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Genetics
Why You Look the Way
You Do
George Radcliffe
Centreville Middle School
April, 2008
CONTENTS
• Vocabulary
• Asexual Reproduction
• Sexual Reproduction
• Mitosis vs. Meiosis
• The Gene
• Genotypes and Phenotypes
• Punnett Square – Predicting Offspring
• Practice Problems
• Review Quiz
VOCABULARY YOU NEED TO BE ABLE
TO SPEAK “GENETICS”
• Asexual reproduction • Allele
• Sexual reproduction • Dominant
• Mitosis – normal cell division • Recessive
• Meiosis • Genotype
• Gamete • Phenotype
• Fertilization • Homozygous
• Pollination • Heterozygous
• Gene • Punnett Square
• Trait
Asexual Reproduction
• Thegene is a strand of DNA that makes a particular
protein, enzyme, or pigment.
• In asexual reproduction, all of the DNA comes from
your 1 parent.
• If an asexually reproducing plant had yellow flowers,
what percentage of its offspring will have yellow
flowers?
• Answer: 100% of the offspring will have yellow
flowers.
Contents
The Gene and Sexual Reproduction
• Thegene is a strand of DNA that makes a particular
protein, enzyme, or pigment.
• ½ of your DNA comes from the mother’s egg.
• ½ comes from the father’s sperm.
• These half cells are produced by the process of meiosis.
• Each of these ½ cells (egg, sperm or pollen) is called a
gamete.
• When the two halves combine (fertilization) you have a
complete set of DNA. (½ + ½ = 1)
• Thus you actually have two of each gene. Vocabulary
Plants and Sexual Reproduction
• Plants usually reproduce sexually, too.
• The female half of the DNA is found in the egg.
• The male half comes from the pollen.
• Insects or wind carries the pollen from one flower to
another. This pollination leads to fertilization of the egg.
• This fertilized egg is called a zygote. The zygote has a
complete set of DNA.
Contents Vocabulary
Mitosis vs. Meiosis
• Go to the following PBS website
listed below. Either run the flash
animation, or use the non-Flash
version.
• Answer: how is meiosis different
from mitosis?
• Link: PBS Meiosis/Mitosis Site
Mitosis vs. Meiosis
• How is meiosis different from
mitosis?
• Answer: In the beginning, they are
the same, one cell dividing into two
complete cells. In meiosis the two
cells then each divide into 2 half
cells.
• If you missed this, go back (hit
“P”), and run the animation again.
Comparison of meiosis and mitosis
Mitosis Meiosis
Normal Cell Division Makes gametes
Makes 2 copies of any cell Makes eggs, sperm, pollen
Produces two cells Produces four gametes
All the DNA is found in ½ of the parent DNA is
each cell. found in gamete.
46 chromosomes 23 chromosomes
23 pairs 1 from each pair
Let’s See if We’ve Got it All Straight.
• Question: What is the process that produces normal cells?
• Answer: Mitosis
• Question: What is the process that produces eggs, sperm, and
pollen?
• Answer: Meiosis
• Question: What is a gamete?
• Answer: a sex cell: egg, sperm, or pollen
• Question: How much DNA does a pollen or egg have?
• Answer: ½ of the normal DNA
• Question: How much DNA does a zygote have?
• Answer: full amount of DNA
The Gene
• Let’s see how genes work.
• Let’s look at human eye color.
• The gene is represented by the letter B.
• There are 2 alleles for this gene; that means that there
are 2 ways that eye color can be.
• Eye color can be brown; we show this gene with a B.
• Eye color can also be blue; we show this gene with a b.
• Remember: You get a gene from each parent; so you
have 2.
Contents Vocabulary
Which Gene Does Your Cell Listen to?
• If you have 2 brown genes (BB), your eyes are obviously brown.
• If you have 2 blue genes (bb), your eyes are obviously blue.
• But what happens if you have one of each gene?
• The dominant gene wins out.
