Theoretical Genetics

Topic 4.3

Assessment Statements4.3.1 Define genotype, phenotype, dominant allele, recessive allele,

codominant alleles, locus, homozygous, heterozygous, carrier and test cross.

4.3.2 Determine the genotypes and phenotypes of the offspring of a monohybrid cross using a Punnett grid.

4.3.3 State that some genes have more than two alleles (multiple alleles).

4.3.4 Describe ABO blood groups as an example of codominance and multiple alleles.

4.3.5 Explain how the sex chromosomes control gender by referring to the inheritance of X and Y chromosomes in humans.

4.3.6 State that some genes are present on the X chromosome and absent from the shorter Y chromosome in humans.

4.3.7 Define sex linkage.4.3.8 Describe the inheritance of colour blindness and hemophilia as

examples of sex linkage.4.3.9 State that a human female can be homozygous or heterozygous

with respect to sex-linked genes.4.3.10 Explain that female carriers are heterozygous for X-linked

recessive alleles.4.3.11 Predict the genotypic and phenotypic ratios of offspring of

monohybrid crosses involving any of the above patterns of inheritance.

4.3.12 Deduce the genotypes and phenotypes of individuals in pedigree charts.

Gregor Mendel• Austrian monk

who published results of garden pea plants inheritance in 1865

• Used artificial pollination in a series of experiments by using a small brush to place the pollen on the reproductive parts of the flowers

Key terminology1. Genotype – symbolic representation of

pair of alleles possessed by an organism, typically represented by two letters

• Ex: Bb, GG, tt

2. Phenotype – characteristics or traits of an organism

• Ex: five fingers on each hand, color blindness, type O blood

3. Dominant allele – an allele that has the same effect on the phenotype whether it is paired with the same allele or a different one; always expressed in phenotype

• Ex: Aa give dominant trait A b/c the a allele is masked; the a allele is not transcribed and translated during protein synthesis

4. Recessive allele – an allele that has an effect on the phenotype only when present in the homozygous state

• Ex: aa gives rise to the recessive trait b/c no dominant allele is there to mask it

5. Codominant allele – pairs of alleles that both affect the phenotype when present in a heterozygote

• Ex: parent with curly hair and parent with straight hair can have children with different degrees of curliness as both alleles influence hair condition when both are present in the genotype

6. Locus – particular position on homologous chromosomes of a gene

7. Homozygous – having two identical alleles of a gene

• Ex: AA is a genotype which is homozygous dominant whereas aa is the genotype which is homozygous recessive

8. Heterozygous – having two different alleles of a gene

• Ex: Aa is a heterozygous genotype

9. Carrier – an individual who has a recessive allele of a gene that does not have an effect on their phenotype

10.Test cross – testing a suspected heterozygote plant or animal by crossing it with a known homozygous recessive (aa). Since a recessive allele can be masked, it is often impossible to tell if an organism is AA or Aa until they produce offspring which have the recessive trait.

Punnett grid

• Shows all possible combinations of genetic information for a particular trait

• Monohybrid cross shows the results for one trait only

• Don’t use Cc, Ff, Oo, Pp, Ss, Uu, Vv, Ww, Yy, Zz

Constructing a Punnett grid1. Choose and

indicate a letter to show the alleles

2. Determine the parents’ genotypes

3. Determine the gametes which the parents could produce

4. Draw a Punnett grid

5. Deduce the chances for each genotype and phenotype

BREED!!!

• http://www2.edc.org/weblabs/Punnett/punnettsquares.html

Multiple alleles

• Sometimes there are three or more alleles for the same gene

• This is the case for the alleles which determine the ABO blood type in humans

• 4 possible phenotypes: A, B, AB, O

• Three alleles of the gene which can produce six different genotypes

• Gene represented by letter I

The sex chromosomes: X and Y

• Determines gender• X much longer and contains

many more genes• Female: XX• Male: XY• Always 50% chance of each

gender• Chances remain the same no

matter how many boys or girls already exist

Genes carried on the sex chromosomes• Sometimes alleles present on

the X chromosome have nothing to pair up with

• X and Y are not alleles, so terms such as dominant and recessive do not apply

Sex linkage

• Any genetic trait whose allele has its locus on the X or Y chromosome is said to be sex linked

• Colorblindness– Inability to distinguish certain

colors (often green and red)– Two colors look the same

• Hemophilia– Disorder in which blood does not

clot properly

Alleles and genotypes of sex-linked traits• Since the alleles for both color

blindness and hemophilia are found only on the X chromosome, the letter X is used in representing them:– Xb = allele for colorblindness– XB = allele for ability to distinguish

colors– Xh = allele for hemophilia– XH = allele for the ability to clot blood

• In both cases, there is no allele on the Y chromosome

• What are the possible genotypes for color blindness?

• Can men be carriers for X-linked alleles?

Dragon Genetics

TOK moment

• Mendel was not recognized or commended for his work during his lifetime

• Why is it that the scientific community is so reluctant to take on new ideas and paradigm shifts?

• Does this have a positive or negative impact on the advancement of science?

• Is it acceptable that a scientist’s new ideas are not adopted just b/c he or she is not a well-known person with an established reputation?

Pedigree Charts

• Pedigree refers to the record of an organism’s ancestry

• Constructed to show biological relationships

• Used to show how a trait can pass from one generation to the next

The Royal Disease Pedigree

Homework

• Create a pedigree chart of your family

• Include yourself, siblings, parents, maternal and paternal aunts and uncles, cousins, maternal and paternal grandparents

Huntington’s disease

• Caused by a dominant allele (H)• Genetic condition causes

severely debilitating nerve damage

• Symptoms do not show until the person is about 40– Why is this relevant to natural

selection?

• Within a few years the person loses complete control of his or her muscles and dies and early death

Pedigree chart showing members of a family affected by Huntington’s disease

?

Codominance in flower color

• Can create more than two colors• Pedigree can help keep track of

how the offspring got their phenotypes

• C refers to the gene that codes for flower color in snapdragons

• Superscript refers to the specific color R or W

• CRCR makes red flowers• CWCW makes white flowers• CRCW makes pink flowers

Pedigree chart for flower color in snapdragons

A B

D F

H J

C E G

I K

Codominance in the shape of rbcs

• Hb refers to the gene that codes for hemoglobin

• Superscript letter for the typical shape of hemoglobin is A for normal and S for the shape that causes sickle cells

• HbAHbA = normal• HbSHbS = sickle cell anemia• HbAHbS = some of each type

TOK

• If someone is a carrier, should they have children knowing that they might transmit the gene?

• How would it feel knowing – or not knowing – if you had the gene?

• How might parents feel when the find out they have passed the gene on to their offspring?

• What about marriage? Career? Life insurance?

• Should your individual genetic makeup be disclosed?