Mendel’s Laws of Heredity

Heredity Heredity- the passing on of characteristics from parent to offspring Characteristics that are inherited are called traits Genetics- the branch of biology that studies heredity

Gregor Mendel In the mid-19th century, Gregor Mendel, an Austrian monk carried out important studies of heredity He was the first to predict how traits are transferred from one generation to the next

Mendel’s Pea Plants His experiments were conducted using pea plants because: The reproduce sexually through the formation of gametes He could control fertilization, by pollinating the plants himself

Monohybrid Crosses A hybrid is the offspring of parents that have different forms of a trait, such as tall and short heights Mendel’s 1st experiments are called monohybrid crosses because he looked at only one trait

Parental Generation Parental generation- P1- each parent plant was from other plants that displayed the same traits He cross-pollinated the tall plant with the short plant

First Generation The offspring of the P1 cross are called the first generation or F1 generation In this generation, all of the pea plants would be tall In the F1 generation, it was as if the shorter parent had never existed

Second Generation Mendel allowed the tall plants from the F1 generation to self-pollinate After the seeds formed, he planted them and counted more than 1000 plants in the second generation or F2 Mendel found that ¾ of the plants were tall and ¼ of the offspring were short

Rule of Unit Factors Mendel concluded that each organism has two factors that control each of its traits We now know that these factors are genes and that they are located on chromosomes Genes exist in alternative forms, we call these different gene forms alleles One allele is inherited from the mother and one is from the father

Rule of Dominance In Mendel’s crosses, the observed trait was referred to as dominant (T) The trait that seemed to disappear was called recessive (t) Dominant alleles are capitalized and recessive alleles are lower-case

Law of Segregation The law of segregation states that every individual has two alleles of each gene and when gametes are produced, each gamete receives one of these alleles During fertilization, these gametes randomly pair to produce four combos of alleles

Phenotypes The way an organism looks and behaves is called its phenotypes Therefore, two organisms can look alike but have different underlying allele combos, Tt or TT, either one will display a dominant phenotype

Genotypes The allele combination an organism contains is known as its genotype The genotype of a tall plant that has two alleles for tallness is TT The genotype of a tall plant that has one allele for tallness and one for shortness is Tt An organism’s genotype can not always be seen

Homozygous or Heterozygous An organism is homozygous for a trait if its two alleles for the trait are the same -homozygous dominant (TT)- both code for tallness -homozygous recessive (tt)- both code for shortness An organism is heterozygous for a trait if its two alleles for the trait differ from each other (Tt)

Mendel’s Dihybrid Crosses Mendel preformed another set of crosses in which he used peas that differed from each other in two traits rather than one A cross involving two different traits is called a dihybrid cross

Parental Generation Mendel took true-breeding pea plants that had round, yellow seeds (RRYY) and crossed them with true-breeding pea plants that had wrinkled, green seeds (rryy)

First Generation He already knew that the F1 generation would have all plants displaying the dominant phenotypes; round, yellow seeds

Second Generation Mendel then let the F1 plants self-pollinate As expected the recessive reappeared When Mendel sorted and counted the plants of the F2 generation, he found they appeared in a definite ratio of phenotypes -9 round yellow: 3 round green: 3 wrinkled yellow: 1 wrinkled green

Law of Independent Assortment Mendel’s second law states that genes for different traits- for example, seed shape and seed color- are inherited independently of each other

Punnett Squares In 1905, Reginald Punnett, an English biologist, devised a shorthand way of finding the expected proportions of possible genotypes in the offspring of a cross This method is called a Punnett Square

Probability Punnett squares are good for showing all the possible combos of gametes and the likelihood that each will occur In reality, however, you don’t get the exact ratio of results shown in the square Genetics is more like flipping a coin-it follows the rules of chance