• Brown (B) is the dominant gene; that’s why it is
represented with a capital letter.
• Thus a person with Bb genes has brown eyes.
• The blue gene (b) is recessive; that means it is hidden or
masked by the dominant gene, BUT it can be passed on to
the offspring.
Contents Vocabulary
Genotypes and Phenotypes
• Let’s show this in a chart.
• Trait: Eye color
• Alleles: B and b
Genotype Phenotype
(the genes) (what it looks like)
BB Brown
bb Blue
Bb Brown
Contents Vocabulary
Genotypes and Phenotypes
• Let’s look at another trait.
• Trait: Height of a pea plant
• Alleles: T and t
• T – gene for tallness t – gene for short plant
Genotype Phenotype
(the genes) (what it looks like)
TT Tall plant
tt Short plant
Tt Tall plant
Let’s Review
TT Tall plant
tt Short plant
Tt Tall plant
• Question: What is the trait?
• Answer: height of pea plant
• Question: What are the alleles for height?
• Answer: T and t
• Question: What is the dominant allele?
• Answer: T
Let’s Review
TT Tall plant
tt Short plant
Tt ???? plant
• Question: What is genotype for a short plant?
• Answer: tt
• Question: What is the phenotype for TT?
• Answer: tall
• Question: What is the phenotype for Tt?
• Answer: tall since T is dominant Contents
Genotype and Phenotype Practice
Let’s do some practice using the 4 pairs of
pea plant genes shown at the right. For T = tall
each genotype listed, give the phenotype t = short
(what it looks like).
G = green leaves
Genotype Phenotype g = yellow leaves
P = purple flowers
Gg green leaves p = white flowers
gg yellow leaves R = round peas
r = wrinkled peas
Genotype and Phenotype Practice
Genotype Phenotype T = tall
Pp purple flowers
t = short
G = green leaves
rr wrinkled peas g = yellow leaves
Rr round peas P = purple flowers
p = white flowers
TtGg tall plants with green
leaves R = round peas
Pprr purple flowers and
r = wrinkled peas
wrinkled peas
TTggppRr tall plants, yellow peas, white
flowers, and round peas
Contents
Homozygous and Heterozygous
TT (homozygous) Tall plant
Tt (homozygous) Short plant
Tt (heterozygous) Tall plant
• Let’s add 2 more words to the mix.
• Homozygous means both genes are the same.
Vocabulary
• Heterozygous means both genes are different,
Contents
• “Homo” = the same “Hetero” = different
• “zygous” refers to the zygote, the first cell formed from the
fertilization of the egg by the sperm.
• Another word for a heterozygous (Tt) is hybrid, one of each gene).
Putting it All Together
TT Tall plant
tt Short plant
Tt Tall plant
• What is the homozygous genotype for a tall plant?
• Answer: TT
• What is the heterozygous genotype for a tall individual?
• Answer: Tt
• What is the heterozygous genotype for a short individual?
• Answer: impossible; Tt is can’t be short.
Monohybrid inheritance
Let the allele for round seeds be: R (dominant allele)
Let the allele for wrinkled seeds be: r (recessive allele)
Parents phenotype round seeds x round seeds
genotype RR RR
Gametes (pollen, eggs) R R R R
Offspring Gametes
Gametes
We call this a
Punnett Square.
We’ll use this to show the 4 ways that the
gametes from the 2 parents can combine.
Monohybrid inheritance
• Let’s watch this in action.
• With these gametes, there are 4 possibilities; that’s
why there are 4 boxes in the Punnett Square.
Possibility # 1
Gametes
Gametes R R
R
R
Monohybrid inheritance
• Let’s watch this in action.
• With these gametes, there are 4 possibilities; that’s
why there are 4 boxes in the Punnett Square.
Possibility # 2
Gametes
Gametes R R
R
R
Monohybrid inheritance
• Let’s watch this in action.
• With these gametes, there are 4 possibilities; that’s
why there are 4 boxes in the Punnett Square.
