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Gregor Mendel Pea Plants and Inheritance Patterns
Mendel At the Monastery…
Mendel’s Hypothesis • There are alternative forms for genes, the units that determine inheritable characteristics (AA or Aa or aa) • For each inherited characteristic, an organism has two alleles, one inherited from each parent. • A sperm or egg carries only one allele (A or a)for each inherited characteristic, because allele pairs separate from each other during meiosis. • When the two alleles of a pair are different, one is fully expressed (dominant) and the other is completely masked (recessive). • The members of the pair may be identical (homozygous) or non-identical (heterozygous).
Revisiting Meiosis Principle of Independent Assortment: The assortment of one pair of genes into gametes is independent of the assortment of another pair of genes. The alignment of one pair of homologs is independent of any other.
PP pp x X P purple white F1 all purple Making crosses • Can represent alleles as letters • flower color alleles P or p • true-breeding purple-flower peas PP • true-breeding white-flower peas pp Pp
PP 25% male / sperm P p Pp 50% 75% P Pp female / eggs pp p 25% 25% Punnett squares Aaaaah, phenotype & genotypecan have different ratios 1st generation (hybrids) Pp x Pp % genotype % phenotype PP Pp Pp pp 1:2:1 3:1
Extending Mendelian genetics • Mendel worked with a simple system • peas are genetically simple • most traits are controlled by single gene • each gene has only 2 version • 1 completely dominant (A) • 1 recessive (a) • But its usually not that simple!
Incomplete dominance • Hybrids have “in-between” appearance • RR = red flowers • rr = white flowers • Rr = pink flowers • make 50% less color RR WW RW RR Rr rr
100% pink flowers 1st generation (hybrids) 100% 25% red 50% pink 25% white 1:2:1 2nd generation Incomplete dominance X true-breeding red flowers true-breeding white flowers P self-pollinate
RR 25% 25% male / sperm R W RW 50% 50% R RW female / eggs WW W 25% 25% Incomplete dominance RW x RW % genotype % phenotype RR RW RW WW 1:2:1 1:2:1
Codominance • Equal dominance • human ABO blood groups • 3 version • A, B, i • A& B alleles are codominant • both A& B alleles are dominant over i allele • the genes code for different sugars on the surface of red blood cells • “name tag” of red blood cell
Blood donation clotting clotting clotting clotting clotting clotting clotting
One gene: many effects • The genes that we have covered so far affect only one trait • But most genes are affect many traits • 1 gene affects more than 1 trait • dwarfism (achondroplasia) • gigantism (acromegaly)
Inheritance pattern of Achondroplasia Aa x aa Aa x Aa a a A a Aa A Aa A AA Aa a a aa aa Aa aa 50% dwarf:50% normal or1:1 67% dwarf:33%normalor2:1
Many genes: one trait • Polygenic inheritance • additive effects of many genes • humans • skin color • height • weight • eye color • intelligence • behaviors
Human skin color • AaBbCc x AaBbCc • can produce a wide range of shades • most children = intermediate skin color • some can be very light & very dark
Albinios Johnny & Edgar Winter albinoAfricans melanin = universal brown color
Coat color in other animals • 2 genes: E,e and B,b • color (E) or no color (e) • how dark color will be: black (B) or brown (b) eebb eeB– E–bb E–B–
Environment effect on genes • Phenotype is controlled by both environment & genes Coat color in arctic fox influenced by heat sensitive alleles Human skin color is influenced by both genetics & environmental conditions Color of Hydrangea flowers is influenced by soil pH
Genetics of sex • Women & men are very different, but just a few genes create that difference • In mammals = 2 sex chromosomes • X & Y • 2 X chromosomes = female: XX • X & Y chromosome = male: XY X X X Y
XXh XXh X XY XY Xh male / sperm X Y XY X X female / eggs Y Xh XhY Sex-Linked Traits 2 normal parents, but mother is carrier x XX XXh
Sex-Linked Traits • Sex chromosomes have other genes on them, especially the X chromosome • Hemophilia in humans • Blood doesn’t clot • Duchenne muscular dystrophy in humans • Loss of muscle control • Red-green color blindness • See green & red as shades of grey
Polydactyly • Individuals are born with extra fingers and toes • The allele for 6+ fingers and toes is dominant,while the allele for 5 digits is recessive • Recessive is far more common! (1:500 have polydactyly)