Punnett Squares: Monohybrid Cross | Genetics


In this video we are going to look at what
a monohybrid cross is. A monohybrid cross is when we are looking
at just one trait. For example if we have a plant and look at
just one trait such flower colour. This trait is going to have two possible alleles
a dominant big R that codes for red flowers, and a recessive little r that codes for white
flowers. Genotype refers to the genetic make-up, in
other words the specific genes or alleles an individual has. There are 3 genotypes for this trait, big
R big R (RR), big R little r (Rr) and little r little r (rr). Even though there 3 genotypes, there are only
two phenotypes. Phenotype refers to the physical expression
of the genes in an individual, in this case either red flowers or white flowers. Those with two big R alleles only have alleles
that code for red flowers and express this. Since big R is dominant over the recessive
little r, in the big R little r plant, they would also express as having red flowers. Those with two recessive little r’s (rr)
would have white flowers. Those that have the genotype big R big R (RR)
and little r little r (rr) are considered homozygous as they have two of the same alleles. Those that have one big R and one little r
(Rr) are considered heterozygous as they have two alleles that are different. For a monohybrid cross we want to cross two
individuals based on this one trait of flower colour. For example we could do a monohybrid True
Breeding cross of two homozygous parents plants. This is known as the P or parental generation. This cross would cross one that is homozygous
dominant, so has the alleles big R big R (RR), with one that is homozygous recessive, having
the alleles little r little r (rr). This is when a punnet square is utilised. For a monohybrid cross, the punnett square
is going to be 2 by 2 and have 4 squares. You put the alleles for the first parent plant
(RR) along the top of the square, and the alleles of the second parent plant along the
side (rr). Since offspring get one allele from each parent
you combine the corresponding allele from the top and the side of each square, with
the capital letter always first until all four squares have two alleles in them. This gives you your potential offspring from
this cross. In this cross all the potential offspring
have the same genotype of big R little r (Rr) and all have the same phenotype of having
red flowers. The offspring of this first cross would be
considered the F1 generation. We could then cross two individuals from this
F1 generation in another monohybrid cross where we would be crossing two heterozygous
Big R little r (Rr) parents. Again we have a punnet square with four squares,
and one parents alleles goes on the top of the square, the other along the side, and
we go through the process again. These offspring from this second cross would
be considered the F2 generation. This time though we get 3 different genotypes,
RR, Rr and rr with a genotype ratio of 1:2:1. We also get two different phenotypes; plants
with red flowers and plants with white flowers, with a phenotype ratio of 3:1. Thank you for watching, if you found this
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