solving genetics problems biology unit 6 powerpoint #2 / chapter 11 mr. velekei
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SOLVING GENETICS PROBLEMS
BiologyUnit 6Powerpoint #2 / Chapter 11Mr. Velekei
Vocabulary
9. Phenotype
10.Genotype
11.Dominant
12.Recessive
13.Homozygous
14.Heterozygous
Genotype vs. phenotype• Difference between how an organism “looks” & its
genetics
• phenotype • description of an organism’s trait• the “physical”
• 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
Notation
A capital letter represents the dominant allele.
A lower case letter represents the recessive allele.
Example: Pea plant height –
Tall: TShort: t
Allele: A variety of a gene• A gene for hair color could have the
allele: _______ for Brown hair or _______ for Blonde hair
• A gene for flower color could have allele: ______for blue flower or ________ for red flower
• A gene for height could have the alleles: _______ for tall or _______ for short
Bb
Bb
T t
Solving Genetics Problems
Homozygous: organism with two identical alleles for a trait
Heterozygous: organism with two different alleles for a trait
Homozygous dominant: organism with two dominant alleles
Homozygous recessive: organism with two recessive alleles
Solving Genetics Problems
• Dominant: An allele that causes its phenotype in a heterozygous genotype.
Examples: TT, Tt (Tall)
• Recessive: An allele that causes a phenotype only seen in a homozygous genotype.
Example: tt (short)
Remember…
A B C D E F G H
Plant A:
a. Phenotype:
b. Genotype:
A B C D E F G H
Plant B:
a. Phenotype:
b. Genotype:
A B C D E F G H
Plant C:
a. Phenotype:
b. Genotype:
F2generation
3:1
75%purple-flower peas
25%white-flower peas
????
Looking closer at Mendel’s work
PX
true-breedingpurple-flower peas
true-breeding white-flower peas
PP pp
100%F1generation(hybrids)
100%purple-flower peas
Pp Pp Pp Pp
phenotype
genotype
self-pollinate
Solving Genetics Problems
We need a method to predict the traits of the offspring, and we
have it, its called the
Punnett Square!
Solving Genetics Problems
Genetics Problems
Goal: to predict the traits of offspring
1. Identify trait(s) and assign a letter to each (capital letter for dominant, lower case letter for recessive)
Solving Genetics Problems
Genetics Problems
Goal: to predict the traits of offspring
1. Identify trait(s) and assign a letter to each (capital letter for dominant, lower case letter for recessive)
2. Determine parents’ genotypes
Solving Genetics Problems
Genetics Problems
Goal: to predict the traits of offspring
1. Identify trait(s) and assign a letter to each (capital letter for dominant, lower case letter for recessive)
2. Determine parents’ genotypes
3. Draw Punnett square and fill in
Solving Genetics ProblemsGenetics Problems
Goal: to predict the traits of offspring1. Identify trait(s) and assign a letter to each (capital
letter for dominant, lower case letter for recessive)
2. Determine parents’ genotypes3. Draw Punnett square and fill in4. Determine the probabilities for offspring of each
genotype and phenotype
Solving Genetics ProblemsExample:In pea plants, the gene for tall height is dominant to
the gene for short height. A short pea plant is cross pollinated with a true breeding tall pea plant.
1. Assign letters: tall = T short = t
2. Parents’ genotypesTrue breeding tall: TT True breeding short = tt
Solving Genetics Problems
3. Draw Punnett Square
Solving Genetics Problems
3. Draw Punnett Square
t t
T
T
Solving Genetics Problems
3. Draw Punnett Square
Tt Tt
Tt Tt
t t
T
T
Solving Genetics Problems
4. Calculate probabilities
TT = _____
Tt = ________
tt = _________
Tall = _________
Short = _________ Tt Tt
Tt Tt
t t
T
T
Solving Genetics Problems
4. Calculate probabilities
10) TT = 0/4
Tt = 4/4
tt = 0/4
11) Tall = 4/4
Short = 0/4 Tt Tt
Tt Tt
t t
T
T
Punnett squaresPp x Pp
P pmale / sperm
P
pfem
ale
/ egg
s
PP
75%
25%
3:1
25%
50%
25%
1:2:1
%genotype
%phenotype
PP Pp
Pp pp pp
Pp
Pp
F1generation(hybrids)
Aaaaah,phenotype & genotypecan have different ratios
Genotypes • Homozygous = same alleles = PP, pp• Heterozygous = different alleles = Pp
homozygousdominant
homozygousrecessive
heterozygous
Phenotype vs. genotype• 2 organisms can have the same phenotype but have
different genotypes
homozygous dominantPPpurple
Pp heterozygouspurple
How do you determine the genotype of an individual withwith a dominant phenotype?
Can’t tellby lookin’at ya!
Test cross• Breed the dominant phenotype —
the unknown genotype — with a homozygous recessive (pp) to determine the identity of the unknown allele
ppis itPP or Pp?
x
PP pp
How does a Test cross work?
p p
P
P
p p
P
p
Pp pp
x x
Pp
Pp Pp
Pp
100% purple
Pp
pp
Pp
50% purple:50% white or 1:1
pp
Am I this?
Or am I this?
Mendel’s 1st law of heredity•Law of segregation
• during meiosis, alleles segregate• homologous chromosomes separate
• each allele for a trait is packaged into a separate gamete
PP
P
P
pp
p
p
Pp
P
p
Segregation of alleles and fertilization as chance events
DIHYBRID (2 FACTOR) CROSSES
+INCOMPLETE AND CO-
DOMINANCE
Vocabulary
15.Co-dominance
16. Incomplete dominance
17.Probability
Monohybrid cross• Some of Mendel’s experiments followed the
inheritance of single characters • flower color• seed color • monohybrid crosses
Dihybrid (2 factor) cross• Other of Mendel’s experiments
followed the inheritance of 2 different characters
• seed color and seed shape• dihybrid (2 factor) crosses
Mendelwas working out
many of the genetic rules!
