Question

Which of the following variants are most likely to be subject to negative selective pressure? (select two answers)

a variant at the HEXA gene that leads to Tay-Sachs disease, a fatal childhood neurodegenerative disease

a variant at the APOE gene that causes Alzheimer’s disease, an adult-onset neurodegenerative disorder

a deletion on the Y chromosome with no health consequences except for a decrease in fertility

a variant in the BRCA1 gene that leads high risk of developing breast cancer from the mid-30’s onward

PLEASE EXPLAIN WITH REFERENCES. THANKS

Answer 1

Positive selective pressure is also called Darwinian pressure. Positive selection refers to mode of natural selection in process of biological evolution promoting the passage of beneficiary allelles.

Negative selection is also called purifying selection. Negative selection refers to mode of natural selection in process of biological evolution obstructing the passage of harmful or deleterious allelles.

The following variants  are most likely to be subject to negative selective pressure:

• a deletion on the Y chromosome with no health consequences except for a decrease in fertility- A Y chromosome infertility is a condition that affects the production of sperm leading to azoospermia or oligospermia . So when a deletion is made on it (purifying) results in correction of Y chromosome , the Y chromosome infertility is resolved and not carried to next generation.
• a variant in the BRCA1 gene that leads high risk of developing breast cancer from the mid-30’s onward-BRCA 1 is a deleterious gene causing breast cancer and ovarian cancer in women . So when that DNA carrying BRCA 1 is removed that prevents the disease .

Answer 2

Ultimately, selective pressure is determined by the relative number of offspring that individuals with and without the variant have. A disease that causes infertility or a decrease in fertility, such as the Y chromosome mutation described above, will decrease the number of offspring produced; these types of conditions often arise from de novo germline mutations rather than being passed down. In the case of a fatal childhood condition, such as Tay-Sachs disease, an affected individual will not survive to reproductive maturity to have offspring. However, late onset diseases, such as breast cancer and Alzheimer’s disease, occur after reproductive maturity, and therefore will not be subject to negative selection.