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What is an allele?

A married form of the same gene and are found in the same place on a set of homologous chromosomes


What does an allele do?

It codes for a specific trait of a certain gene.


What are multiple alleles?

Are circumstances where 3 or more allelic forms of one gene are known but an individual will still only have 2 of these possible multiple alleles


What is a gene for which only 2 alleles known called?



Give an example of multiple alleles.

ABO blood groups


What is codominance?

This is when 2 alleles affect the phenotype in separate, distinguishable ways.


Give an example of codominance.

A and B blood groups


What are polygenic traits?

When a trait is controlled by more than one gene. This means a dominant allele will ad to the next dominant allele.


Give an example of polygenic traits.

Height and skin colour.


What happens in autosomal dominant disorder?

The normal allele is recessive and the mutated or abnormal allele is dominant.


Give an example of an autosomal dominant disorder.

Achondroplasia, a type of dwarfism.


How would you identify an autosomal dominant disorder in a pedigree analysis chart?

The phenotype appears in every generation and affected fathers and mothers pass on the phenotype to both sons and daughters. It also shows parents with the disorder passing it onto their children and/or having unaffected children.


What happens in autosomal recessive disorders?

The affected phenotype is determined by the homozygosity for a recessive allele and the unaffected phenotype is determined by the presence of the corresponding dominant allele.


Give an example of an autosomal recessive disorder.

Phenylketonuria (PKU) and cystic fibrosis (CF).


How would you identify an autosomal recessive disorder in a pedigree analysis chart?

It affects both males and females equally and seen by an affected child coming from 2 unaffected parents.


What happens in X- linked recessive disorders?

The affected gene is on the X chromosome which men only have one of so if they have the chromosome with the affected gene they will have the condition but if a female has an affected chromosome they may not have the condition because it can be over-ridden by having a dominant allele which will be unaffected.


Give an example of X-linked recessive disorder.

Colour blindness. Haemophilia and duchess muscular dystrophy which is fatal.


How would you identify a X-linked recessive disorder in a pedigree analysis chart?

It is displayed in more maps than females and none of the offspring of the affected male are affected.


Give an example of X=linked dominant disorder?

Vitamin D resistant rickets.


How would you identify a X-linked dominant disorder in a pedigree analysis chart?

Affected males pass the condition on to all of their daughters but none of their sons and females with the condition married to an unaffected male will pass it on to half of their sons and daughters.


What are mitochondrial genes?

Genes that are inherited only through to egg,


What happens with mitochondrial genetic diseases?

There are relatively few genetic diseases caused by mitochondrial mutations but they will always be passed from a mother onto the children.


How would you identify a mitochondrial genetic disease in a pedigree chart?

All children of an affected female will be affected but none of an affected male.


What is incomplete dominance?

Neither trait is dominant so an intermediate phenotype is apparent.


What is pleiotropy?

Most genes are, they affect more than one phenotypic character.