Chapter 15: The Chromosomal Basis of Inheritance PDF

**Chapter 15** ============== THE CHROMOSOMAL BASIS OF INHERITANCE ==================================== \>\>\>\>\>\>Mendelian inheritance has it physical basis in the behavior of chromosomes during sexual life cycles. -Chromosomes and genes are both paired in diploid cells. -Chromosomes separate...

**Chapter 15** ============== THE CHROMOSOMAL BASIS OF INHERITANCE ==================================== \>\>\>\>\>\>Mendelian inheritance has it physical basis in the behavior of chromosomes during sexual life cycles. -Chromosomes and genes are both paired in diploid cells. -Chromosomes separate during the formation of gametes and allele pair segregation. -Fertilization restores diploid condition for both chromosomes and genes. \>From these observations the **chromosomal theory of inheritance** arose. A genetics and cytology time line ----------------------------------------------------------------------------------------------------------------------------- -------------------------------------------- GENETICS CYTOLOGY 1860\'s Mendel proposes discrete inherited factors segregate and assort independently during gamete formation 1875 Cytologist work out basics of mitosis 1890 Cytologist work out basics of meiosis 1900 Mendel's work "rediscovered" 1902 Cytology and genetic converge as behavioral similarities between inherited factors and the chromosomes become apparent ----------------------------------------------------------------------------------------------------------------------------- -------------------------------------------- \>\>\>\>\>\>**Linked genes** tend to be inherited together because they are located on the same chromosome. \-\--**Linked genes** -Since independent assortment doesn't occur, the 9:3:3:1 ratio is not seen **Linking Genes to Chromosomes: The Work of Morgan** \>\>\>\>\>\>Thomas H. Morgan traced a gene to a specific chromosome Morgan worked with fruit flies, *Drosophila melanogaster* **Note on genetic symbols:** -An allele is named after the non-**wild-type**, or **mutant phenotype** -The wild-type allele is dominant and the mutant allele is recessive -The wild-type trait is designated with a superscript + sign. \-\--**Wild-type** \-\--**Mutant phenotypes** \>The discovery of a **sex-linked gene (see fig. 15.3)** Morgan crossed a white-eyed (w) male with a red-eyed (w^+^) female In the F~1~ the white-eyed trait disappeared, therefore the red-eyed trait is dominant. In the F~2~ generation the white-eyed trait reappeared BUT ONLY IN THE MALES! **Morgan's Conclusions** Morgan deduced that the gene for eye color is located on the sex chromosomes \-\--**Sex-linked genes** Morgan found the evidence for linked genes in a dihybrid testcross between flies with autosomal recessive traits and heterozygous wild-type flies. The expected 1:1:1:1 phenotype did not occur. Instead more flies had the parental phenotype. Morgan proposed that this shift toward the parental phenotype was due to linkage of the genes. \>\>\>\>\>\>Independent assortment of chromosomes and crossing over cause **genetic recombination** \-\--**Genetic recombination** Remember that recombination of unlinked genes is a result of independent assortment. Back to Mendel Test cross (YyRr x yyrr) =\> 2 **parental types**, 2 **genetic recombinants** Yr yr yR yr ---- -------------- ----------------- ---------- ---------- yr 1/4 YyRr 1/4 yyrr 1/4 yyRr 1/4 Yyrr 1/2 Parental 1/2 Recombinant -50% recombination expected when 2 genes are located on different chromosomes because of INDEPENDENT ASSORTMENT \-\--**Parental types** \-\--**Genetic recombinants** **Crossing Over and Recombination: A Tool for Mapping Genes Using Recombination Frequencies** \>\>\>Back to the flies and their linked genes (see fig 15.9) If the genes were linked, then they should not recombine into assortments not found in parents. Why aren't ALL of the offspring parental types? Test cross: b^+^b vg^+^vg x bb vgvg 965 grey/long 944 black/vest. 206 black/long 185 grey/vest. \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ Parental Recombinants This cross has a 17% recombination frequency (see fig 15.10) The recombination was due to crossing over **Sex-Linked Traits in Humans** \>\>\>\>\>\>The chromosomal basis of sex produces unique patterns of inheritance Sex is a phenotypic character determined by inherited chromosomes. \>There are different systems for chromosomal sex determination (see fig 15.6) **The X-Y and X-0 systems** -In these systems the male is **heterogametic** and the female is -The sex of the offspring is determined at fertilization -The male to female ratio is 1:1 **Heterogametic** versus **Homogametic** \-\--**Heterogametic** - two types of gametes \-\--**Homogametic** - one type of gamete =\> X-Y: Drosophila, humans, other mammals -sex determined by whether male gamete (sperm) bears X or Y -for the somatic cells the female is XX and the male is XY =\> X-0: Grasshoppers, roaches, other insects -sex determined by number of X chromosomes, 1/2 of the male gametes -for the somatic cells the female is XX and the male is X0 **The Z-W systems** -birds, some fish and insects (butterflies) -the female is the heterogametic sex and the male is homogametic -for the somatic cells the female is ZW and the male is ZZ **The Haplo-diploid system** -ants and bees -no sex chromosomes present -queen lays eggs and only some are fertilized -females from fertilized eggs (diploid) \>\>\>SEX-LINKED INHERITANCE -traits unrelated to sex are on the sex chromosomes -in humans, \"sex-linked\" usually mean \"X chromosome linked\" -X chromosome is bigger than the Y chromosome and therefore -most X chromosomes have no homologous loci on the Y \>\>\>Transmissions of sex-linked genes (see fig. 15.7) -Mothers pass X to sons and daughters -recessive sex-linked traits appear in males twice as often as in **Problems in Heredity** \>\>\>\>\>\>Alterations of chromosome number or structure Errors during meiosis or the effect of mutagens can result in major chromosomal changes \>\>\>Changes in chromosomal number: **Aneuploidy** and **polyploidy** Changes in chromosomal number results from a **nondisjunction** during meiosis (I or II) or mitosis (see fig. 15.13) \-\--**Nondisjunction** \-\--**Aneuploidy** -Trisomy =\> 2N + 1 \-- Down\'s syndrome \"Trisomy 21\" (see fig 15.15) -Monosomy =\> 2N - 1 -Mitosis transmits the anomaly \-\--**Polyploidy** -Tetraploidy (4N) -Polyploidy is common in plants