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Genetics

Genetics. 10.2 and 10.3. 10.2.2. Autosomes: any chromosome which is not a sex chromosome (22 pairs in humans) Autosomal traits/genes/inheritance Sex chromosomes: X and Y chromosomes which determine sex (1 pair in humans) Sex-linked traits/genes/inheritance. DIHYBRID CROSS.

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Genetics

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  1. Genetics 10.2 and 10.3

  2. 10.2.2 • Autosomes: any chromosome which is not a sex chromosome • (22 pairs in humans) • Autosomal traits/genes/inheritance • Sex chromosomes: X and Y chromosomes which determine sex • (1 pair in humans) • Sex-linked traits/genes/inheritance

  3. DIHYBRID CROSS • Mendel already knew from previous crosses that Round (R) was dominant to wrinkled (r) • Also he knew that Yellow (Y) was dominant to green (y) • Not surprisingly when he crossed peas that were always Round and Yellow with wrinkled green peas ...What did he predict of the F1 peas? • All Round and Yellow ...

  4. DIHYBRID CROSS • Here is the cross to produce the F1 generation...

  5. THE F2 CROSS • Mendel expected the F2 cross to produce a 3:1 ratio of Round Yellow Peas to wrinkled green peas • This is the way it worked in monohybrid crosses... X 3 Round Yellow : 1 wrinkled green

  6. SURPRISE !!! • Mendel did NOT get what he expected • He found a 9:3:3:1 ratio among four phenotypes instead of the 3:1 among two phenotypes • He found - 9 round yellow 3 round green 3 wrinkled yellow 1 wrinkled green

  7. Law of Independent Assortment • These results caused Mendel to propose another law called INDEPENDENT ASSORTMENT • This law states that at the time of reproduction the members of two or more pairs of unit characters follow the law of segregation independently of each other and assort themselves at random into the gametes

  8. IN OTHER WORDS .. If a parent has this genotype RrYy These are the Gametes RY Ry rY ry

  9. THE CROSS RY Ry rY ry RY RRYY RRYy RrYY RrYy Ry RRYy RRyy RrYy Rryy rY RrYY RrYy rrYY rrYy ry RrYy Rryy rrYy rryy

  10. 10.2.3 Linkage Group • Linkage Group - Genes that occur on the same chromosome • Example: Fly body color and wing shape • Usually flies inherit gray bodies (b+) and normal wings (vg+) • Mutant = black bodies (b) and vestigial wings (vg) • Morgan bred females with males (=testcross) • Expected 1 gray vestigial: 1 gray normal: 1 black vestigial: 1 black normal  b vg ___ __ +b +vg b vg ___ __ b vg

  11. Linkage Group • HOWEVER—he found more parental genotypes than expected! Why? • Deduced that these genes are inherited together because they are on the same chromosome (=linked) • Independent assortment assumes that genes are on different chromosomes (not traveling ‘together’). Morgan’s genes were linked.

  12. 10.2.3 Crossing over Crossing over between non-sister chromatids results in exchange of alleles

  13. Crossing Over • Linked genes do not segregate independently since they are on the same chromosome. • How can they become unlinked? • Crossing over! • The closer the genes are together, the more likely they will be passed on together. • Example: black bodies and vestigial wings in fruit flies • If the genes cross over in prophase I, then the ‘linked’ genes can get separated, creating recombinants.

  14. 10.2.5 – Cross between linked genes • Parental types: have the same phenotype as the parents • Recombinants: have different phenotypes than the parents • Example: Peas Y = yellow R = round y = green r = wrinkled • cross: YyRr x yyrr • Most of offspring will be yellow-round or wrinkled green (parental type) • Much fewer will be green-round or yellow-wrinkled (recombinants) • NOTE: If 50% of the offspring are recombinants, then the genes are located on different chromosomes. (unlinked)

  15. b+vg+ b+vg and b vg and b vg+ 10.2.6 • In flies: one parent is b+b vg+vg b+vg+ b vg • The gametes will be: • Recombinants will be:

  16. POLYGENIC INHERITANCE • This type of inheritance involves ..... “many genes” influencing the inheritance of one trait or characteristic • Human Skin Color, Height and Intelligence are examples • Expect a “Curve of Normal Distribution” for this type of inheritance

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