4. DEGREES OF DOMINANCE Flashcards
- What are the three degrees of dominance?
- Complete Dominance
- Incomplete Dominance
- Co-dominance
- What is Complete Dominance?
THIS OCCURS WHEN:
- the phenotypes of the heterozygote are identical to
the phenotypes of the dominant homozygote
- the dominant allele is expressed in the phenotype
- What is Incomplete Dominance?
THIS OCCURS WHEN:
- the phenotype of the heterozygous F1 Hybrid is
somewhere in between the phenotypes of the 2
parental phenotypes
- the phenotype is a mix of the 2 parental varieties
- neither parental phenotype is distinguishable
- neither allele is expressed
- What is the phenotype associated with Incomplete Dominance known as?
- an Intermediate Phenotype
- What is Co-dominance?
- 2 dominant alleles affect the phenotype
- they do this in distinguishable ways
- the parental phenotypes are distinguishable
- both alleles are expressed
- Provide 2 examples of Incomplete Dominance .
- PALOMINO HORSES
- this is when a brown and a white horse mate
- the offspring is beige in colour - CARNATION FLOWERS
- this is when a red and white flower are genetically
crossed
- the offspring are pink in colour
NB:
- the offspring are heterozygous
- they have an intermediate phenotype
- Provide an example of Co-dominance?
HUMAN BLOOD GROUP TYPES:
- these consist of 3 alleles
- Blood Group A are dominant alleles
- Blood Group B are dominant alleles
- Blood Group O are recessive alleles
- Why are Blood Group A and B considered dominant alleles?
- they have carbohydrates on the surface membrane of the red blood cell
- Why is Blood Group O considered recessive?
- there is no carbohydrate present on the surface membrane of the red blood cell
- What happens when we have an AA or AO Blood Group Genotype?
- the phenotype is Blood Group A
- What happens when we have an BB or BO Blood Group Genotype?
- the phenotype is Blood Group B
- What happens when we have an OO Blood Group Genotype?
- the phenotype is Blood Group O
- What happens when we have an AB Blood Group Genotype?
- the phenotype is Blood Group AB
- both alleles are dominant
- the alleles are co-dominant
- both alleles are expressed in the phenotype
- How do most genes exist in populations?
- most genes exist in more than 2 allelic forms
EG: human blood group types
- What are the 4 phenotypes of the ABO blood group
(A, B, AB, O) in humans determined by?
THEY ARE DETERMINED BY:
- three alleles
- these alleles exist for the enzyme (I)
- this enzyme attached A or B carbohydrates to red
blood cells
(IA, IB, IO/i)
- Which enzyme adds the A carbohydrate to the red blood cells?
- the enzyme encoded by the IA allele
- Which enzyme adds the B carbohydrate to the red blood cells?
- the enzyme encoded by the IB allele
- Which enzyme adds neither A or B carbohydrates to the red blood cells?
- the enzyme encoded by the i (IO) allele
- Do alleles interact?
What does this result in?
- NO
- this means that a dominant allele does not subdue a
recessive allele
- What are alleles variations in?
- they are variations in a gene’s nucleotide sequence
- What does the relationships of dominant and recessive alleles depend on for any character?
- it depends on the level at which we examine the
phenotype
- Are dominant alleles more common in populations than recessive alleles?
- no
- Provide an example of a recessive allele that is more common than a dominant allele.
THE RECESSIVE ALLELE FOR 5 FINGERS/TOES:
- this is more common
- the dominant allele is for 6 fingers
- this is less common
NB:
- 1 out of every 400 babies in the US is born with extra
toes or fingers
- What is Pleiotropy?
- this is the property that most genes have
- it allows them to have multiple phenotypic effects
- Provide an example of Pleiotropic alleles?
- Pleiotropic alleles are responsible for the multiple
symptoms of certain hereditary diseases - EG: Cystic Fibrosis
: Sickle- Cell Disease
- What are some traits controlled by?
- they are controlled by 2 or more genes
- What are 2 examples of traits that are controlled by
2 or more genes?
- Epistasis
- Polygenic Inheritance
- What is Epistasis?
- this is a phenomenon where the effects of 1 gene are
modified by 1 or several other genes - this is when a gene at one locus alters the phenotypic
expression of a gene at a second locus
- What is Polygenic Inheritance?
- this is the reverse of Pleiotropy
- this is when 2 or more genes control the expression of
a single phenotype - they have additive effects
- this means that the phenotype expresses the sum of
the effects of the numerous genes
- What is the result of Epistasis?
- 2 different gene sets affect the same phenotypic
characteristic
- Provide an example of Epistasis in animals?
- their coat colour
- it depends on two genes
ONE GENE:
- determines the pigment colour
- the allele B is for black
- the allele b is for brown
THE SECOND GENE:
- determines whether the pigment will be deposited
into the hair
- the allele C is for colour
- the allele c is for no colour
- Provide an example of Epistasis in humans?
- Albinism
- What is Albinism?
- this is a congenital autosomal recessive disorder
IT IS CHARACTERISED BY:
- a complete or partial absence of pigment
in the skin, hair and eyes
- What causes Albinism?
- it is caused by an absence or defect of Tyrosinase
- this is the enzyme responsible for the production of
melanin
- What happens when a person has an albinism allele?
- this allele will inhibit the expression of specific genes
- these genes are responsible for hair and eye colour
- What is Polygenic Inheritance usually characterised by?
- it is usually characterised by quantitative variation
- Provide a definition of Quantitative Characters?
- these are the characters that vary in population
- they vary along a continuous variation
- this is known as a Continuum
- What is an example of Polygenic Inheritance?
- height
- skin colour
