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

2nd

generation

3:175%purple-flower peas

25%white-flower peas

Looking closer at Mendel’s work

Parents

100%1st

generation(hybrids)

100%purple-flower peas

Xtrue-breedingpurple-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

homologouschromosomes

I’ll speak for both of us!

allele producingfunctional 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 F1

P X

purple white

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

PpF1

P X

purple white

all purple

Punnett squaresPp x Pp

P pmale / sperm

P

p

fem

ale

/ eg

gs

PP

75%

25%

3:1

25%

50%

25%

1:2:1

%genotype

%phenotype

PP Pp

Pp pppp

Pp

Pp

1st

generation(hybrids)

Aaaaah,phenotype & genotype

can have different ratios

Any Questions??

2007-2008

Beyond Mendel’s Lawsof 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

RR Rr rr

RR

WW

RW

Incomplete dominance

true-breedingred flowers

true-breeding white flowers

XP

100%

100% pink flowers1st

generation(hybrids)

self-pollinate

25%White

2nd

generation

25%Red 1:2:1

50%Pink

Incomplete dominance

R Wmale / sperm

R

Wfem

ale

/ eg

gs

RR

RW WW

RW

25%

1:2:1

25%

50%

25%

1:2:1

%genotype

%phenotype

RR

RW

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 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)

Acromegaly: André the Giant

Aa x aa

Inheritance pattern of Achondroplasia

a a

A

a

A a

A

a

Aa x Aa

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

X Y

X X

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

X Y

X X

Dominant ≠ most common allele

Polydactylydominant 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