Download 2.2 Theoretical genetics 1

 Sources of variability include:
 Crossing over during meiosis
 Independent assortment during meiosis (random orientation)
 Fusion of gametes from different parents
 Australian monk
 Studied inheritance in garden pea plants
 Used artifial pollination of different pea plant traits.
 Previous to Mendel studies it was believed that inheritance was
blended (a mix).
 Mendel named inheritance as particulate, and the particles of
inheritance as factors, which now we know as alleles.
 Published his theories of inheritance in 1865
 Genotype: the symbolic representation of a pair of alleles possessed by an organism, typically
represented by two letters. Example: Bb, GG, tt.
 Phenotype: the observable characteristics or traits of an organism. Example: blood type.
 Homozygous: having two identical alleles of a gene. Example: AA or aa.
 Heterozygous: having two different alleles of a gene. Example: Aa.
 Dominant allele: the allele that always is expressed in the phenotype. Example: in Aa, A will be
expressed over a.
 Recessive allele: an allele that is only expressed in its homozygous form .
 Locus: particular position on homologous chromosomes of a gene.
 Carrier: An individual who has a recessive allele of a gene that does not have effect on their
phenotype. Example: Aa carries the gene for albinism but has pigmented skin.
 Test cross: testing a suspected heterozygote by crossing it with a known homozygous recessive
(aa).
 Short or tall?
T= tall
t = short
TT: homozygous tall
F1
Heigth
t
tt: homozygous short
Tt: ____?
t
T
T
 Short or tall?
T= tall
t = short
TT: homozygous tall
F1
Heigth
T
T
t
Tt
Tt
t
Tt
Tt
tt: homozygous short
Tt: heterozygous tall
 Short or tall?
T= tall
t = short
TT: homozygous tall
F2
Heigth
T
t
T
TT
Tt
t
Tt
tt
tt: homozygous short
Tt: heterozygous tall
Genotypic ratio: ???
Phenotypic ratio: ????
 Short or tall?
T= tall
t = short
TT: homozygous tall
F2
Heigth
T
t
T
TT
Tt
t
Tt
tt
tt: homozygous short
Tt: heterozygous tall
Genotypic ratio: 1:2:1
Phenotypic ratio: 3:1
In the early years of the 20th century, many crossing experiments were done in a
similar way to those of Mendel. The French genetist Lucien Cuénot used the house
mouse, Mus musculus, to see whether the principles that Mendel had discovered
also operated in animals. He crossed normal grey-colored mice with albino mice.
The hybrid mice that were produced where all grey. These grey hybrids were
crossed together and produced 198 grey and 72 albino offspring.
1. Calculate the ratio between grey and albino offspring, showing your working (2)
2. Deduce the color of coat that is due to a recessive allele, with two reasons for
your answer (2)
3. Choose suitable symbols for the alleles for grey and albino coat and list the
posible genotypes of mice using your symbols, together with the phenotype and
each genotype (3)
4. Using the headings shown to the right, explain how the observed ratio of grey
and albino mice was produced (5)
5. Suggest how one gene can determine wheter the mice had grey fur and black
eyes or White fur and red eyes (2)
Key to alleles:
Parental phenotypes
Parental genotypes
Alleles in gametes
Hybrid phenotype
Hybrid genotype
Alleles in gametes
Genotypes and
phenotypes of offspring
of hybrid mice, shown
using a punnett grid.
 Work in groups of 3
 Each group will be provided with a bag that contains 10 green peas (G) and 10
yellow peas (g).
 One person will hold the bag, the other person will extract one pea and the third
person will extract a second pea.
 Write the genotype. Ex: (Gg)
 Each time return the peas to the bag
 Repeat the procedure 50 times and write the results.
 Make a data table with the frequencies of observed phenotypes
 Evaluate your data using a Chi-squared analysis
 Write a conclusion
Table of observed and expected frequencies of green and yellow peas
(+/-1 pea)
fo
fe
Green peas
?
