Presentation on theme: "Chapter 15: The Chromosomal Basis of Inheritance"— Presentation transcript:
1 Chapter 15: The Chromosomal Basis of Inheritance
2 The Chromosomal Theory of Inheritance Genes have specific loci on chromosomes and chromosomes undergo segregation and independent assortment
3 Chromosomal Linkage Thomas Morgan (early 20th century) Drosophilia melanogaster(fruit flies)Associated a specific gene with a specific chromosome
4 P1: Mated white eyed male with red eyed female Morgan’s ExperimentP1: Mated white eyed male with red eyed femaleF1: 100% red eyedF1 generation matedF2: 3 red : 1 whiteHowever???
5 Morgan’s Experiment: All females were red eyes: Half the males were redThe other half were white
6 Morgan’s Conclusion: 1. Eye color was linked to sex 2. specific genes are carried on specific chromosomes3. genes located on sex chromosomes exhibit unique inheritance patterns
7 Sex-linkage: genes located on a sex chromosome Linked Genes:Sex-linkage: genes located on a sex chromosomeLinked genes: genes located on the same chromosome that tend to be inherited together
8 Another Morgan Experiment: This time he observed body color and wing size:Wild type = gray body (b+) and normal wings (vg+)Mutant type = black body (b) and vestigial wings (vg)
9 First Cross: true breeding wild type wit black vestigial wings b+b+vg+vg+ X bbvgvgF1 = all wild type phenotype(b+bvg+vg)
10 Second Cross: female dihybrids vs true breeding reeessive males b+ bvg+vg X bbvgvg(test cross)2300 offspring were scored
11 Results: -High proportion of parental phenotypes (965 wild type, 944 black vestigial)-Low proportions of non- parental phenotypes(206 gray vestigial, 185 black normal)
12 Conclusion: 1. Body color and wing size are usually inherited together (genes must be on the same chromosome??)
13 2. Body color and wing size are only partially linked: Conclusion:2. Body color and wing size are only partially linked:
14 Explaining Morgan’s Results: Recombination of unlinked genes vs. linked genes:Unlinked genes = independent assortmentLinked genes = crossing over
15 Recombination:Production of offspring with combinations of traits different from those found in either parent!
16 Genetic Mapping:Genetic maps are an ordered list of the genetic loci along a particular chromosome
17 Genetic mapping: Recombination frequencies depend on: *Distances between genes on a chromosome
18 Recombination frequency refers to the percentage of recombinants occurring in the offspring
19 *crossing over is a random event Alfred Sturtevant:*crossing over is a random event*the farther apart the genes on a chromosome, the higher the probability that crossing over will occur, so the higher the recombination frequency
20 Sturtevant Reasoning: The further apart two genes are, the more points between them where crossing over can occur.
21 *experimental crosses reveal recombination frequencies Linkage Map:Probability of crossover between two genetic loci is proportional to the distance separating the two loci.*experimental crosses reveal recombination frequencies
22 Example: DrosophilaBody color (b)Wing size (vg)Cinnabar (cn)
23 Map units: Distance between genes on a chromosome 1 map unit = 1% recombination frequencyDo not correspond to physical frequencies
24 Seed and flower color in pea plants: Genes that are very far apart on the chromosomeCrossing over is almost certain.
25 Frequency of crossing over is not uniform over the length of the chromosome
26 Map units do not portray order of genes on a chromosome
27 Genetic recombination Crossing over Genes that DO NOT assort independently of each otherGenetic maps The further apart 2 genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequencyLinkage maps Genetic map based on recombination frequencies
28 Human sex-linkageSRY gene: gene on Y chromosome that triggers the development of testesFathers= pass X-linked alleles to all daughters only (but not to sons)Mothers= pass X-linked alleles to both sons & daughtersSex-Linked Disorders: Color-blindness; Duchenne muscular dystropy (MD); hemophilia X-inactivation: 2nd X chromosome in females condenses into a Barr body (e.g., tortoiseshell gene gene in cats)
29 Sex Linked Genes:Fathers= pass X-linked alleles to all daughters only (but not to sons)Mothers= pass X-linked alleles to both sons & daughters
31 Chromosomal Errors:Nondisjunction: members of a pair of homologous chromosomes do not separate properly during meiosis I or sister chromatids fail to separate during meiosis II
32 Chromosomal Errors: Aneuploidy: chromosome number is abnormal Monosomy: missing chromosomeTurner Symdrome -XOTrisomy : extra chromosomeDown syndrome- Trisomy- 21Kleinfelters Syndrome- XXYPolyploidy: extra sets of chromosomes
33 Chromosomal Errors: Alterations of chromosomal structure: Deletion: removal of a chromosomal segmentDuplication: repeats a chromosomal segmentInversion: segment reversal in a chromosomeTranslocation: movement of a chromosomal segment to another
34 Point mutations: affect protein structure and function Base pair substitution: one nucleotide pair replacing anotherMissense vs. Nonsense mutationsMissense = altered codon still codes for an amino acid – not necessarily the right oneNonsense = changes the codon to a stop codonPremature termination leading to malfunctional proteins.
35 Insertions and Deletions: Adding or losing a nucleotide pairDisastrous effect on the proteinCauses a Frame Shift:Nucleotides down stream of the mutation will be improperly grouped into codons that will likely produce a non- functional protein
36 Genomic imprinting 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
38 Inheritance of Organelles: Some genes are considered extranuclear:That is not found in nucleusBut, in organelles such as mitochondria and chloroplastsThese genes do not follow mendelian inheritance patternsRandomly assorted to gametes and daughter cells
39 Inheritance of Organelles: Organelles are inherited maternallySperm only contributes genetic information
40 Mutations:Plant variegation: due to mutations in the genes that control plant pigmentsPattern of variegation is determined by the ratio of wild type allele vs. mutant type allele for pigmentation
41 Mitochondria DNA mutations: Heteroplasmy: when a cell contains both wild type and mutant type mtDNA.Disorders usually affect nervous and muscular systemsThey require the most energy from ATP
42 Mitochondria DNA mutations: Disorders of the optic nerve (leber’s neuropathy) and other eye defects.Kearns- Sayre Syndrome- abnormal heart rate and central nervous system disorderMitochondrial myopathy: muscle deterioration, intolerance to exercise