What is genetics? G ENETICS is the branch of Biology that studies heredity. H EREDITY is… the passing on of characteristics from parents to offspring. Mendelian Genetics

Traits and Genes T RAITS are CHARACTERISTICS that are INHERITED. Genes control the expression of traits. Genes are found on regions of chromosomes Mendelian Genetics

G REGOR M ENDEL Gregor Mendel was the first person to study genetics. Mendel was a monk from Austria. He did his genetics experiments using pea plants. Mendelian Genetics

What did Mendel observe and conclude? Mendel observed that there were DIFFERENT FORMS OF A TRAIT (controlled by a gene) called alleles. Each parent donated one allele for each trait, each offspring had two alleles for each trait. Mendelian Genetics

P1 and F1 generations In a cross (mating) between organisms, the parent generation = P1 generation. The first generation = F1 generation. The second generation = F2 generation. Mendelian Genetics

Genotype and Phenotype The phenotype an organisms appearance and behavior (how the trait expresses itself). The genotype for an offspring or parent is the gene combinations or the set of two alleles (BB or Bb) that they have for each trait. Mendelian Genetics

Homozygous and Heterozygous If the alleles are the same for a trait then the organism’s genotype is homozygous. If the organism has two different alleles for each trait then its genotype is heterozygous (these are often called hybrids). Mendelian Genetics

Dominant and Recessive Alleles Mendel came up with his rule of dominance : an allele that masks the expression of another allele is dominant. The dominant allele is usually capitalized. The other allele is recessive and it is written lower case. Mendelian Genetics

How it works…. For example, the allele for a tall pea plant is dominant over the allele for short pea plants. The dominant allele is expressed when the organism is: – TT or Tt The recessive allele for a trait is expressed only when the organism is – tt Mendelian Genetics

Genotype and phenotype questions. For example, for the height trait, if tallness (T) is dominant over shortness (t) in pea plants, what is the genotype of a homozygous tall pea plant? – _______________________ What is the phenotype of a pea plant with the genotype Tt for its height? – ________________________ What is the phenotype of a pea plant with the genotype tt? What do we call this genotype? – _______________________ Mendelian Genetics

Punnett Square problem In pea plants, tallness (T) is dominant over shortness (t) with respect to height. If two heterozygous pea plants (each have genotype Tt) are crossed, what are the expected numbers of each phenotype and genotype? ________ Punnet Squares

Continued…. What is the probability that the offspring pea plant will be tall? _________ What percentage of the offspring are tall? _________ What is the ratio of tall offspring to short offspring? ________ If the pea plants produce four offspring, how many of them will be short?___________ Punnet Squares

Do we see these same results in nature? In Mendel’s experiments, he saw phenotypic ratios that very closely matched the THEORETICAL result of the Punnett squares (invented after his death). His results would have been closer to the expected results if he had used more pea plants and created more offspring from them. Mendel’s Observations

L AW OF I NDEPENDENT A SSORTMENT The law of independent assortment states that allele pairs separate independently during the formation of gametes. –Therefore, traits are transmitted to offspring independently of one another. This is shown in metaphase I of meiosis due to the fact that the homologous chromosomes line up independently of one another and then separate. These laws do not always hold true in all cases, however they provide a foundation for the basis of our understanding of genetics. The law of independent assortment

The Law of Segregation The law of segregation states that allele pairs separate or segregate during gamete formation, and randomly unite at fertilization. This makes sense according to meiosis because homologous chromosomes separate during anaphase I of meiosis. Thus, each parent gives one allele, for a given trait, to its offspring. This is due to the fact that the parent’s gametes only contain one copy of an allele for a certain trait. The law of segregation

D IHYBRID crosses Mendel also performed experiments where he looked at the variation of two traits amongst pea plants. He crossed pea plants that differed from each other in two traits. Dihybrid crosses

Mendel carefully noted his results and the numbers of each offspring with certain phenotypes. From these, he was able to create his two laws of heredity. Dihybrid crosses

Dihybrid punnet square You must then make the possible allele combinations that the parent’s gametes can have. There will be four possible allele combinations for dihybrid crosses. For example, the genotype TtRr would make the following gametes: – TR, Tr, tR, tr. – What gametes would the genotype TTrr make?________ Dihybrid crosses

Dihybrid cross problem In pea plants, tallness (T) is dominant over shortness (t) with respect to plant height. Yellow seeds (Y) are dominant over green seeds (y) with respect to seed color. If you cross two pea plants that are heterozygous tall and heterozygous for yellow seeds (TtYy), what are the expected genotypes and phenotypes? Dihybrid crosses

Dihybrid Cross Problem What is the probability that the offspring will be tall and have yellow seeds? What is the probability that an offspring will be short and have yellow seeds?______ If the plants produce 16 offspring, how many of them will be short and have green seeds? _____ How many will be tall and have green seeds?_________ Dihybrid crosses