Ch14 Mendel

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Mendel

P generation – true-breeding

• Why pea plants?• distinct heritable

features, or characters (such as flower color); character variants (such as purple or white flowers) are called traits

• He could control mating between plants

• Small• Matures quickly

Results:• 1. Mendel produced 14

pure strains by self-pollination

• ------> P generation

• 2. cross-pollinated plants w/contrasting traits

• -----> F1 generation

• 3. self-pollinated F1 generation

• -----> F2 generation

• 4. counted and found ratios of traits

Figure 11.3-3

P Generation

Experiment

(true-breedingparents)

F1 Generation

F2 Generation

(hybrids)

Purple flowers White flowers

All plants had purple flowers

Self- or cross-pollination

705 purple-floweredplants

224 white-floweredplants

Would these results seem strange to you if you were Mendel?

Table 11.1

Good experiments have large sample sizes

What we now know

Figure 11.4

Allele for purple flowers

Pair of homologouschromosomes

Allele for white flowers

Locus for flower-color gene

Law of segregation• Each gamete only gets one allele for a gene

law of independent assortment• - alleles for different genes distribute to gametes

independently• i.e. dominant traits don't always get passed on with other

dominant traits

Are the alleles on the same chromosome or different chromosomes?

Punnett Square

Copy this down!For every problem, you must:

1. Define alleles

2. Define cross

3. Show work:  Punnett Square

4. Box final answer

monohybrid cross• A cross between individuals that are heterozygous for one

character

Use your white board to solve• A brown eyed man whose mother was blue-eyed marries a

blue-eyed woman. • What is genotypic ratio of their potential offspring?• What are their phenotypes? • What is the probability that they will have two blue-eyed

children?

• Define alleles.• Define cross.• Show work/punnett square.• Box final answer.

Answer:• A brown eyed man whose mother was blue-eyed marries a blue-eyed

woman. • What is genotypic ratio of their potential offspring?• What are their phenotypes? • What is the probability that they will have two blue-eyed children?

• Define alleles.• Define cross.• Show work/punnett square.• Box final answer.

How can we tell the genotype of an individual with the dominant phenotype?

Figure 11.7 Technique

Predictions

Dominant phenotype,unknown genotype:

PP or Pp?

Eggs

Sperm

½ offspring purple and

½ offspring white

Recessive phenotype,known genotype:

pp

If purple-floweredparent is PP

If purple-floweredparent is Pp

Eggs

Sperm

All offspring purple

Results

or

or

p p

P

p

Pp

pp

Pp

pp

p p

P

PPp

Pp

Pp

Pp

• Possible genotypes?

• testcross: breeding the mystery individual with a homozygous recessive individual

• If any offspring display the recessive phenotype, the mystery parent must be heterozygous

Let’s try…• Imagine your candy preference is genetic.

• S = Snicker’s allele• s = Twix allele

• 7 kids like Snicker’s; 1 kid likes Twix

Answer:• Imagine your candy

preference is genetic.• S = Snicker’s allele• s = Twix allele

• 7 kids like Snicker’s; 1 kid likes Twix

dihybrid cross• A cross

between individuals that are heterozygous for two characters

Independent Assortment• Assume characters on different chromosomes

Memorize this pattern:

Memorize this pattern:

Answer:

You try:

• In humans, curly hair and freckles are dominant. A man with curly hair whose mom had straight hair mates with a woman who has straight hair. Both parents are heterozygous for freckles.

• What are the chances of having a child with straight hair and freckles?

• What would be the chance of a child having curly hair and no freckles?

• What about having two children with this phenotype?

Answer:• In humans, curly hair and freckles are dominant. A man with curly hair whose mom

had straight hair mates with a woman who has straight hair. Both parents are heterozygous for freckles.

• What are the chances of having a child with straight hair and freckles?

• What would be the chance of a child having curly hair and no freckles?

• What about having two children with this phenotype?

Probability:Figure 11.9

R r

R

r

Rr

Segregation ofalleles into eggs

Eggs

Sperm

¼

½ ½

½

½

Segregation ofalleles into sperm

Rr Rr

RR

¼

rr

¼

Rr

¼

Rule of Multiplication: “and”

• What is probability of getting:

• Heads and then a Tails

• Heads three times in a row

• A of hearts and then a Black Jack (w/replacement)

Rule of Addition: “or”• What is probability of getting:

• Heads or a Tails

• A of hearts or a Black Jack (w/replacement)

Let’s try a 2nd method for solving dihybrids:•CcFf x ccFf• What are the chances of

having a child with straight hair and freckles?

• What would be the chance of a child having curly hair and no freckles?

Trihybrids and beyond…

•AaBbCc x AABbCC• What is the probability of getting AaBbCc or AABBCC?