Question:

Which of the following is incorrect regarding $ZW$-$ZZ$ type of sex determination ?

Updated On: Jul 7, 2022

It occurs in birds and some reptiles
Females are homogametic and males are heterogametic
$1 :1$ sex ratio is produced in the offsprings
All of these

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The Correct Option is B

Solution and Explanation

In $ZW$ - $ZZ$ type of sex determination, the male has two homomorphic sex chromosomes $(ZZ)$ and is homogametic, and the female has two heteromorphic sex chromosomes $(ZW)$ and is heterogametic. There are, thus, two types of eggs : with $Z$ and with $W$, and only one type of sperms, i.e., each with $Z$. Fertilisation of an egg with $Z$ chromosome by a sperm with $Z$ chromosome gives a zygote with $ZZ$ chromosomes (male). Fertilisation of an egg with $W$ chromosome by a sperm with $Z$ chromosome yields a zygote with $ZW$ chromosomes (female). This mechanism operates in some vertebrates (fishes, reptiles and birds).

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Notes on Principles of Inheritance and Variation

Concepts Used:

Principles of Inheritance and Variation - Mutation

A Mutation is a change in the sequence of our DNA base pairs caused by numerous environmental stimuli such as UV light or mistakes during DNA replication. Germline mutations take place in the eggs and sperm and can be passed onto offspring, whereas somatic mutations take place in body cells and are not passed on.

Types of Mutations

There are three types of mutations, which are as follows:

Silent mutation

It refers to any change in DNA sequence that has no effect on the amino acid sequence in a protein or the functions that a protein performs. There is no phenotypic indication that a mutation has occurred.

Nonsense mutation

When there is a change in the sequence of base pairs due to a point mutation, that results in a stop codon. This leads to a protein that is either shortened or non-functional.

Missense mutation

A missense mutation occurs when a point mutation causes a change in the codon, which then codes for another amino acid.

The mutation is caused by the following factors:

Internal Causes

When DNA copies incorrectly, the majority of mutations occur. Evolution occurs as a result of all of these mutations. DNA makes a copy of itself during cell division. When a copy of DNA isn't flawless, it's called a mutation since it differs somewhat from the original DNA.

External Causes

When certain chemicals or radiations are used to break down DNA, it causes the DNA to break down. The thymine dimers are broken by UV radiation, resulting in altered DNA.