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1 4 Chapter 15~ The Chromosomal Basis of Inheritance.

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1 1 4 Chapter 15~ The Chromosomal Basis of Inheritance

2 2 Mendel’s Inheritance w/ Independent Assortment 2

3 3 The Chromosomal Theory of Inheritance 4 Genes have specific loci on chromosomes and chromosomes undergo segregation and independent assortment

4 4 Chromosomal Linkage 4 Morgan- used Drosophilia melanogaster »XX (female) vs. XY (male) 4 Discovered: 4 Sex-linkage: genes located on a sex chromosome 4 Linked genes: genes located on the same chromosome that tend to be inherited together

5 5 Chromosomal Linkage 4 Sex - Linked –Genes on the X chromosome will only have 1 copy in males 5

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8 8 Genetic recombination 4 Crossing over –Genes that DO NOT assort independently of each other 4 Genetic maps –The further apart 2 genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency 4 Linkage maps –Genetic map based on recombination frequencies

9 9 Recombination Calculations 9

10 10 Human sex-linkage 4 SRY gene: gene on Y chromosome that triggers the development of testes 4 Fathers= pass X-linked alleles to all daughters only (but not to sons) 4 Mothers= pass X-linked alleles to both sons & daughters Sex-Linked Disorders: Color-blindness; Duchenne muscular dystropy (MD); hemophilia

11 11 Sex-Linked Disorders 11

12 12 X- inactivation 4 X-inactivation: 2nd X chromosome in females condenses into a Barr body (e.g., tortoiseshell gene gene in cats) 4 A different X will be randomly be inactive in different cells. (The Barr body can be viewed in the nucleus.) 12

13 13 Chromosomal errors, I 4 Nondisjunction: members of a pair of homologous chromosomes do not separate properly during meiosis I or sister chromatids fail to separate during meiosis II 4 Aneuploidy: chromosome number is abnormal – Monosomy~ missing chromosome – Trisomy~ extra chromosome (Down syndrome) – Polyploidy~ extra sets of chromosomes

14 14 Chromosomal Errors - Turner’s (XO) 14

15 15 Chromosomal Errors - Metafemale 15

16 16 Chromosomal Errors - Klinefelter (XXY) or (XXXY) 16

17 17 Chromosomal Errors - Down’s syndrome (Trisomy 21) 17

18 18 Chromosomal errors, II 4 Alterations of chromosomal structure: 4 Deletion: removal of a chromosomal segment 4 Duplication: repeats a chromosomal segment 4 Inversion: segment reversal in a chromosome 4 Translocation: movement of a chromosomal segment to another

19 19 Genomic imprinting 4 Def: a parental effect on gene expression 4 Identical alleles may have different effects on offspring, depending on whether they arrive in the zygote via the ovum or via the sperm. 4 Fragile X syndrome: higher prevalence of disorder and retardation in males (partially caused by maternal imprint)

20 20 Mitochondrial DNA 4 Small circular DNA located in mitochondria 4 Inherited from the mother (maternally) 20


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