Transcript Slide 1
Instructor Terry Wiseth PREVIOUS NEXT Genetics is the study of how characteristics are passed from one generation to the next. Characteristics that offspring exhibit are the result of combinations of traits exhibited by their parents and subsequently passed down to their offspring. Traits are determined by the types of genes that are carried by an individual. Genes carried by the chromosomes in the sperm and egg will combine in various combinations during fertilization. PREVIOUS NEXT Traits observed and exhibited by an individual are referred to as phenotypes. The coat color exhibited by a dog for instance would be the phenotype. The actual combination of genes that produce the phenotype is called the genotype. Gregor Mendel has shown us that traits are controlled by pairs of genes, one gene from the mother and the other gene from the father. Mendel also proved that some genes have the ability to mask the effect of other genes. We call these “masking” genes dominant and the masked genes are called recessive. PREVIOUS NEXT We may consider the capability of a dog to bark as a trait controlled by a dominant gene. We can use a capital letter to designate this dominant gene (B). The inability to bark is a mutation in dogs that results from a recessive gene. We can use a lower case letter to represent this recessive gene (b). PREVIOUS NEXT Individuals that possess both dominant genes (BB) or one dominant gene and one recessive gene (Bb) would exhibit the trait of having the ability to bark. A genotype of BB or Bb would result in a phenotype of “barkers”. Another possible genotype would be two recessive genes (bb) which would result in a phenotype of “mute” or nonbarkers. PREVIOUS NEXT One of the tools that scientists often use to analyze the possible combination of genetic traits that we might find in succeeding generations is the Punnet square. In this exercise we will learn how to use a Punnet square in predicting the possible combinations of genes that may result in a specific cross. PREVIOUS D d D D D Dd d D d dd NEXT B bX B b B = Barkers b = Mute PREVIOUS A completed Punnet square for a cross between two individuals each with genotypes of Bb is shown below. Click on NEXT to see how a Punnet square is set up and completed. D d D D D Dd d D d dd NEXT A a X A = Dark fur a = Light fur PREVIOUS A a Consider the following cross of two individuals with the genotypes as shown to the left. NEXT A a X A a Click on each of the gametes (letters) listed to the left to set up the Punnet Square A = Dark fur a = Light fur PREVIOUS NEXT A a X A = Dark fur a = Light fur A a Click on each of the gametes listed in the Punnet Square to place them in the possible combinations of gametes in the offspring A a A a PREVIOUS NEXT A A= 1 Aa = 2 aa = 1 List the possible genotypes for all of the possible offspring by clicking on the Punnet Square This is your GENOTYPE RATIO for this cross (1:2:1) A = Dark fur a = Light fur PREVIOUS A A Aa A a aa NEXT Dark fur = 3 Light fur = 1 Click on each of the genotypes listed in the Punnet square to find the phenotype of the possible combinations This is your PHENOTYPE is one twodominant dominantand recessive genes one RATIO for this cross (3:1) There are recessive which results gene inwhich the recessive dominant results in the dominant characteristic characteristic of Light Dark fur fur of A = Dark fur Dark fur a = Light fur Dark fur A A Dark fur Aa Light fur A a PREVIOUS Dark fur aa NEXT D dX D d D = Long ears d = Short ears PREVIOUS How would you set this cross up on the Punnet square given below? Click on NEXT when you have made your decisions NEXT D dX D d D = Long ears d = Short ears Below is how you might have set the Punnet square up for this cross. How would you fill in the middle squares to show the possible combination of genes. Click on NEXT when you have made your decision D d D d PREVIOUS NEXT D dX D d D = Long ears d = Short ears PREVIOUS Your completed Punnet square should look as the table shows below. What are the genotype and phenotype ratios for this cross? Click on NEXT when you have made your decisions. D d D D D Dd d D d dd NEXT D dX D d D = Long ears d = Short ears PREVIOUS Your Genotype ratio would be: DD = 1, Dd = 2, dd = 1 (1:2:1) Your Phenotype ratio would be: Long ears = 3, Short ears = 1 (3:1) D d D D D Dd d D d dd NEXT DD Xdd D = Long ears d = Short ears Now let us consider a different type of cross. If we were to cross a Long eared dog