 8 Genetic Notes 1/22  9 Lab Activity: Predicting Genetic Ratios 1/22

the branch of biology that studies the transmission of hereditary information from parents to offspring

 Mendel was a monk who was educated--he knew MATH! He also had a lot of free time. As for the peas? Well, they were: * easy to grow * inexpensive * easy to pollinate (either self or cross) * easy to study their TRAITS (characteristics) such as: height, seed pod shape, seed color, seed pod color, seed texture, flower position, and seed shape

 Mendel kept very accurate records for hundreds of individual plants.  He used statistical analysis (MATH) to study the traits of different generations.  Through his findings, he proposed that the traits were passed on by some kind of hereditary factors (now know to us as DNA).

 One trait he studied was plant height. Mendel pollinated all tall plants for many generations to get a pure population of tall plants. He also pollinated all short plants to get a pure population of small plants. See picture below... X means to cross (as in cross-pollinate) tall plants X tall plants as well as short plants X short plants He then cross-pollinated a member of the tall pure population with a member of the short pure population... P1 (pure parent generation) P1 TALL PLANT X P1 SHORT PLANT ALL TALL PLANTS F1 (first filial [family] generation) Where did the short trait go? Mendel then crossed two tall members of the F1 generation. F1 TALL PLANT X F1 TALL PLANT THIS IS CALLED A MONOHYBRID CROSS (hybrids known for one trait) F2 (second filial generation) For every 3 TALL PLANTS there was 1 SHORT PLANT

 So, the short trait did not disappear. It was present in the F1 generation, but IT WAS NOT EXPRESSED (this means it did not show up!) Somehow, it was expressed (visible) in the F2 generation.

 because the tall trait showed up more than short, Mendel call this trait DOMINANT  the short trait, because it seemed 'weaker' than the tall trait, was called RECESSIVE After this discovery...well, um...NOBODY CARED! Later on, when scientists studies meiosis in Drosophila (genus name for fruit flies), they linked together Mendel's factors with the chromosomes in gametes

 Mendel's factors are now called GENES  genes are segments of chromosomes (DNA) that code for a characteristic  these characteristics can be: a) physical traits (can be seen like eye color, hair color, height, etc.) b) chemicals produced in the body (like for hormones or enzymes)

 genes are lined up on chromosomes in a certain order, like beads on a string  homologous (similar) chromosomes have the same order of genes...HOWEVER..  these chromosomes might have the different forms of a gene ex. trait=eye color can be blue, black, brown, hazel, green, etc.  these different forms of a gene are called alleles  in simple patterns of inheritance, there are 2 different forms of a gene (alleles), where one is dominant and one is recessive  DOMINANT IS SHOWN BY CAPITAL LETTERS  recessive is shown by lowercase letters ex. in Mendel's peas T = tall t = short

 diploid (having 2 sets of chromosomes) organisms have 2 copies of genes, one on each chromosome they have  the 2 copies can be the same--HOMOZYGOUS, like TT (homozygous dominant) or tt (homozygous recessive)  the 2 copies can be different--HETEROZYGOUS Tt (heterozygous)

 phenotype: what the appearance of an organism is (how it looks) ph = physical  genotype: what the genetic makeup of an organism is (what genes it has) gen = genes genotype can be: homozygous dominant > TT heterozygous (a hybrid--a mix)---> Tt homozygous recessive > tt  recessive phenotypes can only be expressed (show up) when there are 2 copies of a recessive gene present at the same time...if a dominant gene was there, then that would be expressed, hiding the recessive gene!

 when gametes are formed during meiosis (DIPLOID to HAPLOID) the homologous chromosomes (which contain the many different genes) separate randomly  this is called SEGREGATION

 if the organism has a homozygous (the same) genotype for a given trait (TT or tt), then all gametes will also have that trait TT tt meiosis T or T t or t  if the organism has a heterozygous (not the same) genotype for a given trait (Tt), then half of the gametes will have one trait, and the other half will have the other trait Tt <----diploid meiosis T or t <----haploid  when the haploid gametes fuse during fertilization, the diploid number of chromosomes is restored  this can result in a new combination of genes  this is called RECOMBINATION

 P1 (pure parent generation) cross was: TT X tt  F1 (first filial generation) cross was Tt X Tt  F2 (second filial generation) offspring had genotypes of TT, Tt, tT, and tt

 For the F2 generation what is the PHENOTYPIC RATIO? 3 tall : 1 short  For the F2 generation, what is the GENOTYPIC RATIO? 1 homozygous dominant : 2 heterozygous : 1 homozygous recessive  PHENOTYPE DOES NOT ALWAYS EQUAL GENOTYPE!

 Problem: A homozygous tall pea plant is crossed with a short pea plant. (Remember, tall is dominant over short for pea plants!) What would the expected genotypes (the genotypic ratio) and phenotypes (the phenotypic ratio) of their offspring?

T = tall t = short

TT x tt tall homozygous parent & short parent * remember, to be short, it has to have both short alleles!

 STEP 3: do the Punnett square; that is, put one parent on the top and the other parent on the left of the square...then from a blank box, take the allele on the top and the allele from the left and fill them in

 GENOTYPIC RATIO: Tt, Tt, Tt, Tt  PHENOTYPIC RATIO: 4 Tall  Homework: Complete handout on Monohybrid crosses and online questions. Review notes and be prepared for quiz.

1) Use a Punnett square to show the offspring of a cross between two pea plants that are heterozygous for height (Tt). Give the phenotype and genotypes of the offspring. (That is, what is the phenotypic and genotypic ratios!) 2) Use a Punnett square to show the offspring of a cross between a pea plant that is homozygous tall and one that is heterozygous. Give the phenotypes and genotypes of the offspring. 3) Use a Punnett square to show the offspring of a cross between a pea plant that is heterozygous tall and one that is homozygous recessive for height. Give the phenotypes and genotypes of the offspring.