Classify whether each gene regularly exists in a hemizygous state.
Regularly hemizygous
Not regularly hemizygous
Answer Bank
a transgenic gene inserted into only one chromosome of a mouse
a color-blindness gene on the chromosome in a normal human male
a color-blindness gene on an chromosome in a normal human female
a gene on chromosome 21 in an individual with Down syndrome
The Correct Answer And Explanation is:
Let’s classify the following genes as “Regularly hemizygous” or “Not regularly hemizygous” and then explain the reasoning behind the classifications.
Classification:
Regularly Hemizygous:
- a transgenic gene inserted into only one chromosome of a mouse
- a color-blindness gene on the X chromosome in a normal human male
Not Regularly Hemizygous:
- a color-blindness gene on an X chromosome in a normal human female
- a gene on chromosome 21 in an individual with Down syndrome
Explanation (300+ words):
Hemizygosity refers to the condition in which only one copy of a gene or genetic sequence is present in a normally diploid organism. This contrasts with the usual state of diploidy, where two alleles (one from each parent) are present for a given gene. Genes that are regularly hemizygous are found in regions of the genome where it is normal to have only one copy.
Let’s evaluate each example:
- A transgenic gene inserted into only one chromosome of a mouse – This gene is artificially introduced and inserted into one homologous chromosome. The other homolog does not contain the gene, making it hemizygous by design. Since this is how transgenic genes are typically studied in experimental settings, this qualifies as regularly hemizygous.
- A color-blindness gene on the X chromosome in a normal human male – Males have one X and one Y chromosome (XY), meaning they have only one copy of each gene on the X chromosome. Thus, all X-linked genes in males are regularly hemizygous. If the color-blindness gene is present, it is the only copy.
- A color-blindness gene on an X chromosome in a normal human female – Females have two X chromosomes (XX), so they typically have two copies of X-linked genes. This condition is not hemizygous; it is heterozygous or homozygous depending on the alleles.
- A gene on chromosome 21 in an individual with Down syndrome – Down syndrome is caused by trisomy 21, meaning there are three copies of chromosome 21. This is the opposite of hemizygosity (which involves a single copy), making it not regularly hemizygous.
Summary:
Hemizygosity is typical in sex-linked genes in males and in specific artificial or deletion conditions. It is not seen in autosomes of typical or trisomic individuals.