Possibility # 3
Gametes
Gametes R R
R
R
Monohybrid inheritance
• Let’s watch this in action.
• With these gametes, there are 4 possibilities; that’s
why there are 4 boxes in the Punnett Square.
Possibility # 4
Gametes
Gametes R R
R
R
Monohybrid inheritance
Let the allele for round seeds be: R (dominant allele)
Let the allele for wrinkled seeds be: r (recessive allele)
Parents phenotype round seeds x round seeds
genotype RR RR
Gametes
Gametes R R
Offspring
R RR RR
All possibilities R RR RR
are the same: RR.
Offspring phenotypes 100% plants producing round seeds
Offspring genotypes 100% heterozygotes RR
Monohybrid inheritance – Part 2
Let the allele for round seeds be: R (dominant allele)
Let the allele for wrinkled seeds be: r (recessive allele)
Parents phenotype wrinkled seeds x wrinkled seeds
genotype rr rr
r r r r
Gametes (eggs, pollen)
Gametes
Offspring Gametes r r
r rr rr
r rr rr
Offspring phenotypes 100% plants producing wrinkled seeds
Offspring genotypes 100% heterozygotes rr
Monohybrid inheritance – Part 3
Let the allele for round seeds be: R (dominant allele)
Let the allele for wrinkled seeds be: r (recessive allele)
Parents phenotype round seeds x wrinkled seeds
genotype RR rr
R R r r
Gametes
Offspring
Gametes
Gametes r r
R Rr Rr
R Rr Rr
Offspring phenotypes 100% plants producing round seeds
Offspring genotypes 100% heterozygotes Rr
Contents
Heterozygous Cross
Let the allele for round seeds be: R (dominant allele)
Let the allele for wrinkled seeds be: r (recessive allele)
Parents phenotype round seeds x round seeds
genotype Rr Rr
R r R r
Gametes
Offspring
Gametes
Gametes
Gametes
Gametes RR rr
R
R RR Rr
rr Rr rr
Offspring phenotypes 75% plants producing round seeds
25% plants producing wrinkled seeds
Offspring genotypes 50% Rr 25% RR 25% rr
Prediction vs. Actual
Gametes
Gametes R r
R RR Rr
r Rr rr
Predicted Offspring phenotypes 75% plants producing round seeds
25% plants producing wrinkled seeds
Ratio 3:1 Round seeds: wrinkled seeds
• This is what we predict, just like we predict that 1 out of 2
coin flips will be heads.
• Actual results can be different.
• If we bred the plants above many times, we would at least
get close to a 3:1 ratio. Contents
Prediction vs. Actual
• To see how predicted can vary from what actual happens. Let’s
look at coin flips.
• Because there is a 50:50 chance of heads turning up, ½ or 50% of
coin flips will be heads.
• This obviously doesn’t happen, but if you flip the coin enough, you
will approximate 50%.
• Go to the website below, and flip coins different numbers of time.
Notice that when you flip 10,000 times, you will always get close to
50%.
• Return here when you’re done.
Go to Coin Toss Probability
Contents
How can you tell if a parent is RR or Rr?
To test whether a plant producing round seeds is homozygous RR
or heterozygous Rr it can be crossed with a homozygous rr plant
If plant is homozygous dominant RR If plant is heterozygous Rr
Parents Parents
phenotype round x wrinkled phenotype round x wrinkled
genotype RR rr genotype Rr rr
gametes gametes
R r r r
R R r r
Offspring Offspring
gametes gametes
gametes R R gametes R r
r Rr Rr r Rr rr
r Rr Rr r Rr rr
Offspring Offspring
phenotype 100% round phenotype 50% round/50% wrinkled
Genotype 100% Rr genotype 50% Rr 50% rr
Let’s Try a Couple Problems!
Two plants are cross pollinated: Gg and gg.
What will the resulting offspring look like? Work T = tall
out the problem using a Punnett Square like the
t = short
one below on scrap paper.
