Ch. 3: Genetics: The Science of Heredity What are Genes? What are Genes? Sect. 1: Mendel’s Work...

Ch. 3: Genetics: The Science of

Heredity What are Genes?

Sect. 1: Mendel’s WorkSect. 2: Probability & HereditySect. 3: The Cell & InheritanceSect. 4: The DNA Connection

Sect. 1: Mendel’s Work• Gregor Mendel

– Austrian priest/monk– born July 22, 1822; died January 6, 1884– “The Father of Genetics”– worked with pea plants to determine how traits are

passed from generation to generation.

• Genetics: The scientific study of heredity.– Heredity: The passing of physical characteristics

from parents to offspring.• Traits: the different forms of a characteristic

– Ex.) eye color: blue, brown, green– Ex.) height: short, medium, tall

• Mendel’s experiments– Cross-pollinated the flowers of purebred pea plants.

• Pollination: the transfer of pollen from the pistil of a flower to the stamen of another flower.

– Pistil: plant structure that produces the female sex cell (egg) of a flower

– Stamen: plant structure that produces the male sex cell (sperm) of a flower

• Fertilization: the process by which the egg & sperm cell of an organism combine to form a new organism.

• Purebred: the offspring of many generations with the same traits.

– Ex.) Short parents always produce short offspring.

– Ex.) Blue-eyed parents always produced blue-eyed offspring.

– Mendel removed the pollen-producing structures from a pink flower and then brushed the pollen from a white flower onto the pink flower.

• These initial plants, or parent plants, are called the P generation.

– The plants that are produced from the seeds of the P generation are called the 1st filial or F1 generation.

• Filial comes from the latin words for son “filius” and daughter “filia.”

– The plants that are produced from the seeds of the F1 generation are called the 2nd filial or F2

generation.

– Mendel observed that only 1 form of the trait was present in the F1 generation.

• Ex.) Yellow plants (P generation) crossed with green plants (P generation) always produced yellow plants (F1 generation).

– Mendel further observed that in the F2 generation, ¾ of the plants showed 1 form of the trait while ¼ showed the other “missing” form of the trait.

• Ex.) Yellow plants (F1 generation) crossed with Yellow plants (F1 generation) produced Yellow plants ¾ of the time & green plants ¼ of the time.

– The same results held true for all purebred traits.• Ex.) Seed shape, stem height, pod shape, pod color, seed

coat color.

Mendel’s breeding experiments

Ex.) Seed color

•Parents (P)

•First generation (F1)

Female sex cellsfrom a purebred

yellow-seeded plant

Male sex cells in pollen from a

purebred green-seeded plant

Cross fertilized (crossed)

All seeds produced turned out yellow

© 2007 Paul Billiet ODWS

Mendel’s breeding experiments

Ex. Continued.) Seed color

•1st generation(F1)

•2nd generation (F2)

Female sex cellsfrom a yellow-seeded plant

Male sex cells in pollen from a yellow-seeded

plant

Cross fertilized (crossed)

© 2007 Paul Billiet ODWS

3/4 yellow

¼ green

• Mendel concluded1. Mendel believed that individual factors or sets of genetic

“information” must control the inheritance of traits.2. The factors that control traits exists in pairs.3. Each parent (mother & father) contributes 1 of the factors.4. One factor in the pair can mask or hide the other factor.

• Gene: the factors that control a particular trait.– Ex.) eye color, height, hair color, nose shape, etc.

• Allele: the different forms of a gene– Ex.) eye color: brown, blue, green

– Dominant allele• The form of the gene that will always show up in an

organism if present and working correctly.• Typically represented by the 1st capital letter of the

dominant allele for a trait.– Ex.) Seed shape: round (R)– Ex.) Seed color: yellow (Y)

– Recessive allele• The form of the gene that will only show up if the

dominant allele is not present or working correctly.• Typically represented by the lower case letter of the

dominant allele for a trait.– Ex.) Seed shape: wrinkled (r)– Ex.) Seed color: green (y)

• Dominance versus Prevalence– Just because a particular trait is dominant,

meaning the trait will show up if the gene is present & working properly, does not mean it is more prevalent.

• Prevalent: majority; happens, takes place or is found most often.

– Ex.) Seed color: Yellow is dominant; Green is prevalent– Ex.) Widow’s peak: Having one is dominant; Not having one is

prevalent

– This is good because there are some genetic diseases that are found on a dominant gene but they are less prevalent in society.

