THEORETICAL GENETICS

GREGOR MENDEL 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

KEY TERMS 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).

MENDEL´S PEA PLANTS Short or tall?

T= tall

t = short

TT: homozygous tall

tt: homozygous short

Tt: ____?

F1 Heigth T T

t

t

MENDEL´S PEA PLANTS Short or tall?

T= tall

t = short

TT: homozygous tall

tt: homozygous short

Tt: heterozygous tall

F1 Heigth T T

t Tt Tt

t Tt Tt

MENDEL´S PEA PLANTS Short or tall?

T= tall

t = short

TT: homozygous tall

tt: homozygous short

Tt: heterozygous tall

Genotypic ratio: ???

Phenotypic ratio: ????

F2 Heigth T t

T TT Tt

t Tt tt

MENDEL´S PEA PLANTS Short or tall?

T= tall

t = short

TT: homozygous tall

tt: homozygous short

Tt: heterozygous tall

Genotypic ratio: 1:2:1

Phenotypic ratio: 3:1

F2 Heigth T t

T TT Tt

t Tt tt

DBQ: COAT COLOR IN THE HOUSE MOUSE

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 hte 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 phenotypesParental genotypes

Alleles in gametes

Hybrid phenotypeHybrid genotype

Alleles in gametes

Genotypes and phenotypes of offspring of hybrid mice, shown using a punnett grid.

TEST CROSSES 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):

If the individual is homozygous the resulting phenotypic ratio is….?

If the individual is heterozygous the resulting phenotypic ratio is….?

If homozygou

sB B

b

b

If heterozygo

usB b

b

b

TEST CROSSES 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):

If the individual is homozygous the resulting phenotypic ratio is all white sheep

If the individual is heterozygous the resulting phenotypic ratio is 1:1 50% white, 50% black.

If homozygou

sB B

b Bb Bb

b Bb Bb

If heterozygo

usB b

b Bb bb

b Bb bb

CO-DOMINANCE 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 CR CR

CW

CW

CO-DOMINANCE 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

CO-DOMINANCE 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

CO-DOMINANCE 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: 1:2:1Ç

Red flowers: 1/4 or 25% CR

pink flowers: 2/4 or 50% CRCW

White flowers: 1/4 or 25% CW

F2 CR CW

CR CRCR CRCW

CW CRCW CW 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.

INHERITANCE OF ABO BLOOD GROUPS: AN EXAMPLE OF CO-DOMINASE AND MULTIPLE ALLELES.

INHERITANCE OF ABO BLOOD GROUPS: AN EXAMPLE OF CO-DOMINASE AND MULTIPLE ALLELES. Guess what is my blood type???

IA i

i IAi ii

i IAi ii

MOM

DAD

DETERMINATION OF GENDER 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.

DETERMINATION OF GENDER

X X

X XX XX

Y XY XY

Female

Male

SEX LINKAGE 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.

SEX LINKAGE This is what he observed while crossing Drosophila flies with different eye

colors:

Male Y

Female X

Red color: XR

White color: Xr

Female with White

eyesXr Xr

XR

Y

Male with White eyes XR XR

Xr

Y

SEX LINKAGE 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: 50% female with red eyes 50% females with red eyes

50% male with white eyes 50% males with red eyes

Female with White

eyesXr Xr

XR XRXr XRXr

Y XrY Xr Y

Male with White eyes XR XR

Xr XRXr XRXr

Y XRY XRY

SEX LINKAGE Other examples:

Hemophilia

Red-Green color blindness

GENETIC DISEASES AND CARRIERS 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.

GENETIC DISEASES AND 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:

CRITICAL CONSIDERATIONS: CONSEQUENCES OF GENETIC DISEASE: 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 HbS

HbA

HbS

CRITICAL CONSIDERATIONS: CONSEQUENCES OF GENETIC DISEASE: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

CRITICAL CONSIDERATIONS: CONSEQUENCES OF GENETIC DISEASE: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.

h h

H

h

Female

Male

CRITICAL CONSIDERATIONS: CONSEQUENCES OF GENETIC DISEASE: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.

H h

h

h

Male

Female

COMMING SOON… Pedigree charts

Drosophila virtual lab

Analysis of the inheritance of a human characteristic

SOURCES

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