Give three reasons why Drosophila melanogaster is a useful model organism for studying inheritance.

It has a generation time of only about two weeks, it produces very large numbers of offspring, and it has just four pairs of chromosomes carrying many mutations with easily visible effects.

State the phenotypic ratio expected when two individuals heterozygous for two unlinked genes are crossed.

The expected phenotypic ratio is 9 : 3 : 3 : 1.

A cross between two doubly heterozygous flies gives 320 offspring. Calculate the number expected in each of the four phenotypic classes.

Dividing 320 in the ratio 9 : 3 : 3 : 1 gives 180, 60, 60 and 20 offspring.

OCR A-Level Biology B H422

The Developing Cell: Cell Division and Cell Differentiation

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Patterns of Inheritance

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Population Genetics and Epigenetics

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Transport Systems in Plants

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Patterns of Inheritance

Model organisms, dihybrid crosses and the chi-squared test

3 things to know
Components 01, 02 and 03

Overview

Recall questions on Drosophila melanogaster as a model organism, dihybrid crosses and the 9 : 3 : 3 : 1 ratio, independent assortment, and the chi-squared test with its null hypothesis, formula, degrees of freedom, worked calculation and critical values.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Which features of Drosophila make its mutant phenotypes easy to score?

Key term

Why is Drosophila particularly useful for demonstrating sex linkage?

Key term

What is a dihybrid cross?

Questions from this subtopic

Each one with the model answer underneath.

1
Give three reasons why Drosophila melanogaster is a useful model organism for studying inheritance.
Model answer
It has a generation time of only about two weeks, it produces very large numbers of offspring, and it has just four pairs of chromosomes carrying many mutations with easily visible effects.
2
Which features of Drosophila make its mutant phenotypes easy to score?
Model answer
Mutations such as white eyes, ebony body and vestigial wings alter external features that can be seen under a hand lens, and males and females are easily told apart by their abdominal markings.
3
Why is Drosophila particularly useful for demonstrating sex linkage?
Model answer
It has X and Y chromosomes and carries well-known X-linked mutations such as white eye, so reciprocal crosses give different results in the two sexes and the linkage can be shown within a few weeks.
4
What is a dihybrid cross?
Model answer
A dihybrid cross follows the inheritance of two different genes at the same time.
5
State the phenotypic ratio expected when two individuals heterozygous for two unlinked genes are crossed.
Model answer
The expected phenotypic ratio is 9 : 3 : 3 : 1.
6
A cross between two doubly heterozygous flies gives 320 offspring. Calculate the number expected in each of the four phenotypic classes.
Model answer
Dividing 320 in the ratio 9 : 3 : 3 : 1 gives 180, 60, 60 and 20 offspring.

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The Developing Cell: Cell Division and Cell Differentiation

The Developing Individual: Meiosis, Growth and Development

The Development of Species: Evolution and Classification

Cells and Microscopy

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measles and .

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