Genetics The Work of Mendel Gregor Mendel Modern

Genetics & The Work of Mendel

Gregor Mendel • Modern genetics began in the mid-1800 s 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

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

Mendel collected data for 7 pea traits

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

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

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 X P Explain Mendel’s results using …dominant & recessive …phenotype & genotype purple white F 1 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 X P purple white Pp F 1 all purple

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

Any Questions? ?

Beyond Mendel’s Laws of Inheritance 2007 -2008

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

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

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

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 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

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

Many genes: one trait • Polygenic inheritance – additive effects of many genes – humans • • • skin color height weight eye color intelligence behaviors

Human skin color • Aa. Bb. Cc x Aa. Bb. Cc – can produce a wide range of shades – most children = intermediate skin color – some can be very light & very dark

Albinism melanin = universal brown color Johnny & Edgar Winter

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 ee. B– E–bb E–B–

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

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

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 X X • loss of muscle control – red-green color blindness • see green & red as shades of grey X Y

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

Polydactyly individuals are born with extra fingers or toes the allele for >5 fingers/toes is DOMINANT & the allele for 5 digits is 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)

Hound Dog Taylor