G = green leaves
Gametes
g = yellow leaves
Gametes g g P = purple flowers
G p = white flowers
g R = round peas
r = wrinkled peas
1. What percentage of the plants will be yellow?
2. What is the ratio of green to yellow plants?
Let’s Try a Couple Problems!
Two plants are cross pollinated: Gg and gg.
What will the resulting offspring look like? Work T = tall
out the problem using a Punnett Square like the
t = short
one below on scrap paper.
G = green leaves
Gametes
g = yellow leaves
Gametes g g P = purple flowers
G Gg Gg p = white flowers
g gg gg R = round peas
r = wrinkled peas
1. What percentage of the plants will be yellow? 50%
2. What is the ratio of green to yellow plants?
1:1 ratio of green: yellow
Practice Problem #2
Two plants are cross pollinated: Gg and Gg.
What will the resulting offspring look like? Work T = tall
out the problem using a Punnett Square like the
t = short
one below on scrap paper.
G = green leaves
Gametes
g = yellow leaves
Gametes P = purple flowers
p = white flowers
R = round peas
r = wrinkled peas
1. What percentage of the plants will be green?
2. What is the ratio of green to yellow plants?
Practice Problem #2
Two plants are cross pollinated: Gg and Gg.
What will the resulting offspring look like? Work T = tall
out the problem using a Punnett Square like the
t = short
one below on scrap paper.
G = green leaves
Gametes
g = yellow leaves
Gametes G g P = purple flowers
G GG Gg p = white flowers
g Gg gg R = round peas
r = wrinkled peas
1. What percentage of the plants will be green? 75%
2. What is the ratio of green to yellow plants?
3:1 ratio of green: yellow
Practice Problem #3
Two plants are cross pollinated: GG and gg.
What will the resulting offspring look like? Work T = tall
out the problem using a Punnett Square like the
t = short
one below on scrap paper.
G = green leaves
Gametes
g = yellow leaves
Gametes P = purple flowers
p = white flowers
R = round peas
r = wrinkled peas
1. What percentage of the plants will be yellow?
Practice Problem #3
Two plants are cross pollinated: GG and gg.
What will the resulting offspring look like? Work T = tall
out the problem using a Punnett Square like the
t = short
one below on scrap paper.
G = green leaves
Gametes
g = yellow leaves
Gametes G G P = purple flowers
g Gg Gg p = white flowers
g Gg Gg R = round peas
r = wrinkled peas
1. What percentage of the plants will be yellow? 0%
Practice Problem #4
Two plants are cross pollinated: Pp and Pp.
What will the resulting offspring look like? Work T = tall
out the problem using a Punnett Square like the
t = short
one below on scrap paper.
G = green leaves
Gametes
g = yellow leaves
Gametes P = purple flowers
p = white flowers
R = round peas
r = wrinkled peas
1. What percentage of the plants will be white?
2. What is the ratio of purple to white flowers?
Practice Problem #4
Two plants are cross pollinated: Pp and Pp.
What will the resulting offspring look like? Work T = tall
out the problem using a Punnett Square like the
t = short
one below on scrap paper.
G = green leaves
Gametes
g = yellow leaves
Gametes P p P = purple flowers
P PP Pp p = white flowers
p Pp pp R = round peas
r = wrinkled peas
1. What percentage of the plants will be white? 25%
2. What is the ratio of purple to white flowers?
3:1 ratio of purple: white Contents
Review Questions
• Number a ¼ sheet of paper from 1
to 15.
• Answer the questions with no
assistance.
• Score your paper with the key;
consider 11 or more correct as OK.
• Go back into the PowerPoint to
correct your errors.
Review Questions
1. What process produces the
gametes?
A. fertilization
B. mitosis
C. conjugation
D. meiosis
Review Questions
2. In asexual reproduction
A. Gametes are produced.
B. Fertilization occurs.
C. An identical cell is produced.
D. A cell with ½ of the DNA is
produced.
