Patterns of Inheritance

Intro To Genetics

Gregor Mendel • Mid-1800s• Austrian Monk, Gregor Mendel• Used pea plants to study how traits are

passed from one generation to the next. • “Basic principles of Heredity”

Mendelian Genetics: • For his experiments on inherited traits,

Mendel used pea plants • Because they could produce purebred

individuals (genetically identical)• How? Self-pollination • Once he produced Pure-bred strains, he

began to interbreed them in a controlled environment.

• Mendel crossed pure-bred GREEN-POD (dominant) plants with pure-bred YELLOW-POD (recessive) plants.

• The results are shown below:

Why did all the offspring come out green, if one of it’s parents were yellow?

• Pure-bred= Homozygous= Parent generation

Parent 1- Homozygous Green (GG)Parent 2- Homozygous Yellow (gg)

• When they were interbred, each parent contributed one allele to the offspring (First Generation)= one G and one g!

Offspring 1- Heterozygous offspring (Gg)

SEGREGATION Mendel didn’t just stop after crossing the parent plants,

he had another question… Had the recessive alleles simply disappeared,

or were they still present in the new plants?

P- Parent Generation F1- First (filial) Generation F2- Second (filial) Generation

• The F2 cross shows the recessive alleles reappeared in the second generation. HOW?

• The Reappearance indicated that, at some point, the allele for shortness had separated or segregated from the allele for tallness. HOW?

The alleles for tallness and shortness in the F1 plants must have segregated from each other during the formation of the SEX CELLS, or GAMETES.

FORMATION OF GAMETESP Generation= Tall plant (TT) X short plant (tt)

F1 Generation=All tall plants with genotype (Tt)

F1 (Tt) X F1 (Tt) F2 Generation= 1 Tall plant (TT), 2 Tall plants with (Tt), 1 SHORT plant with (tt)

During GAMETE FORMATION, the alleles for each gene segregate from each other, so that each gamete carries only one allele for each gene.

SUMMARY OF MENDEL’S PRINCIPLES

Mendel’s Principles of Heredity, observed through Patterns of Inheritance, form the BASIS OF MODERN GENETICS.

1. The Inheritance of biological characteristics is determined by individual units called GENES, which are passed from parent to offspring.

2. Where two or more form (alleles) of the gene for a single trait exist, some alleles may be DOMINANT and others may be RECESSIVE.

3. In most sexually reproducing organisms, each adult has TWO COPIES of each gene—one from each parent. These genes SEGREGATE from each other when gametes are formed.

4. Alleles for different genes usually SEGREGATE INDEPENDENTLY of each other.

Many Organisms, including humans, reproduce sexually. • They receive genes from both their parents • Both parents contribute genes for the same traits.

The Genes may be the same, or they may code for different forms of a trait.

Gene= heightTrait (allele)= short or tall

Alleles- are different forms of the gene for a specific trait.

Genotype: • Complete set of genes carried by

an organism • Includes all the alleles that are

not expressed as well as those that are.

• Ex: Rabbit Fur genotype can include black and brown fur (Bb)

Phenotype:• Set of traits that an

organism displays• What you actually express

physically• Ex: Rabbit Fur Phenotype

for (Bb) would be black fur.

Dominant And Recessive Traits

Dominant Traits When an organism has two different alleles for a

trait, the dominant allele is expressed (phenotype).

Uppercase letters EX: Black Fur in rabbits BB (two dominant

alleles)

Recessive Traits Are expressed only when no dominant alleles

are present. Lowercase letters

EX: Brown fur in rabbits bb (two recessive alleles)

Organism that receives two identical alleles for a characteristic shows that

characteristic (phenotype).

Trait Dominant Recessive

Freckles Present Absent

Hairline Widow’s peak

Straight

Earlobe Free Attached

Ability to taste PTC (phenylthiocarbamide)

Tasting Nontasting

Homozygous- two dominant or two recessive

(BB or bb). Heterozygous- two different alleles for a trait

(Bb).

Phenylthiocarbamide has the unusual property that it either tastes very bitter or is virtually tasteless, depending on the genetic makeup of the taster.