Dihybrid crosstrue-breedingyellow, round peas
true-breedinggreen, wrinkled peas
x
YYRR yyrrP
100%F1generation(hybrids)
yellow, round peas
Y = yellowR = round
y = greenr = wrinkled
self-pollinate
9:3:3:1
9/16yellowround peas
3/16greenround peas
3/16yellowwrinkledpeas
1/16greenwrinkledpeas
F2generation
YyRr
What’s going on here?• If genes are on different chromosomes…
• how do they assort in the gametes?• together or independently?
YyRr
YR yr
YyRr
Yr yRYR yr
Is it this?
Or this?
Which systemexplains the
data?
9/16yellowround
3/16greenround
3/16yellowwrinkled
1/16greenwrinkled
Is this the way it works?YyRr YyRr
YR yr
YR
yr
x
YyRr
Yr yRYR yr
YyRr
YR yr
or
YYRR YyRr
YyRr yyrr
Dihybrid crossYyRr YyRr
YR Yr yR yr
YR
Yr
yR
yr
YYRR
x
YYRr YyRR YyRr
YYRr YYrr YyRr Yyrr
YyRR YyRr yyRR yyRr
YyRr Yyrr yyRr yyrr
9/16yellowround
3/16greenround
3/16yellowwrinkled
1/16greenwrinkled
YyRr
Yr yRYR yr
YyRr
YR yr
or
The laws of probability govern Mendelian inheritance
• Rule of Multiplication:• probability that 2+ independent events will occur together in a specific combination multiply probabilities of each event
• Ex. 1: probability of throwing 2 sixes• 1/6 x 1/6 = 1/36
• Ex. 2: probability of having 5 boys in a row• ½ x ½ x ½ x ½ x ½ = 1/32
• Ex. 3: If cross AABbCc x AaBbCc, probability of offspring with AaBbcc is:• Answer: ½ x ½ x ¼ = 1/16
The laws of probability govern Mendelian inheritance
• Rule of Addition:• Probability that 2+ mutually exclusive events will occur add together individual probabilities
• Ex. 1: chances of throwing a die that will land on 4 or 5?• 1/6 + 1/6 = 1/3
Mendel’s 2nd law of heredity
roundwrinkled
• Law of independent assortment• different loci (genes) separate into gametes
independently • non-homologous chromosomes align independently• classes of gametes produced in equal amounts
• YR = Yr = yR = yr
• only true for genes on separate chromosomes or on same chromosome but so far apart that crossing over happens frequently
yellowgreen
: 1 1 : 1:1Yr Yr yR yR YR YR yr yr
YyRr
Review: Mendel’s laws of heredity • Law of segregation
• monohybrid cross • single trait
• each allele segregates into separate gametes• established by Metaphase 1
• Law of independent assortment• dihybrid (or more) cross
• 2 or more traits
• genes on separate chromosomes assort into gametes independently
EXCEPTION linked genes
Beyond Mendel’s Lawsof Inheritance
Extending Mendelian genetics• Mendel worked with a simple system
• peas are genetically simple• most traits are controlled by a single gene• each gene has only 2 alleles, 1 of which
is completely dominant to the other
• The relationship between genotype & phenotype is rarely that simple
Incomplete dominance• Heterozygote shows an intermediate, blended phenotype• example:
• RR = red flowers• rr = white flowers• Rr = pink flowers
• make 50% less color
RR
RRWW
RW
WWRW
Incomplete dominancetrue-breedingred flowers
true-breeding white flowers
XP
100%
100% pink flowersF1
generation(hybrids)
self-pollinate
25%white
F2generation
25%red
1:2:1
50%pink
It’s likeflipping 2 pennies!
Solve the following crosses: Incomplete DominanceIn flowers there is a gene for Purple (P) which has incomplete dominance to the recessive color white (p). The heterozygote plant produced by a white and purple flower is violet (light purple).
a) What are the percentages of genotypes and phenotypes of the offspring of two violet colored plants (draw a punnett square)
Co-dominance• 2 alleles affect the phenotype equally & separately• Not blended phenotype• Both alleles contribute to the phenotype• Example is human ABO blood groups
Solve the following crosses: Co-DominanceIn Cows the trait for coat (fur) color is expressed by R for Red fur and W for white fur. The hybrid of the two is called a Roan color (RW).
a) Describe what you think the Roan cow’s fur will look like:
b) What are the genotype and phenotype percentages of a cross between a Red and a White cow?
Solve the following crosses: Co-DominanceIn mushrooms there is a gene for Purple Spots (P) and a gene for Green spots (G). Cross a Homozygous purple with a Homozygous Green.
a) What do you think their offspring will look like? Genotype? Phenotype?
b) What will the genotype and phenotype percentages be in the F2 have when you cross two from the F1 generation.
Pleiotropy • Most genes are pleiotropic
• one gene affects more than one phenotypic character• 1 gene affects more than 1 trait• dwarfism (achondroplasia) • gigantism (acromegaly)
Acromegaly: André the Giant
Epistasis
B_C_B_C_
bbC_bbC_
_ _cc_ _cc
• One gene completely masks another gene• coat color in mice = 2 separate genes
• C,c: pigment (C) or no pigment (c)
• B,b: more pigment (black=B) or less (brown=b)
• cc = albino, no matter B allele
• 9:3:3:1 becomes 9:3:4
Polygenic inheritance• Some phenotypes determined by additive effects of 2
or more genes on a single character• phenotypes on a continuum• human traits
• skin color• height• weight• intelligence• behaviors
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