Genetics Why do we look the way we do?

Honors Biology Chapters 9 & 12

Inheritance of chromosomes Egg + sperm zygote

egg

sperm

zygote

fertilizationmitosis &

development

meiosis

Inheritance of genes Chromosomes passed from Mom & Dad

to offspring are comprised of genes may be same information may be different information

eye color(blue or brown?)

eye color(blue or brown?)

Effect of genes Gene = region of a chromosome that codes for

a trait Genes come in different versions for each trait

brown vs. blue eyes brown vs. blonde hair A version of a gene = an allele

Genes affect what you look like

XBBbb

Bb Bb Bb Bb

Where did the blue eyes go??

Genes affect what you look like…

XBbbb

Bb Bb bb bb

Why did the blue eyes stay??

Genes affect what you look like…

XBbBb

BB or Bb BB or Bb BB or Bb bb

Where did the blue eyes come from??

What did we show here? Genes come in “versions”

brown vs. blue eye color alleles

Alleles are inherited separately from each parent brown & blue eye colors are separate &

do not blend either have brown or blue eyes, not a blend

Some alleles mask others brown eye color masked blue

How does this work?

eye color(brown?)

hair color

hair color

eye color

(blue?)

Homologous chromosomes have same genes… …but maybe different alleles

Traits are inherited as separate units

1 from Mom

1 from Dad

For each trait, an organism inherits 2 copies of a gene (2 alleles), 1 from each parent

homologous chromosomes

Genetics vs. appearance There can be a difference between

how an organism looks & its genetics Its expressed trait/s = phenotype

brown eyes vs. blue eyes Its alleles, or genetic makeup =

genotype BB, Bb, bb

2 people can have the same phenotype but have different genotypes: BB vs Bb

Genetics vs. appearance

eye color(brown)

eye color

(brown)

eye color(blue)

eye color

(brown)

vs.

BB

B

BBb

B

b

How were these brown eyes made?How were these brown eyes made?

Practice

If G is the allele for pointy ears and g is the allele for floppy ears, what will be the ear shape phenotypes of the puppies with these genotypes?

The dominant allele is _ for the trait ___________

The recessive allele is _ for the trait ___________

Genotype GG = Phenotype __________

Genotype Gg = Phenotype __________

Genotype gg = Phenotype ___________

Practice

G is for pointy ears and g is for floppy ears. Also, H is for a pink nose and h is for a black nose.

Genotype GGHH = Phenotype ______ and ______

Genotype GgHh = Phenotype ______ and ______

Genotype gghh = Phenotype ______ and _______

Genotype GGhh = Phenotype ______ and _______

Genotype Gghh = Phenotype ______ and _______

Genotype ggHH = Phenotype ______ and _______

Practice

Which of these are traits and which are phenotypes?

1. Finger length 2. Blue eyes 3. Long hair 4. Number of leaves 5. Shape of tentacles 6. Warbling song

Practice

Which of these are alleles and which are traits? 1. Eye color 2. Bone integrity 3. i 4. Insulin shape 5. B 6. Na

Practice

Which of these are phenotypes and which are genotypes?

1. Curly hair 2. Jj 3. PP 4. Arthritic knees 5. Type B blood 6. Spotted fur and a pink nose 7. HHGg 8. Purple leaves and spiny stem

Genetics&

The Work of 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

Gregor Mendel

Pollen transferred from white flower to stigma of purple flower

all purple flowers result

Mendel’s work

?

self-pollinate

Bred pea plants cross-pollinate

true breeding parents

allowed offspring to self-pollinate & observed next generation

Mendel collected data for 7 pea traits

2nd

generation

3:175%purple-flower peas

25%white-flower peas

Looking closer at Mendel’s work

Parents

100%1st

generation(hybrids)

100%purple-flower peas

Xtrue-breedingpurple-flower peas

true-breeding white-flower peas

self-pollinate

F1

F2

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

dominant allele functional protein

affects characteristic masks other alleles

recessive allele no noticeable effect allele makes a

non-functioning protein

homologouschromosomes

allele producingfunctional protein

mutant allele malfunctioning

protein

Mendel’s Results and Conclusions

RESULT: Whenever Mendel crossed two P

plants, one of the traits disappeared in the F1 plants.

