1. Genetics & The Work of Mendel

2. 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.

3. 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 (kids)
allowed offspring to self-pollinate & observed next generation (F2- grandkids) P
anthers
removed
all purple flowers result F1
P = parents
F = filial generation
self-pollinate F2

4. 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

5. What did Mendel’s findings mean?
Traits come in alternative forms = alleles
purple vs. white flower color
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

6. 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!
wild type allele producing functional protein
mutant allele producing malfunctioning protein
homologous chromosomes

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

8. 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

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

10. 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!
Anaphase 1

11. Monohybrid cross
Some of Mendel’s experiments followed the inheritance of single characters
flower color
seed color
monohybrid crosses

12. 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

13. phenotype & genotype can have different ratios
Aaaaah,
phenotype & genotype can have different ratios
Punnett squares
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

14. Can’t tell by lookin’ at ya!
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?

15. 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

16. 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

17. Mendel was working out many of the genetic rules!
Dihybrid cross
Other of Mendel’s experiments followed the inheritance of 2 different characters
seed color and seed shape
dihybrid crosses
Mendel was working out many of the genetic rules!

18. 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

19. 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?

20. NO! Is this the way it works? YyRr x YyRr YR yr YR YYRR YyRr yr YyRr
9/16
yellow
round YR yr
3/16
green
round
Well, that’s NOT right! YR
YYRR
YyRr
3/16
yellow
wrinkled yr
YyRr
yyrr
1/16
green
wrinkled

21. YES! Dihybrid cross YyRr x YyRr YR Yr yR yr YR Yr yR yr YYRR YYRr YyRRor
Dihybrid cross
YyRr x
YyRr
9/16
yellow
round YR Yr yR yr YR Yr yR yr
3/16
green
round
YYRR
YYRr
YyRR
YyRr
BINGO!
YYRr
YYrr
YyRr
Yyrr
3/16
yellow
wrinkled
YyRR
YyRr
yyRR
yyRr
1/16
green
wrinkled
YyRr
Yyrr
yyRr
yyrr

22. Mendel’s 2nd law of heredity
Can you think of an exception to this?
Mendel’s 2nd law of heredity
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 or on same chromosome but so far apart that crossing over happens frequently
yellow
green
round
wrinkled
YyRr Yr Yr yR yR YR YR yr yr 1 : 1 : 1 : 1

23. Law of Independent Assortment
Which stage of meiosis creates the law of independent assortment?
Metaphase 1
Remember Mendel didn’t even know DNA —or genes— existed!
EXCEPTION!
If genes are on same chromosome & close together
will usually be inherited together
rarely crossover separately
“linked”

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

25. Review: Mendel’s laws of heredity
Law of segregation
monohybrid cross
single trait
each allele segregates into separate gametes
established by Metaphase 1
Law of independent assortment
dihybrid (or more) cross
2 or more traits
genes on separate chromosomes assort into gametes independently
EXCEPTION!
linked genes
metaphase1

26. Incomplete dominance
Heterozygote shows an intermediate, blended phenotype
example:
RR = red flowers
rr = white flowers
Rr = pink flowers
make 50% less color
RR
WW
RW RR RW WW

27. Incomplete dominance P F1 100% 1:2:1 F2 X true-breeding red flowers
white flowers
100%
100% pink flowers F1
generation
(hybrids)
self-pollinate
25%
white F2
generation
25%
red
1:2:1
50%
pink

28. Codominance
There are two or more alleles that are dominant in a phenotype.
Both alleles are expressed in heterozygous condition.
Ex: Red – RR, white – WW, red and white - RW

29. Multiple Alleles 2 alleles affect the phenotype equally & separately
not blended phenotype
human ABO blood groups
3 alleles
IA, IB, i
IA & IB alleles are co-dominant
glycoprotein antigens on RBC
IAIB = both antigens are produced
i allele recessive to both

30. Epistasis
The phenotypic expression of a gene at one locus alters that of a gene at a second locus.

31. Polygenic inheritance
Some phenotypes determined by additive effects of 2 or more genes on a single characteristic
phenotypes on a continuum
human traits
skin color
height
weight
intelligence
behaviors

32. Environmental effects
Phenotype is controlled by both environment & genes
Human skin color is influenced by both genetics & environmental conditions
Coat color in arctic fox influenced by heat sensitive alleles
The relative importance of genes & the environment in influencing human characteristics is a very old & hotly contested debate
a single tree has leaves that vary in size, shape & color, depending on exposure to wind & sun
for humans, nutrition influences height, exercise alters build, sun-tanning darkens the skin, and experience improves performance on intelligence tests
even identical twins — genetic equals — accumulate phenotypic differences as a result of their unique experiences
Color of Hydrangea flowers is influenced by soil pH

33. Pedigree Analysis
A pedigree is a family tree that describes the interrelationships of parents and children across generations
Inheritance patterns of particular traits can be traced and described using pedigrees

34. (a) Is a widow’s peak a dominant or recessive trait?
Fig b
1st generation
(grandparents) Ww ww ww Ww
2nd generation
(parents, aunts,
and uncles) Ww ww ww Ww Ww ww
3rd generation
(two sisters) WW ww
Figure 14.15a Pedigree analysis or Ww
Widow’s peak
No widow’s peak
(a) Is a widow’s peak a dominant or recessive trait?

35. (b) Is an attached earlobe a dominant or recessive trait?
Fig c
1st generation
(grandparents) Ff Ff ff Ff
2nd generation
(parents, aunts,
and uncles) FF or Ff ff ff Ff Ff ff
3rd generation
(two sisters) ff FF or Ff
Figure 14.15b Pedigree analysis
Attached earlobe
Free earlobe
(b) Is an attached earlobe a dominant or recessive trait?

36. Recessively Inherited Disorders
Organisms that don’t display the phenotype, but have the gene are known as carriers.
Cystic fibrosis
Sickle cell anemia
Tay Sacs disease

37. Dominantly Inherited Disorders
Organisms that have the gene, will show the phenotype.
Dwarfism
Huntington’s disease

38. Genetic Testing Testing for Carriers Fetal testing Newborn screening
Blood test done before conception
Fetal testing
Amniocentesis
Chorionic villus sampling
Newborn screening