1. Chapter 14: Mendel & The Gene Idea

2. The History of Your Inheritance
Identify one or two physical characteristics that seem to be found in most of your family members. Why do you think these particular characteristics are so common?
Are there some inherited characteristics that you have that your parents do not?
Do you have any characteristics that are unique to you?

4. Character vs. Trait
Some people use these two words synonymously, when they are, in fact, different.
Characteristic (Character)
a heritable feature that varies among individuals (e.g.eye colour, flower colour, etc.)
Trait
Each variant of a characteristic (e.g. blue vs. brown eyes, purple vs. white flowers)

5. Gregor Mendel
Modern genetics began in the mid-1800s, when monk named Gregor Mendel documented inheritance in peas
used quantitative analysis
collected data & counted them
excellent example of scientific method
Developed his theory decades before chromosomes were observed under the microscope!
He studied at the University of Vienna from 1851 to 1853 where he was influenced by a physicist who encouraged experimentation and the application of mathematics to science and a botanist who aroused Mendel’s interest in the causes of variation in plants. After the university, Mendel taught at the Brunn Modern School and lived in the local monastery.
The monks at this monastery had a long tradition of interest in the breeding of plants, including peas. Around 1857, Mendel began breeding garden peas to study inheritance.

6. Why the Pea? Reason 1
Peas are available in many varieties, each with easily distinguishable characteristics and traits
These varieties are easy to count

7. Pea Plant Anatomy Stamen: male organ Carpel: female organ
Where pollen (male gametes) produced
Sticky structure that receives pollen during fertilization
anther
stamen
filament
stigma
style
carpel
ovules in
ovary
Female gametes

8. Why the Pea? Reason 2
Can strictly control which plants mated with which

9. Mendel started with True-breeding Plants
are those that produce offspring that are of the same variety when they self-pollinate
E.g. a plant with purple flowers is true-breeding if the seeds produced by self-pollination all give rise to plants that also have purple flowers

10. Mendel’s work Bred pea plants P F1 F2
Pollen transferred from white flower to stigma of purple flower
Bred pea plants
cross-pollinate true breeding parents (P)
P = parental
raised seed & then observed traits (F1)
F = filial
allowed offspring to self-pollinate & observed next generation (F2) P
anthers
removed
all purple flowers result F1
P = parents
F = filial generation
self-pollinate F2

11. Looking closer at Mendel’s work
true-breeding
purple-flower peas
true-breeding
white-flower peas
Hybridization! X P
Where did
the white flowers go?
100% F1
generation
(hybrids)
purple-flower peas
In a typical breeding experiment, Mendel would cross-pollinate (hybridize) two contrasting, true-breeding pea varieties.
The true-breeding parents are the P generation and their hybrid offspring are the F1 generation.
Mendel would then allow the F1 hybrids to self-pollinate to produce an F2 generation.
White flowers came back!
self-pollinate F2
generation
3:1
75%
purple-flower peas
25%
white-flower peas

12. What did Mendel’s findings mean?
Traits come in alternative versions
purple vs. white flower color
alleles
different alleles vary in the sequence of nucleotides at the specific locus of a gene
some difference in sequence of A, T, C, G
purple-flower allele & white-flower allele are two DNA variations at flower-color locus
different versions of gene at same location on homologous chromosomes

13. Traits are inherited as discrete units
For each characteristic, an organism inherits 2 alleles, 1 from each parent
diploid organisms inherit 2 sets of chromosomes, 1 from each parent
homologous chromosomes
Key
Maternal set of
chromosomes (n = 3)
2n = 6
Paternal set of
chromosomes (n = 3)
Pair of homologous
chromosomes
(one from each set)

14. 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
masks other alleles
recessive allele
allele makes a malfunctioning protein
I’ll speak for both of us!
maternal
paternal
wild type allele producing functional protein
mutant allele producing malfunctioning protein
homologous chromosomes

15. 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 F1 P X
purple
white
all purple
Explain Mendel’s results using
…dominant & recessive
…phenotype & genotype

16. PP pp Pp Making crosses x Can represent alleles as letters
flower color alleles  P or p
true-breeding purple-flower peas  PP
true-breeding white-flower peas  pp F1 P X
purple
white
all purple PP x pp
Why is the F1 all purple pea plant not PP? Pp

17. Looking closer at Mendel’s work
true-breeding
purple-flower peas
true-breeding
white-flower peas X
phenotype P PP pp
genotype
100% F1
generation
(hybrids)
purple-flower peas
In a typical breeding experiment, Mendel would cross-pollinate (hybridize) two contrasting, true-breeding pea varieties.
The true-breeding parents are the P generation and their hybrid offspring are the F1 generation.
Mendel would then allow the F1 hybrids to self-pollinate to produce an F2 generation. Pp Pp Pp Pp
self-pollinate
Pp x Pp
75%
purple-flower peas
25%
white-flower peas
3:1 F2
generation ? ? ? ?

