1. Ch. 14/15 Mendelian Genetics Review & Chromosomes

2. Gregor Mendel Who was the “father of genetics”?
Modern genetics began in the mid-1800s in an abbey garden, where a monk named Gregor Mendel documented inheritance in peas
used experimental design
used quantitative analysis
collected data & counted them
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-pollinated 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. A closer look 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.
self-pollinate F2
generation
3:1
(¾ : ¼)
75%: 25%
75%
purple-flower peas
25%
white-flower peas

5. What did these findings mean?
Traits come in alternative forms = alleles (alternate form of a gene)
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/overshadowed 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. Wild type versus mutant
The allele that encodes the phenotype most common in a particular natural population is known as the wild type allele.
Any form of that allele other than the wild type is known as a mutant form of that allele. Here are a few examples.

8. Genotypes Homozygous = same alleles contributed to the zygote = PP, pp
Heterozygous = different alleles contributed to the zygote = Pp
homozygous dominant
heterozygous
homozygous recessive

9. 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 (2 alleles combined)

10. 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

11. 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

12. Monohybrid cross
Some of Mendel’s experiments followed the inheritance of single characters
monohybrid crosses examine the inheritance of one trait only

13. Determining probability
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
100%

14. phenotype & genotype can have different ratios?
Woa-
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

16. I can’t tell just 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
I can’t tell just by lookin’ at ya!
How do you determine the genotype of an individual with with a dominant phenotype?

17. We can’t do this with humans!!
Test cross
Breed the dominant phenotype — the unknown genotype — with a homozygous recessive (pp) to determine the identity of the unknown allele x
We can’t do this with humans!!
is it PP or Pp? pp

18. 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

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

20. Dihybrid cross (probability of inheriting 2 traits at the same time)
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 F2
generation
9/16
yellow
round
peas
3/16
green
round
peas
3/16
yellow
wrinkled
peas
1/16
green
wrinkled
peas

21. 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 2. 1. YR yr YR Yr yR yr
Which system explains the data?

22. #2 is correct! Dihybrid cross YyRr x YyRr YR Yr yR yr YR Yr yR yr YYRR
Phenotypic ratio
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
YYRr
YYrr
YyRr
Yyrr
3/16
yellow
wrinkled
YyRR
YyRr
yyRR
yyRr
1/16
green
wrinkled
YyRr
Yyrr
yyRr
yyrr

23. 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

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

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

26. Gene Linkage
Genetic map: ordered list of the genetic loci (region) along a chromosome
Linkage map: genetic map based on recombination frequencies (** remember, genetic recombination occurs during prophase 1 when chromatids cross over)
Distance between genes= map units

28. ** NOTE about Gene Linkage
Genes found on same chromosome are considered LINKED!
How close or far away are they?
Fewer gamete possibilities the closer they are!
Why? Less possibility for crossing over to occur.
Independent assortment does not apply
No linkage if genes are on separate chromosomes (# of recombinants increases)

29. This shows linked genes with no crossing over and crossing over.
How can we test to see if genes are linked?

30. Chromosome Mapping Calculates the frequency of recombinant offspring.
Recombination frequency = # map units
Ex: 13% recombinant frequency = 13 map units
Greater % = greater distance
Lower % = closer distance

31. Any Questions so far??
Let’ move on!

32. Incomplete dominance (“blending”)
Heterozygote shows an intermediate, blended phenotype
example:
RR = red flowers
rr = white flowers
Rr = pink flowers
make 50% less color RR RW WW

33. 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

34. Codominance (“with”)
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

35. Multiple Alleles 2 alleles affect the phenotype equally & separately
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

38. 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

39. Pleiotropy Pleion= “more”
Occurs when a gene has multiple phenotypic effects.
Ex) in peas, the gene that codes for flower color (purple or white) also affects the color of the seed coating (gray or white)… in humans sickle cell disease

40. Epistasis
Phenotypic expression of a gene at one locus alters a gene at a second locus.
Ex) Labrador retriever coat color
B= black coat
b= brown coat
BUT… 2nd gene determine whether or not the the pigment will be deposited in the fur
E= color deposited (black or brown)
e= no color deposited (yellow)

41. BB/Bb BB/Bb bb bb ee EE/Ee EE/Ee ee
So….
BB/Bb
BB/Bb bb bb ee
EE/Ee
EE/Ee ee
Black Lab
Chocolate Lab
Yellow Lab

43. Sex linked traits Genes are on sex chromosomes
as opposed to autosomal chromosomes
first discovered by T.H. Morgan at Columbia U.
Drosophila breeding
good genetic subject
prolific
2 week generations
4 pairs of chromosomes
XX=female, XY=male

