Genetics & The Work of Mendel 2006-2007

Genetics The study of heredity What’s heredity? The passing of traits from one generation to the next… Serious study of heredity began in the mid 1800s

Ideas about Heredity in the 1800s… Blending Offspring was a BLEND between two parents Like mixing different colors of paint. Should have been easily dismissed as wrong because some pretty major traits are clearly NOT “blended”…

Experiments in heredity in the 1800s POORLY designed For example… How is beauty passed from one generation to the next…? Beauty is subjective Humans are very poor experimental organisms for studies of heredity Cannot control who they mate with Takes too long to see how offspring turn out Studies usually limited to just a few family groups

Gregor Mendel “The Father of Genetics” Modern genetics began in the mid-1800s in an abbey garden, where a monk named Gregor Mendel documented inheritance in pea plants He studied at the University of Vienna from 1851 to 1853 where he was influenced by a physicist who encouraged experimentation and the application of mathematics to science and a botanist who aroused Mendel’s interest in the causes of variation in plants. After the university, Mendel taught at the Brunn Modern School and lived in the local monastery. The monks at this monastery had a long tradition of interest in the breeding of plants, including peas. Around 1857, Mendel began breeding garden peas to study inheritance.

What did Mendel do that was different? Studied PEA PLANTS EASY to grow Small size allows you to grow lots in a relatively small space Relatively quick generation time A few weeks as opposed to years

Breeding Pea Plants allows GREAT CONTROL over the experiment Why? Because pea plants have BOTH reproductive parts AND these are HIDDEN inside flower petals This means that they NORMALLY SELF FERTILIZE Pollen carrying sperm fertilizes eggs within the same plant Plants do NOT generally get random pollen from other pea plants If Mendel wanted to breed two plants, he could open petals and control what pollen reached the female parts of the plant

How Mendel did his crosses… Got 2 pea plants that he wanted to use as parents… “mom” and “dad” Made sure these parents were TRUE BREEDING This means that no matter how many times a plant was allowed to self-fertilize, the types of offspring that resulted were ALWAYS the SAME That is, a pink-flowered plant would ALWAYS have pink-flowered babies… Why true breeding? CONTROL VARIABLES!!!!

If Mendel wanted to breed two different plants together, here’s what he did: Opened petals Cut male parts off of each plant and used them to fertilize female parts of the other plant Waited for seeds to form Planted them to see what traits the next generation would have

Pea Plants were a good choice… Lots of easily distinguishable traits that are either “on” or “off” and no inbetween Either Tall or Short – no “medium” plants Made it easier for Mendel to figure out his results Mendel examined heredity of 7 different traits Plant height Flower color Pea shape Pea color Etc…

Mendel collected data for 7 pea traits

What ELSE did Mendel do that was different? He used true experimental method NOT some very limited and subjective look at a human family… Used QUANTITATIVE data collection and analysis This means he COUNTED the pea plants that resulted from his crosses of parents He ANALYZED the data using statistics He crossed THOUSANDS of plants. Why?? Multiple trials!

A typical cross as done by Mendel: Select 2 parents that differ in ONE trait only Why? LESS CONFUSION Fewer variables Called a MONOHYBRID cross Mono = 1 Hybrid = difference Example = Plant Height One parent TALL One parent SHORT All other characteristics would be the same between the two parents

PHENOTYPE Term for the PHYSICAL APPEARANCE of a trait in an organism The phenotype (physical appearance) of a tall plant is TALL The phenotype (physical appearance) of a short plant is SHORT

A typical cross as done by Mendal: The plants selected by Mendel would always be TRUE BREEDING Tall parent always had tall offspring if left to self fertilize Short parent always had short offspring if left to self fertilize

The cross was performed… Mendel would cross the two parents Called this generation the P (parent) generation P = true breeding parents

Mendel would wait and observe offspring that resulted First generation of offspring is called F1 generation Mendel noticed the F1 generation always turned out TALL How could this be? What happened to the short parent? He had to figure this out!

