Fundamentals of Genetics

Gregor Mendel  Gregor Mendel was a monk in mid 1800’s who discovered how genes were passed on.  He used peas to determine the pattern of heredity

7 Characteristics of Pea Plants  1. height of plant (tall and short stem)  2. color of pea pods (green or yellow)  3. pod shape (inflated or wavy)  4. feel of peas (smooth or wrinkle)  5. color of peas (green or yellow)  6. flower positions (on sides or end of stem)  7. flower colors (purple or white)

7 Characteristics of Pea Plants  Mendel noticed that the plants carried different characteristics, or traits  He further noticed that there were two strains for each trait.

Mendel had to pollinate the peas  pollination: combining pollen from the male part of the plant to the female part of the plant (i.e., mating the plant)  pollen comes from the anthers: male reproductive organ of plants. Stigma: female reproductive organ of plants

Obtaining the Pure strain  self-pollination: mating male & female of the same plant or flower  cross-pollination: mating from flowers of 2 different plants  By using these techniques, Mendel achieved plants that were pure for each given strain

Mendel’s Experiments  The pure plants for each strain were called the Parent or P 1 generation  The offspring of the P 1 generation were called the first filial, or F 1 generation  The offspring of the F 1 generation, the second filial, were called the F 2 generation. These would be the “grandchildren” of the P 1 generation

Mendel’s Results and Conclusions  In one of his experiments, Mendel crossed a plant pure for green pods with a plant pure for yellow pods

Mendel’s Results and Conclusions  To his surprise, all of the offspring, or F 1 generation, had green pods

Mendel’s Results and Conclusions  However, when he crossed plants from the F 1 generation, the resulting F 2 generation had about three-fourths green pods and one-fourth yellow pods

Mendel’s Results and Conclusions  Mendel concluded that something within the pea plants controlled characteristics, which he called factors  Since each characteristic had two forms, he concluded their must be a pair of factors controlling each trait

Mendel’s Results and Conclusions  Whenever Mendel crossed strains, one of the P 1 strains failed to appear in the F 1 plants.  However, in every case, that trait reappeared in a ratio of about 3:1 in the F 2 generation

Mendel’s Results and Conclusions  Mendel reasoned that the trait appearing in the F 2 generation was controlled by a dominant factor because it masked, or dominated the other factor

Mendel’s Results and Conclusions  Since the trait reappeared in the F 2 generation, it must be controlled by a recessive factor  Thus, a trait controlled by a recessive factor had no observable effect on an organism’s appearance when it was paired with a trait controlled by a dominant factor

Law of Segregation  The paired factors separate during the formation of sex cells.  Each reproductive cell receives only one factor from each cell  When the gametes combine during fertilization, the organism will again have two factors controlling each trait

Law of Independent Assortment  Mendel also found that no relationship existed between different characteristics.  Thus, factors for different characteristics are distributed to gametes independently

Chromosomes and Genes  Mendel’s studies agree with modern molecular genetics.  A gene is a segment of a chromosome that controls a particular trait  Since chromosomes occur in pair, genes also occur in pairs  What Mendel called “factors” are now known as alleles

Chromosomes and Genes  Letters are used to represent alleles  Capital letters refer to dominant alleles  Lowercase letters refer to recessive alleles  The letter used is not important

Chromosomes and Genes  Each trait would be represented by a pair of letters, each representing an allele from a parent

Checking For Understanding  Heredity: the genes that are passed on to the offspring  Pure bred: plants that will always produce the same offspring  Genetics is the study of how genes are passed from parents to offspring.  Strain (breed): plants that has a specific or unique trait  P (or P 1 ) generation: the parent generation (the first mating)

Checking For Understanding  F 1 generation: the first offspring generation (children)  F 2 generation: the second offspring generation (grandchildren)  self-pollination: mating male & female of the same plant or flower  cross-pollination: mating from flowers of 2 different plants

Checking For Understanding  Dominant genes: those genes that are expressed and seen  Recessive genes: genes that are shaded over and do not show  Every trait that you have must have a dominant and/or recessive gene  The two genes do not separate during mitosis  Dominant and recessive genes only separate during meiosis or making gametes

9.2 Genetic Crosses Objectives  Explain how probability is used to predict the results of genetic crosses  Use a Punnett square to predict the results of monohybrid and dihybrid crosses  Explain how a testcross is used to show the genotype of an individual whose phenotype is dominant  Differentiate a monohybrid cross from a dihybrid cross

