GENETICSGENETICS

88thth Grade Science Grade Science

VocabularyTrait – physical characteristic of an organismHeredity – the passing of traits from parent to offspringPurebred – having the exact same traits as the parentGene – factor that controls traitsAllele – the different forms of genes that control a

particular traitDominant allele – one whose trait shows up in an

organism Recessive allele – one whose trait is hidden or covered

upHybrid – having two different alleles for the same

characteristic

Mendel – Father of Modern Genetics

• Augustinian monk who taught natural science in high school

• Interested in plants, meteorology, and theory of evolution

• Came from a poor family – entered monastery at 21 – was then able to attend university and conduct research

• After observing plants decided to experiment by crossing pea plants

• Saw the traits were inherited in certain ratios

Took 7 yrs to cross and record data from plants in order to prove laws of Inheritance.Became first person to trace characteristics of successive generations of a living thingsPublished Experiments with Plant Hybrids which is the most enduring and

influential publication in history of science

Mendel continuedFrom studies came up with certain basic laws of heredity: 1. Heredity factors do not combine – are passed intact 2. Each member of parent generation gives only half of its hereditary factors to each offspring (some “dominant” over others) 3. Different offspring of same parents receive different sets of hereditary factorsWork was ignored for 30 years, but was rediscovered after his death.

Abbey of St. Thomas Library

Photo: © Stepan Bartos

Pea Plant Experiment

Choose pea plants because they were available and easy to track generationsPollen was easy to see and cross-pollination was controlledSeveral physical traits were easy to establishedFor 2 years grew different varieties to make offspring always the sameNoticed some traits disappeared in 1st generation – called them “recessive”Those traits that appeared called “dominant”Later generations would have recessive traits reappear in a mathematically predictable patternGrew about 28,000 pea plants over 8yrs. before publishing results in 1864

Illustration: Greg Mercer

Pea Plant Experiment

Illustration: Greg Mercer

Vocabulary

Punnett square – chart showing all possible outcomes of a genetic combinationPhenotype – physical appearance or observable traits of an organismGenotype – the genetic makeup of an organismHomozygous – an organism that has 2 identical

alleles for a traitHeterozygous – an organism that has 2 different alleles for a traitCo-dominance – alleles are neither dominant nor recessive, neither allele is masked and both are expressed

Mendel recognized that principles of probability can be used to predict results of genetic crossUse Punnett cross to show / predict outcomes

Cell Reproduction and Inheritance

Meiosis – process which allows chromosomes to divide to form

sex cellsMessenger RNA – copies coded message from DNA in nucleus

and carries the coded message into the cytoplasmTransfer RNA – carries amino acids and adds them to growing proteinMutation – any change that occurs in gene or chromosomeMultiple alleles – a human trait controlled by a single gene with

more than 2 alleles Sex-linked gene – genes on X and Y chromosomes, alleles are

passed from parent to offspring on a sex chromosomeCarrier – person who has one recessive allele for a trait and one

dominant allelePedigree – chart or “family tree” that tracks traits of a familyAmniocentesis – removal of fluid surrounding a developing baby Karyotype – picture of all the chromosomes in a cell

Chromosomes are made of many genes joined together; each one contains a large number of genes due to the number of traits

of an organismHumans have 23 pairs or 46 chromosomes, dogs 78, silkworms 56

– size of organism does not determine number of chromosomes

Remember, one chromosome from each pair in an organism comes from the mother and one from the father

Inheritance

Inside chromosome is replicated DNADNA contains the code to determine the size, shape, and other traits of an organismDNA is made up of 4 different nitrogen bases – adenine (A), thymine (T), guanine (G), cytosine (C)This forms the rungs of the ladder of DNAAdenine (A) ALWAYS pairs with thymine (T)Guanine (G) ALWAYS pairs with cytosine (C)One gene may contain anywhere from several

hundred to a million or more basesThe order of the bases determines the structure of proteins that make up amino acidsAre only 20 common amino acids – can be combined in different ways to form

thousands of different proteinsThe order of the nitrogen bases along a gene forms the code that determines what type of protein will be produced –called protein synthesisBefore protein synthesis occurs a “messenger” must carry the genetic code from

the DNA inside the nucleus into the cytoplasm – is called ribonucleic acid or RNA

Though RNA resembles DNA has some differences:-alnost always looks like one side or strand of DNA- contains different sugar molecule - does not contain thymine in bases, replaces it with

uracil base

Transfer RNA carries amino acids and adds them to growing proteinA 3 base code indicates a specific amino acid

Translating the Code

First DNA unzips between its base pairs, then one of the strands of DNA directs production of a strand of messenger RNA

The RNA bases pair up with the DNA, with uracil pairing up with adenineMessenger RNA then leaves the nucleus and attaches to a ribosome in the

cytoplasm, there transfer of RNA picks up the amino acid specifiedby 3 letter code

Each transfer RNA molecule puts an amino acid in the correct order along the growing protein chain

Process continues for each section between 3 letter code

Mutations

Any mistake in the transfer can result in a mutationCauses a cell to produce an incorrect protein during protein synthesisSome are result of small change in hereditary material such as substitution

of single base pair for anotherCan occur during DNA replication processSome occur when chromosomes don’t separate correctly during meiosisWill cause too few or too many chromosomesIf mutation occurs in body cell then mutation only affects the cell that carries

it, if in a sex cell then mutation able to be passed on to offspring andwill affect offspring’s phenotype

Mutation can cause change in organism that may or may not help it to surviveSome neither helpful nor harmfulGenetic disorders causes by mutation or change in DNA

CELL DIVISION: CELL DIVISION: MITOSIS AND MEIOSISMITOSIS AND MEIOSIS

MitosisMitosis• Occurs in cells of the Occurs in cells of the

bodybody• Produces identical Produces identical

offspring cellsoffspring cells• Produces two cellsProduces two cells• Cells have same Cells have same

number of number of chromosomes as chromosomes as parent cell parent cell

MeiosisMeiosis• Occurs only in sex Occurs only in sex

cellscells• Produces non-Produces non-

identical offspring identical offspring cellscells

• Produces four cellsProduces four cells• Cells have half the Cells have half the

number of number of chromosomes as chromosomes as parent cellparent cell

Sex Cells

Sex of offspring determined by sex chromosome – are the only pair of chromosomes that may not match

Sex –linked genes produce sex-linked traitsMales are more apt to have a sex-linked trait due to having only 1 X chromosomeA person can be a carrier for a sex-linked trait but not show any sign for that

traitUse pedigree to trace inheritance patterns of traits

http://www.bioweb.uncc.edu/1110Lab/notes/notes1/lab6.htm

http://www.fieldmuseum.org/mendel/story_discovered.asp

Resources and pictures used for this presentation:

http://search.live.com/images/results.aspx?q=Chromosome&mkt=en-US#

University of Texas Medical Branch

Cell Biology Graduate Program

http://www.ngsgenealogy.org/images/Pedchart.jpg

Science Explorer Grade 8 – Prentice Hall