Chapter 15~ The Chromosomal Basis of Inheritance

Mendel’s Inheritance w/ Independent Assortment 2

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

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

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

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Genetic recombination Crossing over Genes that DO NOT assort independently of each other 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 Linkage maps Genetic map based on recombination frequencies 8

Recombination Calculations 9

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

Sex-Linked Disorders 11

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

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

Chromosomal Errors - Turner’s (XO) 14

Chromosomal Errors - Metafemale 15

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

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

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

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

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