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Transcript
Midterm Friday - or - Monday? Covers Chapters 1,2 and 3.6 If yed, it will mean moving Exam 2 to May 6. Sex Chromosomes ... ‘X’ and ‘Y’ chromosomes that determine the sex of an individual in many organisms, Females: XX Males: XY a A a Differential Region Differential Region Paring Region Paring Region male XX: female a hemizygous: condition where gene is present in only one dose. male X Linkage …the pattern of inheritance resulting from genes located on the X chromosome. X-Linked Genes …refers specifically to genes on the Xchromosome, with no homologs on the Y chromosome. P x Blue Female Gametes Pink Male or Gametes or F1 Blue Female Blue Male F1 x Blue Female Gametes or Blue Male or Gametes or or F2 Blue Female Blue Male Blue Female Pink Male F2 Blue Female Blue Male Blue Female 3:1 Pink Male P x Pink Female Gametes Blue Male or Gametes or F1 Blue Female Pink Male Gametes or or F2 Pink Female Pink Male Blue Female Blue Male F2 Pink Female 1 Pink Male Blue Female Blue Male 1 1 1 Sex Linkage to Ponder • Female is homozygous recessive X-linked gene, – what percentage of male offspring will express? – what percentage of female offspring will express if, • mate is hemizygous for the recessive allele? • mate is hemizygous for the dominant allele? • Repeat at home with female heterozygous Xlinked gene! Sex-Linked vs. Autosomal • autosomal chromosome: non-sex linked chromosome, • autosomal gene: a gene on an autosomal chromosome, • autosomes segregate identically in reciprocal crosses. X-Linked Recessive Traits Characteristics • Many more males than females show the phenotype, – female must have both parents carrying the allele, – male only needs a mother with the allele, • Few (or none) of the offspring of affected males show the disorder, – all of his daughters are carriers, • half of the sons born to these daughters are carriers. X-Linked Dominant • Affected males married to unaffected females pass the phenotype to their daughters, but not to their sons, • Heterozygous females married to unaffected males pass the phenotype on to half their sons and daughters, • Homozygous dominant females pass the phenotype on to all their sons and daughters, Autosomal Dominant • Phenotypes appear in every generation, • Affected males and females pass the phenotype to equal proportions of their sons and daughters. Pedigree for Very Rare Trait 1/2 1/2 Recessive? ---> Yes! X-Linked? Autosomal? ---> Yes! 1/2 x 1/2 x 1/2 ? = 1/8 Linkage • Genes located on the same chromosome segregate together and are said to be linked. Independent Assortment A A a B B b A b 2n = 4 a B a b Meiosis Prophase I cont. A a A a B B b b Chiasmata and crossing over 2n = 1 Meiosis No Cross Over A a B b Parent Cell A A a a B B b b Daughter Cells Have Parental Chromosomes 2n = 1 Meiosis With Cross Over A a B b Parent Cell A A a a B b B b Daughter Cells Have Recombinant Chromosomes Gamate Formation in F1 Dihybrids P: GGWW x ggww F1 Genotype: GgWw G g W w alleles GW Gw gW gw gametes .25 .25 .25 .25 probability How do you test for assortment of alleles? F1: GgWw GW Gw gW gw .25 .25 .25 .25 Test Cross • phenotypes of the offspring indicate the genotype of the gametes produced by the parent in question. Test Cross GgWw x ggww GW (.25) gw (1) GgWw (.25) Gw (.25) gw (1) G gww (.25) gW (.25) gw (1) ggWw (.25) gw (.25) gw (1) ggww (.25) Test Cross GgWw x ggww GW (.25) gw (1) GgWw (.25) P Gw (.25) gw (1) Ggww (.25) R gW (.25) gw (1) ggWw (.25) R gw (.25) gw (1) ggww (.25) P parental types GgWw and gwgw recombinant types Ggww and ggWw P: GGWW and ggww Recombination Frequency …or Linkage Ratio: the percentage of recombinant types, – if 50%, then the genes are not linked, – if less than 50%, then linkage is observed. Linkage • Genes located on the same chromosome do not recombine, – unless crossing over occurs, • The recombination frequency gives an estimate of the distance between the genes. Recombination Frequencies • Genes that are adjacent have a recombination frequency near 0%, • Genes that are very far apart on a chromosome have a linkage ratio of 50%, • The relative distance between linked genes influences the amount of recombination observed. Dihybrid Cross P yellow/round GGWW green/wrinkled x GW F1 ggww gw GgWw phenotype genotype gametes genotype Questions • Don’t rely on the answers in the back of the book to solve your problems… – Don’t just solve them, but understand the principles needed to solve them. • More practice questions are online. No answers are available, i.e. confirm your own answers a b f c e d