Male birds have two identical sex chromosomes (ZZ), while females have two different sex chromosomes (ZW). The feather colour gene in parakeets is a sex-linked recessive gene on the Z chromosome. What percentage of the offspring of a green male and a yellow female will be green?
(a) Only the males
(b) Only the females
(c) All of the offspring
(d) None of the offspring
VerifiedHint: Recall the basics of genetics and sex-linked inheritance. Sex-linked genes are found on the sex chromosomes and their expression is influenced by the individual's gender.
Complete Step-by-step Answer: A gene is considered sex-linked if it is located on the sex chromosomes, which in birds are the Z and W chromosomes. Females have one Z chromosome and one W chromosome (ZW), while males have two Z chromosomes (ZZ). Sex-linked genes are always expressed in males because they only have one copy of the sex chromosome, while females may be carriers of the gene but may not express the trait if they have one normal copy of the gene and one mutated copy on the Z chromosome.
In this case, the cinnamon gene is sex-linked recessive, which means that the cinnamon trait must be expressed by both copies of the gene. A cinnamon male has two copies of the mutated cinnamon gene (ZZc), while a normal female has two copies of the gene (ZW+).
When a cinnamon male is crossed with a normal female, the offspring will have the following genotypes:
Male Parent | Female Parent | Offspring Genotypes | Phenotypes |
ZZc | ZW+ | Zc+ | Normal female |
ZZc | ZW+ | ZcW+ | Carrier female |
ZZc | ZW+ | ZZc | Cinnamon male |
ZZc | ZW+ | ZWc | Normal male |
As can be seen, all male offspring will have at least one copy of the mutated cinnamon gene and will thus be cinnamon, whereas females will have a 50% chance of being carriers and a 50% chance of being normal.
Hence the Correct Answer is Option A
Note: The inheritance of sex-linked genes can be more complex than this, especially when there are multiple sex chromosomes or when there are multiple genes involved in a trait. However, the basic principles are the same: sex-linked genes are inherited differently by males and females, and this can affect the frequency of certain traits in a population.
