Genetics & The Work of Mendel

Modern genetics began in the mid-1800s in an abbey garden, where a monk named Gregor Mendel documented inheritance in peas –used good experimental design –used mathematical analysis collected data & counted them –excellent example of scientific method Gregor Mendel

Pollen transferred from white flower to stigma of purple flower all purple flowers result Mendel’s work ? self-pollinate Bred pea plants –cross-pollinate true breeding parents –raised seed & then observed traits –allowed offspring to self-pollinate & observed next generation

Mendel collected data for 7 pea traits

2 nd generation 3:1 75% purple-flower peas 25% white-flower peas Looking closer at Mendel’s work Parents 100% 1 st generation (hybrids) 100% purple-flower peas X true-breeding purple-flower peas true-breeding white-flower peas self-pollinate

What did Mendel’s findings mean? Some traits mask others –purple & white flower colors are separate traits that do not blend purple x white ≠ light purple purple masked white –dominant allele functional protein –affects characteristic masks other alleles –recessive allele no noticeable effect allele makes a non-functioning protein homologous chromosomes I’ll speak for both of us! allele producing functional protein mutant allele malfunctioning protein

Genotype vs. phenotype Difference between how an organism “looks” & its genetics –phenotype description of an organism’s trait –genotype description of an organism’s genetic makeup Explain Mendel’s results using …dominant & recessive …phenotype & genotype F1F1 P X purplewhite 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 x pp PpPp F1F1 P X purplewhite all purple

Punnett squares Pp x Pp Pp male / sperm P p female / eggs PP 75% 25% 3:1 25% 50% 25% 1:2:1 % genotype % phenotype PP PpPp PpPp pp PpPp PpPp 1 st generation (hybrids) Aaaaah, phenotype & genotype can have different ratios

Any Questions??

Beyond Mendel’s Laws of Inheritance

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 RRRrrr  RR  WW  RW

Incomplete dominance true-breeding red flowers true-breeding white flowers X P 100% 100% pink flowers 1 st generation (hybrids) self-pollinate 25% White 2 nd generation 25% Red 1:2:1 50% Pink

Incomplete dominance RW male / sperm R W female / eggs RR RW WW RW 25% 1:2:1 25% 50% 25% 1:2:1 % genotype % phenotype RR RW WW 25% 50% RW x RW

Codominance Equal dominance –human ABO blood groups –3 versions 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

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)

Acromegaly: André the Giant

Aa x aa Inheritance pattern of Achondroplasia aa A a Aa A a Aa x Aa Aa aa Aa 50% dwarf:50% normal or 1:1 AA aa Aa 67% dwarf:33% normal or 2:1 Aa

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

Albinism Johnny & Edgar Winter 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) E–B–E–bbeeB–eebb

Phenotype is controlled by both environment & genes Color of Hydrangea flowers is influenced by soil pH Human skin color is influenced by both genetics & environmental conditions Environment effect on genes Coat color in arctic fox influenced by heat sensitive alleles

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 XY XX

Sex chromosomes

Sex-linked traits Sex chromosomes have other genes on them, too –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 XY XX

Dominant ≠ most common allele Polydactyly dominant allele Because an allele is dominant does not mean… –it is better, or –it is more common

Polydactyly recessive allele far more common than dominant  only 1 individual out of 500 has more than 5 fingers/toes  so 499 out of 500 people are homozygous recessive (aa) the allele for >5 fingers/toes is DOMINANT & the allele for 5 digits is recessive individuals are born with extra fingers or toes

Hound Dog Taylor