37.5
Yellow peas
?
12.5
50
50
2
X =
Considering Mendel´s monohybrid cross proportion 3:1
(fo-fe)2/fe
Df= (number of columns -1) (number of rows – 1)
P= ?
If
homozygous
B
B
B
b
b
 Used to determine the posible genotypes of a
gene
b
 Crosses the unknown individual with a known
recesive
In a cross beweent a white sheep (B__) and a black
sheep (bb):
If
heterozygous
 If the individual is homozygous the resulting
b
 If the individual is heterozygous the resulting
b
phenotypic ratio is….?
phenotypic ratio is….?
If
homozygous
B
B
b
Bb
Bb
b
Bb
Bb
If
heterozygous
B
b
 If the individual is homozygous the resulting
b
Bb
bb
 If the individual is heterozygous the resulting
b
Bb
bb
 Used to determine the posible genotypes of a
gene
 Crosses the unknown individual with a known
recesive
In a cross beweent a white sheep (B__) and a black
sheep (bb):
phenotypic ratio is all white sheep
phenotypic ratio is 1:1 50% white, 50% black.
 In the inheritance of the flower color of Mirabilis jalapa, a red flowered plant is
crossed with a White flowered plant. The offspring results in pink flowers. This is
the result of the presence of two dominan alleles of the same gene.
Red flowers: CR
White
flowers: CW
F1
CW
CW
CR
CR
 In the inheritance of the flower color of Mirabilis jalapa, a red flowered plant is
crossed with a White flowered plant. The offspring results in pink flowers. This is
the result of the presence of two dominan alleles of the same gene.
Red flowers: CR
White
flowers: CW
100% pink
flowers: CRCW
F1
CR
CR
CW
CRCW
CRCW
CW
CRCW
CR CW
 In the inheritance of the flower color of Mirabilis jalapa, a red flowered plant is
crossed with a White flowered plant. The offspring results in pink flowers. This is
the result of the presence of two dominan alleles of the same gene.
Red flowers: CR
White
flowers: CW
F2
CR
CW
CR
CW
 In the inheritance of the flower color of
Mirabilis jalapa, a red flowered plant is
crossed with a White flowered plant. The
offspring results in pink flowers. This is the
result of the presence of two dominan
alleles of the same gene.
Phenotypic ratio: 1:2:1
F2
CR
CW
Red flowers: 1/4 or 25% CR
CR
CRCR
CRCW
CW
CRCW
C W CW
Ç
pink flowers: 2/4 or 50% CRCW
White flowers: 1/4 or 25% CW
 The ABO blood groups have three alleles:
 Type A: IA causes the production of glycoprotein in the membrane of
red blood cells.
 People who doesnt have it has anti-A antibodies
 Type B: IB causes the production of a different glycoprotein in the red
blood cell membrane
 People who doesn´t have it has anti-B andibodies
 Type AB: IA IB produces both glycoproteins, so neither anti-A, nor anti-B
are produced.
 Type O: i is recessive because it does not cause the production of
gycoprotein.
 Guess what is my blood type???
MOM
DAD
IA
i
i
IA i
ii
i
IA i
ii
 Gender in humans is determined at the moment of fertilization by one chromosome
carried in the sperm.
 This can either be an X or a Y chromosome.
 Because X and Y determine gender, they are called the sex chromosomes.
 The x chromosome is large with many genes essential in both male and female.
 The Y chromosome is much smaller with far fewer genes.
 One gene in particular is called TDF, only found in the Y chromosome to cause male
development
 Females have two X chromosomes and males have an X and a Y chromosome.
Female
X
X
X
XX
XX
Y
XY
XY
Male
 Drosophyla melanogaster is a fruit fly used for the study of genetics because it is
easy to handle and has a life cycle of 2 weeks.
 Thomas Morgan worked with these flies finding in most of the cases the same ratios
as Mendel with pea plants, but in some cases, there were differences.
