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In a monohybrid cross two plants or animals, which differ at only one gene, are bred together.
By looking at alleles of the genes that the parents have we can tell how the offspring will turn out.
This can be very useful when carrying out selective breeding of plants or animals.
In humans with inherited diseases it is extremely useful to know about the alleles since we can inform them about the likely effect if they want to have children.
However, like everything else in biology, it is important to be absolutely clear about what we are talking. So we need to sort out some descriptive words.
Homozygous and heterozygous
Chromosomes come in pairs. Each chromosome in a pair will have a gene at the same point on the chromosome. There can be more than one alternative form of the gene at that point. These alternative forms are called alleles.
Both chromosomes in a pair have one allele for the gene. If the two alleles are the same we say that the individual is 'homozygous' for that gene. It they are different the individual is 'heterozygous'.
So if 'H' is an allele for a height gene, and it produces tall offspring, then another allele might be 'h' which would make the individual small.
So if you had the two alleles 'HH' or 'hh' you would be homozygous for that gene. But, if you had 'Hh' you would be heterozygous.
Dominant and recessive
With the 2 alleles we thought about above, the allele for tallness (H) gives quite different instructions to the plant cells than does the allele for smallness (h).
What happens in the heterozygous case (Hh) when they are together?
You find that the effects of the H allele would be shown more strongly in the offspring than the h.
We say that in this case, the tallness allele H dominates the smallness allele h.
Therefore we talk about the dominant allele (H) and the recessive allele (h).
(To make things clear the dominant allele is always shown as an uppercase letter).
HH = Homozygous dominant
hh = Homozygous recessive
Hh = Heterozygous
Genotype and phenotype
When you look at someone or at a plant, you can only consider what they looklike. You can't work out which alleles they have for a particular gene. You are considering their phenotype. This is the outward effects of the genes - what you see.
Knowing their actual combination of alleles - for example, whether they are homozygous recessive - is to know their genotype. To know what genes they carry.
Think about plants with a genotype that is heterozygous (Hh).
Their phenotype will be to be tall plants - since the tallness allele (H) dominates the smallness allele (h).
The last thing! You would soon get confused about which plants or animals you are talking about. There are the parents, then their offspring, and their offspring, etc. etc.
So, to make it nice and easy we give each generation a name.
The first plants or animals bred together are called the Parental generation, or P1 generation.
Their offspring are called the First Filial generation, or F1 generation.
Their offspring are called the Second Filial generation, or F2 generation.
And so on. And so on.
Suppose we wanted to cross two of the tall plants we thought about earlier.
The allele for tallness is H and is dominant to that for smallness, h.
If the two plants are heterozygous, they will have a genotype, which contains the alleles Hh.
Remember: the gametes of any individual contain only half of the chromosomes. So only one of the alleles will be present in each gamete cell.
What will the offspring be like?
So there will be 3 tall plants for every 1 small plant. Or to put it another way, there is a 75% chance that each F1 (offspring) plant will be tall.
What percentage of tall plants would you get in this cross?