1. Introduction to Human Genetics
But what happens when meiosis goes wrong?
What when wrong?
Happens meiosis wrong?
When wrong?
What meiosis goes wrong?
Boehm

2. Goals of Meiosis What do you think they are? Boehm
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

3. Normal Meiosis Starting cell chromosomes Phase showing chromosomes
in Meiosis I
End of Meiosis I
Phase showing chromosomes
in Meiosis II
End of Meiosis II
Boehm

4. Meiosis leads to genetic variability
The three sources of genetic variability in a sexually reproducing organism are:
Independent assortment of homologous chromosomes and sister chromatids (Punnett square).
Crossing over.
Random fertilization of an ovum by a sperm.
All three mechanisms reshuffle the various genes carried by individual members of a population.
BUT mutations are what ultimately create a population’s diversity of genes
some mutations can be beneficial, while others can be harmful to an individual.

5. The process of meiosis isn’t perfect
Human disorders can result from mistakes in meiosis
These disorders can be caused by:
Chromosomal number abnormalities (too many or too few chromosomes)
Sections of entire chromosomes can be deleted, added, or swapped.

6. Diagnosis of Chromosomal Abnormalities
Tests are available to determine in utero if a child has a particular chromosomal disorder.
One technique, amniocentesis, can be used beginning at the 14th to 16th week of pregnancy.
Fetal cells extracted from amniotic fluid are cultured and karyotyped to identify some disorders.
Fig a
Boehm
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

7. Example of a Karyotype
Boehm

8. Alterations of chromosome number cause some genetic disorders
Boehm

9. Abnormal Meiosis Resulting in Chromosome Number Issues
Starting cell chromosomes
Phase showing chromosomes
in Meiosis I
End of Meiosis I
Phase showing chromosomes
in Meiosis II
End of Meiosis II
Boehm

10. Source of Chromosome Number Abnormalities
Nondisjunction occurs when problems with the meiotic spindle cause errors in daughter cells.
Homologous chromosomes do not separate during meiosis I.
Alternatively, sister chromatids may fail to separate during meiosis II.
Fig
Boehm
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

11. Consequences of Nondisjunction
Some gametes receive two of the same type of chromosome and another gamete receives no copy.
Offspring results from fertilization of a normal gamete with one after nondisjunction will have an abnormal chromosome number or aneuploidy.
Trisomic cells = 2n + 1(extra) total chromosomes.
Monosomic cells = 2n - 1(too few) chromosomes.
If the organism survives, aneuploidy typically leads to a distinct phenotype.
Boehm
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

12. Consequences of Nondisjunction
Several serious human disorders are due to alterations of chromosome number and structure.
Although the frequency of aneuploid zygotes may be quite high in humans, most embryos are spontaneously aborted before birth.
Certain aneuploid conditions upset the balance less, leading to survival to birth and beyond.
Boehm
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

13. Down Syndrome
One aneuploid condition, Down syndrome, is due to three copies of chromosome 21.
It affects one in 700 children born in the United States.
This trisomy severely alters an individual’s phenotype in specific ways.
Fig
Boehm
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

14. Trisomy Frequency
Trisomies of other chromosomes also increase in incidence with maternal age, but it is rare for infants with these autosomal trisomies to survive for long.
Why do you also think survival of a trisomy 21 human is more likely than most other, fatal trisomies?
Boehm
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

15. Sex Chromosome Aneuploidy
Nondisjunction of sex chromosomes produces a variety of aneuploid conditions in humans.
Unlike autosomes, this aneuploidy upsets the genetic balance less severely.
Why do you think this is true?
Boehm
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

16. Map of Genes on Human Chromosomes

17. Sex Chromosome Aneuploidy
Klinefelter’s syndrome, an XXY male, occurs once in every 2000 live births.
These individuals have male sex organs, but are sterile.
There may be feminine characteristics, but their intelligence is normal.
Males with an extra Y chromosome (XYY) tend to be somewhat taller than average, and are fertile.
Trisomy X (XXX), which occurs once in every 2000 live births, produces fertile, healthy females.
Monosomy X or Turner’s syndrome (X0), which occurs once in every 5000 births, produces phenotypic, but immature, sterile females.
Boehm
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

18. Other types of errors in meiosis
Sometimes parts of entire chromosomes are deleted, added, or moved.
Boehm

19. Chromosome Structure Abnormalities
A deletion occurs when a chromosome fragment lacking is lost during cell division.
This chromosome will be missing certain genes.
A duplication occurs when a fragment becomes attached as an extra segment to a sister chromatid.
Fig a & b
Boehm
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

20. Chromosome Structure Abnormalities
An inversion occurs when a chromosomal fragment reattaches to the original chromosome but in the reverse orientation.
In translocation, a chromosomal fragment joins a nonhomologous chromosome
Fig c & d
Boehm
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

21. Chromosome Structure Abnormalities
Deletions and duplications are common in meiosis.
Homologous chromatids may break and rejoin at incorrect places, such that one chromatid will loose more genes than it receives.
When do you think these errors could occur in the overall process of meiosis?
Boehm
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

22. Chromosome Structure Abnormality:
Deletion
Deletions cause severe physical and mental problems.
One syndrome, cri du chat, results from a specific deletion in chromosome 5.
These individuals are mentally retarded, have a small head with unusual facial features, and a cry like the mewing of a distressed cat.
This syndrome is fatal in infancy or early childhood.
Boehm
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

23. Chromosome Structure Abnormality:
Translocation
Chromosomal translocations can occur between nonhomologous chromosomes.
Chromosomal translocations have been implicated in certain cancers, including chronic myelogenous leukemia (CML).
CML occurs when a fragment of chromosome 22 switches places with a small fragment from the tip of chromosome 9 (commonly referred to as the “Philadelphia chromosome”).
Boehm
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

24. Now can you go diagnose a patient?
Well, we wouldn’t really trust you to do that after just this one presentation, but…
Use the class handout of three karyotypes to diagnose the patients as “normal” or “abnormal” in terms of chromosome structure and number.