Genetics

Monohybrid Cross

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Ben Whitten

Learning Objectives

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What is a monohybrid cross?
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A monohybrid cross involves the fusion of gametes from two monohybrids, who are parents consisting of one dominant and one recessive allele, that differ in only one characteristic; i.e. only one gene is being investigated.

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How are monohybrid crosses performed?
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A Punnett square can be used to predict the genotypes and phenotypes of particular offspring (and in some cases to make predictions of the genotypes/phenotypes of the parents). There are different steps in working out a monohybrid cross.


Context


We are investigating the inheritance of one trait; albinism in guinea pigs. If a guinea pig is not an albino, then they are said to be either homozygous dominant (AA) or heterozygous (Aa); this means that albinism is an autosomal recessive trait, meaning that a guinea pig would need two recessive alleles (aa) to be albino.


To begin, a key must be formed. In this case, the key would be;

  • A = non-albino

  • a = albino

The cross which will be peformed in this case (a monohybrid cross) will be Aa × aa. The Punnett square which is formed is shown below.

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Monohybrid cross for albinism
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From this cross, we can see the genotype ratio and therefore the phenotype ratio.


The genotype ratio is 2 Aa : 2 aa = 1 Aa : 1 aa, or 50% Aa : 50% aa


The phenotype ratio is 2 non-albino : 2 albino = 1 non-albio : 1 albino, or 50% non-albino, 50% albino

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Introduction to Genetics
Monohybrid Cross
Test Cross
Multiple Alleles
Dihybrid Cross
Polygenic Inheritance
Dominance Patterns
Autosomal Inheritance
Sex-linked Inheritance
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