3.3 – Theoretical Genetics

3.3 – Theoretical Genetics

3.3.1 – Define genotype, phenotype, dominant allele, recessive allele, codominant alleles, locus, homozygous, heterozygous, carrier and test cross

Genotype – The alleles of an organism Phenotype – The characteristics of an organism

Phenotype – The characteristics of an organism Dominant Allele – An allele that has the same effect on the phenotype whether it is present

Dominant Allele – An allele that has the same effect on the phenotype whether it is present in the homozygous or heterozygous state Recessive Allele – An allele that only has an effect on the phenotype when present in the

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

Codominant Alleles – Pairs of alleles that both affect the phenotype when present in a heterozygote

Locus – The particular position on homologous chromosomes of a gene

Homozygous – Having two identical alleles of a gene

Heterozygous – Having two different alleles of a gene

Carrier – An individual that has one copy of a recessive allele that causes a genetic disease in individuals that are homozygous for this allele

Test Cross – Testing a suspected heterozygote by crossing it with a known homozygous recessive

3.3.2 – Determine the genotypes and phenotypes of the offspring of a monohybrid cross using a Punnett grid

Genetics crosses allow us to determine the genotype and phenotype of offspring, using the Punnett grid method.

Cross = Heterozygote (Rr) x Heterozygote (Rr)

R = Dominant gene r = Recessive gene

In this example, there are three genotypes in the ratio:

1RR : 2Rr : 1rr

The phenotypes for the offspring with the dominant gene will all express this gene in their phenotype. Only the genotype rr results in a phenotype with the recessive gene. Hence, there are two phenotypes in the ratio:

3 Dominant : 1 Recessive

3.3.3 – State that some genes have more than two alleles (multiple alleles)

There are some genes that have more than two alleles. However, an individual can only possess two alleles. Multiple alleles increase the number of different phenotypes in a given population. Multiple alleles can be dominant, codominant or recessive.

3.3.4 – Describe ABO blood groups as an example of codominance and multiple alleles

In the human ABO blood group system, the blood group can be determined by three different alleles, meaning that there a multiple alleles. These are A, B and O. ABO antigens are sugars attached to the surface of red blood cells. Each allele codes for enzymes that join theses sugars together.

O produces a non-functioning enzyme that does not make any changes to the basic molecule. A and B are codominant (expressed equally), producing different antigens. These antigens react with antibodies present in the blood from other people, which must be matched for transfusion.

These can also be written IA, IB and i. I stand for immunoglobin

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

Gender is determined by the sex chromosome inherited from each parent. Males are referred to as the heterogametic sex. Each somatic cell has one X chromosome and one Y chromosome. Females are homogametic. Somatic cells with two X chromosomes. X chromosomes are longer than Y chromosomes. Sex chromosomes are the 23rd pair.

The only possible phenotypes from a cross are male and female – XX or XY. The ratio of karyotypes is therefore:

1XX : 1XY

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

There is a size difference between the X and Y chromosomes. A male will only have one allele for genes that occur in the non-homologous region of the X chromosome. Genes in the homologous region will have to alleles, functioning like other genes. Females always have two alleles because the complete length of X chromosome has a homologous pair.

3.3.7 – Define sex linkage

Genes carried on only one of the sex chromosomes and which therefore show a different pattern of inheritance in crosses where the male carries the gene from where the female carries the gene

When a gene is sex linked, the characteristic is usually seen in the heterogametic sex (human males). Examples include red-green colour blindness and haemophilia.

3.3.8 – Describe the inheritance of color blindness and hemophilia as example of sex linkage

Colour Blindness

Red-Green colour blindness is produced by a sex-linked, recessive allele. The gene loci is on the non-homologous region of the X chromosome. Males must inherit the gene from their mothers. Males cannot pass the gene onto their sons. A female can carry the gene without expressing it.


Haemophilia is produced by sex-linked, recessive alleles. Males are always affected, but females can be carrier. It is inherited from the mother. Haemophilia is a condition in which blood clotting factor cannot be produced, causing uncontrolled bleeding. It is more common in men than women.

3.3.9 – State that a human female can be homozygous or heterozygous with respect to sex-linked genes

Homozygous – having two identical alleles of a gene

Heterozygous – having two different alleles of a gene

Females can be homozygous for sex-linked alleles, with both alleles have the same gene. On the other hand, they can be heterozygous, with each chromosome carries a different gene.

3.3.10 – Explain that female carriers are heterozygous for X-linked recessive alleles

Carriers for recessive alleles have both the dominant and the recessive allele. The disease is recessive, so it is not expressed in the carrier’s phenotype. They have two different alleles of a gene.

3.3.11 – Predict the genotypic and phenotypic ratios of offspring and monohybrid crosses involving any of the above pattern of inheritance

Cystic fibrosis is found on the autosomal chromosomes. It is a recessive disorder in which the allele key is dominant CF and Cf for the recessive allele.

Therefore the ratio of phenotypes is:

3 No Disease : 1 With Disease

Therefore, the child has a 75% chance of not having the disease.


3.3.12 – Deduce the genotypes and phenotypes of individuals in pedigree charts

Geneticists collect information about individuals and relatives within a family. They construct diagrams of inheritance, or family trees, which are called pedigrees. Circles are females, squares are males, diamonds are for unknown, small black circles means died at infancy. Carriers are marked with a small black dot in the centre. Identical and fraternal twins are shown as:

Black means the individual has the condition, white means unaffected. Mating is indicated by a horizontal line

For dominant and recessive alleles, upper case and lower case letter should be used. Letter representing alleles should be chosen with care to avoid confusion between the cases. For co-dominance, the main letter should relate to the gene, and the suffix to the allele

i.e. Sickle cell anaemia: HbA and Hbsb

Individuals are identified by their generation and order number. Generations are written I, II, III, IV, etc. Orders are simply 1,2,3. The propositus is the person through whom the pedigree is discovered.