Mendel’s next step: Cross the F1 plants with…. THEMSELVES! Actually allowed them to self-fertilize Wait and observe the offspring that resulted Second generation of offspring (grandchildren of original parents) is called F2 generation

How did the F2 generation turn out? ¾ were TALL ¼ were SHORT

SUMMARY of Mendel’s Results In F1 generation, one trait APPEARED to be LOST In the F2 generation, the “lost” trait always reappeared In the F2 generation, the ratio of offspring ALWAYS was: ¾ possessed the “unlost” trait ¼ possessed the “lost” trait

Mendel EXPLAINED these strange result. He said… Every trait is determined by 2 “factors” possed by the cells of the organism Factor = GENE Male parent contributes 1 factor to offspring Female parent contributes 1 factor to offspring

Mendel’s explanation… Factors (genes) may exist in alternate forms Alternate forms called alleles Example: the gene for pea plant height exists in two forms (ALLELES) Tall allele Short allele

Mendel’s explanation… In the cases Mendel studied, one form (allele) seemed “stronger” than the other form Term for an allele that can MASK another allele DOMINANT Term for an allele that is MASKED by another allele RECESSIVE

How it all works… What’s dominant in pea plant height? Let’s say that we have a cross as follows: Male = Tall Female = Short

Genetic notation rules: Usually the first letter of the dominant trait is used to symbolize the gene being studied Uppercase letter = dominant Lowercase letter = recessive Example: T = tall t = short

How it works… Each parent possesses two alleles for the trait. Since Mendel started with TRUEBREEDING plants, he said their genetic make-ups were as follows: Male = TT Female = tt

GENOTYPE The letter combinations showing an organisms genetic make-up with regard to a particular trait for example: TT or tt

How it works… Each parent sends ONE of its alleles The process that assures that only ONE allele from each parent will be sent to the offspring is… MEIOSIS Divides parent’s genetic material in HALF

How it works… At fertilization these alleles are COMBINED Since the parents are true breeding, there is only one type of allele that each can send: Male’s gametes (sperm) can only contain T allele for the height gene Female’s gametes (egg) can only contain the t allele for the height gene Offspring has NO CHOICE but to be Tt

Now we cross the F1 plants: Cross of F1s is different because each F1 plant contains both T and t alleles! Thus, each F1 plant can send EITHER a T or a t to the next generation There is a 50% chance that T could be sent from one parent in an F1 cross There is a 50% chance that t could be sent by the other parent in an F1 cross There is NO CONTROL over which allele is sent by EITHER parent to the offspring. It is RANDOM.

Possible results: You could get an offspring that gets a T from mom and a T from dad This offspring would be TT This offspring would be TALL

Possible results: You could get an offspring that gets a T from Mom and a t from dad This offspring would be Tt This offspring would also be TALL because T is dominant over t

Possible results: You could get an offspring that gets t from Mom and a T from dad This offspring would also be Tt This offspring would also be TALL because T is dominant to t

Possible results: You could get an offspring that gets a t from mom and a t from dad This offspring would be tt This offspring would be SHORT

Possible results: If you had LOTS of offspring, the LAWS of PROBABILITY (chance) say that: 1 out of 4 of them will be TT 2 out of 4 of them will be Tt 1 out of 4 of them will be tt

Possible Results AND because of these combinations of alleles, we can also say that ¾ of them would appear TALL ¼ of them would appear SHORT THIS is what Mendel observed A 3:1 ratio That is, for every 3 tall plants produced in the F2 generation, there was only one short plant produced.

Other Terms… HOMOZYGOUS HETEROZYGOUS Describes an individual who has two alleles that are the SAME for a trait TT tt HETEROZYGOUS Describes an individual who has two alleles that are DIFFERENT for a trait Tt

Another Example: In humans, tongue rolling is DOMINANT over non tongue rolling Cross a male homozygous tongue roller with a female non tongue roller

What symbol to use? R = roller r = non roller

What are the genotypes? Male = RR Female = rr

What are the phenotypes? Male = Roller Female = Non roller

Intersecting blocks to get Make a Punnet Square Male genotype F E M A L Combine letters of the Intersecting blocks to get Possible outcomes of offspring

What will be the genotype fo the F1 generation? Rr

What will be the phenotype of the F1 generation? Roller

Now cross the F1s

Define Phenotypic Ratio Number of each phenotype you have What is the phenotypic ratio of the F2s? 3:1

Define genotypic ratio The number of each different gentoype you have What is the genotypic ratio of F2s? 1:2:1

Any Questions?? 2006-2007