44. Discovery of sex linkage
true-breeding
red-eye female
true-breeding
white-eye male X P
Huh! Sex matters?!
100%
red eye offspring F1
generation
(hybrids)
100%
red-eye female
50% red-eye male
50% white eye male F2
generation

45. Genetics of Sex X Y X XX XY X XX XY
In humans & other mammals, there are 2 sex chromosomes: X & Y
2 X chromosomes
develop as a female: XX
gene redundancy, like autosomal chromosomes
an X & Y chromosome
develop as a male: XY
no redundancy X Y X XX XY X XX XY
50% female : 50% male

46. Morgan’s flies… Female have a 100% chance of being red-eyedx
Female have a 100% chance of being red-eyed
Males have a 100% chance of being red-eyed
XRXR
XrY Xr Y XR
XRXr
XRY XR
XRXr
XRY

47. Morgan’s flies… Female have 100% chance of being red-eyedx
Female have 100% chance of being red-eyed
Males have a 50% chance of being red-eyed and a 50% chance of being white-eyed
XRXr
XRY XR Y XR
XRXR
XRY Xr
XRXr
XrY

48. Genes on sex chromosomes
Y chromosome
Have very few genes other than SRY
sex-determining region
master regulator for “maleness”
turns on genes for production of male hormones
X chromosome
Have traits other than sex determination
mutations: (all are recessive)
hemophilia
Duchenne muscular dystrophy
color-blindness
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.

49. Hemophilia XHXh XHY Hh x HH XH XHXh XH Y Xh XHXH XHXH XHY XHY XH Xh XH
sex-linked recessive
Hemophilia
XHXh
XHY
Hh x HH XH
XHXh XH Y
male / sperm Xh
XHXH
XHXH
XHY
XHY XH Xh
female / eggs
Normal female
Normal male XH
XHY
XHXh
XHXh
XhY
XhY
Carrier female
Affeced male Y

50. X-inactivation Female mammals inherit 2 X chromosomes XH XHXh Xh
one X becomes inactivated during embryonic development
condenses into compact object = Barr body
which X becomes Barr body is random
patchwork trait = “mosaic”
patches of black
XH
XHXh
Xh
tricolor cats can only be female
patches of orange

51. 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

52. Chromosomal Genetic Disorders
Nondisjunction:
failure of homologous chromosomes to separate during meiosis I or failure of sister chromatids to separate during meiosis II.

53. Abnormal chromosome numbers
Aneuploidy: when a normal sex cell unites with one containing an abnormal amount of chromosomes… zygote will have an abnormal amount of chromosomes
Monosomy: 2n-1 (45 chromosomes per cell)
Trisomy: 2n+1 (47 chromosomes per cell)
Polyploidy: more than two complete chromosome sets in somatic cells.
Triploidy (3n) and tetraploidy (4n)
Common in plants, uncommon in animals (some fish & amphibians have this)

54. Alterations of chromosome structure
Deletion:
Chromosome fragment is lost
Insertion:
Extra segment of chromosome gets added
Duplication:
Extra segment of chromosome gets added (is the same as the one on the sister chromatid)
Inversion:
Fragment breaks off and attaches in reverse order
Translocation:
Fragment adds to a nonhomologous chromosome

55. insertion
inversion
deletion
translocation
duplication

56. How can we see Genetic Disorders?
Karyotype

57. 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
½ shaded= “carrier” (heterozygote in recessive traits only)

58. (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?
dominant

59. (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
recessive
(b) Is an attached earlobe a dominant or recessive trait?

60. Detection First Trimester Prenatal Screening Tests
Ultrasound test for fetal nuchal translucency (NT). Nuchal translucency screening uses an ultrasound test to examine the area at the back of the fetal neck for increased fluid or thickening.
Two maternal serum (blood) tests. The blood tests measure two substances found in the blood of all pregnant women: 
Pregnancy-associated plasma protein screening (PAPP-A)--a protein produced by the placenta in early pregnancy. Abnormal levels are associated with an increased risk for chromosome abnormality.
Human chorionic gonadotropin (hCG)--a hormone produced by the placenta in early pregnancy. Abnormal levels are associated with an increased risk for chromosome abnormality.

61. Detection Second Trimester Prenatal Screening Tests
Alpha-fetoprotein screening (AFP). AFP is a protein normally produced by the fetal liver and is present in the fluid surrounding the fetus (amniotic fluid), and crosses the placenta into the mother's blood. The AFP blood test is also called MSAFP (maternal serum AFP).
Abnormal levels of AFP may signal the following:
Open neural tube defects (ONTD), such as spina bifida
Down syndrome
Other chromosomal abnormalities
Defects in the abdominal wall of the fetus

62. Detection

63. Detection

64. Ready to practice and move on to even more genetics stuff?