Revision:Theoretical Genetics

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Define

Genotype

The alleles possessed by an organism.

Phenotype

All the characteristics of an organism.

Dominant allele

An allele which has the same effect on the phenotype whether it is present in the homozygous or heterozygous state.

Recessive allele

An allele which only has an effect on the phenotype when present in homozygous state.

Codominant alleelle

Allelees which have a partial effect on the phenotype when present in heterozygotes but a greater effect in homozygotes.

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 a recessive allele of a gene that does not have an effect on the phenotype.

Test Cross

Testing a suspected heterozygote by crossing with a known homozygous recessive.

Punnet Grid

Pedigree Chart

A record of ancestry or purity of breed.

State that some genes have more than two alleles

Describe ABO blood groups as an example of co-dominance and multiple alleles

Co-dominant alleles - alleles which have a partial effect on the phenotype when present in heterozygous state but have a greater effect in homozygotes.

An example is in the alleles controlling the BO blood group system in humans.

The ABO system is controlled by 3 alleles; I , I and I .

I produces A antigens

I produces B antigens

I produces no antigens

There are 2 such alleles present in each person and therefore 6 combinations which are:

I I A, I I A, I I B, I I AB, I I O, I I B

The I and I alleles show equal dominance with respect to one another (i.e. the are co-dominant), but each is dominant to I. Inheritance follows normal Mendelian patterns.

Outline how the sex chromosomes determine gender by referring to the inheritance of x and y chromosomes of humans

Xy male

Xx female

recessive genetic diseases may manifest themselves in the males of the family, as the y chromosome cant carry any genes and therefore only one recessive is needed for the phenotype to develop.

State that some genes are present on the x chromosome and absent from the shorter y chromosome

Define sex linkage

Genes carried on sex chromosomes.

State two examples of sex linkage

• Colour Blindness
• Haemophilia

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

Explain that female carriers are heterozygous for x-linked alleles

Calculate and predict the genotypic and phenotypic ratios of offspring of monohybrid crosses involving any of the above patterns of inheritance

Deduce the genotypes or phenotypes of individuals in pedigree charts

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