2C ­ GENETICS ­ Meiosis Lesson ­ Disorders
Meiosis and Reproduction
Fertilization: egg and sperm unite to form a diploid cell
(the tail of sperm disintegrates after chromosomes are delivered) egg (n) + sperm (n) = zygote (2n) Sex chromosomes = the chromosomes involved in determining gender; the 23rd pair
Autosomes = the chromosomes not involved in determining gender; pairs 1 through 22
When errors occur in meiosis we can get the wrong number of chromosomes in a cell.
• a missing chromosome • an extra chromosome Meiosis
Homologous pairs of chromosomes separate & move toward opposite poles of the cell. Disjunction: the separation of chromosomes during meiosis.
Nondisjunction: the failure of homologous chromosomes or sister chromatids to separate properly
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2C ­ GENETICS ­ Meiosis Lesson ­ Disorders
Errors in Meiosis
In mitosis or meiosis, there can be an error in the processes that results in the change in the number of chromosomes that the daughter cells receive.
There are two types of this error condition:
Monosomy = having only one copy of a chromosome when you should have a pair.
Trisomy = having three copies of a chromosome when you should have a pair.
Errors in Meiosis ‐ Nondisjunction
The consequences of an error in the number of chromosomes passed on to daughter cells shows up during fertilization (egg and sperm cells join).
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2C ­ GENETICS ­ Meiosis Lesson ­ Disorders
Karyotype Chart ­ a picture of an organism's chromosomes • chromosomes are matched according to size, centromere position and banding patterns (location of genes)
• homologous pairs are then arranged from largest to smallest • used to diagnose chromosomal abnormalities
• chromosome pairs 1­22 are the autosomal chromosomes (autosomes)
• chromosome pair 23 are the sex chromosomes
Errors in Meiosis ‐ Nondisjunction
Down Syndrome (Trisomy 21)
• one of the most common trisomic disorders
• results in an individual with an extra chromosome 21 when non disjunction occurs
• individual has 47 chromosomes
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2C ­ GENETICS ­ Meiosis Lesson ­ Disorders
Errors in Meiosis ‐ Nondisjunction
Patau Syndrome (Trisomy 13)
• results in an individual with an extra chromosome 13 when non disjunction occurs
• individual has 47 chromosomes
Errors in Meiosis ‐ Nondisjunction
Edward Syndrome (Trisomy 18)
• results in an individual with an extra chromosome 18 when non disjunction occurs
• individual has 47 chromosomes
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2C ­ GENETICS ­ Meiosis Lesson ­ Disorders
Errors in Meiosis ‐ Nondisjunction
Turner Syndrome
• results when non disjunction of the sex chromosomes occurs
• female with an 1 X chromosome
• individual has 45 chromosomes
Errors in Meiosis ‐ Nondisjunction
Klinefelter's Syndrome
• results when non disjunction of the sex chromosomes occurs
• male with 2 X chromosomes and 1 Y chromosome
• individual has 47 chromosomes
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2C ­ GENETICS ­ Meiosis Lesson ­ Disorders
Errors in Meiosis ‐ Nondisjunction
Jacob Syndrome (Super Male Syndrome)
• results when non disjunction of the sex chromosomes occurs
• male with 1 X chromosome and 2 Y chromosomes
• individual has 47 chromosomes
Why does genetic material change?
Mutations ­ are random changes in DNA that occur when mistakes in DNA replication are not fixed ­ these are spontaneous mutations
Mutagens ­ substances that increase the rate of mutations
> viruses
> chemicals ­ pesticides; food additives
> radiation ­ UV rays; X­rays
• mutations can be harmful, beneficial or have no effect
• There are 2 types: chromosomal ­ affect the entire chromosome as genes are modified
– 5 types: translocations, inversions, deletions, duplications and those caused by non­disjunction
gene ­ deletion, insertion, inversion or substitution of one or more nucleotides
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2C ­ GENETICS ­ Meiosis Lesson ­ Disorders
Chromosomal Muations
• translocation ­ when a segment of a chromosome breaks off and combines with a non­homologous chromosome
• inversion ­ when a segment of a chromosome flips or changes direction; it is located in the same place, but with an inverted direction
• deletion ­ when a segment of a chromosome is lost
• duplication ­ when a segment of a chromosome is doubled; the sequence is repeated
Chromosomal Mutation
Cri-du-Chat Syndrome
• results because of a deletion on the short arm of chromosome 5
• infant cries like a cat due to problems in the larynx and nervous system
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2C ­ GENETICS ­ Meiosis Lesson ­ Disorders
gene Muations
• deletion ­ when one or more nucleotides are lost • insertion ­ when one or more nucleotides are added
• inversion ­ when one or more nucleotides flip or change directions
• substitution ­ when a one or more nucleotides are replaced with other nucleotides
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Attachments
meiosis with crossing over.mp4
meiosis with names of stages.mp4