offspring, because they were heterozygous for that trait, he ended up with some having the homozygous recessive trait, some having the homozygous dominant trait and some continuing the heterozygous trait. In correct breeding terms his first cross between the plants is called the F1 cross or F1 generation.The breeding out of those offspring is called the F2 cross or F2 generation.
Now since he has Ss, ss and SS to work with you could use Punnett squares to determine what the next generations of offspring will look like. Compare your results with what you have learned about ratios and you’ll be able to see how it all fits together.
More on Genetic Frequencies
Take a look at the cross below between two heterozygous parents. If two heterozygous parents are crossed, the frequency ratio of the alleles will be 50% each. Remember the genotype can be Ss, SS or ss, but the allele is either ‘S’ or ‘s.’
S
s
S
SS
Ss
s
Ss
ss
We can see S S S S (4 x S) and s s s s (4 x s). This means that the frequency of the allele ‘S’ is 50% and the frequency of the allele ‘s’ is 50%. See if you can calculate the frequencies of the alleles ‘S’ and ‘s’ in the following crosses for yourself.
S
s
S
S
s
s
S
s
Recall that the Hardy-Weinberg law states that the sum of all the alleles in a population should equal 100% , but the individual alleles may appear in different ratios. There are five situations that can cause the law of equilibrium to fail. These are discussed below.
1. Mutation. A mutation is a change in genetic material, which can give rise to heritable variations in the offspring. Exposure to radiation can cause genetic mutation, for example. In this case the result would be a mutation of the plant’s genetic code that would be transferred to its offspring. The effect is equivalent to a migration of foreign genetic material being introduced into the population.There are other factors that can cause mutations. Essentially a mutation is the result of DNA repair failure at the cellular level. Anything that causes DNA repair to fail can result in a mutation.
2. Gene Migration. Over time, a population will reach equilibrium that will be maintained as long as no other genetic material migrates into the population. When new genetic material is introduced from another population, this is called introgression. During the process of introgression many new traits can arise in the original population, resulting in a shift in equilibrium.
3. Genetic Drift. If a population is small, equilibrium is more easily violated, because a slight change in the number of alleles results in a significant change in genetic frequency. Even by chance alone certain traits can be eliminated from the population and the frequency of alleles can drift toward higher or lower values. Genetic drift is actually an evolutionary force that alters a population and demonstrates that the Hardy-Weinberg law of equilibrium cannot hold true over an indefinite period of time.
4. Nonrandom Mating. External or internal factors may influence a population to a point at which mating is no longer random. For example, if some female flowers develop earlier than others they will be able to gather pollen earlier than the rest. If some of the males release pollen earlier than others, the mating between these early males and females is not random, and could result in late-flowering females ending up as a sinsemilla crop. This means that these late-flowering females won’t be able to make their contribution to the gene pool in future generations. Equilibrium will not be maintained.
5. Natural Selection. With regard to natural selection, the environment and other factors can cause certain plants to produce a greater or smaller number of offspring. Some plants may have traits that make them less immune to disease, for example, meaning that when the population is exposed to disease, less of their offspring will survive to pass on genetic
Now since he has Ss, ss and SS to work with you could use Punnett squares to determine what the next generations of offspring will look like. Compare your results with what you have learned about ratios and you’ll be able to see how it all fits together.
More on Genetic Frequencies
Take a look at the cross below between two heterozygous parents. If two heterozygous parents are crossed, the frequency ratio of the alleles will be 50% each. Remember the genotype can be Ss, SS or ss, but the allele is either ‘S’ or ‘s.’
S
s
S
SS
Ss
s
Ss
ss
We can see S S S S (4 x S) and s s s s (4 x s). This means that the frequency of the allele ‘S’ is 50% and the frequency of the allele ‘s’ is 50%. See if you can calculate the frequencies of the alleles ‘S’ and ‘s’ in the following crosses for yourself.
S
s
S
S
s
s
S
s
Recall that the Hardy-Weinberg law states that the sum of all the alleles in a population should equal 100% , but the individual alleles may appear in different ratios. There are five situations that can cause the law of equilibrium to fail. These are discussed below.
1. Mutation. A mutation is a change in genetic material, which can give rise to heritable variations in the offspring. Exposure to radiation can cause genetic mutation, for example. In this case the result would be a mutation of the plant’s genetic code that would be transferred to its offspring. The effect is equivalent to a migration of foreign genetic material being introduced into the population.There are other factors that can cause mutations. Essentially a mutation is the result of DNA repair failure at the cellular level. Anything that causes DNA repair to fail can result in a mutation.
2. Gene Migration. Over time, a population will reach equilibrium that will be maintained as long as no other genetic material migrates into the population. When new genetic material is introduced from another population, this is called introgression. During the process of introgression many new traits can arise in the original population, resulting in a shift in equilibrium.
3. Genetic Drift. If a population is small, equilibrium is more easily violated, because a slight change in the number of alleles results in a significant change in genetic frequency. Even by chance alone certain traits can be eliminated from the population and the frequency of alleles can drift toward higher or lower values. Genetic drift is actually an evolutionary force that alters a population and demonstrates that the Hardy-Weinberg law of equilibrium cannot hold true over an indefinite period of time.
4. Nonrandom Mating. External or internal factors may influence a population to a point at which mating is no longer random. For example, if some female flowers develop earlier than others they will be able to gather pollen earlier than the rest. If some of the males release pollen earlier than others, the mating between these early males and females is not random, and could result in late-flowering females ending up as a sinsemilla crop. This means that these late-flowering females won’t be able to make their contribution to the gene pool in future generations. Equilibrium will not be maintained.
5. Natural Selection. With regard to natural selection, the environment and other factors can cause certain plants to produce a greater or smaller number of offspring. Some plants may have traits that make them less immune to disease, for example, meaning that when the population is exposed to disease, less of their offspring will survive to pass on genetic
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