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Genetic Dominance

DJ Horvat

on 22 March 2012

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Transcript of Inheritance

As you have learned, humans inherit one set of 23 chromosomes from their mother and one set of 23 chromosomes from their father.
This means we each inherit two ‘doses’ of each gene, such as the gene for eye colour, blood type and hair colour. The two ‘doses’ of a gene are termed alleles.
Phenotype: The physical characteristics (or trait) of an organism...
The phenotype is determined by both genetic make-up and environmental influences.
Eye colour is determined by at least 3 different genes and each of these genes has different alleles that determine eye colour.
These are just two genes on one particular pair of chromosomes; thousands of others will be located on these chromosomes.
Dominant and recessive genes
If the two alleles of a particular gene are the same, the individual is said to be homozygous for the characteristic that gene codes for.
If the two alleles are different, the individual is said to be heterozygous for that characteristic.
Most organisms within a particular species are homozygous for most of their genes. This means there is no competition, as it were, between these genes.
But if the alleles for a particular trait are heterozygous, in many cases one will ‘override’ the other. The allele that overrides the other is described as dominant; the one it overrides is described as recessive.
If someone inherits the allele for free ear lobes from both their parents, they are homozygous for the gene for ear lobe attachment. But if they inherit the allele for free ear lobes from their mother and the allele for attached ear lobes from their father, or vice versa, they are heterozygous for that gene.
An allele is one form of a gene that has 2 or more alternative forms.
Genotype: The genetic makeup of an organism or a group of organism.
A Genotype is also a group of organisms sharing a specific genetic constitution
Co-Dominance is a condition in which both alleles of a gene pair in a heterozygote are fully expressed, with neither one being dominant or recessive to the other.
Neither allele is dominant. Both are expressed in the offspring.
This inheritance pattern is termed co-dominance.
Multiple Alleles
Sometimes a gene has more than two alleles. This case is referred to as multiple alleles.
A very good example of multiple alleles is ABO blood typing. The gene for blood types has three different alleles, represented by A, B and o.
This means that A and B are both dominant over o and co-dominant with each other.
It is crucial that anyone requiring a blood transfusion is given the right type of blood, or they might die.
When it occurs, there are more combinations of genotypes and hence more possible phenotypes.
Prac - Blood Types
Sex Linked inheritance
Sex linked genes are genes that are found in the X chromosome
If a recessive gene is situated on the X chromosome it has far more chance of being expressed in males because that gene may not be found on the Y chromosome
A recessive gene that is found on one X chromosome of a woman will not be expressed if the other X chromosome contains a normal, dominant gene
Incomplete Dominance
Incomplete dominance is a form of intermediate inheritance in which one allele for a specific trait is not completely dominant over the other allele. This results in a combined phenotype.
In a certain species of fish, blue scales and yellow scales are incompletly dominant. Predict the outcome between the blue and yellow homozygous fish...
Did you get:
Genotype - 100% BY
Phenotype - 100% Green
Predict the outcome between two fish of the second generation...
Did you get:
Genotype - 25% BB, 50% BY, 25% YY
Phenotype - 25% Blue, 50% Green, 25% Yellow
In some species of frog, green skin and purple skin are codominant. Predict the outcome between two heter0zygous frogs...
Did you get:
Phenotype - 25% GG, 50% GP, 25% PP
Geneotype - 25% Green, 50% Green with purple, 25% Purple
Pedigrees are charts that represent family trees.
Pedigrees are used to indicate the phenotypes of individuals in successive generations within a family.
Sometimes they include the genotypes of individuals, at other times they are used to infer genotypes.
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