• Hybrid– An organism that has 2 different alleles for a

particular trait • Ex.) Seed shape: Rr• Ex.) Seed color: Yy• Ex.) Pod shape: Ss• Ex.) Stem height: Tt

– The opposite of a purebred organism that has 2 of the same alleles for a particular trait.

• Ex.) Seed shape: RR or rr• Ex.) Seed color: YY or yy• Ex.) Pod shape: SS or ss• Ex.) Stem height: TT or tt

Sect. 2: Probability & Heredity • Probability

– A number that describes how likely it is that an event will occur.

– The laws of probability predict was is likely to occur, not necessarily what will occur.

– Probability is written as• The number of ways of getting a particular outcome to

the total number of possible outcomes.• Can be expressed as a ratio, fraction, decimal or percent• Ex.) Getting heads when flipping a coin

– 1 to 2, ½, .50, or 50%

• Independent versus Dependent events– Independent event

• An event in which the outcome or the likelihood of an outcome is not affected by a 2nd event happening.

– Ex.) Landing on heads after tossing a coin AND rolling a 5 on a single 6-sided die.

– Ex.) Choosing a marble from a jar AND landing on heads after tossing a coin.

– Dependent event• An event in which the outcome or the likelihood of an

outcome is affected by the outcome of the 1st event.– Ex.) Picking 3 Aces in a row from a deck of cards without

replacing the cards.

• Punnett square– Developed by Reginald

Punnett a British geneticist.

– A chart that shows all the possible combinations of alleles that can result from a genetic cross for a specific trait.

– Also called a monohybrid cross when trying to determine the possible allele combinations when dealing with 1 trait or dihybrid cross when dealing with 2 traits.

• Completing a Punnett Square.1. Start by drawing a box & dividing it into 4 squares.

2. Write the male parent’s alleles along the top of the square & the female parent’s alleles along the left side.

R r

R

r

R r

R RR Rr

r Rr rr

R r

R R r

r R r

4. Copy the female parent’s alleles into the boxes to their right.

3. Copy the male parent’s alleles into the boxes beneath them.

5. The completed Punnett square shows all the possible allele combinations for the offspring.

– In a Punnett square, each square is equally to ¼ or 25%.– To determine the total probability of a particular outcome

determine the number of times that outcome appears in the Punnett square & multiply by 25% or ¼.

– Ex.) Rr = 2 squares 2 x 25% 50% or ½.

R r

R RR Rr

r Rr rr

• Genotype– An organisms specific allele combination for a trait

• Seed color: YY, Yy, or yy• Seed shape: RR, Rr, rr

– Categories of genotypes• Heterozygous

– Having 2 different alleles for a trait – Also known as hybrids

» Yy or Rr

• Homozygous– Having the same 2 alleles for a trait– Also known as purebreds

» YY or yy; RR or rr

• Phenotype– The physical appearance or visible traits.

• In human genetics heterozygotes; those who have a dominant and a recessive allele are called carriers.

• This means they are carrying a recessive allele without expressing it physically.

Genotypes Phenotypes

YY Homozygous Yellow

Yy Heterozygous Yellow

yy Homozygous Green

• Codominance or “Incomplete dominance”– The alleles are neither dominant or recessive as a

result both alleles are expressed in the offspring.• Ex.) Roan cattle or Appaloosa horses• Ex.) Hazel or dual colored eyes in humans & animals

– Huskies 1 white & 1 blue eye or 1 blue & 1 brown eye

– Both alleles for the trait are written as superscripts for the trait.

• Ex.) Feathers FBFW Black & White Feathers• Ex.) Eye color EBEg Hazel eyes

Sect. 3 The Cell & Inheritance• Walter Sutton

– American geneticist– developed the Chromosome Theory of Inheritance

1. States that genes are carried from parent to offspring on chromosomes.

– Chromosome» A double rod of condensed chromatin; contains DNA that

carries the genetic information2. Chromosomes also exist in pairs.3. Sex cells have ½ the number of chromosomes typically

found in body cells.4. When fertilization takes place, the joining of sex cells,

the newly created organism ends up with the correct number of chromosomes having received ½ from each parent.

MeiosisThe process by which the number of chromosomes is reduced by half to form sex cells-egg & sperm. During meiosis, the chromosome pairs separate and are distributed to two different cells. The resulting sex cells have only half as many chromosomes as the other cells in the organism.