The missing trait reappeared in the F2 plants in a 3:1 ratio pattern

CONCLUSION: LAW OF DOMINANCE One trait is dominant because it

masked or dominated the other trait One trait is recessive because it “hid”

behind the dominant one. It can only be seen when the plant has no dominant alleles.

CONCLUSION: LAW OF SEGREGATION

Pairs of alleles segregate (separate) during the formation of gametes (meiosis—homologous pairs separate)

A parent only passes one allele for each gene onto a zygote

Mendel’s Results and Conclusions

CONCLUSION: LAW OF INDEPENDENT ASSORTMENT

Factors for different characteristics are distributed to gametes independently or randomly.

Which allele is passed for one one gene doesn’t affect which allele is passed down from other genes

Mendel’s Results and Conclusions

DNA and chromosomes weren’t discovered until many decades after Mendel’s death

Today, we understand the genetic mechanisms that underlie his mathematical discoveries…

Mendel’s Legacy

Gamete Formation Suppose there’s a gene for eye color, with the

alleles B for brown eyes or b for blue eyes. A man has the genotype Bb, which gives him

the phenotype brown eyes. Meiosis produces his gametes…

bb

bbb

b

BB

BBB

B

He can make gametes that are EITHER B or b.Half of his gametes will be one, half will be the other.We simplify, saying that he produces either B or b allele sperm. Equal chance of each.

bB

S P

hase

1st C

ytok

ines

is

2nd C

ytok

ines

is

Normal cell in G1

Four Gametes

Practicing the Law of Segregation

(Some gametes are written with more than one letter. If Dad’s genotype is LTLt, he will make a sperm that has the LT allele or a sperm that has the Lt allele.)

Genotype YY makes what gamete/s?

Genotype Tt makes what gamete/s?

Genotype bb makes what gamete/s?

Genotype Ii makes what gamete/s?

Genotype K1K2 makes what gamete/s?

How do we say it?

BB = brown eyesbb = blues eyes

Bb = brown eyes

2 of the same allele=Homozygous

2 different alleles=Heterozygous

BB

B

B

bb

b

b

Bb

B

b

homozygous dominanthomozygous recessive

Practice

Identify each of these genotypes as being homozygous or heterozygous.GG ____________ Ss ________Yy ___________ Vv

________kk ____________

Practice

Identify each of these genotypes as being homozygous dominant, homozygous recessive, or heterozygous.ee ____________ CC

________QQ ___________ pp ________Ll ____________

Practice

Suppose that the I allele codes for orange fins and the i allele codes for yellow fins. The heterozygous genotype: __ The homozygous dominant genotype: __ The homozygous recessive genotype: __ A fish with yellow fins must have a _____________

genotype. A fish with orange fins could be either

_____________ or ___________________.

Punnett squaresBb x Bb

male / sperm

fem

ale

/ eg

gs

X

BB

Bb bb

BbB

b

B b

Genetics and Probability

Figuring out offspring is a matter of chance.A Punnett Square provides the

probabilities of two parents producing particular zygotes.

An example, using coins:

Punnett Squares

Using the letter H to stand for heads…If you flip a coin that’s heads (H) on both sides,

what are the chances that it will come up heads (H)?

Punnett Squares

If it’s a normal coin, heads (H) on one side and tails (h) on the other…What are the odds that it will come up heads

(H) on a flip?

Punnett Squares

If you flip TWO normal coins, what are the odds that you will get heads (H) on both flips?

Punnett Squares

The first flip will be either heads (H) or tails (h):

Punnett Squares

The second flip will also be either heads (H) or tails (h):

Punnett Squares

These are the possible combinations that you could have produced:

Punnett Squares

These are the possible combinations that you could have produced:

Punnett Squares

These are the possible combinations that you could have produced:

Punnett Squares

These are the possible combinations that you could have produced:

Punnett Squares

These are the possible combinations that he could have produced:

H

H H H H

H

Punnett Squares 1 in 4 possible outcomes would be

both heads (HH). The chance of getting heads on both flips = 1/4 = 25%

H h

H

h

H H H h

H h h h

Punnett Squares What are the odds of getting heads on

one flip, tails on the other?

