1. Chapter 14. Mendel & Genetics

2. Gregor Mendel
Modern genetics began in the mid-1800s when 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
cross-pollinated true breeding parents (P)
raised seed & then observed traits (F1)
filial
allowed offspring to cross-pollinate & observed next generation (F2)
P = parents
F = filial generation

4. Mendel collected data for 7 pea traits

5. Looking closer at Mendel’s work
true-breeding
purple-flower peas
true-breeding
white-flower peas X P
100%
purple-flower peas F1
generation
(hybrids)
100%
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.
self-pollinate
75%
purple-flower peas
25%
white-flower peas
3:1 F2
generation

6. 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
purple-flower allele & white-flower allele are 2 DNA variations at flower-color locus
different versions of gene on homologous chromosomes

7. Traits are inherited as discrete units
diploid organisms
inherits 2 sets of chromosomes, 1 from each parent
homologous chromosomes

8. 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
fully expressed
recessive allele
no noticeable effect
the gene makes a non-functional protein

9. Genotype vs. phenotype phenotype genotype
description of an organism’s trait
what it “looks” like
genotype
description of an organism’s genetic makeup F1 P
Explain Mendel’s results using
…dominant & recessive
…phenotype & gentotype

10. PP pp Pp x Making crosses using representative letters
flower color alleles  P or p
true-breeding purple-flower peas  PP
true-breeding white-flower peas  pp PP x pp Pp

11. Looking closer at Mendel’s work
true-breeding
purple-flower peas
true-breeding
white-flower peas X P PP pp
phenotype
100%
purple-flower peas
100%
purple-flower peas F1
generation
(hybrids)
100%
100% Pp Pp Pp Pp
self-pollinate
75%
purple-flower peas
75%
purple-flower peas
25%
white-flower peas
3:1
3:1 F2
generation ? ? ? ?

12. Punnett squares Pp x Pp PP P p Pp P p PP Pp Pp pp Pp pp 25% 75% 50%
genotype %
phenotype
25%
75% PP P p
male / sperm
50% Pp P p
female / eggs PP Pp Pp
25%
25% pp Pp pp
1:2:1
3:1

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

14. Phenotype vs. genotype
2 organisms can have the same phenotype but have different genotypes PP
homozygous dominant
purple Pp
heterozygous
purple

15. Dominant phenotypes
It is not possible to determine the genotype of an organism with a dominant phenotype by looking at it.
PP?
Pp?

16. Test cross
Cross-breed the dominant phenotype — unknown genotype — with a homozygous recessive (pp) to determine the identity of the unknown allele x
is it PP or Pp? pp

17. Test cross x x PP pp Pp pp p p p p Pp Pp Pp Pp P P 50%:50% 1:1 100% P

18. Mendel’s laws of heredity (#1)PP P
Law of segregation
when gametes are produced during meiosis, homologous chromosomes separate from each other
each allele for a trait is packaged into a separate gamete pp p Pp P p

19. Law of Segregation What meiotic event creates the law of segregation?
Meiosis 1

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

21. Dihybrid cross
Other of Mendel’s experiments followed the inheritance of 2 different characters
seed color and seed shape
dihybrid crosses

22. 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
self-pollinate
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.
9/16
yellow
round
peas
9:3:3:1
3/16
green
wrinkled
1/16 F2
generation

23. What’s going on here?
How are the alleles on different chromosomes handed out?
together or separately?
YyRr
YyRr YR yr YR Yr yR yr

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

25. Mendel’s laws of heredity (#2)
Law of independent assortment
each pair of alleles segregates into gametes independently
4 classes of gametes are produced in equal amounts
YR, Yr, yR, yr
only true for genes on separate chromosomes
YyRr Yr Yr yR yR YR YR yr yr

26. Law of Independent Assortment
What meiotic event creates the law of independent assortment?
Meiosis 1

27. Review: Mendel’s laws of heredity
Law of segregation
monohybrid cross
single trait
each allele segregates into separate gametes
established by Meiosis 1
Law of independent assortment
dihybrid (or more) cross
2 or more traits
each pair of alleles for genes on separate chromosomes segregates into gametes independently

28. Extending Mendelian genetics
Mendel worked with a simple system
peas are genetically simple
most traits are controlled by a single gene
each gene has only 2 alleles, 1 of which is completely dominant to the other
The relationship between genotype & phenotype is rarely that simple

29. Incomplete dominance Heterozygotes show an intermediate phenotype
RR = red flowers
rr = white flowers
Rr = pink flowers
make 50% less color

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

31. Incomplete dominance CRCW x CRCW CRCR CR CW CRCW CRCR CRCW CR CW CRCW%
genotype %
phenotype
CRCR
25%
25% CR CW
male / sperm
50%
50%
CRCW
CRCR
CRCW CR CW
female / eggs
CRCW
CWCW
25%
25%
CRCW
CWCW
1:2:1
1:2:1

