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Unformatted text preview: character segregate into different gametes. Homologous pair of chromosomes Figure 14.4 Allele for white flowers 22. For each character, an individual inherits 2 alleles (1 from each parental gamete)
remarkably, Mendel didn’t even know about chromosomes! 3 3. If two alleles differ, the dominant allele determines character appearance Two alleles in an individual can be identical The other recessive allele has no noticeable effect
P Generation (true-breeding parents) Purple flowers White flowers ! 1 chromosome in pollen 1 chromosome in egg Or, Two alleles in an individual can be different
All plants had purple flowers Both chromosomes - and therefore both alleles - carried in hybrid F1 seed 4 4. Two alleles for a heritable character segregate into different gametes The Punnett Square:
Diagram to account for possible allele composition of offspring P (Purple) p (white) P1 p1 x P2 p2
sperm 1 p1 The Law of Segregation
“During gamete formation, different alleles end up in different gametes” yields 4 possible combinations: F2 Generation 1/4 = 1/4 = MEIOSIS gametes 1/4 = 1/4 = P1 P2 P 1 p2 p1 P2 p 1 p2 1 white 3 purple
P P 2
PP Pp egg
Pp pp This segregation corresponds to the distribution of homologous chromosomes to different gametes in meiosis Random combination of the gametes results in a 3:1 ratio - just like Mendel observed in F2 generation. Figure 14.5 Allele Combinations
An organism’s traits (appearance or function) Phenotype vs. Genotype
PP (homozygous) Pp (heterozygous)
2 1 Genotype
An organism’s genetic make-up 3 Purple Homozygous
Organism has a pair of identical alleles for gene, “true-breeding”
1 Purple Pp (heterozygous) Heterozygous
Organism has a pair of different alleles for gene
Figure 14.6 White pp (homozygous)
Ratio 1:2:1 1 Ratio 3:1 The Testcross
Mendel designed the Testcross to determine an unknown genotype of an individual with the dominant phenotype. Mendel’s Law of Independent Assortment
Mendel identified his second law of inheritance by following two characters at the same time. Unknown genotype?
PP or Pp? ! Known genotype
pp Are two characters inherited: …together? …or separately? …or Test reveals genotype! If PP, All purple:
p p 1/2 If Pp, purple, 1/2 white:
p p P Pp P Pp Pp Pp P Pp Pp p pp pp Figure 14.7 Dihybrid Cross
P Generation YYRR yyrr Gametes YR ! yr Figure 14.8 Mendel’s Law of Independent Assortment Dihybrids: If F1 generation is heterozygous for 2 characters… …then in the F2 generat...
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This note was uploaded on 04/03/2010 for the course BIOLOGY BILD 1 taught by Professor Traver during the Winter '10 term at UCSD.
- Winter '10