H h

H

h

H H H h

H h h h

Punnett Squares

2 of 4 possible outcomes = 1/2 = 50%

H h

H

h

H H H h

H h h h

Punnett Squares

What if you flip two different coins:One coin has two headsThe other is normal, one heads and

one tails

What are the odds of getting heads on both flips?

Punnett Squares

2/4 = 1/2 = 50%

This is called a Punnett Square. Punnett Squares display all possible gametes and

possible offspring.

H H

H

h

H H H H

H h H h

Punnett Squares

The top and side boxes show possible gametes. The middle boxes show possible zygotes (offspring) they would create.

Step-By-Step Instructions

Sample problem: What are the chances that a heterozygous brown-eyed father and a blue-eyed mother would have a blue-eyed child? (Use letters B/b)

1. Figure out Mom and Dad’s genotypes.In the example: Dad = ___, Mom = __

Punnett Squares

2. Figure out Mom and Dad’s gametes. Dad’s gametes = __ and __ Mom’s gametes = __ and __

3. Set up a square. For a monohybrid cross (studying only one gene),

make a normal tic-tac-toe board.

Punnett Squares4. Write Dad’s gametes on one side, and Mom’s

on the other.• It doesn’t matter whether Mom or Dad is on the side vs top, just

keep both eggs together and both sperm together.

Punnett Squares

4. (continued) Like a genotype, if there’s a dominant allele, put it first.

A a

A

a

A

A

a a

Punnett Squares

5. Complete zygote genotypes.Remember to put dominant allele first,

if there is one.

6. Write out all the zygote genotypes as fractions.I.e. 1/4, 2/4, 3/4, 4/4

Punnett Squares

7. Reduce fractions if possible, and convert fractions to percentages.For instance, if two of the four zygotes

are AA, the probability of genotype AA is 2/4 = 1/2 = 50%

8. If applicable, rewrite offspring genotype results as phenotype results.

Punnett Squares

If you did the problem correctly, it should EITHER look like this, OR…

B b

b

b

Bb

Bb

bb

bb

Genotype ProbabilitiesBB = 0/4 = 0%Bb = 2/4 = 1/2 = 50%bb = 2/4 = 1/2 = 50%

Phenotype ProbabilitiesBrown eyes = 2/4 = 1/2 = 50%Blue eyes = 2/4 = 1/2 = 50%

Final Answer50% probability of a blue-eyed child

Punnett Squares

…OR like this. Notice that results are the same.

b b

B

b

Bb

bb

Bb

bb

Genotype ProbabilitiesBB = 0/4 = 0%Bb = 2/4 = 1/2 = 50%bb = 2/4 = 1/2 = 50%

Phenotype ProbabilitiesBrown eyes = 2/4 = 1/2 = 50%Blue eyes = 2/4 = 1/2 = 50%

Final Answer50% probability of a blue-eyed child

Practicing with Punnett Squares

Parents – Tt and ttT t

t Tt tt

t Tt tt

Offspring:50% Tt – tall50% tt – short

Parents – TT and TtT T

T TT TT

t Tt Tt

Offspring:100% TT or Tt – tall

Practicing Punnett Squares

Show a monohybrid cross, with all genotype and phenotype probabilities, for parents who are HH and hh.

Practicing Punnett Squares

Show a monohybrid cross, with all genotype and phenotype probabilities, for parents who are Ll and Ll.

Practicing Punnett Squares

If Dad’s genotype is Rr and Mom is homozygous recessive, what are the odds of having homozygous dominant offspring?

Practicing Punnett Squares

If both parents are heterozygous, and they have ten offspring, how many of those offspring would you predict will be homozygous recessive?

Practicing Punnett Squares

Suppose black fur is dominant and white fur is recessive. Two parents, one with black fur and one with white fur, have many offspring. Roughly half of their babies are black-furred, and half are white-furred. What were the genotypes of the parents? Hint: when a question asks you to figure out parental

genotypes, make test crosses, Punnett Squares for every possibility, then see which one gives you offspring results that fit.

Testcross

Dominant phenotypes can have either BB or Bb genotypes. How do you know?

Perform a test cross, or a cross with a known recessive genotype (bb) If any recessive offspring appear,

unknown must be heterozygous If no recessive offspring appear,

unknown can be homozygous.