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Mendelian Genetics
Biology – Premed
Windsor University School of Medicine
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CHAPTER 22
GENETICS&
THE WORK OF MENDEL
There is more to lectures than the power point slides!
Engage your mind
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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
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• 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”
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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
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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
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• 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
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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
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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
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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
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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
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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
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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
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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!
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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
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• 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
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Incomplete dominance• Hybrids have “in-between” appearance
• RR = red flowers• rr = white flowers• Rr = pink flowers
• make 50% less color
RR Rr rr
RRWWRW
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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
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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
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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:
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• 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.
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• 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.
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• 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.
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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
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Blood donation
clotting clotting
clotting clotting
clotting clotting clotting
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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)
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Many genes: one trait
• Polygenic inheritance
• additive effects of many genes• humans
• skin color• height• weight• eye color• intelligence• behaviors
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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
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AlbinismJohnny & Edgar Winter
albinoAfricans
melanin = universal brown color
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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
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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
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Sex chromosomes
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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
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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
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Dominant ≠ most common allele• Because an allele is dominant does not mean…• it is better, or• it is more common
Polydactylydominant allele
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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