What is the key to the recognition of codominance? the alleles affect more than one trait. the heterozygote expresses the phenotype of both homozygotes. the trait exhibits a continuous distribution. the phenotype of the heterozygote falls between the phenotypes of the homozygotes. the dominant allele is not always expressed.

The heterozygote expresses the phenotype of both homozygotes.

The reason is codominance occurs when neither of the alleles has dominance over the other one and so both alleles are expressed.  The key to the recognition of codominance is recognizing that the heterozygote is expressing both of the homozygous alleles.  To determine this, first you must know what the homozygous alleles are.  Let's say that a cow has the allele for red hair and the allele for white hair.  A homozygous red hair cow would have red hair. A homozygous white hair cow would have white hair.  If the cow is heterozygous and it has both red and white hair, then the cow is exhibiting codominance.  Neither the allele for red hair nor the allele for white hair has dominance over the other, and both are expressed. A form of dominance in which the alleles of a gene pair in a heterozygote are fully expressed thereby  resulting in offspring with a phenotype that is neither dominant nor recessive.

the level of progesterone is low at the pre-ovulatory phase of the cycle but it gradually increases and the levels are high at the luteal phase. if pregnancy occurs, the level of progesterone will be high throughout the pregnancy.

if egg is fertilized and the mother gets pregnant, progesterone starts preparing the uterus for the implantation. it starts building up the lining of uterus and make it hostile of the period of pregnancy.