F2 Pink FemalePink MaleBlue FemaleBlue Male Gametes or
F2 Pink FemalePink MaleBlue FemaleBlue Male 11 11
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 X- linked 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,
Recessive?---> Yes! Pedigree for Very Rare Trait Autosomal?X-Linked?---> Yes! 1/2 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.
A b 2n = 4 Independent Assortment a A A B B b a B a b
Meiosis Prophase I cont. Chiasmata and crossing over AaAa BBbb aA
Meiosis No Cross Over Parent Cell Daughter Cells Have Parental Chromosomes Aa Bb A B a b a b A B 2n = 1
Meiosis With Cross Over Parent Cell Daughter Cells Have Recombinant Chromosomes Aa Bb A B A b a b a B 2n = 1
Gamate Formation in F1 Dihybrids P: GGWW x ggww G g W w GWGwgWgw alleles gametes probability.25 F1 Genotype: GgWw
How do you test for assortment of alleles? GWGwgWgw.25 F1: GgWw
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 (.25) gW (.25) gw (.25) gw (1) G gww (.25) GgWw (.25) ggWw (.25) ggww (.25)
Test Cross GgWw x ggww GW (.25) Gw (.25) gW (.25) gw (.25) gw (1) Ggww (.25) GgWw (.25) ggWw (.25) ggww (.25) P P parental types GgWw and gwgwP: GGWW and ggww R R recombinant types 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 yellow/round green/wrinkled GGWW x ggww GW gw GgWw phenotype genotype gametes genotype P F1
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