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how to work out expected ratios with phenotypes

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how to work out expected ratios with phenotypes

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YolandeMaster · Tutor for 5 years

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## Working Out Expected Ratios with Phenotypes<br /><br />To work out expected ratios with phenotypes, you need to understand the principles of Mendelian genetics. Gregor Mendel, the father of genetics, discovered that traits are inherited in predictable patterns. These patterns can be represented as ratios.<br /><br />### Steps to Calculate Expected Ratios<br /><br />1. **Identify the Parental Genotypes**: Determine the genotypes of the parents involved in the cross. A genotype is the set of genes an organism carries for a particular trait.<br /><br />2. **Determine Possible Gametes**: Each parent can produce gametes (sex cells) that contain one allele for each trait. Use the Punnett square method to determine the possible combinations of alleles in the gametes.<br /><br />3. **Perform a Cross**: Cross the gametes from each parent to determine the possible genotypes of the offspring.<br /><br />4. **Determine the Phenotypes**: Based on the genotypes of the offspring, determine their phenotypes (observable traits).<br /><br />5. **Calculate the Ratio**: Count the number of each phenotype and express this as a ratio.<br /><br />### Example<br /><br />Let's consider a simple example of a monohybrid cross between two heterozygous pea plants (Tt) for the trait of height, where 'T' represents the dominant allele (tall) and 't' represents the recessive allele (short).<br /><br />1. **Parental Genotypes**: Tt x Tt<br /><br />2. **Possible Gametes**: T, t from each parent<br /><br />3. **Perform a Cross**:<br /><br /> ```<br /> | | T | t |<br /> |----|----|----|<br /> | T | TT | Tt |<br /> | t | Tt | tt |<br /> ```<br /><br />4. **Determine the Phenotypes**: TT and Tt are tall (dominant trait), tt is short (recessive trait)<br /><br />5. **Calculate the Ratio**: The phenotypic ratio is 3 (tall) : 1 (short)<br /><br />Remember, this is a simplified example. Real-life genetics can be more complex due to factors like co-dominance, multiple alleles, and environmental influences.
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