 Morgan deduced that this inheritance could be due to the gene being located on
the X chromosome.
 This is what he observed while crossing Drosophila flies with different eye colors:
Male Y
Female X
Female with
White eyes
Red color: XR
White
color: Xr
Xr
Xr
Male with
White eyes
XR
Xr
Y
Y
XR
XR
 This is what he observed while crossing Drosophila flies with different eye colors:
Male Y
Female X
Red color: XR
White color: Xr
Phenotypic ratio:
Female with
White eyes
Xr
Xr
XR
XRXr
XRXr
Y
XrY
Xr Y
50% female with red eyes
50% male with white eyes
Male with
White eyes
XR
XR
Xr
XRXr
XRXr
Y
XRY
XRY
50% females with red eyes
50% males with red eyes
 Other examples:
 Hemophilia
 Red-Green color blindness
 A genetic disease is an illness that is caused by a gene. There are over 4000
genetic diseases in humans
 Cystic fibrosis
 Phenylketonuria (PKU)
 Tay-Sachs disease
 Marfan´s síndrome
 Most genetic diseases are caused by a recessive allele of a gene, so it only
expresses in individuals that are homozygous for that gene.
 Heterozygous individuals do not show the disease, but they can pass on the gene to
their offspring. These individuals are called carriers.
 A small proportion of genetic diseases are caused by a dominant allele.
 It is not posible to be carrier of these diseases.
 Most genetic diseases reduce the chance of survival and reproduction, so these
alleles are not usually passed on to offspring and remain very rare.
 This is the case of sickle cell anemia:
 There are many issues for families in which there is genetic disease. Consider the
scenarios below and discuss what advice should be given.
1. A man and a woman are planning to get married. Both have had genetic screening
to find whether they have the allele for sickle-cell anemia. Both are carriers of the
allele.
HbA
HbA
HbS
HbS
2. A gene called BRCA2 is linked to a igh risk of breast cancer. A screening program
is being planned, to find out which women have the gene. An ethics committe has to
decide who should be able to find out the results of the screening:
 The women who have been tested
 The women´s doctors
 Medical researchers investigating breast cancer
 Life insurance companies
 Companies who are hiring workers
3. A teenage boy´s mother has just died as a result of Huntington´s disease. This
disease is due to a dominant allele. The onset of the disease is not usually until the
age of 35. The boy isn´t sure whether to agree to have genetic screening to find out
whether he has the allele for Huntington´s disease.
Male
h
H
Female
h
h
4. A 25 year old woman would like to have a baby. Her husband, who is 30 years old,
has had genetic screening and has found out that he has the dominant allele for
Huntington´s disease.
Male
H
h
Female
h
h
 Pedigree charts
 Drosophila virtual lab
 Analysis of the inheritance of a human characteristic
 Allot, A., & Mindorff, D. (2007). IB Diploma Programme Biology Course Companion. New York: Oxford
Press.
 Damon, A., McGonegal, R., & Tosto, P. (2007). Biology Standard Level. New Jersey: Pearson.
 All world images, Gregor Mendel, http://allworldimages.blogspot.com/2011/07/gregor-mendel.html
 4b. Genetics, http://karimedalla.wordpress.com/2013/02/03/4-3b-genetics/
 Exploring nature educational resource (2014) Gregor Mendel´s genetic discoveries with peas.
http://www.exploringnature.org/db/detail.php?dbID=22&detID=54
 Perkepi (2014) Incomplete dominance and codominance http://www.perkepi.com/incomplete-and-
codominance/
 Platform 2 (2014) Gender Inequality, http://www.myplatform2.com/node/329
 Huss, M. (2006) Notes Set 3. http://www.clt.astate.edu/mhuss/BiSci%20Notes%20Set%203.htm
 New Science Biology, Color Blindness, Hemophilia, Brown Teeth, Drowophila melanogaster, linkage.
http://newsciencebiology.blogspot.com/2014/03/color-blindness-hemophilia-brown-teeth.html