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Transcript
1. Why do organisms need haploid cells? 2. What would happen if “crossing over” didn’t occur during Meiosis? 3. What are gametes? Heredity • Heredity is the passing on of characteristics from parents to offspring. These characteristics are called traits Gregor Mendel • It was not until the mid-nineteenth century that Gregor Mendel, an Austrian monk, carried out important studies of heredity. • Mendel argued that parents pass on to their offspring factors (now called genes) that are responsible for inherited traits. • Mendel was the first person to succeed in predicting how traits are transferred from one generation to the next. Mendel chose his subject carefully • Mendel chose to use the garden pea in his experiments. • Garden pea plants reproduce sexually, which means that they produce male and female sex cells, called gametes. Mendel wanted to pick his own parent plants Remove male parts • When Mendel wanted to breed, or cross, one plant with another, he opened the petals of a flower and removed the male organs. • He then dusted the female organ with pollen from the plant he wished to cross it with. Pollen grains • He only wanted to study one trait at a time at first. SHORT TALL • So he picked two plants that were the same in every way, except height. • The tall pea plants he worked with were from populations of plants that had been tall for many generations and had always produced tall offspring. • They were said to be pure-bred tall“pure-bred” means that it comes from two parents with the same form of a trait. • Likewise, the short plants he worked with were pure-bred for shortness. • A hybrid is the offspring of parents that have different forms of a trait, such as tall and short height. Parents Hybrid Offspring Monohybrid crosses • Mendel’s first experiments are called mono-hybrid crosses • Mono means “one” • Hybrid means “parents with different forms of a trait” • So we are crossing two parent plants that differ from each other by a one trait— which in this case is height. The First Generation • He cross-pollinated the tall pea plant with pollen from a short pea plant. • All of the offspring grew to be as tall as the taller parent. The Second Generation • Mendel allowed the tall plants in this first generation of offspring to pollinate. P1- Parents F1- first generation of offspring F2- second generation of offspring What happened? • Mendel concluded that each organism has two genes that control each of its traits. Gene • We now know that these genes are located on chromosomes. • Genes exist in alternative forms. We call these different gene forms alleles. Gene for Hair Texture Mom’s allele codes for Straight hair Gene for Hair Texture Dad’s allele codes for Curly hair Chromosome # 1 Chromosome # 1 Inherited from mom Inherited from dad • An organism’s two alleles are located on their homologous chromosomes—one inherited from the female parent and one from the male parent. b B Bb Rule of Dominance • Mendel called the over-powering allele “dominant” and the trait that sometimes disappeared “recessive”. Bb • Mendel concluded that the allele for tall plants is dominant to the allele for short plants. • It is customary to use the same letter for different alleles of the same gene. T t for Tall for Short The letter “T” equals height. Short plant Tall plant t T T t T F1 Big T is Tall Little T is Short t All tall plants T t • An uppercase letter is used for the dominant allele and a lowercase letter for the recessive allele. Short plant Tall plant t T T t T • The dominant allele is always written first for each trait. Tt t F1 All tall plants T t The Law of Segregation • The law of segregation states: every individual has two alleles of each gene. (one inherited from mom, one inherited from dad) But, when gametes (or sex cells) are produced, each gamete only receives one of these alleles to pass on. Bb bb B b b b Phenotypes and Genotypes • The way an organism looks and behaves is called its phenotype. (brown eyes) • The allele combination an organism contains