PUNNETT SQUAREUseful for finding the PROBABILITY of a simple genetic cross. • Parent’s alleles are written across the top and side of the

square. • Combining these alleles give the POSSIBLE genotypes of the

offspring

PRACTICE: Create a Punnett Square in your notes:

1. Mom has Blue eyes (bb)2. Dad has dark brown eyes (Bb)

What is the possible genotypes and phenotypes of their unborn baby?

MOM DAD

Monohybrid Cross(One-Factor Cross)

Dihybrid Cross (Two-Factor Cross)

Step 1: Write the Genotypes for the parents Ex: Pea Plant Height (Tt) and/or Color (Gg)

Tt and Tt TtGg and TtGg

Step 2: What alleles would be found in all possible gametes of the parents

Tt—T and tTt—T and t

TtGg—TG, Tg, tG, and tgTtGg—TG, Tg, tG, and tg

Step 3: Draw a table with enough squares for each pair of gametes from each parent

Step 4: Fill in the table by combining the gametes’ genotypes.

Step 5: Determine the genotype and phenotype of each offspring. Calculate the percentage or ratio of each.

T tTt

TG tG Tg tg

tG

TG

tg

Tg

TtGgTT

Intermediate Traits

Incomplete Dominance- the result is a BLEND of the two forms of the trait

Ex: Flower color

Codominance- Condition in which both alleles are expressed in the same organism.

Ex: Chicken Feathers & ABO Blood group

Multiple Alleles- although each organism has only two alleles for the trait, more than two possible alleles may exist in the population. Ex: Human Blood Types (A, B, AB, or O)

Polygenic Traits- are controlled by two or more genesEx: Height in humans

Blood Transfusions

Blood Type of Donor

Blood Type of Recipient

A B AB O

A YES X YES X

B X YES YES X

AB X X YES X

O YES YES YES YES

Intermediate Traits

Sex-Linked Inheritance- Because X and Y chromosome determines sex, the genes located on them show a pattern of inheritance.

Sex-linked gene- gene located on a sex chromosome

• Genes on the Y chromosome are found ONLY in MALES and are passed directly from father to son.

• Genes located on the X chromosome are found in both sexes, but the fact that men have just one X chromosome leads to some interesting consequences.• Ex: three genes responsible for color vision, all

located on X chromosome • Males- defective allele for any of these genes results

in colorblindness • In order for a RECESSIVE allele to be EXPRESSED in

FEMALES, it must be present in TWO copies—one on each of the X chromosomes.

RECESSICE PHENOTYPES of a Sex-Linked genetic disorders tends to be much more COMMON among MALES than among females.

PedigreesShows the presence or absence of a trait according to the relationships between parents, siblings, and

offspring.

• By analyzing a pedigree, we can often GUESS the genotypes of family members.• Based on a pedigree, you can often determine if an allele for a trait is dominant or

recessive, autosomal or sex-linked.

Human Genetic Disorders“It runs in the family”

What does that really mean?

• Changes in a gene’s DNA sequence can change proteins by altering their amino acid sequences, which may directly affect one’s phenotype.

Disorders Caused by individual Genes:

Sickle Cell Disease: o Disorder caused by a defective allele for beta-

globin, one of two polypeptides in hemoglobin. o The defective polypeptide makes HEMOGLOBIN less

soluble, causing them to STICK TOGETHER. o This causes the cell to get a SICKLE-SHAPE. o Because of its shape it gets stuck in the capillaries

Huntington’s Disease:o Caused by a dominant allele for a protein found in

brain cells. o The allele for this disease contains a long string of

bases in which the codon CAG (which codes for Glutamine) repeats over and over again.

o Symptoms include mental deterioration and uncontrollable movements

Central Dogma of Biology

DNAmRNAprotein DNA TRANSCRIBES to mRNA

Process is called transcription

mRNA TRANSLATES to proteins Process is called translation mRNA actually makes amino acids, which come together

to make proteins

Nucleic Acids

DNA Double strand Deoxyribose sugar A=T G=C

RNA Single Strand Ribose sugar A=U G=C Uracil is the nitrogenous base

used instead of THYMINE

DNA Replication Simplified

Unzip parent DNAAdd nucleotides to the 2 template strands of DNA

DNA parent strand makes 2 daughter strands…one fast, smart daughter strand (leading) and one, slower, no-fast daughter strand (lagging)

