Question

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.

Answer 1

Nondisjunction during meiosis leads to gametes with abnormal chromosome numbers, resulting in disorders like Down syndrome. Aneuploidy causes a variety of disorders, including those that involve the X chromosome, such as Klinefelter syndrome. Errors in chromosome structure can occur via inversions and translocations, potentially causing genetic disorders and reduced fertility.

Step-by-step explanation:

Explain how nondisjunction leads to disorders in chromosome number

Nondisjunction is an error that occurs during meiosis, when homologous chromosomes or sister chromatids fail to separate properly. This results in gametes with an abnormal number of chromosomes. For instance, during meiosis I, if homologous chromosomes do not separate, two gametes will lack that chromosome and two will have two copies. In meiosis II, if sister chromatids do not separate, one gamete will lack the chromosome, two will be normal, and one will have two copies. Down syndrome, or trisomy 21, is an example of a disorder arising from nondisjunction where there are three copies of chromosome 21.

Compare disorders that aneuploidy causes

Aneuploidy is the presence of an abnormal number of chromosomes in a cell. Disorders caused by aneuploidy range from Down syndrome (trisomy 21) to Klinefelter syndrome (an extra X chromosome in males). Disorders involving the X chromosome are more common and often less severe due to a phenomenon called X inactivation.

Describe how errors in chromosome structure occur through inversions and translocations

Chromosome structure can be altered through inversions, where a segment of a chromosome is reversed end to end, and through translocations, where segments of two different chromosomes are exchanged. These structural changes can lead to genetic disorders, often due to abnormal gene function and reduced fertility, as they can disrupt the normal alignment and separation of chromosomes during meiosis.

Answer 2

Answer: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.