TOC

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

1/22

GENETICS

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

GREGOR MENDEL AND HIS PEA PLANTS EXPERIMENTS (1857-1865)

WHY MENDEL? WHY PEAS?

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).

HERE'S BASICALLY WHAT HE DID 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 PLANTTHIS 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.

MENDEL'S PRINCIPAL OF DOMINANCE

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 RECESSIVEAfter 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)

NOTICE THAT WHAT THE ORGANISM LOOKS LIKE AND THE KIND OF GENES IT HAS ARE TWO SEPARATE WAYS TO CLASSIFY AN ORGANISM

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)---> Tthomozygous 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!

MENDEL'S PRINCIPLE OF SEGREGATION AND RECOMBINATION

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 <----diploid----> ttmeiosis

T or T <----haploid----> 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 <----diploidmeiosis

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

LET'S LOOK BACK AT MENDEL'S FIRST FEW PEA PLANT CROSSES

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

NOW, COMPARE THE DIFFERENT PHENOTYPES (PHYSICAL APPEARANCE) AND GENOTYPES (GENETIC MAKEUP).

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!

TO SHOW SEGREGATION (SEPARATION OF GENES/CHROMOSOMES) AND RECOMBINATION (DURING

FERTILIZATION), WE USE PUNNETT SQUARES

PUNNETT SQUARE METHOD

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?

STEP 1: SET UP THE KEY THAT WILL SHOW HOW EACH ALLELE (FORM OF A GENE) WILL BE SHOWN

T = tallt = short

STEP 2: SET UP THE PARENTS THAT WILL BE CROSSED; USE THE INFORMATION FROM THE WORD PROBLEM ITSELF

TT x tttall 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

STEP 4: SHOW THE RESULTS (RATIOS) OF THE CROSS

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.

TRY THESE PUNNETT SQUARE PROBLEMS

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.