1. Genetics & The Work of Mendel

2. Do you know these terms? alleles Homozygous Heterozygous Phenotype
Genotype
Test cross
Back cross
Mendel’s Laws

3. Genetics The study of inheritance
how genes are passed on through generations
What is a gene?
A section of DNA that codes for a protein

4. Gregor Mendel
Modern genetics began in the mid-1800s in an abbey garden, where a monk named Gregor Mendel documented inheritance in peas
used experimental method
used quantitative analysis
collected data & counted them
excellent example of scientific method
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.

5. 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)
Pure or homozygous
raised seed & then observed traits (F1)
Hybrid or heterozygous
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

6. Generations P - parents. Usually pure breeding – ex. BB or bb
F1 - are hybrid – Bb since they get a B from one parent and b from the other due to meiosis
F2 - the result of crossing two Bb. The ratio is usually 3:1 dominant:recessive

7. Mendel collected data for 7 pea traits

8. Looking closer at Mendel’s work
true-breeding
purple-flower peas
true-breeding
white-flower peas 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

9. What did Mendel’s findings mean?
Traits come in alternative versions
purple vs. white flower color
Alleles are genes for the same trait
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

10. Traits are inherited as discrete units
For each characteristic, an organism inherits 2 alleles, 1 from each parent
diploid organism
inherits 2 sets of chromosomes, 1 from each parent

11. 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
_Wild type(Dominant)
functional protein
masks other alleles
_Mutatant(recessive)
allele makes a malfunctioning protein
wild type allele producing functional protein
mutant allele producing malfunctioning protein
homologous chromosomes

12. 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

13. PP pp Pp x 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 F1 P X
purple
white
all purple PP x pp Pp

14. Looking closer at Mendel’s work
true-breeding
purple-flower peas
true-breeding
white-flower peas X
phenotype P PP pp
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
self-pollinate
75%
purple-flower peas
25%
white-flower peas
3:1 F2
generation ? ? ? ?

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

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

17. Phenotype vs. genotype
2 organisms can have the same phenotype but have different genotypes
homozygous dominant PP
purple Pp
heterozygous
purple
How do you determine the genotype of an individual with with a dominant phenotype?

18. Test cross
Cross 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

19. 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 p p
100% purple
50% purple:50% white or 1:1

20. RESULTS If any recessive offspring result, the parents were hybrid
If all (of many offspring) show the dominant trait, the parents probably are pure dominant for that trait, but you can’t prove it.

21. Back Cross
If you know the parents, you can determine the offspring genotypes (look back at the parents)
If any parents have the recessive trait, the offspring (with the dominant trait) MUST BE HYBRID.

22. Black shows recessive trait
Pedigree chart
B-Brown allele
b-blue allele
What is the
genotype
here? bb B_ Bb
We know that
The blues must be
pure 1 2 bb bb 3 4 5 6 bb 7 8
Black shows recessive trait

23. Mendel’s laws of heredity
Segregation
single trait
each allele segregates into separate gametes
established by Metaphase 1
Independent Assortment
2 or more traits
genes on separate chromosomes assort into gametes independently Tt
T t
EXCEPTION
linked genes
metaphase1

24. Law of Dominance
When parents with different expressions of a trait (brown/blue, red/white) are crossed, the trait that is shown in the F1 is the dominant trait, and the hidden trait is recessive
NOTE: This is not always true but it was for the traits Mendel studied.

25. Law of Segregation-explanationPP P
Law of Segregation-explanation
during meiosis, alleles segregate
(homologous chromosomes separate)
each allele for a trait is packaged into a separate gamete pp p Pp P p

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

27. Monohybrid cross
Some of Mendel’s experiments followed the inheritance of single characters
flower color
seed color
Monohybrids

28. Dihybrid cross
Other of Mendel’s experiments followed the inheritance of 2 different characters
seed color and seed shape
Dihybrids
Mendel was working out many of the genetic rules!
Yellow/green, Round/wrinkled

29. Dihybrid cross P YYRR yyrr 100% F1 YyRr 9:3:3:1 F2 x true-breeding
yellow, round peas
true-breeding
green, wrinkled peas x
YYRR
yyrr
Y = yellow
R = round
y = green
r = wrinkled
100% F1
generation
(hybrids)
yellow, round peas
YyRr
Wrinkled seeds in pea plants with two copies of the recessive allele are due to the accumulation of monosaccharides and excess water in seeds because of the lack of a key enzyme. The seeds wrinkle when they dry.
Both homozygous dominants and heterozygotes produce enough enzyme to convert all the monosaccharides into starch and form smooth seeds when they dry.
self-pollinate
9:3:3:1 F2
generation
9/16
yellow
round
peas
3/16
green
round
peas
3/16
yellow
wrinkled
peas
1/16
green
wrinkled
peas

30. Which system explains the data?
What’s going on here?
If genes are on different chromosomes…
how do they assort in the gametes?
together or independently?
YyRr
Is it this?
Or this?
YyRr YR yr YR Yr yR yr
Which system explains the data?

31. Dihybrid cross YyRr x YyRr YR Yr yR yr YR Yr yR yr or 9/16 yellow
round YR Yr yR yr YR Yr yR yr
3/16
green
round
3/16
yellow
wrinkled
1/16
green
wrinkled

32. 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
yellow
green
round
wrinkled
YyRr Yr Yr yR yR YR YR yr yr 1 : 1 : 1 : 1

33. Law of Independent Assortment
Which stage of meiosis creates the law of independent assortment?
Remember Mendel didn’t even know DNA —or genes— existed!
EXCEPTION
If genes are on same chromosome
will usually be inherited together
Linkage

34. The chromosomal basis of Mendel’s laws…
Trace the genetic events through meiosis, gamete formation & fertilization to offspring

35. Linkage – Morgan used fruit flies
Genes on the same chromosome are usually inherited together. Ex. red hair and freckles in humans
Linked genes are changed by crossing over
More crossing over occurs between genes that are far apart on the chromosome
Chromosome maps are based on percentage of crossing over.

36. ------------40--------------
Chromosome maps
Problem: What is the relative position of the genes ABCD on the chromosome if cross-over experiments show the following: (% of cross-over)
A-B 40%
C-B 10%
D-C 10%
D-B 20%
A-D 20% A D
--10--
C--10-- B
Answer: ADCB or BCDA

37. Linkage and Crossing Over
Example Tall, Yellow hybrids are crossed TtYy X TtYy
If Linkage occurs (genes are on the same chromosome), you expect 75% Tall Yellow and 25% short green in the F2
If crossing over occurs, you expect most offspring to look like either parent, and a small number to have a different combination than the parents.

38. Mendel chose peas wisely
Pea plants are good for genetic research
available in many varieties with distinct heritable features with different variations
flower color, seed color, seed shape, etc.
Mendel had strict control over which plants mated with which
each pea plant has male & female structures
pea plants can self-fertilize
Mendel could also cross-pollinate plants: moving pollen from one plant to another

39. Mendel chose peas luckily
Pea plants are good for genetic research
relatively simple genetically
most characters are controlled by a single gene with each gene having only 2 alleles,
one completely dominant over the other

40. Any Questions??