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- Define the terms:
- Genotype as the genetic makeup of an organism in terms of the alleles present (e.g. Tt or GG)
- Phenotype as the physical or other features of an organism due to both its genotype and its environment (e.g. tall plant or green seed)
- Homozygous as having two identical alleles of a particular gene (e.g. TT or gg). Two identical homozygous individuals that breed together will be pure-breeding
- Heterozygous as having two different alleles of a particular gene (e.g. Tt or Gg), not pure-breeding
- Dominant as an allele that is expressed if it is present (e.g. T or G)
- Recessive as an allele that is only expressed when no dominant allele of the gene is present (e.g. t or g)
Learn the definitions 😊
- Calculate and predict the results of monohybrid crosses involving 1 : 1 and 3 : 1 ratios.
Monohybrid inheritance involves the study of how a single gene is passed from parent to child.
It should be noted that for each gene, the dominant allele is usually denoted using a capital letter, e.g. T, and the recessive allele is usually denoted using a small letter, e.g. t.
You should also know that in some diagrams, they use the terms F1 and F2.
F1 is the first generation of offspring acquired when you cross a homozygous dominant organism and a homozygous recessive organism. This means that all F1 organisms are heterozygous. (I’ll show you how that works in a moment). Sometimes, people use the term F1 to describe the generation of offspring resulting from a cross, although this is technically incorrect.
F2 is the generation resulting from a cross between two F1 organisms. As F1 organisms are heterozygous, F2 organisms can be homozygous dominant, heterozygous or homozygous recessive.
Before I get to the monohybrid cross diagrams, let me show you what a Punnett grid is:
We use these to help us calculate all the possible offspring genotypes and the likelihood of each genotype being born.
Note that in diploid organisms, one set of chromosomes contain one of the alleles for a particular trait, and the other set of chromosomes will get the other allele.
This is why gametes will have one allele each.
Here’s a cross resulting in F1 organisms:
AA and aa are the parent genotypes.
As a result, all the offspring genotypes are Aa (heterozygous).
Since A is the dominant allele, all offspring will display the phenotype resulting from A.
To help you understand, here’s an example:
The allele A codes for normal and the allele a codes for albinism (a condition in which the body cannot produce the pigment melanin – which is what gives skin its brownish colour. Asians and Africans tend to have a higher density of melanin in their skin than Whites. Albinism results in white patches or completely white skin, and sometimes pale blue or pale pink irises.)
A is dominant, so wherever A is present, the organism will be normal.
This means one of the parents is normal and the other an albino, and all the offspring are normal. Therefore, both the genotypic and phenotypic ratios of the offspring are 1.
Here’s a cross between two F1 organisms. Note that this cross results in a 3:1 phenotypic ratio and a 1:2:1 genotypic ratio:
Both parents are heterozygous (they are normal).
When you cross them, the results are one homozygous dominant offspring (normal), two heterozygous offspring (also normal) and one homozygous recessive organism (albino).
This means the phenotypic ratio normal : albino is 3:1 and the genotypic ratio is 1:2:1.
So, there is a ¾ chance of the cross resulting in a normal offspring if only one child is born, and a ¼ chance of an albino offspring if one child is born.
The last cross we should look at is a cross involving a 1:1 ratio:
Note that this happens when one of the parents are heterozygous and the other parent homozygous recessive.
Here, both the phenotypic ratio and the genotypic ratio is 1:1.
On the other hand, if you cross a homozygous dominant organism with a heterozygous organism, the genotypic ratio is still 1:1, but the phenotypic ratio becomes 1.
As a summary, you can use the following rules as a cheat sheet instead of having to draw out the Punnett grid every time:
- homozygous dominant x homozygous dominant give offspring with a genotypic ratio of 1 (all AA) and a phenotypic ratio of 1 (all display dominant trait).
- homozygous dominant x homozygous recessive give offspring with a genotypic ratio of 1 (all Aa) and a phenotypic ratio of 1 (all display dominant trait).
- homozygous dominant x heterozygous give offspring with a genotypic ratio of 1:1 (½ are AA and ½ are Aa) and a phenotypic ratio of 1 (all offspring display dominant trait).
- homozygous recessive x heterozygous give offspring with a genotypic ratio of 1:1 (½ are Aa and ½ are aa) and a phenotypic ratio of 1:1 (½ display dominant trait, ½ display recessive trait).
- heterozygous x heterozygous give offspring with a genotypic ratio of 1:2:1 (¼ are AA, ½ are Aa and ¼ are aa) and a phenotypic ratio of 3:1 (¾ display dominant trait and ¼ display recessive trait).
Notes submitted by Sarah.
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