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Meiosis and Chromosomal Inheritance
Chromosomes• Karyotype – all
chromosomes in an organism
• Homologous chromosomes – specific chromosome pair
• Gamete‐ sex cell,
Ex: sperm and egg
• Somatic cells‐ all cells except for gametes
General Life Cycle
• Haploid- cell has 1 copy of each chromosome (n)
• What type of cells would have this?
• Diploid- cell has 2 copies of each chromosome (2n)
• What type of cells would have this?
• What does the (n) signify?
Mitosis vs. Meiosis
Goals of Meiosis
• Produce a cell that has half of the genetic material as somatic cells
– Reduction Division – random separation of homologous chromosomes
• Increase variation in those sex cells produced
– Segregation of Sister Chromatids – random separation of sister chromatids
– Independent Assortment
– Crossing over
Meiosis Stages
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Meiosis I
Book uses bivalents I use tetrads (usually makes more sense to my students)
Meiosis II
Meiosis – General Process
• Goal of Meiosis is to produce a cell that has half of the genetic material as somatic cells and to increase variation in those sex cells produced
• Reduction Division – random separation of homologous chromosomes
• Segregation of Sister Chromatids – random separation of sister chromatids
When Meiosis Goes Wrong
What is the difference between these karyotypes?
Nondisjunction• The failure of homologous chromosomes to separate properly during meiosis.
Normal: Nondisjunction:
The results
• If the abnormal gametes is fertilized the results
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For the Test
• You don’t need to know specifics about each disease.
• You will need to know what can happen when Meiosis does not occur correctly
Down Syndrome: Trisomy 21
• 1 in 900 live births
• Distinctive physical appearance
• Development: slow growth, developmental delays
• Health Impact; 40% have heart defects, Prone to respiratory infection, High leukemia rate.
Trisomy of chromosome 18: Edwards Syndrome (47, XX +18 or 47 XY +18)
•mental retardation •small head, small eyes, small lower jaw •congenital heart defects (90% of individuals) •Infants have a 5% chance of surviving•Occur in 1:8000 live births
This baby only lived 5 days
Trisomy 13: Patau syndrome• Occurs 1 in 6, 000 live births• Congenital heart defects• Mental retardation, severe • Seizures • Small head and eyes • Scalp defects (absent skin) • Cleft lip and/or palate • Eyes close set ‐‐ eyes may actually fuse together into one • Iris defects • Ear Abnormalities• Extra digits (polydactyly) • Hernias• Skeletal (limb) abnormalities
Monosomy• The zygote is lacking a chromosome
• Organisms lacking one or more chromosomes rarely survive
Turner Syndrome
• Female (45 total chromosomes, 1 sex chromosome (X))
• Only monosomy that is not lethal
• Don’t develop normal at time of puberty, underdeveloped breasts, rudimentary ovaries
• Infertile
• Treatment with hormone supplements can help these women lead normal lives
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Klinefelter Syndrome
• Male (XXY)
• May have some learning difficulties
• Affected male may not develop normally at puberty
• In severe cases: rudimentary testes & prostate gland, sparse facial and pubic hair, long arms and legs, breast development may occur
• Infertility
Super Male Syndrome: XYY
• frequency is approximately 1 of 1000 males
• often are more physically active
• tendency to a delayed mental maturation
• an increased tendency for learning‐problems in school, this means a need for early and adequate stimulation.
XXX syndrome (Trisomy X)
• 1 in 1,000 females
• Fertile
• Most are normal
• May have mild retardation
Goals of Meiosis
• Produce a cell that has half of the genetic material as somatic cells
– Reduction Division – random separation of homologous chromosomes
• Increase variation in those sex cells produced
– Segregation of Sister Chromatids – random separation of sister chromatids
– Independent Assortment
– Crossing over
Structures of Chromosomes
• Locus – location of a gene on a chromosome
Types of Chromosomes Autosomes = chromosomes 1‐22
Sex chromosomes = X and/or Y XX = female
XY = male
Even though these chromosomes determine sex, only one gene on the Y chromosome actually determines sex
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Law of Independent Assortment• Chromosomes randomly distributed during meiosis
• Genes on different chromosomes inherited independently
Genetic Linkage
• Genes on same chromosome inherited together
• Law of Independent Assortment is increasing variation with the idea that a chromosome is a complete “package” which is true
• However we can take apart that package too!
Example: Fruit Flies
• Genetic cross of fruit flies with known genotypes
• Expected 1:1:1:1 ratio
• Observed 5:5:1:1 ratio
• Parental phenotypes more common than non‐parental phenotypes
Cross‐over
• Exchange of genes between homologous chromosomes
• Gametes possess unique combination of alleles
Result: 4 different types of gametes
Example: Mendel’s Peas
• Flower color and seed color genes both located on chromosome 1
• Distance between loci great enough that genes assort independently
• In other words they don’t seem to be linked because so much crossing over occurs
Flower Color
Seed Color
Example: Fruit Flies• From this data, do you think that these genes are located closer or further apart on the chromosome? Explain.
G
W
G
W
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Variation
• Independent assortment of chromosomes– 8,388,608 possible
combinations with 23 different chromosomes
• Cross‐over– think about crossing over in
combination with the independent assortment of chromosomes… 8,388,608 combinations becomes infinitesimal!
• Fertilization‐ which sperm makes it!
Chromosomal Abnormalities
• Changes in chromosome number or structure
– Deletions
– Inversions
– Translocations
– Duplications
Example: Cri du chat
• Deletion of part of chromosome 5
• Name based on the infant’s cry, which is high-pitched and sounds like a cat
Why Sex?
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