4.3: Theoretical genetics

Question 1

Define genotype, phenotype, dominant, recessive and codominant alleles, locus, homozygous, heterozygous, carrier and test cross.

Answer 1

• Genotype: the alleles 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 in the homozygous or heterozygous state.
• 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.

Question 2

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

Answer 2

Some genes have more than two alleles for a given trait (e.g. the ABO blood group system).

Question 3

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

Answer 3

The ABO blood group is a good example of codominance and multiple alleles. There are three allele that control the ABO blood groups. If there are more than two allele of a gene then they are called multiple allele. The allele IA corresponds to blood group A (genotype IAIA) and the allele IB corresponds to blood group B (genotype IBIB). Both of these are dominant and so if IA and IB are present together they form blood group AB (genotype IAIB). Both allele affect the phenotype since they are both codominant. Codominant allele are pairs of allele that both affect the phenotype when present together in a heterozygote. The allele i is recessive to both IA and IB so if you have the genotype IA i you will have blood group A and if you have the genotype IB i you will have blood group B. However if you have the genotype ii then you are homozygous for i and will be of blood group O. Below is a table to summaries which genotypes give which phenotypes.

Phenotype = Genotype

A = IAIA or IAi

B = IBIB or IBi

AB = IAIB

O = ii

Question 4

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

Answer 4

There are two chromosomes which determine gender. These are called the sex chromosomes and there are two types, the X and the Y chromosome. Females have two X chromosomes whereas males have one X and one Y chromosome. The X chromosome is relatively large compared to the Y (which is much smaller) and contains many genes. The Y chromosome on the other hand only contains a few genes. The female always passes on to her offspring the X chromosome from the egg (female gamete). The male can pass on either the Y or the X chromosome from the sperm (male gamete). If the male passes on the X chromosome then the growing embryo will develop into a girls. If the male passes on the Y chromosome then the growing embryo will develop into a boy. Therefore gender depends on whether the sperm which fertilises the egg is carrying an X or a Y chromosome.

Question 5

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

Answer 5

The Y chromosome is much shorter than the X chromosome and contains only a few genes.
- Includes the SRY sex-determination gene and a few others (e.g. hairy ears gene)

The X chromosome is much longer and contains several genes not present on the Y chromosome.
- Includes the genes for haemophilia and red-green colour blindness.

Question 6

Define sex linkage:

Answer 6

When the gene controlling the characteristic is located on the sex chromosome and so we associate the characteristic with gender.

Question 7

Describe the inheritance of colour blindness and hemophilia as example of sex linkage.

Answer 7

• Colour blindness and haemophilia are both examples of X-linked recessive conditions
• The gene loci for these conditions are found on the non-homologous region of the X chromosome (they are not present of the Y chromosome)
• As males only have one allele for this gene they cannot be a carrier for the condition
• This means they have a higher frequency of being recessive and expressing the trait
• Males will always inherit an X-linked recessive condition from their mother
• Females will only inherit an X-linked recessive condition if they receive a recessive allele from both parents

When assigning alleles for sex-linked traits the convention is to write the allele as a superscript to the sex chomosome (usually X).
+ Haemophilia: XH = unaffected ; Xh = affected
+ Colour Blindness: XA = unaffected ; Xa = affected

Question 8

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

Answer 8

A human female can be both homo- and heterozygous.

Question 9

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

Answer 9

• An individual with a recessive allele for a disease condition that is masked by a normal dominant allele is said to be a carrier
• Carriers are heterozygous and can potentially pass the trait on to the next generation, but do not suffer from the defective condition themselves
• Females can be carriers for X-linked recessive conditions because they have two X chromosomes - males (XY) cannot be carriers
• Because a male only inherits an X chromosome from his mother, his chances of inheriting the disease condition from a carrier mother is greater.