32. Co-dominance
2 alleles affect the phenotype in separate, distinguishable ways
ABO blood groups
3 alleles
IA, IB, i
both IA & IB are dominant to i allele
IA & IB alleles are co-dominant to each other
determines presences of oligosaccharides on the surface of red blood cells

33. Blood type IA IA IA i type A IB IB IB i type B IA IB type AB i i
genotype
phenotype
status
IA IA IA i
type A
type A oligosaccharides on surface of RBC __
IB IB IB i
type B
type B oligosaccharides on surface of RBC
IA IB
type AB
both type A & type B oligosaccharides on surface of RBC
universal recipient
i i
type O
no oligosaccharides on surface of RBC
universal donor

34. Blood compatibility 1901 | 1930 Matching compatible blood groups
critical for blood transfusions
A person produces antibodies against oligosaccharides in foreign blood
wrong blood type
donor’s blood has A or B oligosaccharide that is foreign to recipient
antibodies in recipient’s blood bind to foreign molecules
cause donated blood cells to clump together
can kill the recipient
Karl Landsteiner
( )

35. Blood donation

36. Pleiotropy Most genes are pleiotropic
one gene affects more than one phenotypic character
wide-ranging effects due to a single gene:
dwarfism (achondroplasia)
gigantism (acromegaly)
The genes that we have covered so far affect only one phenotypic character, but most genes are pleiotropic

37. Acromegaly: André the Giant

38. Pleiotropy
It is not surprising that a gene can affect a number of organism’s characteristics
cystic fibrosis
mucus build up in many organs
sickle cell anemia
sickling of blood cells

39. Epistasis One gene masks another coat color in mice = 2 genes
pigment (C) or no pigment (c)
more pigment (black=B) or less (brown=b)
cc = albino, no matter B allele
9:3:3:1 becomes 9:3:4

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

41. Nature vs. nurture 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

42. Sex-linked traits
Although differences between women & men are many, the chromosomal basis of sex is rather simple
In humans & other mammals, there are 2 sex chromosomes: X & Y
2 X chromosomes develops as a female: XX
redundancy
an X & Y chromosome develops as a male: XY
no redundancy

43. autosomal chromosomes
Sex chromosomes
autosomal chromosomes
sex chromosomes

44. Genes on sex chromosomes
Y chromosome
SRY: sex-determining region
master regulator for maleness
turns on genes for production of male hormones
pleiotropy!
X chromosome
other traits beyond sex determination
hemophilia
Duchenne muscular dystrophy
color-blind
Duchenne muscular dystrophy affects one in 3,500 males born in the United States.
Affected individuals rarely live past their early 20s.
This disorder is due to the absence of an X-linked gene for a key muscle protein, called dystrophin.
The disease is characterized by a progressive weakening of the muscles and loss of coordination.

45. Human X chromosome Sex-linked usually X-linked
more than 60 diseases traced to genes on X chromosome

46. Map of Human Y chromosome?
< 30 genes on Y chromosome
SRY

47. Sex-linked traits XHXh XHY Hh x HH XHXh XH Xh XH Y XHXH XHXH XHY XHY
sex-linked recessive
XHXh
XHY
Hh x HH
XHXh XH Xh XH Y
male / sperm
XHXH
XHXH
XHY
XHY XH Xh
female / eggs
XHY Y XH
XHXh
XHXh
XhY
XhY

48. Sex-linked traits summary
follow the X chromosomes
males get their X from their mother
trait is never passed from father to son
Y-linked
very few traits
only 26 genes
trait is only passed from father to son
females cannot inherit trait

50. Hemophilia is a sex-linked recessive trait defined by the absence of one or more clotting factors.
These proteins normally slow and then stop bleeding.
Individuals with hemophilia have prolonged bleeding because a firm clot forms slowly.
Bleeding in muscles and joints can be painful and lead to serious damage.
Individuals can be treated with intravenous injections of the missing protein.

51. X-inactivation Female mammals inherit two X chromosomes
one X becomes inactivated during embryonic development
condenses into compact object = Barr body

52. X-inactivation & tortoise shell cat
2 different cell lines in cat

53. Mechanisms of inheritance
What causes dominance vs. recessive?
genes code for polypeptides
polypeptides are processed into proteins
proteins function as…
enzymes
structural proteins
hormones

54. Prevalence of dominance
Because an allele is dominant does not mean…
it is better
it is more common
Polydactyly:
dominant allele

55. Polydactyly recessive allele far more common than dominant
individuals are born with extra fingers or toes
dominant to the recessive allele for 5 digits
recessive allele far more common than dominant
 399 individuals out of have only 5 digits
 most people are homozygous recessive (aa)

56. Hound Dog Taylor