18. Punnett squares Pp x Pp F1 P p PP Pp P p PP Pp Pp Pp pp pp
Aaaaah,
phenotype & genotype can have different ratios
Pp x Pp F1
generation
(hybrids) %
genotype %
phenotype P p
male / sperm PP
25%
75% Pp
50% P p
female / eggs PP Pp Pp Pp pp
25%
25% pp
1:2:1
3:1

19. Genotypes Homozygous = same alleles = PP, pp
Heterozygous = different alleles = Pp
homozygous dominant
heterozygous
homozygous recessive

20. Phenotype vs. genotype
2 organisms can have the same phenotype but have different genotypes
homozygous dominant PP
purple Pp
heterozygous
purple
Can’t tell by lookin’ at ya!
How do you determine the genotype of an individual with with a dominant phenotype?

21. Test cross
Breed the dominant phenotype — the unknown genotype — with a homozygous recessive (pp) to determine the identity of the unknown allele x
How does that work?
is it PP or Pp? pp

22. How does a Test cross work?x x
Am I this?
Or am I this? PP pp Pp pp p p p p P P Pp Pp Pp Pp P p Pp Pp pp pp
100% purple
50% purple:50% white or 1:1

23. Mendel’s 1st law of heredityPP P
Law of segregation
during gamete formation, alleles segregate
homologous chromosomes separate during meiosis
each allele for a trait is packaged into a separate gamete pp p Pp P p

24. Whoa! And Mendel didn’t even know DNA or genes existed!
Law of Segregation
Which stage of meiosis creates the law of segregation?
Metaphase 1
Whoa! And Mendel didn’t even know DNA or genes existed!

25. Chapter 14 Probability & Genetics

26. Genetics & Probability
Mendel’s laws reflect same laws of probability that apply to tossing coins or rolling dice

27. Probability & genetics
Calculating probability of making a specific gamete is just like calculating the probability in flipping a coin
probability of tossing heads? 50%
probability making a P gamete… Pp P p
50% PP P
100%

28. Probability & genetics
Outcome of 1 toss has no impact on the outcome of the next toss (i.e. they are independent events!)
probability of tossing heads each time?
probability making a P gamete each time?
50% Pp P p
50%

29. Calculating probability
Pp x Pp
sperm
egg
offspring P PP
1/2 x
1/2 =
1/4 P p
male / sperm P p Pp
1/2 x
1/2 =
1/4 P p
female / eggs PP Pp p P Pp
1/2 x
1/2 =
1/4 Pp pp p pp
1/2 x
1/2 =
1/4

30. Rule of multiplication (AND)
Chance that 2 or more independent events will occur together (AND)
probability that 2 coins tossed at the same time will land heads up
probability of Pp x Pp  pp
1/2 x 1/2 = 1/4
1/2 x 1/2 = 1/4

31. Rule of addition (OR)
Chance that an event can occur 2 OR more different ways
sum of the separate probabilities
probability of Pp x Pp  Pp
sperm
egg
1/2
offspring = x
1/4 P p Pp
1/4 +
1/2

33. Sample Problem
In humans the allele for albinism is recessive to the allele for normal skin pigmentation. If two heterozygotes have children,
(a) What is the genotypic and phenotypic ratios for their offspring?
(b) What is the chance that a child will have normal skin pigment?
(c) What is the chance that the child will be albino?

34. Solution (a) Aa x Aa A (1/2) a (1/2) AA A (1/2) a AA Aa Aa Aa Aa aa aa
Let A be dominant allele for normal skin pigmentation, let a be the recessive allele for albinism
Aa x Aa %
genotype %
phenotype
A (1/2)
a (1/2)
sperm
25% or
1/4
75% or
3/4 AA A
(1/2) a
eggs
50% or
1/2 AA
(1/2 x 1/2
=1/4) Aa Aa
(1/2 x 1/2
=1/4) Aa Aa
(1/2 x 1/2
=1/4) aa
(1/2 x 1/2
=1/4)
25% or
1/4
25% or
1/4 aa
1:2:1
3:1

35. Solution (b) and (c) Aa x Aa A (1/2) a (1/2) A (1/2) a AA Aa Aa aa
Let A be dominant allele for normal skin pigmentation, let a be the recessive allele for albinism
Aa x Aa
Chances that the child will have normal skin pigmentation (i.e. they must be AA OR Aa OR Aa):
Chance of AA + Chance of Aa
= 1/4 + 1/4 + 1/4
= 3/4 or 75%
Chances that the child will be albino (i.e. they must have aa genotype)
Chance of aa = 1/4
A (1/2)
a (1/2)
sperm A
(1/2) a
eggs AA
(1/2 x 1/2
=1/4) Aa
(1/2 x 1/2
=1/4) Aa
(1/2 x 1/2
=1/4) aa
(1/2 x 1/2
=1/4)

36. Challenge: A (1/2) a (1/2) A (1/2) a AA Aa Aa aa
If the child is normal, what is the chance that it is a carrier for the albino allele?
Normal individuals are either AA or Aa
For it to be a carrier, it must have Aa genotype:
2/3 or 67%
A (1/2)
a (1/2)
sperm A
(1/2) a
eggs AA
(1/2 x 1/2
=1/4) Aa
(1/2 x 1/2
=1/4) Aa
(1/2 x 1/2
=1/4) aa
(1/2 x 1/2
=1/4)

37. Any Questions?

38. Practice
Pg. 135 # 1-9, 13, 16
Worksheet