Understanding Inheritance

1. Understanding Inheritance Basic Mendelian Genetics

2. What is a gene? 1. Chromosomes are threadlike structures that are found in the nuclei of cells. These chromosomes contain genetic information for organisms.

3. Genes and Alleles • A gene is a section on a chromosome that has • genetic information for an organism. Remember that each parent contributes one gene for each trait. For instance, in Mendel’s pea plants each parent plant contributed one gene for color. • The genes don’t have to be the same. Traits for each organism can be the same or be different. These traits are called alleles. Alleles are the different forms that a gene might take.

4. Genes and Alleles (cont’) The chromosome pair has traits for all of the characteristics for each trait. 1. Think about Mendel’s peas. What traits could be found on the chromosomes of a pea plant?

5. Genotype The genotype consists of the two genes for each trait. Remember what we said about the pea color??? PP represented the dominant trait for purple color and the pp represented the recessive trait for white color of pea flowers. Capital letters represent dominant traits while lower case letters represent recessive traits.

6. 2. Write the genotype for two purple pea flowers. Then write the genotype for two white pea flowers. How many genotypes represent purple flowers? Check your answers on the next slide? How many did you get correct?

7. Answers PP--------------------------------purple pea flowers pp--------------------------------white pea flowers Pp--------------------------------purple pea flowers There are two different genotypes for purple pea flowers and one genotype for white pea flowers.

8. Phenotypes The phenotype differs from the genotype in that the genotype is an abstract idea that is written to show what makes the traits show up. The phenotype is the trait that is expressed for each characteristic. For instance, the letters PP mean that this is the dominant allele for purple pea flowers but the phenotype is the purple color of the pea flowers.

9. Homozygous and Heterozygous Genotypes When the two alleles of a gene are the same, its genotype is homozygous. RR and rr are both homozygous genotypes. When the two alleles of a gene are different, its genotype is heterozygous. Rr is a heterozygous gene.

10. Identify the following as homozygous or heterozygous. RR _____________________________ Yy _____________________________

11. Punnett Squares and Pedigrees The Punnett square is a model used to predict the different combinations of alleles (genotypes) and the possible phenotypes of offspring. The Model for the Punnett Square is on page 162 in the text book. Look at the genotypes on the next square and complete the Punnett square.

12. Punnett Square Write the male alleles at the top of the Punnett Square. Write the female alleles down the side of the square. Cross the male alleles and the female alleles and write them in the box.

13. Punnett Square Create a Punnett square for the cross of one tall plant with a genotype of Tt and the genotype tt.

14. Complex Genetics Monohybrid crosses are very simple to do with the Punnett square. But some genetic traits are neither dominant nor recessive. These traits can be expressed differently. For instance, cross breeding some organisms such as plants can result in neither trait being dominant, so neither trait shows up and a new trait is expressed. This is called incomplete dominance.

15. Complex Traits Another complex trait that can be expressed occurs when both traits are dominant and so can be shared. In this case both traits are expressed. This is called codominance.

16. Multiple alleles and Human Blood Type Some genes have more than two alleles , or multiple alleles. A good example of multiple alleles is human blood type. The human blood type alleles are A, B, and O, ABO. This results in four blood types—A, B, AB, and O. Sex chromosomes determine an organism’s gender. Females have two X chromosomes and males have an X and a Y chromosome.