solving genetics problems biology unit 6 powerpoint #2 / chapter 11 mr. velekei

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


• 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


We need a method to predict the traits of the offspring, and we

have it, its called the

Punnett Square!


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)


Genetics Problems



2. Determine parents’ genotypes


Genetics Problems



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


3. Draw Punnett Square



t t

T

T



Tt Tt

Tt Tt

t t

T

T


4. Calculate probabilities

TT = _____

Tt = ________

tt = _________

Tall = _________

Short = _________ Tt Tt

Tt Tt

t t

T

T


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

solving genetics problems biology unit 6 powerpoint #2 / chapter 11 mr. velekei

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