Genotype and Phenotype  Phenotype is what the organism looks like, that is, its outward appearance  Genotype is the genetic makeup of an organism –For example, an organism could show the phenotype for brown eyes, yet have either the BB or Bb genotype

Homozygous Heterozygous  An organism is homozygous for a trait if the genotype is either pure dominant (e.g., BB) or pure recessive (e.g., bb) –Thus, BB would be called homozygous dominant, and bb would be referred to as homozygous recessive  An organism is heterozygous for a trait if the allele is mixed, as in Bb

Probability  Probability tells what to expect Probability = Number of times an event is expected to happen Number of opportunities for the event to happen

Predicting Results of Monohybrid Crosses  A cross between two individuals that only involves one trait is called a monohybrid cross  We use what is called a Punnett square to predict the results

Punnett Squares  First determine the genotypes for the two parents (the P 1 generation)  For this demonstration we will use T for tall (the dominant allele) and t for short (the recessive allele).  Mother is short (homozygous recessive) so we will use tt  Father is tall (heterozygous), so we will use Tt

Start by drawing a box with four squares

Take the father’s genotype and split the letters on the side

Take the mother’s genotype and split the letters on the top

Copy the letters down and across to fill in the first box

And the next box…

And the last box!

Determining Phenotype Ratios  We see that we have 50% tall and 50% short, or 2 out of 4 for each.  We could write this as 2:2, using the dominant trait first.  With phenotypes, we can also use percentages, such as 50:50

Determining Genotype Ratios  We always write genotype ratios in the following format: DD:Dd:dd, or homozygous dominant: heterozygous: homozygous recessive.  In this square we have a ratio of 0:2:2  Always include the zero

Example 1 Homozygous x Homozygous  This cross shows a cross between a plant homozygous dominant for purple flowers with a plant homozygous recessive for white flowers  G RATIO = 0:4:0  P RATIO = 4:0

Example 2 Homozygous x Heterozygous  This cross shows a cross between a guinea pig homozygous dominant for black coat with one heterozygous  G RATIO = 2:2:0  P RATIO = 4:0

Example 3 Heterozygous x Heterozygous  Here both rabbits show the dominant black coat, even though they are heterozygous for the trait. Notice how the offspring shows the recessive trait  G RATIO = 1:2:1  P RATIO = 3:1

Example 4 Test Crosses  Sometimes you have an organism with a recognizable phenotype, such as curly hair (A). However, you cannot tell if the organism is homozygous dominant (AA) or heterozygous (Aa)  A test cross can help you determine the organism’s genotype as well  Cross the organism with another organism homozygous recessive (aa) for that trait

AA x aa  If the organism is AA, a cross with aa will give all offspring showing the dominant trait, regardless of how many offspring are generated Genotype ratio 0:4:0 Phenotype ratio 4:0

Aa x aa  If, however, about half of the offspring show the recessive trait, then you can be certain that the genotype of the test organism was heterozygous Genotype ratio 0:2:2 Phenotype ratio 2:2

Example 5 Incomplete Dominance  Sometimes neither allele is completely dominant over the other  This results in the Rr showing both factors in its phenotype

Example 6 Codominance  Codominance occurs when both alleles for a gene are expressed in a heterozygous offspring  In codominance, neither allele is dominant, nor do the alleles blend as in incomplete dominance The coat of this cow consists of both red and white hairs. Both the red and white phenotypes show

Dihybrid Crosses  Place all four alleles for one parent along the top, and those for the other parent along the side For this cross both parents will be heterozygous for round yellow (RrYy) peas

Dihybrid Cross  Combine the parent alleles to create the possible combinations for the offspring.  Remember!! Each offspring must have 4 letters (two alleles for each trait)

Checking for Understanding  The genotype is the genetic makeup (e.g., DD, Dd, dd) of an organism  The phenotype is the outward appearance (brown hair, blue eyes)  Probability is the likelihood that an event will take place  A Punnett square can be used to predict outcomes of genetic crosses

Checking for Understanding  A cross involving only one trait (one pair of alleles) is a monohybrid cross  A cross involving two traits (two pairs of alleles) is a dihybrid cross  A testcross can determine if an organism is DD or Dd  Complete dominance occurs when DD and Dd have the same phenotype

Checking for Understanding  Incomplete dominance occurs when the phenotype shows a blend of both traits  Codominance occurs when both traits show without blending in the phenotype