1. Mendelian Genetics Biology – Premed Windsor University School of Medicine

2. CHAPTER 22 Genetics & The Work of Mendel
There is more to lectures than the power point slides!
Engage your mind

3. Gregor Mendel used good experimental design used mathematical analysis
Modern genetics began in the mid-1800s in an abbey garden, where a monk named Gregor Mendel documented inheritance in peas
used good experimental design
used mathematical 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.

4. True Breeding: When self-fertilized, only produces offspring with the same traits
Eg. A plant with purple flowers is true-breeding if the seeds produced by self-pollination in successive generations all gave rise to purple flowers
Cross-pollinate: Eg. Breeding purple-flowered plants and white-flowered plants -- “Hybridization”

5. Mendel’s work ? cross-pollinate two true breeding parents
Pollen transferred from white flower to stigma of purple flower
Bred pea plants
cross-pollinate two true breeding parents
Eg. Purple flower plants & White-flowered plants
raised seed & then observed traits
allowed offspring to self-pollinate & observed next generation
all purple flowers result
P = parents
F = filial generation
self-pollinate ?

6. Looking closer at Mendel’s work
true-breeding
purple-flower peas
true-breeding
white-flower peas
Parents
(P) X
100%
1st
generation
(hybrids)
(F1)
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
2nd
Generation
(F2)
3:1
75%
purple-flower peas
25%
white-flower peas

7. Results: Both purple-flowered and white-flowered plants appeared in the F2 generation, in a ratio of approximately 3: 1
What is an allele?
Alternate versions of a genes
For each character, an organism inherits two copies (2 alleles) of a gene from each parents
Eg. Purple flower allele and white flower allele are two DNA sequence variations

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 in F1
dominant allele
functional protein
affects characteristic
masks other alleles
recessive allele
no noticeable effect
allele makes a non-functioning protein
I’ll speak for both of us!
allele producing functional protein
mutant allele malfunctioning protein
homologous chromosomes

9. Genotype vs. Phenotype F1 P X
purple
white
all purple
Difference between how an organism “looks” & its genetics
phenotype
description of an organism’s trait
genotype
description of an organism’s genetic makeup
2 people can have the same appearance but have different genetics: BB vs Bb

10. PP pp Pp Making crosses x flower color alleles  P or p
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

11. Traits are inherited as separate units
For each trait, an organism inherits 2 copies of a gene, 1 from each parent
a diploid organism inherits 1 set of chromosomes from each parent
diploid = 2 sets of chromosomes
1 from Mom
homologous chromosomes
1 from Dad

12. Making gametes Remember meiosis! B BB = brown eyes bb = blues eyes BB
 brown is dominant over blue
 blue is recessive to brown B b Bb

13. How do we say it? 2 of the same alleles B Homozygous BB
BB = brown eyes
bb = blues eyes bb b
homozygous dominant
homozygous recessive
2 different alleles Heterozygous Bb B b
Bb = brown eyes

14. Extending Mendelian genetics
Mendel worked with a simple system
peas are genetically simple
most traits are controlled by single gene
each gene has only 2 version
1 completely dominant (A)
1 recessive (a)
But its usually not that simple!

15. Punnett Square
Handy diagram device for predicting the allele composition of offspring from a cross between indivials of known genetic makeup
“Capital Letter” to symbolize a Dominant allele
“Small Letter” to sumbolize a Recessive allele

16. Alleles can show different degrees of dominance and recessiveness in relation to each other
For some genes neither allele is completely dominance and the F1 hybrids have a phenotype somewhere between the parent varieties == Incomplete Dominance

17. Incomplete dominance RR WW RW RR Rr rr RR = red flowers
Hybrids have “in-between” appearance
RR = red flowers
rr = white flowers
Rr = pink flowers
make 50% less color
RR
WW
RW RR Rr rr

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

19. Co-dominance human ABO blood groups 3 version A, B, i
Equal dominance
human ABO blood groups
3 version
A, B, i
A & B alleles are codominant
both A & B alleles are dominant over i allele
the genes code for different sugars on the surface of red blood cells
“name tag” of red blood cell

