of chromosome 14 and one that has a piece of chromosome 21 attached to it
the extra material from chromosome 21 causes Down Syndrome (like Trisomy 21)

9. Sometimes translocations result in deletions
Deletion: a fragment of a chromosome is lost
Much of our DNA does not contain genes, therefore small deletions are not usually harmful
However, if the lost fragment contains one or more genes then the instructions for making certain proteins may be lost
Those proteins have specific functions in the body
Ex: Prader-Willi Syndrome – deletion of chromosome 15

10. Duplication: part of a chromosome is repeated
The repeated region sometimes includes an entire gene
Interestingly, if a region is only duplicated once, the duplication may have no effect
The duplication is most likely to have an effect when duplicated multiple times
Ex: Three or more duplications on chromosome 15 results in developmental delays and seizures

11. Inversion: reverses a fragment of the original chromosome
Most of the genes are still present in their normal number and some may function
However, inversions can cause problems with the synapsis of chromosomes during meiosis
Therefore someone with a chromosomal inversion may have difficulties conceiving a baby
Summary: figure 4.39 on page 119

12. Genetic Testing
Modern technologies offer many ways to obtain information about the genetic makeup of an individual
Important part of diagnosing, treating or preventing illness
In addition to karyotype analysis, genetic testing can include screening for specific gene sequences

13. Carrier testing: can determine if an individual carries a copy of a mutation that his or her children could inherit
A person with a family history of a genetic condition may want to undergo genetic testing to find out his or her risk of developing the condition
This type of testing is called preventative or presymptomatic testing and is usually done for disorders that appear after birth or later in a person’s life

14. A human embryo becomes a fetus about seven weeks after fertilization
Diagnostic genetic testing is used to confirm a diagnosis when symptoms for a particular genetic condition are present
A human embryo becomes a fetus about seven weeks after fertilization
Prenatal testing is used to detect small-scale mutations or chromosomal alterations in a fetus
Amniocentesis: between 14 and 20 weeks of pregnancy; extraction of some amniotic fluid that surround fetus
Chorionic Villus Sampling: as early as week 8 of pregnancy; extraction of some chorionic villi from the tissue around the fetus

15. Phenylalanine accumulation can lead to severe developmental delays
Newborn screening can detect some genetic disorders at birth through simple tests
Ex: shortly after birth, newborns are screened for phenylketonuria (PKU)
PKU is an inherited disorder that results in the inability of children to break down the amino acid phenylalanine
Phenylalanine accumulation can lead to severe developmental delays
Therefore the detection of this condition can prevent retardation
Homework: page 123 #1-4, 6-9, 12