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Presentation on theme: "Meiosis."— Presentation transcript:

1 Meiosis

2 Homologous Chromosomes
In the body cells, or somatic cells, of animals and most plants, chromosomes occur in pairs. One from mom, one from dad Pairs are called homologous chromosomes Normal body cells are diploid cells because the nave the full number of chromosomes, or n. In humans, 2n = 46.

3 Organisms that reproduce sexually need special sex cells that contain half the normal number of chromosomes. Sex cells are called gametes. These gametes are haploid cells and have the n amount of chromosomes. In humans, n = 23.

4 Chromosome numbers Do all organisms have the same number of chromosomes?

5 Homologous Chromosomes
sex chromosomes autosomes How many autosomes are found in the human the body cell? 44 (22 pairs) Match up the homologous chromosomes

6 Meiosis Meiosis is a two-part cell division process that produces gametes or sex cells. four daughter cells all daughter cells are haploid chromosomes are shuffled in the process, so that each daughter cell has a unique combination of genes (crossing over)



9 Crossing Over To make things more interesting, crossing over can occur. During prophase, the homologous chromosomes form tetrads.

10 Crossing Over Crossing over takes place when two chromosomes break and then reconnect but to different end piece. Crossing over increases the number of variations that can exist in offspring.

11 Crossing Over Which of the following did crossing over ONLY take place? B A C

12 Fertilization Male sex cells are called sperm.
Female sex cells are called eggs. When two sex cells unite, fertilization occurs. A fertilized egg is called a zygote. This is the first cell of the new offspring!

13 Nondisjunction Although the events of meiosis usually proceed accurately, sometimes an accident occurs and the chromosomes fail to separate correctly. This is known as nondisjunction.


15 Nondisjunction The zygote that has been created will have an extra or missing chromosome. This would result in an inherited genetic disorder. In most cases, the presence of an additional chromosome or missing chromosome results in the death of the zygote.

16 Nondisjunction In humans, n = 23. So the normal body cell is 2n = 46 chromosomes. What would happen if a gamete containing an extra chromosome(n+1) were to be paired with a gamete containing the correct number of chromosomes (n)? The zygote that has been created will have and diploid number of chromosomes n+1 = 47 chromosomes. n+1 n

17 Karyotypes When you are born, doctors take a sample of your blood and create a karyotype. This will enable the doctor to see if you have any additional or missing chromosomes. A karyotype is a standardized arrangement of all the chromosomes of a cell.

18 Karyotypes Homologous chromosomes are paired up, then they are put in order from the LARGEST chromosome pair to the smallest.

19 Karyotypes 2 x chromosomes = GIRL autosomes sex chromosomes

20 Karyotypes Everything okay here?? What gender??
2 different sized sex chromosomes = XY = male

21 Down’s syndrome Everything okay here?? trisomy 21 trisomy 21
decreased muscle tone at birth asymmetrical or odd-shaped skull round head with flat area at the back of the head slanting eyes small mouth with protruding tongue single crease on the palm slowed growth and development delayed mental and social skills trisomy 21 trisomy 21

22 Turner’s syndrome Everything okay here?? monosomy X monosomy X
short stature (affects almost all girls with Turner, to different degrees) failure of ovaries to develop (90-95% of girls) webbed neck (25%) or short neck (40%) abnormal fingernails and toenails (70%) hearing disorders (50-90%) frequent ear infections in childhood (75%) monosomy X monosomy X monosomy X

23 Turner’s syndrome XO XO XO XO

24 Kleinfelter’s syndrome
Everything okay here?? XXY XXY XXY

25 Kleinfelter’s syndrome
No symptoms - some men are unaware they have an extra chromosome Enlarged male breasts - only severe in about 10% of cases Sparse facial hair Sparse body hair Inability to produce sperm Tallness - more likely to be taller than non-XXY males. Thinness Normal intelligence Language impairment Delayed language XXY XXY

26 Kleinfelter’s syndrome

27 Patau’s syndrome trisomy 13 trisomy 13 trisomy 13 trisomy 13
Everything okay here?? trisomy 13 trisomy 13 trisomy 13 trisomy 13

28 Patau’s Syndrome Mental retardation, severe Seizures Small head
Scalp defects (absent skin) Small eyes (microphthalmia) Cleft lip and/or palate


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