# Genetics & The Work of Mendel

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Genetics & The Work of Mendel

Gregor 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 He studied at the University of Vienna from 1851 to 1853 where he was influenced by a physicist who encouraged experimentation and the application of mathematics to science and a botanist who aroused Mendel’s interest in the causes of variation in plants. After the university, Mendel taught at the Brunn Modern School and lived in the local monastery. The monks at this monastery had a long tradition of interest in the breeding of plants, including peas. Around 1857, Mendel began breeding garden peas to study inheritance.

Mendel’s work Bred pea plants ? cross-pollinate true breeding parents
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 P = parents F = filial generation self-pollinate ?

Mendel collected data for 7 pea traits

Looking closer at Mendel’s work
true-breeding purple-flower peas true-breeding white-flower peas X Parents 100% 1st generation (hybrids) purple-flower peas In a typical breeding experiment, Mendel would cross-pollinate (hybridize) two contrasting, true-breeding pea varieties. The true-breeding parents are the P generation and their hybrid offspring are the F1 generation. Mendel would then allow the F1 hybrids to self-pollinate to produce an F2 generation. self-pollinate 2nd generation 3:1 75% purple-flower peas 25% white-flower peas

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 I’ll speak for both of us! 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 F1 P X purple white all purple Explain Mendel’s results using …dominant & recessive …phenotype & genotype

PP pp Pp x 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 F1 P X purple white all purple PP x pp Pp

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

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 WW RW
RR = red flowers rr = white flowers Rr = pink flowers make 50% less color RR WW RW RR Rr rr

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

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

Genetics of Blood type A A A or A i B BB or B i AB O i i pheno-type
genotype antigen on RBC antibodies in blood donation status A A A or A i type A antigens on surface of RBC anti-B antibodies __ B BB or B i type B antigens on surface of RBC anti-A antibodies AB both type A & type B antigens on surface of RBC no antibodies universal recipient O i i no antigens on surface of RBC anti-A & anti-B antibodies universal donor

Blood donation 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) The genes that we have covered so far affect only one phenotypic character, but most genes are pleiotropic

Acromegaly: André the Giant

Inheritance pattern of Achondroplasia
Aa x aa Aa x Aa a a A 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 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 albino Africans
melanin = universal brown color

OCA1 albino Bianca Knowlton

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 The relative importance of genes & the environment in influencing human characteristics is a very old & hotly contested debate a single tree has leaves that vary in size, shape & color, depending on exposure to wind & sun for humans, nutrition influences height, exercise alters build, sun-tanning darkens the skin, and experience improves performance on intelligence tests even identical twins — genetic equals — accumulate phenotypic differences as a result of their unique experiences 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

Sex chromosomes

Sex-linked traits Sex chromosomes have other genes on them, too X X X
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 X X Duchenne muscular dystrophy affects one in 3,500 males born in the United States. Affected individuals rarely live past their early 20s. This disorder is due to the absence of an X-linked gene for a key muscle protein, called dystrophin. The disease is characterized by a progressive weakening of the muscles and loss of coordination. X Y

Sex-linked traits HH XHY XHXh Hh x XHY Y XH XH Y XHXH XHXH XHY XHY XH
sex-linked recessive HH XHY XHXh Hh 2 normal parents, but mother is carrier x XHY Y XH XH Y male / sperm XHXH XHXH XHY XHY XH Xh female / eggs XHXh XH Xh XHXh XHXh XhY XhY

Hemophilia is a sex-linked recessive trait defined by the absence of one or more clotting factors.
These proteins normally slow and then stop bleeding. Individuals with hemophilia have prolonged bleeding because a firm clot forms slowly. Bleeding in muscles and joints can be painful and lead to serious damage. Individuals can be treated with intravenous injections of the missing protein.

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

Polydactyly recessive allele far more common than dominant
individuals are born with extra fingers or toes the allele for >5 fingers/toes is DOMINANT & the allele for 5 digits is recessive 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

Any Questions?