20. Quick Review;
Johann Gregor Mendel ( )
Father of Genetics
Gregor Mendel, through his work on pea plants, discovered the fundamental laws of inheritance. He deduced that genes come in pairs and are inherited as distinct units, one from each parent. Mendel tracked the segregation of parental genes and their appearance in the offspring as dominant or recessive traits. He recognized the mathematical patterns of inheritance from one generation to the next. Mendel's Laws of Heredity are usually stated as:

21. 1) The Law of Segregation: Each inherited trait is defined by a gene pair. Parental genes are randomly separated to the sex cells so that sex cells contain only one gene of the pair. Offspring therefore inherit one genetic allele from each parent when sex cells unite in fertilization.

22. 2) The Law of Independent Assortment: Genes for different traits are sorted separately from one another so that the inheritance of one trait is not dependent on the inheritance of another.

23. 3) The Law of Dominance: An organism with alternate forms of a gene will express the form that is dominant.
The genetic experiments Mendel did with pea plants took him eight years ( ) and he published his results in During this time, Mendel grew over 10,000 pea plants, keeping track of progeny number and type. Mendel's work and his Laws of Inheritance were not appreciated in his time. It wasn't until 1900, after the rediscovery of his Laws, that his experimental results were understood.

24. Genetics of Blood type A A A or A i B BB or B i AB O i i Pheno- type
Genotype
antigen on RBC
antibodies in blood
donation status A
A A or A i
type A antigens on surface of RBC
anti-B antibodies __ B
BB or B i
type B antigens on surface of RBC
anti-A antibodies AB
both type A & type B antigens on surface of RBC
no antibodies
universal recipient O
i i
no antigens on surface of RBC
anti-A & anti-B antibodies
universal donor

25. Blood donation clotting clotting clotting clotting clotting clotting

26. One gene: many effects
The genes that we have covered so far affect only one trait
But most genes affect many traits
1 gene affects more than 1 trait
dwarfism (achondroplasia)
gigantism (acromegaly)
The genes that we have covered so far affect only one phenotypic character, but most genes are pleiotropic

27. Many genes: one trait additive effects of many genes humans skin color
Polygenic inheritance
additive effects of many genes
humans
skin color
height
weight
eye color
intelligence
behaviors

28. Human skin color can produce a wide range of shades
AaBbCc x AaBbCc
can produce a wide range of shades
most children = intermediate skin color
some can be very light & very dark

29. Albinism melanin = universal brown color Johnny & Edgar Winter
albino Africans
melanin = universal brown color

30. Environment effect on genes
Phenotype is controlled by both environment & genes
Coat color in arctic fox influenced by heat sensitive alleles
Human skin color is influenced by both genetics & environmental conditions
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

31. Genetics of sex
Women & men are very different, but just a few genes create that difference
In mammals = 2 sex chromosomes
X & Y
2 X chromosomes = female: XX
X & Y chromosome = male: XY X X X Y

32. Sex chromosomes

33. Sex-linked traits especially the X chromosome hemophilia in humans
Sex chromosomes have other genes on them, too
especially the X chromosome
hemophilia in humans
blood doesn’t clot
Duchenne muscular dystrophy in humans
loss of muscle control
red-green color blindness
see green & red as shades of grey X X
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. X Y

34. Sex-linked traits XHXh HH XHY x Hh XH Y XHY Y XH XHXH XHXH XH Xh XHY
sex-linked recessive
XHXh HH
XHY x Hh
2 normal parents,
but mother is carrier XH Y
male / sperm
XHY Y XH
XHXH
XHXH XH Xh
female / eggs
XHY
XHY
XHXh XH Xh
XHXh
XHXh
XhY
XhY

36. 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.

37. Dominant ≠ most common allele
Because an allele is dominant does not mean…
it is better, or
it is more common
Polydactyly
dominant allele

38. Polydactyly  only 1 individual out of 500
individuals are born with extra fingers or toes
the allele for >5 fingers/toes is DOMINANT & the allele for 5 digits is recessive
recessive allele far more common than dominant
 only 1 individual out of 500
has more than 5 fingers/toes
 so 499 out of 500 people are homozygous recessive (aa)