GENETICS 8 th Grade Science

Vocabulary Trait – physical characteristic of an organism Heredity – the passing of traits from parent to offspring Purebred – having the exact same traits as the parent Gene – factor that controls traits Allele – the different forms of genes that control a particular trait Dominant allele – one whose trait shows up in an organism Recessive allele – one whose trait is hidden or covered up Hybrid – 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 things Published Experiments with Plant Hybrids which is the most enduring and influential publication in history of science

Mendel continued From 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 factors Work 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 generations Pollen was easy to see and cross-pollination was controlled Several physical traits were easy to established For 2 years grew different varieties to make offspring always the same Noticed 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 pattern Grew 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 combination Phenotype – physical appearance or observable traits of an organism Genotype – the genetic makeup of an organism Homozygous – an organism that has 2 identical alleles for a trait Heterozygous – an organism that has 2 different alleles for a trait Co-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 cross Use Punnett cross to show / predict outcomes

Cell Reproduction and Inheritance Meiosis – process which allows chromosomes to divide to form sex cells Messenger RNA – copies coded message from DNA in nucleus and carries the coded message into the cytoplasm Transfer RNA – carries amino acids and adds them to growing protein Mutation – any change that occurs in gene or chromosome Multiple 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 chromosome Carrier – person who has one recessive allele for a trait and one dominant allele Pedigree – chart or “family tree” that tracks traits of a family Amniocentesis – 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 organism Humans 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 DNA DNA contains the code to determine the size, shape, and other traits of an organism DNA is made up of 4 different nitrogen bases – adenine (A), thymine (T), guanine (G), cytosine (C) This forms the rungs of the ladder of DNA Adenine (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 bases The order of the bases determines the structure of proteins that make up amino acids Are only 20 common amino acids – can be combined in different ways to form thousands of different proteins The order of the nitrogen bases along a gene forms the code that determines what type of protein will be produced –called protein synthesis Before 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 protein A 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 adenine Messenger RNA then leaves the nucleus and attaches to a ribosome in the cytoplasm, there transfer of RNA picks up the amino acid specified by 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 mutation Causes a cell to produce an incorrect protein during protein synthesis Some are result of small change in hereditary material such as substitution of single base pair for another Can occur during DNA replication process Some occur when chromosomes don’t separate correctly during meiosis Will cause too few or too many chromosomes If 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 and will affect offspring’s phenotype Mutation can cause change in organism that may or may not help it to survive Some neither helpful nor harmful Genetic disorders causes by mutation or change in DNA

CELL DIVISION: MITOSIS AND MEIOSIS Mitosis Occurs in cells of the bodyOccurs in cells of the body Produces identical offspring cellsProduces identical offspring cells Produces two cellsProduces two cells Cells have same number of chromosomes as parent cellCells have same number of chromosomes as parent cellMeiosis Occurs only in sex cellsOccurs only in sex cells Produces non-identical offspring cellsProduces non-identical offspring cells Produces four cellsProduces four cells Cells have half the number of chromosomes as parent cellCells have half the number of chromosomes as parent 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 traits Males are more apt to have a sex-linked trait due to having only 1 X chromosome A person can be a carrier for a sex-linked trait but not show any sign for that trait Use pedigree to trace inheritance patterns of traits

Resources and pictures used for this presentation: University of Texas Medical Branch Cell Biology Graduate Program Science Explorer Grade 8 – Prentice Hall