Presentation on theme: "Chromosomal Basis of Inheritance"— Presentation transcript:
1Chromosomal Basis of Inheritance Section 10.6 and Chapter 11
2Chromosomes are the Physical Basis of Mendelian Inheritance chromosomal behavior accounts for Mendel's laws and ratiosmovement of chrom. during meiosis Mendel’s laws and ratioschromosome theory of inheritancehuman chromosomes22 pairs of autosomal chromosomesnormal body chromosomes1 pair of sex chromosomes (X and Y)males: XY; females: XXall chromosomes contain thousands of genesLinked Genesgenes located on the same chromosomegenes located specifically on sex chromosomes = sex-linkedlinked genes tend to be inherited together – why?they do not assort independentlycrosses involving them deviate from Mendel's laws and ratiosparental pheno. are disproportionately represented in offspringeven with linked genes, some offspring have traits different from parentscrossing over
5Genetic Recombination recombinant offspringoffspring with traits different from their parentsindependent assortmentMendel's lawrecombination of unlinked genesgenes on different chromosomestakes place during meiosis I (metaphase)yields typical Mendelian ratioscrossing over (crossover)recombination of linked genestakes place during meiosis I (prophase)genes located farther apart are more likely to crossovergene mappingused to determine the order and position of genes on chrom.mapping techniques make use of many chrom. featuresseveral different kinds of maps and many uses of gene mapping
7Sex Chromosomes and Sex-Linked Genes sex of any organism has a chromosomal basisvaries by type of organism involvedsex is an inherited trait determined by certain chrom.X-Y system females: XX; males: XYX-0 system females: XX; males: XZ-W system females: ZW; males: ZZhaploid-diploid system females: 2n; males: nsex-linked geneshave unique patterns of inheritancesex chrom. carry most genes related to sexalso carry genes unrelated to sexalmost all sex-linked genes are carried only on the X-chrom.no corresponding gene on the Ythus, sex-linked traits are said to be “X-linked”males have only one copy of these X-linked genes – why?females have 2 copies of these X-linked genes – why?females can be heterozygous for any X-linked trait, whereas males cannot be
8several X-linked genes are the cause of sex-linked disorders most of these disorders are recessivefound much more frequently in males than in females – why?females must inherit 2 copies of the recessive gene, while males need only inherit one – why?sex-linked genetics problems (see handout)hemophilia: X-linked recessive trait, causes blood to clot improperlyH = normal allele; h = hemophilia allelegenotypes:XHXH and XHXh = normal femaleXHXh = carrier femaleXhXh = hemophiliac femaleXHY = normal maleXhY = hemophiliac male
9Sex-Linked GenesProblem: In humans, hemophilia is an X-linked recessive trait. A hemophiliac man has a daughter with the normal phenotype. She meets a man who is also normal for the trait. What are the genotypes of everyone involved? What is the probability that the couple will have a hemophiliac daughter? A hemophiliac son? If the couple has 3 sons, what chance is there that all of them will have hemophilia?Answer: Part 1: Determine the genotypes of everyone involved.hemophiliac man = XhY (by definition)normal man the daughter meets = XHY (by definition)normal daughter = she must be XHXh, regardless if her mother was XHXH or XHXhCheck this with Punnett Squares:
11Part 2: Determine the possibilities for the couple's offspring. The F1 cross is XHXh x XHYPart 3: State these possibilities as probabilities.normal daughter (XHXH or XHXh) = 50% = 1/2 = (1 in 2)carrier daughter (XHXh) = 25% = 1/4 = (1 in 4)hemophiliac daughter (XhXh) = 0%normal son (XHY) = 25% = 1/4 = (1 in 4)hemophiliac son (XhY) = 25% = 1/4 = (1 in 4)chance of 3 sons being hemophiliacs (use Rule of Multiplication):1/4 x 1/4 x 1/4 = 1/64 = 1.6 % chanceXH XhXHXHXHXhXHYXhYXHY
13Errors in Chromosomal Inheritance genetic disorders can be caused by:recessive alleles on any chromosome, esp., X-linked recessivesphysical/chemical disturbances that damage chrom. or alter inheritanceerrors in meiosis that alter inheritancenondisjunction – an error during meiosiscan occur in two ways:homologous chromosomes fail to separate (meiosis I)sister chromatids fail to separate (meiosis II)one gamete receives two of the same chrom., the other receives no copyabnormal gamete unites with normal one at fertilizationaneuploidy
15trisomyaneuploid cell has a chromosome in triplicate (2n + 1)trisomy 21 = Down's Syndrometrisomy 18 = Edward’s SyndromePoly-X (XXX)Klinefelter's Syndrome (XXY)Jacob’s Syndrome (XYY)monosomyaneuploid cell has only 1 copy of a certain chrom. (2n - 1)almost all cases are lethalmonosomy X (X0) = Turner’s Syndrometetrasomy (2n + 2), pentasomy (2n + 3), etc.rare and usually involve only sex chrom.FigA child with Down’s Syndrome. Note the karyotype showing an extra chromosome #21
16Fig. 10.12 Turner’s Syndrome (XO) and Klinefelter’s Syndrome (XXY)
17polyploidyorganism possesses more than two complete sets of chrom.triploidy (3n) and tetraploidy (4n)common in plant kingdom; very rare in animalscan result from complete nondisjunction during meiosispolyploids are more nearly normal than aneuploids – why?mosaicismchrom. abnormalities that do not show up in every cellonly present in some cells and tissuesan ind. has two populations of cells with different genotypesboth came from a single fertilized eggusually results from mutations in mitosis, early in embryonic devel.symptoms less severe than if all cells are affected
19structural alterations of chromosomes alterations in the physical/chemical structure of chrom.most have harmful effects; but some beneficialdeletionsduplicationsoften have beneficial effects major evol. mechanisminversionstranslocations
21Fig. 10. 14. The results of a deletion Fig The results of a deletion. When chromosome #7 loses an end piece, the result is Williams Syndrome. These children, although unrelated, have the same appearance, health, and behavioral problems
22Another result of a deletion Another result of a deletion. When a group of genes are accidentally deleted from chromosome #5, the result it Cri du Chat syndrome.
23Fig. 10. 15 The results of a translocation Fig The results of a translocation. When chromosomes #2 and #20 exchange segments, the result is Alagille Syndrome. Individuals have distinctive facial features because the translocation disrupts an allele on chromosome #20.