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Mendelian Genetics Biology – Premed Windsor University School of Medicine.

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Mendelian Genetics Biology – Premed Windsor University School of Medicine
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Page 1: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

Mendelian Genetics

Biology – Premed

Windsor University School of Medicine

Page 2: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

CHAPTER 22

GENETICS&

THE WORK OF MENDEL

There is more to lectures than the power point slides!

Engage your mind

Page 3: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

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

Page 4: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

• True Breeding: When self-fertilized, only produces offspring with the same traits

Eg. A plant with purple flowers is true-breeding if the seeds produced by self-pollination in successive generations all gave rise to purple flowers

• Cross-pollinate: Eg. Breeding purple-flowered plants and white-flowered plants -- “Hybridization”

Page 5: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

Pollen transferred from white flower to stigma of purple flower

all purple flowers result

Mendel’s work• Bred pea plants

• cross-pollinate two true breeding parentsEg. Purple flower plants & White-flowered plants

• raised seed & then observed traits

allowed offspring to self-pollinate & observed next generation

?

self-pollinate

Page 6: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

2nd

Generation(F2)

3:175%purple-flower peas

25%white-flower peas

Looking closer at Mendel’s work

Parents(P)

100%1st

generation(hybrids)(F1)

100%purple-flower peas

Xtrue-breedingpurple-flower peas

true-breeding white-flower peas

self-pollinate

Page 7: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

• Results: Both purple-flowered and white-flowered plants appeared in the F2 generation, in a ratio of approximately 3: 1

What is an allele? • Alternate versions of a genes• For each character, an organism inherits two copies (2 alleles)

of a gene from each parents

Eg. Purple flower allele and white flower allele are two DNA sequence variations

Page 8: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

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 in F1

• 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 malfunctioningprotein

Page 9: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

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

2 people can have the same appearance but have different genetics: BB vs Bb

Page 10: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

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

Page 11: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

Traits are inherited as separate units

• For each trait, an organism inherits 2 copies of a gene, 1 from each parent• a diploid organism inherits 1 set of chromosomes from each parent• diploid = 2 sets of chromosomes

1 from Mom

1 from Dad

homologous chromosomes

Page 12: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

Making gametes

BB = brown eyesbb = blues eyesBb = brown eyes

BB

bb

Bb

brown is dominant over blue blue is recessive to brown

Remember meiosis!

B

B

b

b

B

b

Page 13: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

How do we say it?

BB = brown eyesbb = blues eyes

Bb = brown eyes

2 of the same allelesHomozygous

2 different alleles Heterozygous

BB

B

B

bb

b

b

Bb

B

b

homozygous dominanthomozygous recessive

Page 14: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

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!

Page 15: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

Punnett Square• Handy diagram device for predicting the allele composition

of offspring from a cross between indivials of known genetic makeup

• “Capital Letter” to symbolize a Dominant allele• “Small Letter” to sumbolize a Recessive allele

Page 16: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

• Alleles can show different degrees of dominance and recessiveness in relation to each other

• For some genes neither allele is completely dominance and the F1 hybrids have a phenotype somewhere between the parent varieties == Incomplete Dominance

Page 17: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

Incomplete dominance• Hybrids have “in-between” appearance

• RR = red flowers• rr = white flowers• Rr = pink flowers

• make 50% less color

RR Rr rr

RRWWRW

Page 18: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

Incomplete dominance

true-breedingred flowers

true-breeding white flowers

XP

100%

100% pink flowers

1st

generation(hybrids)

self-pollinate

25%white

2nd

generation

25%red 1:2:1

50%pink

Page 19: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

Co-dominance

• 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

Page 20: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

Quick Review;• Johann Gregor Mendel (1822-1884)• Father of Genetics• Gregor Mendel, through his work on pea plants,

discovered the fundamental laws of inheritance. He deduced that genes come in pairs and are inherited as distinct units, one from each parent. Mendel tracked the segregation of parental genes and their appearance in the offspring as dominant or recessive traits. He recognized the mathematical patterns of inheritance from one generation to the next. Mendel's Laws of Heredity are usually stated as:

Page 21: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

• 1) The Law of Segregation: Each inherited trait is defined by a gene pair. Parental genes are randomly separated to the sex cells so that sex cells contain only one gene of the pair. Offspring therefore inherit one genetic allele from each parent when sex cells unite in fertilization.

Page 22: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

• 2) The Law of Independent Assortment: Genes for different traits are sorted separately from one another so that the inheritance of one trait is not dependent on the inheritance of another.

Page 23: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

• 3) The Law of Dominance: An organism with alternate forms of a gene will express the form that is dominant.

• The genetic experiments Mendel did with pea plants took him eight years (1856-1863) and he published his results in 1865. During this time, Mendel grew over 10,000 pea plants, keeping track of progeny number and type. Mendel's work and his Laws of Inheritance were not appreciated in his time. It wasn't until 1900, after the rediscovery of his Laws, that his experimental results were understood.

Page 24: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

Genetics of Blood typePheno-

typeGenotype

antigenon RBC

antibodiesin blood

donationstatus

A A A or A itype A antigens

on surface of RBC

anti-B antibodies __

B BB or B itype B antigens

on surface of RBC

anti-A antibodies __

AB ABboth type A &

type B antigens on surface

of RBC

no antibodies universal recipient

O i ino antigens on surface

of RBC

anti-A & anti-B antibodies

universal donor

Page 25: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

Blood donation

clotting clotting

clotting clotting

clotting clotting clotting

Page 26: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

One gene: many effects

• The genes that we have covered so far affect only one trait

• But most genes affect many traits • 1 gene affects more than 1 trait

• dwarfism (achondroplasia)• gigantism (acromegaly)

Page 27: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

Many genes: one trait

• Polygenic inheritance

• additive effects of many genes• humans

• skin color• height• weight• eye color• intelligence• behaviors

Page 28: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

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

Page 29: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

AlbinismJohnny & Edgar Winter

albinoAfricans

melanin = universal brown color

Page 30: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

Environment effect on genes

• 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

Coat color in arctic fox influenced by heat sensitive alleles

Page 31: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

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

Page 32: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

Sex chromosomes

Page 33: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

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 greyX Y

X X

Page 34: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

Sex-linked traits

XH Ymale / sperm

XH

Xhfe

mal

e /

eggs

XHXH

XHXh

XHY

XhYXHXh

XH

Xh

XHYY

XH

XHXH XHY

XHXh XhY

sex-linked recessive

2 normal parents,but mother is carrier

HH HhxXHY XHXh

Page 35: Mendelian Genetics Biology – Premed Windsor University School of Medicine.
Page 36: Mendelian Genetics Biology – Premed Windsor University School of Medicine.
Page 37: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

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

Polydactylydominant allele

Page 38: Mendelian Genetics Biology – Premed Windsor University School of Medicine.

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


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