with a genotype of DD with a short eared dog with a genotype of dd, what would the genotype and phenotype ratios be? d d D D PREVIOUS NEXT DD Xdd D = Long ears d = Short ears PREVIOUS The genotype ratio for this cross would be all Dd The phenotype ratio for this cross would be all Long ears d d D Dd Dd D Dd Dd NEXT Genotypes that exhibit both genes as identical are called homozygous or purebred. Examples of homozygous genotypes would be BB or bb. BB would represent a homozygous dominant genotype while bb would represent a homozygous recessive genotype. Genotypes that exhibit genes that are not identical are called heterozygous or hybrid. An example of a heterozygous genotype would be Bb. When we consider only a single trait, such as ear length or coat color by itself it is considered to be a monhybrid cross. “Mono” in this case refers to considering only a single trait. If we consider two traits at the same time such as ear length and coat color together it is considered to be a dihybrid cross. “Di” refers to considering two traits at the same time. Our next exercise will involve performing a dihybrid cross. PREVIOUS NEXT The dihybrid cross we will consider is that of a dog with dark fur color with long ears mated to a dog with light fur color with short ears. The genotypes of the two dogs are listed below. AADD dark fur long ears X aadd light fur short ears Set up a Punnet square for this cross. You will need to use a Punnet square that has a total of 16 boxes (4 X 4) instead of the smaller 4 box square that you used in the monohybrid crosses. Click on NEXT to see how the Punnet square will be set up. PREVIOUS NEXT AADD AADD X aad a dd Another of Mendel’s discoveries is that during the formation of the sperm and egg cells of an indivdual, the genes will separate independently. This means that the two genes that determine a trait will not end up in the same sperm or egg cell. Click on the individual genes to set up the Punnet square. Possible sperm or egg cell genotypes PREVIOUS Possible sperm or egg cell genotypes NEXT We can now see how the possible combinations of genes can occur in the offspring. Fill in the squares as you had performed earlier. It is best to match the homologous chromosomes in each square. Click on NEXT to see if you have filled in the squares correctly. a d ad a d ad Possible sperm or egg cell genotypes PREVIOUS Possible sperm or egg cell genotypes AD AD AD AD NEXT Note how the distribution of the genes have occurred in each of the squares. Click on the Punnet square to find the Phenotype and Genotype ratios. Genotype ration = All AaDd Phenotype ratio = All Dark fur, Long ears a d ad a d ad Possible sperm or egg cell genotypes PREVIOUS AD AD AD AD Possible sperm or egg cell genotypes AaDd AaDd AaDd AaDd AaDd AaDd AaDd AaDd AaDd AaDd AaDd AaDd AaDd AaDd AaDd AaDd NEXT The next dihybrid cross we will consider is that of a dog with dark fur color with long ears mated to a dog with dark fur color with long ears. The genotypes of the two dogs are listed below. Both of these dogs are hybrids. AaDd X AaDd dark fur long ears dark fur long ears Set up a Punnet square for this cross. Click on NEXT to see how the Punnet square will be set up. PREVIOUS NEXT AaDd X AaDd Now that you have set up the Punnet square you can fill out each of the inside squares. Click on NEXT to see if you have filled it out correctly. A D a D Ad a d Possible sperm or egg cell genotypes PREVIOUS Possible sperm or egg cell genotypes AD aD Ad ad NEXT AaDd X AaDd How does your finished Punnet square compare with the one displayed below.? Now determine the genotype and phenotype ratios. Click on NEXT when you have made your determination. A D a D Ad a d Possible sperm or egg cell genotypes PREVIOUS AD aD Ad ad Possible sperm or egg cell genotypes AADD Aa DD AADd Aa Dd Aa DD a a DD Aa Dd a a Dd AADd Aa Dd AAd d Aa d d Aa Dd a a Dd Aa d d a a d d NEXT AaDd X AaDd Genotype ratio AADD=1 aaDD=1 AaDD=2 aaDd=2 AADd=2 Aadd=1 AaDd=4 Aadd=2 aadd=1 Phenotype ratio Dark fur Long ears=9 Dark fur Short ears=3 Light fur Long ears=3 Light fur Short ears=1 A D a D Ad a d Possible sperm or egg cell genotypes PREVIOUS AD aD Ad ad Possible sperm or egg cell genotypes AADD Aa DD AADd Aa Dd Aa DD a a DD Aa Dd a a Dd AADd Aa Dd AAd d Aa d d Aa Dd a a Dd Aa d d a a d d NEXT Sites of Interest for Punnet Squares http://curriculum.calstatela.edu/courses/builders/lessons/less/les4/casino/cas1ck .html http://www.athro.com/evo/gen/punexam.html