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Chapter 15~ The Chromosomal Basis of Inheritance

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Mendel’s Inheritance w/ Independent Assortment

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The Chromosomal Theory of Inheritance

Genes have specific loci on chromosomes and chromosomes undergo segregation and independent assortment

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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

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Chromosomal Linkage

Sex - Linked– Genes on the X

chromosome will only have 1 copy in males

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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

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Recombination Calculations

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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

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Sex-Linked Disorders

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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.)

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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

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Chromosomal Errors - Turner’s (XO)

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Chromosomal Errors - Metafemale

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Chromosomal Errors -Klinefelter (XXY) or (XXXY)

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Chromosomal Errors - Down’s syndrome (Trisomy 21)

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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

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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)

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Mitochondrial DNA

Small circular DNA located in mitochondria

Inherited from the mother (maternally)

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