Unformatted text preview: rozygous
allele; recessive trait) and never in a heterozygous
Every person has two copies of every gene on autosomal
(non-sex determining) chromosomes, one from mother and
sex determining) chromosomes one from mother and
one from father. If a genetic trait is recessive, a person
needs to inherit two copies of the gene for the trait to be
expressed Thus both parents have to be carriers of
expressed. Thus, both parents have to be carriers of a
recessive trait in order for the offspring to express that trait.
If both parents are carriers, there is a 25% chance with
each offspring to show the recessive trait
each offspring to show the recessive trait.
Mendelian Ratios homozygous parent X homozygous parent Incomplete dominance:
color One allele of a pair is
not fully dominant over
its partner, so the
phenotype of the
somewhere in between
the phenotypes of the
F2 offspring 3 phenotypes
in 1:2:1 ratio
16 See IVLE animation Multiple
Multiple Alleles ABO Blood Group Inheritance
(A and B are Codominance; O is recessive)
Genotype Protein Phenotype AA A Full effect of A AO
AO A Full effect of A BB B Full effect of B BO B Full effect of B AB A, B Full effect of AB OO Neither (no protein) O (no effect) Important for blood
Important for blood
See IVLE animation and
Heredity & Traits>Genes & Blood Type Mechanisms of dominance
• The states of complete dominance and incomplete dominance result not from genetics, but from biochemistry.
• The “completeness” of dominance depends only on how effective the protein is at doing its job.
• Incomplete dominance simply means that half as many molecules (in the heterozygote) are not as effective as the number produced by the homozygote. This result in the display of intermediate, blended phenotype of the alleles with degree of color shades or severity in the het...
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