10.2 dihybrid crosses and gene linkage
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10.2 Dihybrid Crosses and Gene Linkage Learning Targets: Calculate and predict offspring of dihybrid crosses. Define linkage group and identify recombinant offspring. Gregor Mendel (1822-1884) Austrian monk who bred pea plants and studied inheritance - PowerPoint PPT PresentationTRANSCRIPT
10.2 Dihybrid Crosses and Gene Linkage
Learning Targets: Calculate and predict offspring of dihybrid crosses. Define linkage
group and identify recombinant offspring.
• Gregor Mendel
(1822-1884)
• Austrian monk who bred pea plants and studied inheritance
• He started with “true-breeding” plants (when self-fertilized, all offspring will have same traits = homozygous), and followed traits through three generations
• P1 generation – parental
• F1 generation – first filial (kids)
• F2 generation – second filial (grandkids)
Mendel’s Hypothesis:
• Different alleles (versions of genes) produce different traits (*although Mendel didn’t actually know about genes)
• Each organism inherits two alleles for every trait; one from each parent
Mendel’s Hypothesis:
• If two alleles are different, the dominant allele will be expressed, the recessive allele will have no noticeable effect (*Dominance is NOT linked to frequency in a population)
• Alleles (on chromosomes) separate/segregate during meiosis – each parent only passes on one copy for each trait (This is Mendel’s “Law of Segregation”)
Homozygous – two identical alleles: TT, ttHeterozygous – two different alleles: Tt
Homozygous dominant
Heterozygous Homozygous recessive
Genotype – allele
combination
Phenotype – physical
appearance
Homozygous dominant
Heterozygous Homozygous recessive
Genotype – allele
combination
TT Tt tt
Phenotype – physical
appearance
Tall Tall Short
Homozygous – two identical alleles: Tt, ttHeterozygous – two different alleles: Tt
Testcross – in order to determine the genotype of a dominant phenotype, cross a homozyogous recessive with the unknown genotype
Ex) If P = purple, and p = white, what are the possible genotypes of a purple flower?
• Testcross: If the offspring phenotype ratio is 1 purple : 1 white, what is the genotype of the purple flower?
• Prove it with a Punnett square:
Genetics Problems Steps
• Make a key of dominant and recessive alleles and assign letters
• Determine parent genotypes and record Determine alleles for possible gametes
• Solve (often using a Punnett square) and report offspring genotypes and phenotypes.
Punnett Squares:• Show all possible
combinations of alleles that result from a genetic cross
• Follow probability rules• Possible gametes go
around outside of square
• Offspring genotypes are filled in inside squares
Parent genotypes:
TT x tt
Punnett Squares:• Show all possible
combinations of alleles that result from a genetic cross
• Follow probability rules• Possible gametes go
around outside of square
• Offspring genotypes are filled in inside squares
Parent genotypes:
TT x tt
T T
t
t
Punnett Squares:• Show all possible
combinations of alleles that result from a genetic cross
• Follow probability rules• Possible gametes go
around outside of square
• Offspring genotypes are filled in inside squares
Tt Tt
Tt Tt
T T
t
t
Parent genotypes:
TT x tt
• Genotypes of offspring: All Tt
• Phenotypes of offspring: All Tall Tt Tt
Tt Tt
T T
t
t
Parent genotypes:
TT x tt
• Mendel hypothesized that if two traits are followed, they will assort independently and offspring could get any possible combination (This is Mendel’s “Law of Independent Assortment”)
This is supported with a dihybrid cross:
Ex) Two parents are heterozygous for hairy fingers (hairy fingers, H, is dominant) and widow’s peak (widow’s peak, W, is dominant)
Parent genotypes: ________ x _______
Parent genotypes: HhWw x HhWw
Possible gametes parents can pass on:
___, ___, ___, ___ (Hint: FOIL)
Parent genotypes: HhWw x HhWw
Possible gametes parents can pass on:
HW, Hw, hW, hw
HW Hw hW hw
Parent genotypes: HhWw x HhWw
Possible gametes parents can pass on:
HW, Hw, hW, hw
HW Hw hW hw
HW Hw
hW
hw
Parent genotypes: HhWw x HhWw
Possible gametes parents can pass on:
HW, Hw, hW, hw
HHWW HHWw HhWW HhWw
HW Hw hW hw
HW Hw
hW
hw
Parent genotypes: HhWw x HhWw
Possible gametes parents can pass on:
HW, Hw, hW, hw
HHWW HHWw HhWW HhWw
HHWw HHww HhWw Hhww
HhWW HhWw hhWW hhWw
HhWw Hhww hhWw hhww
HW Hw hW hw
HW Hw
hW
hw
Parent genotypes: HhWw x HhWw
Possible gametes parents can pass on:
HW, Hw, hW, hw
HHWW HHWw HhWW HhWw
HHWw HHww HhWw Hhww
HhWW HhWw hhWW hhWw
HhWw Hhww hhWw hhww
HW Hw hW hw
HW Hw
hW
hw
Genotypes of offspring:
Phenotypes of offspring:
Parent genotypes: HhWw x HhWw
Possible gametes parents can pass on:
HW, Hw, hW, hw
HHWW HHWw HhWW HhWw
HHWw HHww HhWw Hhww
HhWW HhWw hhWW hhWw
HhWw Hhww hhWw hhww
HW Hw hW hw
HW Hw
hW
hw
Genotypes of offspring:1HHWW: 2HHWw:
Parent genotypes: HhWw x HhWw
Possible gametes parents can pass on:
HW, Hw, hW, hw
HHWW HHWw HhWW HhWw
HHWw HHww HhWw Hhww
HhWW HhWw hhWW hhWw
HhWw Hhww hhWw hhww
HW Hw hW hw
HW Hw
hW
hw
Genotypes of offspring:1HHWW: 2HHWw: 1HHww: 2HhWW: 4HhWw: 2Hhww: 1hhWW: 2hhWw: 1hhww
HHWW HHWw HhWW HhWw
HHWw HHww HhWw Hhww
HhWW HhWw hhWW hhWw
HhWw Hhww hhWw hhww
HW Hw hW hw
HW Hw
hW
hw
Genotypes of offspring:1HHWW: 2HHWw: 1HHww: 2HhWW: 4HhWw: 2Hhww: 1hhWW: 2hhWw: 1hhww
Phenotypes of offspring:9 hairy fingers, widow’s peak:
HHWW HHWw HhWW HhWw
HHWw HHww HhWw Hhww
HhWW HhWw hhWW hhWw
HhWw Hhww hhWw hhww
HW Hw hW hw
HW Hw
hW
hw
Genotypes of offspring:1HHWW: 2HHWw: 1HHww: 2HhWW: 4HhWw: 2Hhww: 1hhWW: 2hhWw: 1hhww
Phenotypes of offspring:9 hairy fingers, widow’s peak: 3 hairy fingers, no widow’s peak: 3 hairless fingers, widow’s peak: 1 hairless fingers, no widow’s peak
• Now we understand, however, that while chromosomes assort independently, genes may be linked if they are on the same chromosome.
Linkage group – any two genes on the same chromosome. These are often passed on together in gametes and do not show predictable (Mendelian) ratios.
The recombinant chromatids show new combinations. Circle the recombinants.
Record the IB notation below:AB AB Ab aB abab
Linked gene example:
If the genes for traits X and Y are linked, identify the recombinants if a heterozygote is crossed with a homozygous recessive.
Parent genotypes:XY x xyxy xy
Recombinant offspring:Xy and xYxy xy
Possible gametes:XxYy XY, Xy, xY, xyxyxy xy