genetics how are traits passed from parents to offspring?
TRANSCRIPT
Gregor Mendel
• Austrian monk• Experimented with pea plants– Produce lots of offspring– Readily observable traits– Mating is easily controlled– Pure-breeding strains were available
• Studied several traits, but only one at a time
• Phenotype – physical appearance of an organism
• Tall or short• Round or wrinkled• Yellow or green seed
• Parents – purple flowers and white flowers – Both were pure breeding
• F1 – first offspring generation– All had purple flowers– These plants were allowed to self-fertilize
• F2 – second offspring generation– 3 purple-flowered plants– 1 white-flowered plant
• Where was the white-flowered trait in the parents (all purple)?
• Why do the crosses produce a 3:1 ratio of purple to white?
• Mendel concluded that the white trait must be hidden in the purple flowers
• The trait that is hidden or masked is said to be recessive
• The trait that is expressed is said to be dominant
• Purple is dominant over white
Law of Segregation
• The 3:1 ratio suggested to Mendel that each organism carried two factors (genes) for the trait flower color
• These factors separate (segregate) during the formation of sex cells (pollen and eggs) so that each sex cell only has one factor (gene) for a trait
Genotype
• Genetic make-up of an organism• Allele – forms of a gene– Ex- flower color alleles are purple and white
• Homozygous –having two of the same allele for a gene
• Heterozygous- having one dominant and one recessive allele
Representing the genotype
• Dominant allele is represented by capital letter
• Recessive allele is represented lower-case letter
• Ex – Flower color– P – purple– p - white
F1 self fertilizes
• Pp x Pp
• Sex cells P p P p
• Offspring PP Pp Pp pp
• Sex cells combine randomly
• Write the sex cells from one individual along the left-hand side of the square
• Write the sex cells from the second individual along the top
• Combine row and column to produce the offspring genotypes
Dihybrid crosses
• Monohybrid crosses follow one trait at a time• Dihybrid crosses follow two traits
• P Round seeds X wrinkled seeds
• F1 all round
• F2 3 Round : 1 wrinkled• Conclusions : round is dominant over wrinkled
• P Yellow seed x green seed
• F1 yellow seed
• F2 3 Yellow : 1 green• Conclusion: Yellow is dominant over green
• P Round-Yellow x wrinkled-green
• F1 all round-yellow
• F2 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green
• Note: 12 round : 4 wrinkled• 12 yellow : 4 green
The F2 phenotype ratios are unchanged
How did Mendel explain these observations?
Law of independent assortment
• The inheritance of one trait has no effect on the inheritance of a second trait
• Ex- the shape of a seed does not affect its color and vice versa
• When sex cells are formed, a cell receives one allele for each trait being studied
Results
• 9 round yellow: 3 round green: 3 wrinkled yellow: 1 wrinkled green
• 1 RRYY: 2 RRYy: 1 RRyy• 2 RrYY : 4 RrYy : 2 Rryy• 1 rrYY : 2 rr Yy : 1 rryy
Incomplete dominance
• Two or more alleles influence the phenotype producing an individual that is intermediate between the two alleles
• Ex. Four-o-clocks• Red Flowers X White Flowers
Pink Flowers
• I A - blood cell carries polysaccharide A• I B - blood cell carries polysaccharide B• i – neither A nor B is present
• Traits such as hemophilia and red-green color blindness occur more often in males than females
• Traits are found on the sex chromosome (X)• Females - XX• Males - XY– Males only receive one allele for these traits
Polygenic Inheritance
• Many genes influence a trait• Examples – height, eye color, skin color• Graded variation in the trait