Leading strand (runs 3’ to 5’) Lagging strand (runs 5’ to 3’)

Attach fragments on lagging strand

DNA Polymerases Enzymes Make covalent bonds between nucleotides of the new strands Fast, accurate process

Error only one in a billion nucleotides

Brings over nucleotides to unzipped DNA strand and drops them off

More Players in DNA Replication

DNA polymerase can only read a strand that is running 3-prime to 5-prime… DNA polymerase works non-stop

adding nucleotides onto the strand that runs in the 3’ to 5’ direction

Therefore, Only one strand is made by a smooth, and continuous process…

The other strand is put together in bits and pieces… Each little section of nucleotides is

called an “Okazaki Fragment” These are then “glued” together to

make one, continuous strand in the

end by another enzyme… DNA Ligase

OKAZAKI FRAGMETS!!!are short, newly synthesized DNA fragments that are formed on the lagging template strand during DNA replication.

Important Enzymes

DNA Helicaseunzips

DNA PolymeraseAdds nucleotides

DNA LigaseAttaches/glues okazaki

DNAmRNAamino acids/polypeptide chain (Proteins)

DNA codes for an RNA strand The every 3 bases on the RNA strand

code for a specific amino acid CODON: three sequential bases that

code for a specific a.a. (20 a.a. total) Amino acid are strung together to

make a protein (primary structure) Change DNA will change RNA which

will change amino acids, which change protein

Ala: Alanine  Cys: Cysteine  Asp: Aspartic acid  Glu: Glutamic acidPhe: Phenylalanine  Gly: Glycine His: Histidine  Ile: Isoleucine Lys: Lysine Leu: Leucine  Met: Methionine Asn: AsparaginePro: Proline Gln: Glutamine Arg: Arginine Ser: SerineThr: Threonine Val: Valine Trp: Tryptophane Tyr: Tyrosisne

DNAmRNAProtein

Transcription Different form of the same message DNA makes single stranded RNA (U replaces T) RNA leaves nucleus

Translation Translate from nucleic acid language to amino acid language Uses codons, 3-base “word” that codes for specific a.a.

“code” for an amino acid Several codons make a “sentence” that translates to a

polypeptide (protein)

The Genetic Code

Am. Biochemist Marshall Nirenberg began to crack the genetic code in the 1960s Built RNA model with Uracil, called poly U, conducted

experiments with it and figured out UUU coded for amino acid phenylalanine

Scientists used his procedures to figure out the other amino acids represented by codons

Stop codons: UAA, UGA, UAG SIGNAL END OF GENETIC MESSAGE

Start codon: AUG SIGNAL TO START TRANSLATING an RNA transcript

Start Codons

UAAUGAUAG

Stop Codons

AUG

Three Types of RNA

mRNAtRNArRNA

Three Types of RNA…

Three Types of RNA… #1

mRNA (messanger RNA) RNA transcribed from DNA template RNA polymerase (enzyme) links RNA nucleotides together Modified in nucleus before if exits

RNA splicing: process in which Introns are removed and exons re joined together to make a continuous coding mRNA molecule

Introns Internal non-coding regions of DNA and mRNA Space fillers They are cut out of mRNA before it is allowed to leave the nucleus Process is called RNA splicing (processing)

Exons Coding region of DNA and mRNA that will be translated (Expressed) VERY important part of mRNA…it is carrying the message from DNA (def can’t cut this out)

Three Types of RNA…#2

tRNA (transfer RNA) The interpreter Translate 3-letter base codes into

amino acids Carries anti-codon on one end

(three letters opposite of what is on mRNA)

Carries amino acid on other end Anti-codon recognizes codon and

attaches

Three Types of RNA…#3

rRNA (ribosomal RNA) Found in ribosome Ribosome composed of 2 subunits:

Small subunit for mRNA to attach Large Subunit for two tRNAs to

attach “P” site: holds the tRNA carrying

the growing polypeptide chain “A” site: holds the tRNA that is

carrying the next a.a. to be added to the chain

When stop codon (UAA, UAG, UGA) is reached, translation ends and polypeptide is released from tRNA by hydrolysis