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4.3 Theoretical Genetics
4.3.1 Definitions
4.3.2 Monohybrid
Crosses
Monohybrid genetic crosses: genetics
involving one gene.
• Example: Pea plants and the
texture of their seed coats.
• The characteristic of seed coat
texture is controlled by one gene
with two alleles.
• The seed coat can be either
smooth or rough.
• Smooth coat is dominant to rough
coat.
• One parent is homozygous
dominant and the other is
homozygous recessive.
4.3.2 Monohybrid
Crosses
• F1 Cross = F1
(heterozygote) x F1
(heterozygote)
• Phenotype = Smooth
coat seed x Smooth
coated seed:
4.3.3 Multiple Alleles
•
•
•
•
Some genes have more than two alleles.
An individual can only possess two alleles.
The population may contain many alleles for a given gene.
Multiple alleles increases the number of different
phenotypes.
• Multiple alleles can be dominant, recessive or codominant to each other.
• Example: Rabbit coat colour(C) has four alleles which have
the dominance hierarchy: C > cch > ch> c
• This produces 5 phenotypes, Dark(C_) , Chinchilla (cchcch),
light grey (cchch ,cchc), Point restricted (ch ch, chc) and
albino (cc)
4.4.4 Codominance and Multiple
Alleles
• The ABO blood group
system is an example of
both a multiple allele and
codominance condition.
• There are three alleles the
base letter = I stands for
immunoglobulin
• IA and IB are codominant to
each other. Both these
alleles are dominant to i
• The Allele hierarchy is IA =
IB > i
4.3.5 Sex Chromosomes
• Gender in humans is controlled by the
23rd pair of chromosomes.
• XX is female and XY is male.
• Female - two X chromosomes: one
inherited from the father the other
from the mother. They are both the
longer chromosomes
• Male - one X chromosome inherited
from the mother and the much shorter
Y chromosome inherited form the
father.
• The image represents the difference in
the XX and XY combination.
4.3.6 Sex Chromosomes and Genes
Male: Some genes are present on the X-chromosome
but missing on the shorter Y.
• The image of the male 23rd pair of homologous
chromosomes represents the size difference in the
two chromosomes.
• In the non-homologous region of the Xchromosome, a male will only have one allele for
any gene in this region.
• Genes in the homologous region have two alleles
per gene and function just as other genes already
described.
Female: The complete length of the X-chromosome
has a homologous pair on the other Xchromosome.
• Genes on the x-chromosome of female therefore
have two alleles just like another gene on the
other chromosomes.
4.3.7 Sex Linkage
• Genes on the non-homologous region of the X chromosome are said to be sex linked.
• Phenotypes associated with recessive alleles are
more common in males
• The recessive allele (a) is found on the nonhomologous region of the X-chromosome.
• Males only get one allele for this gene.
• Males have a 50% chance of being recessive.
• Female have a lower risk (33.3 %) since they
always receive 2 alleles.
• 'Recessive' males can pass on this condition ( Xchromosome) to the 'daughter'.
• Cannot pass these conditions to the 'sons' as
they pass the y-chromosome with no alleles.
4.3.8 Colorblindness