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Chapter 14 Mendel
genetics
Gregor Mendel
• Pea plants–Advantages
•Variety of characteristics–Seed color
•Many different traits–Yellow, green
To begin
• Self pollination –True breed–Asexual–P generation
• Cross pollination –Hybrid–F1 generation
Next
• F1 generation• Self pollinate• F2 generation
–the law of segregation– the law of independent
assortment.
Law of segregation
• Dominant alleles–The one expressed
• Recessive alleles–The one masked
–Alleles appear on certain locus on a certain chromosome
Law of segregation
• Where do the alleles come from?
• If the alleles differ, than one becomes dominant and the other recessive
Law of Segregation
• States: That the two alleles for a heritable
character separate and segregate during gamete production and end up in different gametes
(meiosis – homologous chromosomes)
Punnett Squares
Mendel used the probability chart to guess the genetic outcome
Dominant – big letter
Recessive- small letter
Vocabulary
• Homozygous – identical alleles for a character
• Heterozygous – different alleles for a character
• Genotype – genetic make up • Phenotype – physical traits• Monohybrid – one allele type• Dihybrid – two allele types
Law of independent assortment • Dihybrids produced a 9:3:3:1 ratio• States
Each pair of alleles segregates independently during gamete formation
*We are talking about genes on nonhomologous chromosomes
Multiplication Rule
• The outcome of one coin toss has no impact on the next coin toss–Independent …just like
distribution of alleles
Multiplication Rule
• 1. compute the probability of each independent event
• 2. multiply the individual probabilities to get the probability of both occurring at the same time
practice
• What is the probability that you will get a TTRR- tall red plant in a dihybrid
Rule of addition
• The probability of an event can be made in two or more different ways
• 1. compute each independently • 2. add the probabilities
We can combine the two
• What is the probability of an offspring having two recessive phenotypes for at least two or three traits resulting from a trihybrid between PpYyRr and Ppyyrr
• What are all the possible genotypes?
Use rule of multiplication first
• Start with ppyyRr• pp= • yy= • Rr=
• All together =
Rule of multiplication
• Now do it for each genotype set: • DO YOU TRUST ME
ppYyrr= 1/16 • Ppyyrr= 1/8• Ppyyrr= 1/16• Ppyyrr= 1/16
Now addition
• The chance that a given offspring will have at least two recessive traits is
1/16 + 1/8 + 1/16 + 1/16 + 1/16 = 6/16
Deviations from Mendelian patterns
• Some alleles behave differently in relation to each other–Codominance
• Two alleles affect the phenotype–Incomplete dominance
• Two heterozygous alleles show an intermediate phenotype
Common misconceptions
• T or F because the allele is dominant it is more prevalent that the recessive allele
• T or F the nucleotide sequence of the dominant allele and recessive allele interact on the chromosome
Extra Information
• Pleitropic- most genes affect more than one phenotypic character–One gene can affect a number of
characteristics –Example : sickle cell
• Swollen hands and feet, prone to infections, growth delays, vision problems
Extra Information
• Epistasis- a gene at one locus alters the phenotypic expression of a gene at a second locus –Example: albinism
Pedigrees