A) Identify a human disorder with developmental limitations that results from changes in chromosome number. Explain how nondisjunction leads to changes in chromosome number. B) Mutations result in changes to genotype that can result in changes in phenotype. Explain how a mutation causes a change in the genotype of an organism AND may result in a change in the phenotype of an organism.

A) Patau Syndrome (Trisomy 13) causes intellectual and physical disability, such as underdeveloped eyes, extra fingers/toes, heart defects. Survival beyond the first year is uncommon.

B)

Explanation: Trisomy (or aneuploidy in general) can result when chromosomes to do not separate correctly during mitosis or meiosis. In meiosis, non-separation of a chromosome pair results in a gamete with two copies of the chromosome (and consequently one gamete lacking the chromosome.

At fertilisation the embryonic cell will have three copies of the chromosome (and an embryonic cell with one copy only). These are serious mutations, many of which are non-viable and usually abort spontaneously. Others results in abnormal development and poor survival rates. Only one autosomal trisomy (21) and sex chromosome trisomies (XXX, XXY etc) result in babies that survive into adulthood, albeit with physical and developmental abnormalities (trisomy 21 abd trisomy X). Only one monosomy (XO) is viable.

Other mutations can be less inimical, and may not result in any variation from normal. Mutations in genes coding for proteins or ribosomes may result in potential loss of function. As there are two copies of each gene, the mutated gene is paired with a normal gene and if that is expressed there will be no loss of function.

There are cases where the mutated gene is dominant it will change the phenotype. An example is Marfan Syndrome, an autosomal (chromosome 15) dominant mutation resulting in connective tissue abnormalities and long bones.

An autosomal recessive mutation is not expressed but is carried. If both parents are carriers, the probability of offspring to be doubly recessive and have an altered phenotype is 25%. Cystic fibrosis is an example.

A third possibility is where two alleles are co-dominant. An example of this is sickle cell disease. Consider the three allele pairs SS, Ss and ss.

Those with ss have sickle cell anaemia, a painful and debilitating condition. Those with SS have normal blood cells. Those with Ss have sickle cell trait, which has some mild deficits because a proportion of blood cells are abnormal.