Review Questions
3. In sexual reproduction, meiosis
produces
A. A zygote.
B. A cell with ½ of the DNA
C. A cell with double the DNA
D. A cell with the same DNA as the
parent’s cell.
Review Questions
4. A plant with Tt genes reproduces
asexually. What will its offspring
look like?
A. None will be tall.
B. 50% will be tall.
C. 75% will be tall.
D. 100% will be tall.
Review Questions
5. What is the genotype of a T = tall
homozygous tall plant? t = short
G = green leaves
A. TT g = yellow leaves
B. Tt P = purple flowers
p = white flowers
C. tt
R = round peas
D. None of the above r = wrinkled peas
Review Questions
6. What is the phenotype of a T = tall
Gg plant? t = short
G = green leaves
A. Green leaves g = yellow leaves
B. Yellow leaves P = purple flowers
p = white flowers
C. Green and yellow leaves
R = round peas
D. It depends on who gave r = wrinkled peas
the G gene.
Review Questions
7. Which of the below could T = tall
be the genotype for a t = short
plant with purple flowers? G = green leaves
g = yellow leaves
A. PP P = purple flowers
B. Pp p = white flowers
C. Either PP or Pp R = round peas
r = wrinkled peas
D. Neither PP or Pp
Review Questions
8. Which of the below could T = tall
be the genotype for a t = short
short plant with purple G = green leaves
flowers? g = yellow leaves
P = purple flowers
A. TtPP p = white flowers
B. ttPp R = round peas
C. TTpp r = wrinkled peas
D. Ttpp
Review Questions
9. A cutting is taken from a pea T = tall
plant. Thus the plant t = short
reproduces asexually. The
parent has TtGg genes. What G = green leaves
percentage of the offspring g = yellow leaves
will be tall with yellow leaves? P = purple flowers
p = white flowers
A. 100%
R = round peas
B. 50%
r = wrinkled peas
C. 25%
D. 0%
Review Questions
10. If a homozygous tall and T = tall
heterozygous tall plant are t = short
cross pollinated, what
G = green leaves
percentage of the offspring
g = yellow leaves
will be short?
P = purple flowers
A. 0% p = white flowers
B. 25% R = round peas
C. 50% r = wrinkled peas
D. 75%
Review Questions
11. What will be the ratio of T = tall
round peas to wrinkled t = short
peas if a Rr plant is bred G = green leaves
with a rr plant? g = yellow leaves
P = purple flowers
A. 4:1 p = white flowers
B. 3:1 R = round peas
C. 2:1 r = wrinkled peas
D. 1:1
Review Questions
12. Predict the result of T = tall
breeding: t = short
Gg and Gg G = green leaves
g = yellow leaves
A. 100% will be green. P = purple flowers
B. 25% will be yellow.
p = white flowers
R = round peas
C. 50% will be green. r = wrinkled peas
D. None will be yellow.
Review Questions
13. If you breed a Rr plant with a
RR plant,
T = tall
A. You will get all homozygous t = short
offspring. G = green leaves
B. You will get no wrinkled plants
g = yellow leaves
since round is dominant.
C. 75% of the offspring will be P = purple flowers
round since wrinkled is p = white flowers
recessive. R = round peas
D. You will get all heterozygous r = wrinkled peas
offspring.
Review Questions
14. Which of the below gene
set represents a gamete? T = tall
t = short
A. TtGg G = green leaves
B. Tt g = yellow leaves
C. TT P = purple flowers
p = white flowers
D. T
R = round peas
r = wrinkled peas
Review Questions
15. Challenge Question: How
many tall and green plants
will result from breeding T = tall
Ttgg with TTgg? t = short
G = green leaves
A. 100%
g = yellow leaves
P = purple flowers
B. 50%
p = white flowers
C. None
R = round peas
D. Depends on other r = wrinkled peas
variables
Answer Key
1. D 9. D
2. C 10. A
3. B 11. D
4. D 12. B
5. A 13. B
6. A 14. D
7. C 15. C
8. B
Contents Vocabulary
