17.3 - Probability And Genetic Crosses Flashcards
What is a ratio, and how is it expressed?
- A ratio is a measure of the relative size of two groups or classes, expressed as a proportion.
- For example, if a group of 60 humans includes 40 males and 20 females, the ratio of males to females is 40 to 20.
- This can be simplified to 2 to 1 and is written as 2:1.
- Ratios can also be expressed in decimal form by dividing the value of the smallest group into the larger group.
How is a ratio simplified, and why is this done?
- Ratios are simplified by dividing the value of the smallest group into the value of each larger group.
- This allows for easier comparison between groups.
- For example, if there are 35 males and 25 females, the ratio of males to females is 7:5. To further simplify, you divide 7 by 5, resulting in 1.4, and the ratio is then expressed as 1.4:1.
What ratio did Mendel expect in the F₂ generation for dominant and recessive traits, and why was this not exact?
- Mendel expected a 3:1 ratio of dominant to recessive traits in the F₂ generation, but he never obtained an exact 3:1 ratio in any of his experiments.
- These discrepancies are due to statistical error, which arises because genetic events like the fusion of gametes are subject to chance, similar to how each toss of a coin is an independent event and doesn’t guarantee a perfect 1:1 outcome of heads to tails.
Why do statistical errors occur in genetic crosses?
- Statistical errors occur in genetic crosses because the fusion of gametes is determined by chance, much like tossing a coin.
- Each gamete fusion is an independent event and is not influenced by previous fusions.
- As a result, the actual ratios of offspring may deviate from the expected ratios, such as the 3:1 ratio of dominant to recessive traits
How is the process of gamete fusion similar to tossing a coin?
- The fusion of gametes is similar to tossing a coin because both are independent events determined by chance.
- Just as a coin has a 50% chance of landing heads or tails regardless of previous outcomes, gametes randomly fuse without being influenced by past fusions.
- This randomness explains why the actual outcomes in genetic crosses may differ from expected ratios.
What is the significance of sample size in genetic experiments?
- Sample size is important in genetic experiments because larger samples are more likely to produce results that closely match theoretical ratios, such as Mendel’s expected 3:1 ratio of dominant to recessive traits.
- The larger the sample, the more representative and accurate the results tend to be.
- Mendel’s closest ratios to 3:1 occurred in experiments with the largest sample sizes, while the experiment with the smallest sample size had the furthest deviation from the expected ratio.
How does sample size affect the accuracy of genetic cross results?
- Larger sample sizes increase the likelihood that the actual results will closely match the theoretical ratios.
- This is because the effects of chance are minimized with larger numbers, producing more reliable and representative data.
- In Mendel’s experiments, the results with the largest sample sizes were the closest to the expected 3:1 ratio.
What role does chance play in determining genetic outcomes?
- Chance plays a critical role in determining which gametes fuse during reproduction, influencing the genetic outcomes of offspring.
- For example, in a cross between a heterozygote (Gg) and a homozygous recessive (gg), half of the gametes from the heterozygote will carry the dominant allele (G), and half will carry the recessive allele (g).
- The fusion of these gametes occurs randomly, leading to variation in the offspring’s traits.
Why are large numbers of organisms important in genetic crosses?
- Large numbers of organisms are important in genetic crosses because they help reduce the influence of chance, allowing the observed ratios of traits to more closely match the theoretical ratios.
- Larger samples provide more accurate and reliable results, which is why Mendel’s experiments with larger sample sizes were closer to the expected 3:1 ratio than those with smaller sample sizes.
Across was carried out between a pea plant producing green pods and one producing yellow pods. The seeds from this cross were germinated and, of the 63 plants grown, all produced green pods:
A) State the probable genotype of the parent plant with green pods.
B) Explain why we cannot be absolutely certain of the parent plant’s genotype.
A) homozygous dominant (GG)
B)
- We cannot be absolutely certain because if the unknown genotype were heterozygous (Gg) the gametes produced would contain alleles of two types: either dominant (G) or recessive (g).
- It is a matter of chance which of these gametes fuses with those from our recessive parent — all these gametes have a recessive allele (g).
- It is just possible that, in every case, it is the gametes with the dominant allele that fuse and so all the offspring show the dominant character.
- Provided the sample of offspring is large enough, however, we can be reasonably sure that the unknown genotype is homozygous dominant.