is known as its genotype. (Bb) • An organism’s genotype can’t always be known by its phenotype. Bb BB • An organism is homozygous for a trait if its two alleles for the trait are the same. TT • The pure-bred tall plant that had two alleles for tallness (TT) would be homozygous for the trait of height. • An organism is heterozygous for a trait if its two alleles for the trait differ from each other. Tt • Therefore, the tall plant that had one allele for tallness and one allele for shortness (Tt) is heterozygous for the trait of height. What is the phenotype of the pea plant below? What is the genotype of the pea plant ? tt So therefore, what would you call this trait? Homozygous short Punnett Squares • A Punnett square is a quick way to find the possible allele combinations in offspring. • Lets say: one parent has a genotype of Tt the other parent also has Tt T t T TT Tt t Tt tt Remind me of what the Law of Segregation states. DISCUSS WITH THOSE AROUND YOU: WHAT ARE TRAITS AND ALLELES, AND HOW THEY ARE PASSED DOWN Question 1 What is the relationship between TRAITS and ALLELES? DISCUSS WITH THOSE AROUND YOU: WHAT DOES “MONO-HYBRID CROSS” MEAN Question 2 What is a Mono-hybrid cross? DISCUSS WITH THOSE AROUND YOU: MENDEL’S LAW OF SEGREGATION Question 3 In your own words, explain the law of segregation? How are alleles segregated and passed to offspring? Bb B b 1. Using the picture above, explain Mendel’s Law of Segregation. T 2. Why is t this Punnett square set up incorrectly? S s TT 3. Why is this Punnett Tt square set up incorrectly? Tt TT Di-hybrid crosses • Remember, Mendel performed experiments on pea plants that only differed in one trait: HEIGHT • Mendel then decided to perform another set of crosses in which he used peas that differed from each other only in TWO traits rather than one. Di-hybrid crosses These experiments were called Di-hybrid crosses: • Di means “two” - because there were only two differences • Hybrid because the parents were different from each other (in two ways) • Cross because he chose the parents to breed with each other The pea plants were the same in every way, EXCEPT PEA SHAPE (round, wrinkled) and PEA COLOR (yellow, green) THE TWO DIFFERENCES • In pea plants: Yellow is Dominant “Y” and Green is Recessive “y” Round is Dominant “R” and Wrinkled is Recessive “r” RR YY rr yy Mendel took pure-bred plants with round yellow peas and crossed them with pure-bred plants with wrinkled green seeds. • What happened with the first generation of offspring? P1- Parents F1- first generation of offspring F2- 2nd generation of offspring What happened with the second generation of offspring? The Law of Independent Assortment • The Law of Independent Assortment states: genes for different traits—for example, eye shape and eye color— are inherited independently of each other. bb Bb rr RR Bb Rr Punnett Square of Dihybrid Cross Gametes from RrYy parent Gametes from RrYy parent RY Ry rY ry RRYY RRYy RrYY RRYy RRYy RrYy Rryy RrYY RrYy rrYY rrYy RrYy Rryy rrYy rryy Dihybrid crosses RrYy RY Ry rY ry • A Punnett square for a dihybrid cross will need to be four boxes on each side for a total of 16 boxes. • Remember with the Law of Segregation in a mono-hybrid cross, you had to separate the alleles because you can only pass ONE ALLELE FOR EACH TRAIT on to your offspring. T t In a di-hybrid, you still only pass one allele PER trait to your offspring…. But remember: in a di-hybrid you have two RR YY traits…. RY T TT Tt t Tt tt RY RY RY Male parent is Heterozygous for Brown eyes and Homozygous recessive for almond Let’s practice one together, theneyes: you will do one on your own. Bb rr Female parent is Homozygous recessive for Blue eyes and Heterozygous for Round eyes: bb Rr Br Br br br bR Bb Rr Bb Rr bb Rr bb Rr br Bb rr Bb rr bb rr bb rr bR Bb Rr Bb Rr bb Rr bb Rr br Bb rr Bb rr bb rr bb rr Father, CcSs (Curly hair, Slanted eyes) CS Cs cS cs CS Mother, CcSs (Curly hair, Slanted eyes) CCSS CCSs CcSS CcSs Cs CCSs CCss CcSs Ccss cS CcSS CcSs CcSs Ccss ccSS ccSs cs ccSs ccss Head, C-curly, c-straight Eyes, S-slanted, s-straight