Meiosis and Punnett Squares

• Punnett Square will show the allele possibilities.

A Lineup of Genes• Chromosomes are made up of many

genes joined together like beads on a string.

• In Figure 15, one chromosome in the pair came from the female parent. The other chromosome came from the male parent. Notice that each chromosome in the pair has the same genes. The genes are lined up in the same order on both chromosomes. However, the alleles for some of the genes might be different. For example, the organism has the A allele on one chromosome and the a allele on the other. As you can see, this organism is heterozygous for some traits and homozygous for others.

Sect. 4: DNA Connection• Nucleic acids

– Organic molecule that contains the instructions cells need to carry out all of the functions of life.

– Composed of the elements Carbon, Oxygen, Hydrogen, Nitrogen & Phosphorus in various combinations.

– There are 2 types of nucleic acids• Deoxyribonucleic acid

– More commonly known by the acronym, DNA

• Ribonucleic acid– More commonly known by the acronym,

RNA

• Steps of DNA replication– Step 1: The bonds connecting nitrogen base pairs

dissolves.– Step 2: DNA unzips – Step 3: RNA brings in & attaches new nitrogen

bases to the now exposed nitrogen bases; • As a result 2 identical sets of nitrogen base pairs are

created • Adenine bonds to Thymine bonds to Adenine only• Guanine bonds to Cytosine bonds to Guanine only

– Step 4: Deoxyribose & Phosphate are brought in to complete the sides resulting in 2 exact copies of the DNA molecule.

• Additional names for DNA are– Double Helix – Twisted ladder – Code of Life

• RNA is responsible for carrying the genetic code from the nucleus to Ribosomes.– Ribosomes are the cell organelle located within the cytoplasm

that is responsible for protein production.• Proteins are large organic molecules made of carbon, hydrogen, oxygen,

nitrogen & in certain cases, sulfur.– Amino acids are the building blocks of proteins.

• Proteins serve several functions within the body…– Building blocks for most cell organelles & body structures.

• Hair, skin, fingernails, muscles, etc.– Proteins form parts of various cells membranes.

– Enzymes are specialized proteins that speed up or slow down chemical reactions within the body.

• There are 2 types of RNA.– Type 1: Messenger RNA (mRNA)

• mRNA: responsible for taking a copy of the DNA code from the chromosomes in the nucleus to the appropriate ribosome.

– Type 2: Transfer RNA (tRNA)• tRNA: responsible for bring the appropriate

amino acid to the ribosome to build the protein.

• RNA, which stands for ribonucleic acid, contains the same nitrogen bases as DNA with the exception of one and is only 1 strand unlike the 2 in DNA.– Thymine is replaced with Uracil and

matches up with Adenine.

• Steps to Protein synthesis1. Messenger RNA production

• DNA unzips• RNA bases match up along one of

the unzipped DNA strands.• Genetic information is transferred

from the DNA strand to the RNA strand.

2. Messenger RNA attaches to a Ribosome

• The ribosome moves along the messenger RNA strand reading the genetic code 3 bases (codons) at a time.

– Codon: a 3 letter combination of the 4 nitrogen bases that code for & match up to a specific amino acid.

» Stop codons: 3 letter combination that tells the ribosome to stop adding amino acids to a growing protein chain.

3. Transfer RNA attaches to Messenger RNA

• Based on the specific 3 letter combination (codon) that is read the transfer RNA retrieves the amino acid that the codon matches.

• The amino acid is then connected to the previous amino acid.

4. Protein production finishes• The protein chain continues to grow

with each amino acid that is added.• The ribosome reaches a STOP

codon or 3 letter combination that tells the ribosome to stop adding amino acids to the protein chain.

• The protein chain is then released and transport to the part of the body that it is needed.

• Mutations– A mutation is any “change” in the DNA; gene or chromosome.– There are 3 types of DNA mutations

• Type 1: Substitution: a DNA base or bases are exchanged for another.

• Type 2: Deletion: a DNA base or bases are deleted from the code for a gene.

• Type 3: Addition: a DNA base or bases are added to the code for a gene.

– Mutations can lead to diseases such as cancer if they occur during Mitosis.

– Mutations that take place during Meiosis can lead to inherited genetic disorders.

• Sickle cell anemia, Hemophilia, Colorblindness, etc.

• An organisms environment determines if the mutation is harmful or helpful.– Ex.) Albinism within a forest environment would

be a harmful mutation as it makes the organism visible to potential prey or predators.