Section 9-2 Genetic Crosses

1. Section 9-2 Genetic Crosses

2. Basic Terminology An organism’s genetic makeup is called the genotype. It consists of the alleles that the organism inherits from both parents. If both alleles are the same they are referred to as homozygous. Two capital letters are homozygous dominant and two lower case letters are homozygous recessive. These are pure genotypes. A heterozygous genotype has one capital and one lower case letter.

3. Basic Terminology The organism’s physical appearance is called the phenotype. It is possible to know the phenotype of an organism but not the genotype if it involves the dominant form of a trait. When the genotype is recessive there is only one possible phenotype.

4. Probability Probability is the likelihood that an event will occur. It is calculated by the following equation: Probability = the number of times an event is expected to happen the number of times an event could happen

5. Probability The number of times an event occurs is important in determining the actual vs. expected probability. For example, when flipping a coin the odds are ½ heads or ½ tails. You can flip it ten times and get 8 heads and 2 tails instead of 5 each. If you flip the coin 100 times, you are likely to get closer to 50-50.

6. Probability Just because a genetic trait is likely to be passed down more frequently, doesn’t mean it always will be. A couple might have a 75% chance of passing on a dominant trait like freckles. They can have five kids and none of them will have freckles because there is still a 25% chance of getting the recessive trait.

7. Patterns of Inheritance Patterns of inheritance show if a trait is found on an autosome or sex chromosome and how genotypes correspond to phenotypes. The simplest pattern is called complete dominance. Mendel was lucky that all seven traits he studied fit this pattern. It occurs on autosomes where the dominantallele totally masks the recessive allele.

8. Patterns of Inheritance This means that there are three possible genotypes but only two possible phenotypes. This is because both the homozygous dominant and heterozygous allele combinations result in the samephysical trait. For example having a Widow’s peak is dominant over not having one. So both the WW and Ww genotypes show this trait.

9. Patterns of Inheritance Another pattern of inheritance is called incomplete dominance. It occurs on autosomes where neither allele is totally dominant over the other so there is no masking of one form of the trait. The heterozygous form produces a third phenotype that is a blending of each equally dominant allele.

10. Patterns of Inheritance This pattern is most often seen in the cases of colorand size of features where an in between possibility can occur. The same species of beetle can be small, medium or large in size. Another pattern is called codominace. There is still a third phenotype but it involves both of the forms expressed together and not blended. For example, red and white peonies make variegated flowers.

11. Patterns of Inheritance An inheritance pattern that involves the sex chromosomes is called sex-linked. Genes occur only on the X or Y chromosome. It is similar to complete dominance where the dominantallele totally masks the recessive allele. The alleles are written as a superscript above the X or Y.

12. Patterns of Inheritance A gene found on the Y chromosome is only present in males. A gene found on the X chromosome will have one copy in males but two in females. This results in more recessive traits showing up more often in males.