How is sex determined in birds? In which gender would you expect to see a Barr b
ID: 151032 • Letter: H
Question
How is sex determined in birds? In which gender would you expect to see a Barr body in birds? How is sex determined in insects, and how do they balance gene dosage differences between the two genders for the X chromosome? What is the cause of Down Syndrome? Why is trisomy 21 more common than other types of trisomies? How is sex determined in birds? In which gender would you expect to see a Barr body in birds? How is sex determined in insects, and how do they balance gene dosage differences between the two genders for the X chromosome? What is the cause of Down Syndrome? Why is trisomy 21 more common than other types of trisomies? How is sex determined in birds? In which gender would you expect to see a Barr body in birds? How is sex determined in insects, and how do they balance gene dosage differences between the two genders for the X chromosome? What is the cause of Down Syndrome? Why is trisomy 21 more common than other types of trisomies?Explanation / Answer
In birds, sex is determined by ZW-ZZ system which is a chromosomal system of sex determination. Males are homogametic(ZZ) while females are heterogametic(ZW). Z chromosome is larger in birds and has more genes. It is similar to X chromosome in XY system.
Since there are two Z chromosomes one is inactivated and results in the formation of a Barr body. During interphase, the number of Barr bodies visible is only one in birds like human females.
2- Sex determination in insects
In insects, there is XX-XO system of sex determination. Males have only one X chromosome while as females have two. Dosage compensation is balancing of expression of X-chromosomal genes in Males and Females. In insects Dosage compensation involves Hyperactivation of X-chromosome genes in males so that expression of one chromosomal gene in males is equal to the expression of two chromosomal genes in females.
Downs Syndrome
Downs syndrome is caused due to nondisjunction of the 21st chromosome during egg or sperm formation. As a result of this, there is one extra copy of the 21st chromosome forming 47, XX +21 for females and 47, XY +21 for males.
Why Trisomy 21 is the most common?
There are other trisomies as well but most of these are lethal in the embryo and we don't see any individual with these abnormalities.
Chromosome 21 is very small as compared to other chromosomes containing only 48 million base pairs and about 220 protein-coding genes. This means that this chromosome is not of much importance as compared to other chromosomes, and nondisjunction of this chromosome is not lethal. That is the reason we see trisomy 21 as most common.