Punnet Squares Explained: The Probability of Inheritance
Probability of Inheritance
Next, you put the genotype of one parent across the top and that of the other parent down the left side. For example, if parent pea plant genotypes were YY and GG respectively, the setup would be:
Note that only one letter goes in each box for the parents. It does not matter which parent is on the side or the top of the Punnett square.
In this example, 100% of the offspring will likely be heterozygous (YG). Since the Y (yellow) allele is dominant over the G (green) allele for pea plants, 100% of the YG offspring will have a yellow phenotype, as Mendel observed in his breeding experiments.
An offspring's genotype is the result of the combination of genes in the sex cells or gametes (sperm and ova) that came together in its conception. One sex cell came from each parent. Sex cells normally only have one copy of the gene for each trait (e.g., one copy of the Y or G form of the gene in the example above). Each of the two Punnett square boxes in which the parent genes for a trait are placed (across the top or on the left side) actually represents one of the two possible genotypes for a parent sex cell. Which of the two parental copies of a gene is inherited depends on which sex cell is inherited--it is a matter of chance. By placing each of the two copies in its own box has the effect of giving it a 50% chance of being inherited.
If a carrier (Aa) for such a recessive disease mates with someone who has it (aa), the likelihood of their children also inheriting the condition is far greater (as shown below). On average, half of the children will be heterozygous (Aa) and, therefore, carriers. The remaining half will inherit 2 recessive alleles (aa) and develop the disease.It is likely that every one of us is a carrier for a large number of recessive alleles. Some of these alleles can cause life-threatening defects if they are inherited from both parents. In addition to cystic fibrosis, albinism, and beta-thalassemia are recessive disorders.
Some disorders are caused by dominant alleles for genes. Inheriting just one copy of such a dominant allele will cause the disorder. This is the case with Huntington disease, achondroplastic dwarfism, and polydactyly. People who are heterozygous (Aa) are not healthy carriers. They have the disorder just like homozygous dominant (AA) individuals.
http://anthro.palomar.edu/mendel/images/Punnett6.gif http://anthro.palomar.edu/mendel/ima...nant_trait.gif If only one parent has a single copy of aPunnett squares are standard tools used by genetic counselors. Theoretically, the likelihood of inheriting many traits, including useful ones, can be predicted using them. It is also possible to construct squares for more than one trait at a time. However, some traits are not inherited with the simple mathematical probability suggested here.
dominant allele for a dominant disorder,
their children will have a 50% chance of
inheriting the disorder and 50% chance
of being entirely normal.
When dealing with a simple recessive trait, you base your expected results on an initial assumption (we assume that primary inherited epilepsy is a simple autosomal recessive gene). The "results" here would be the affected pups of the litter. In this case we also assume that both dog and bitch are carriers of the recessive trait, that is, they are ’Ne’ (N = dominant normal and e = recessive epileptic). Then by use of the Punnett square diagram we have:
It all depends on the mode of inheritance. Punnet squares get much more complicated. All right lets see if I can break it down for you.
You have a male who is a carrier for kink tail. K represents normal and k represents kink. Because he is a carrier his allele is Kk. The female is also a carrier meaning she is also Kk.
This is what your suares will look like
Now, what we have is Dad’s gametes (say) across the top and Mom’s (we assume) down the side. Notice that Dad himself has the genotype Kk – two alleles. So does Mom. What we have got across the top and down the side of the Punnett square are the types of gametes (sperm cells) that Dad could possibly make – he can give each sperm cell just one letter and either it will be a big K or a little k. Same for Mom with her egg cells.
Now, obviously, what we have in the square is one KK puppy, two Kk puppies, and one kk puppy. The KK and Kk puppies will be normal and the kk puppy will have a kink. You do not expect each litter to have exactly four puppies, three normal and one kink. You expect roughly a 3 normal : 1 kink ratio of puppies. Put another way: every puppy born to this kind of cross has a ¼ chance (25%) of having a kink, versus a ¾ chance (75%) of being normal.
Now say that the sire is not a carrierand the mother has a kink then all puppies will be carriers
If the sire is not a carrier and the mother is a carrier then 50% will be normal and 50% will be carriers
That's weird. We studied that at biology class but I did not understand it before but when you showed it I now understand it.
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