1. 4.2-Sexual Reproduction cont’d Genetic Variation
SBI3U

2. Recall-Check your understanding..
At which of the following stages of meiosis do diploid cells become haploid?
Prophase I
Metaphase I
Anaphase I
Metaphase II
Anaphase II

3. Recall-Check your understanding..
Which of the following is NOT true of mitosis and meiosis?
Mitosis produces 2 daughter cells; meiosis produces 4
In metaphase chromosomes line up at cell equator; in meiosis I chromosomes pair up at equator
Mitosis produces identical daughter cells; meiosis produces genetically varied cells
Mitosis produces haploid cells; meiosis produces diploid cells

4. Genetic Variation
Changes in genetic sequences create different versions of genes.
Mutations are the original source of genetic diversity.

5. During meiosis, genetic variation is ensured in 2 ways:
Independent Assortment of Chromosome
Crossing Over

6. 1. Independent Assortment
This occurs during metaphase I, when each homologous chromosome is randomly oriented towards one of the poles.
This alone can produce over 8 million different chromosome combinations.(2^n)

7. 1. Independent Assortment

8. Test your understanding…
What would be the potential combination of chromosomes for a donkey who has 62 chromosomes (31 pairs)? a) b)
c) 231
d) 262
e) b and c

9. 2. Crossing Over . Occurs during Prophase I
genetic material between maternal and paternal chromosomes is exchanged
DNA molecules of two non-sister chromatids are broken and rejoined at the same place. .

10. Errors in Meiosis: Changes in Chromosome Structure
When chemical bonds are re-formed during crossing over, errors can occur that can result in genetic disorders. There are 4 types.
1)Deletion
2) Duplication
3) Inversion
4)Translocation
Two major types of Mistakes:
1) Nondisjunction
2) Alteration in chromosome structure

11. table 4.1 (pg. 177) * know symptoms for each disorder

12. Deletion A chromosomal fragment lacking a centromere is lost.
The affected chromosome is missing genes

13. Ex. Cri Du Chat Deletion on chromosome 5
The child is mentally disabled, small head with unusual facial features.
Children with this disorder generally die in infancy or early childhood.

14. Duplication
A deleted fragment from another chromosome, may be attached as an extra segment on a sister or non-sister chromatid.

15. Inversion
A chromosomal fragment can reattach to the original chromosome but in reverse order.

16. Translocation
A segment moves from one chromosome to a non- homologous chromosome
Reciprocal translocation: both chromosomes exchange information
Nonreciprocal: the other chromosome does not exchange information.

17. Ex. Chronic Myelogenous Leukemia (CML)
Reciprocal translocation between chromosome 22 and chromosome 9
The chromosome 22 is abnormally short.

18. Errors in Meiosis: Changes in Chromosome Number
Non disjunction- homologous pairs or sister chromatids do not separate properly
This Occurs when :
Homologous pairs don’t separate successfully during anaphase I
Sister chromatids do not separate during anaphase II

19. Nondisjunction

20. Examples of Non disjunction
Trisomy: If the fertilized egg has an extra chromosome
( 2n + 1)
Monosomy: If the fertilized egg has one less chromosomes
( 2n -1 )
Polyploidy: More than two complete chromosome sets.
(3n, 4n)

21. Ex. Trisomy Down Syndrome : due to an extra chromosome 21
Total of 47 chromosomes.

22. Ex. Trisomy Klinefelter Syndrome: an extra X chromosome in Males (XXY)
Contain male sex organs, but the testes are abnormally small and the man is sterile.
The extra X chromosome is inactivated, but there can still be the development of breasts.

23. Ex. Trisomy Trisomy X (XXX): females have an extra X chromosome
These females are healthy and cannot be distinguished from other XX females.

24. Ex. Monosomy Turner Syndrome: Females missing an X chromosome
They have female characteristics, they are sterile because the sex organs do not mature.

25. Prenatal Genetic Testing
Pregnant women of all ages may request or be referred for prenatal genetic testing.
Procedures are covered by the Ontario Heath Insurance Plan. (OHIP)
Deciding to have prenatal testing can be a difficult decision since there are many factors to consider, including the possibility of pregnancy termination with some procedures.