Presentation on theme: "The Chromosomal basis of Inheritance"— Presentation transcript:
1 The Chromosomal basis of Inheritance It’s where the genes are….
2 The Chromosomal Theory of Inheritance Genes have specific loci on chromosomesChromosomes undergo segregation and independent assortment
3 Chromosomal Linkage Morgan Drosophilia melanogaster XX (female) vs. XY (male)Sex-linkage: genes located on a sex chromosomeLinked genes: genes located on the same chromosome that tend to be inherited together
4 Genetic recombination Crossing over of linked genes- Now they DO 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 frequenciesAnimate!
5 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; Duchene muscular dystrophy (MD); hemophiliaX-inactivation: 2nd X chromosome in females condenses into a Barr body (e.g., tortoiseshell gene gene in cats)
6 Chromosomal Alterations Non-disjunction: members of a pair of homologous chromosomes do not separate properly during meiosis I or sister chromatids fail to separate during meiosis II.Aneuploidy: abnormal number of a certain chromosome. Examples:Monosomy X ~ Turner Syndrome(only viable human monosomy)Trisomy 21~ Down SyndromeTrisomy 13 ~ Patau SyndromeTrisomy 18 ~Edward’s SyndromePolyploidy: more than 2 complete sets of chromosomes.Comes from non-disjunction of all chromosomesCommon in Plant Evolution.
7 Alterations in Chromosome Structure Can result from non-reciprocal crossovers or weak area of chromosomes.Deletion: removal of a chromosomal segmentDuplication: repeats a chromosomal segment (a fragment may join a homolog)Inversion: segment reversal in a chromosome (reattaches in reverse)Translocation: movement of a chromosomal segment to another
8 Various effects of Chromosomal Alterations: -Homozygous deletions are usually lethal-Duplications & translocations usually deleted-Some reciprocal translocations can alter phenotype because of“POSITION EFFECT”:-The influence on a gene’s expressionbecause of its location among neighboring genes.
9 Genomic imprintingIdentical alleles may have different effects on offspring, depending on whether they arrive in the zygote via the ovum or via the sperm.“Something” happens to the gene after they arrive to form the zygoteMay involve degrees of methylization (CH3 groups)Example:Fragile X syndrome: higher prevalence of disorder and in males
10 GENOMIC IMPRINTING Prader-Willi / Angelman Syndrome- Another example ofGENOMIC IMPRINTINGPrader-Willi / Angelman Syndrome-Both disorders are caused by a deletion onChromosome 15 but:If the altered chromosome came from Dad:Prader-Willi-Mentally handicapped, short, obeseIf the altered chromosome came from Mom:Angelman’s Syndrome-motor/mental symptoms, jerky, uncontrolled laughter
11 Most (but not all) Disorders are recessively Inherited. Defective alleles code for either a malfunctioning protein….or no protein at all.Heterozygotes can be phenotypically normal ifone of the normal copies is all that is needed.They are, of course, carriers.Most people with recessive disorders are born to“normal” parents. (carriers)Probability is ¼ that the mating of 2 carriers will producea homozygous recessive.…and 2/3 that (s)he will be a carrier.Consanguinity: condition that results from shared ancestryResults in more homozygous recessives. (Stearns County Syndrome)REMEMBER!- not all disorders are recessivesome are “late-acting” & occur after sexual maturity.Example- Huntington’s Disease
12 Some other Genetic disorders: Cri-du-Chat: A specific deletion in Chromosome 5mentally handicapped, distorted face, cry that sounds like a catChronic Myologenous Leukemia-Portion of Chromosome 22 translocates with portion of chromosome 15Patau Syndrome: Trisomy 13Severely mentally handicapped, cleft lipEdward’s Syndrome: Trisomy 18Low ears,deformed fingers, heart problems