DNA, Genes and inheritanceI got it from my mama!

DNA is a double helix

discovered by Watson and Crick in 1953

they won a Nobel prize for their work

It was Watson, playing with cardboard cutouts, who realized that the bases could hydrogen bond specifically with one another, G with C and A with T, and stack up in parallel between the phosphate sugar backbones!

The structure of DNAuncoiled diagramdiagramspace-filling model

The structure of DNA

PPP

PPP

GC

TA

SSSSSS

The base pairs

CytosineCGuanineG

Hydrogen bond

ThymineTAdenineA

Chargaffs RuleAdenine must pair with Thymine

Guanine must pair with Cytosine

because they always pair together, their amounts in a DNA molecule are the same

GC

TA

Question:If there is 30% Adenine in a strand of DNA, how much Cytosine is present?

Answer:there would be 20% Cytosine:

Adenine (30%) = Thymine (30%)Guanine (20%) = Cytosine (20%) (50%) = (50%)

GenesDNA is not a sequence of random information, it is broken up into areas with different functions

sections of DNA with particular functions are called genes

CHROMOSOMES

CHROMOSOMES

start of human chromosome 1These are all genes!

Allelesdifferent versions of the same gene are called alleles

e.g. the gene for eye pigment in fruit flies (Drosophila) has a red allele, a white allele, and a brown allele

chromosomes are always paired, so it is possible to have two different alleles for any given gene

eye colour alleles in Drosophila

Ishihara Colour Blindness Test

everyone sees 12

normal = 8red-green = 3total = nothing

normal = 74red-green = 21total = nothing

normal = 6red-green = nothingtotal = nothing

Gregor Mendel: father of geneticsdiscovered heredity the passing on of characteristics from parent to offspring

bred pea plants until they were true-breeding, the offspring always resembled the parents

Mendels experimenthe took one plant of each type, such as purple and white flowers, and bred them together:

all the offspring were purple

took two of these offspring and bred them to make a second generation: 3 purple offspring / 1 white offspring

cross-fertiliseParental generationFirst generation (F1)

true-breeding purple flowerstrue-breeding white flowersall offspring purple

First generation (F1)self-fertilizeSecond generation (F2)all offspring purple3/4 purple1/4 white

Mendels Conclusionsparents pass on elements to their offspring; these elements remain unchanged during life

offspring receive one element from each parent; therefore offspring have a pair

each parent can pass on a different version of the element

Mendels elements are now called GENES.

Number of genesCarsonella ruddii (bacterium)180

Streptococcus pneumoniae (bacterium)2,300

Escherichia coli (bacterium) 4,400

Saccharomyces cerevisiae (yeast)5,800

Drosophila melanogaster (fruit fly)13,700

Caenorhabditis elegans (nematode) 19,000

Strongylocentrotus purpuratus (urchin) 23,300

Homo sapiens (human)27,000

Mus musculus (mouse)29,000

Oryza sativa (rice)50,000

Dominant vs. recessivedominant = when one allele overwrites the expression of another allelerecessive = when one allele is masked by the expression of another allelethe dominant allele is given a capital letter (P), the recessive allele a lower case letter (p)

Dominant and recessive traits in peas

Human dominant/recessive traits:Cleft in chinNo cleft dominant, cleft recessive

Hairline Widow peak dominant, straight hairline recessive

Eyebrow sizeBroad dominant, slender recessive

Eyebrow shapeSeparated dominant, joined recessive

Eyelash lengthLong dominant, short recessive

DimplesDimples dominant, no dimples recessive

EarlobesFree lobe dominant, attached recessive

FrecklesFreckles dominant, no freckles recessive

Tongue rollingRoller dominant, nonroller recessive

Tongue foldingInability dominant, ability recessive

Finger mid-digital hairHair dominant, no hair recessive

Bent little fingerBent dominant, straight recessive

Interlaced fingersLeft thumb over right dominant, right over left recessive

Hair on back of handHair dominant, no hair recessive

Heterozygous vs. homozygousHeterozygous = having two different alleles for a particular gene (e.g. Pp)Homozygous = having two of the same allele for a particular gene (e.g. PP or pp)

Genotype vs. phenotypegenotype = the genetic characteristics of an organismeg. PP or Pp

phenotype = the visible physical features of an organism e.g. white flowerPhenotypeGenotype

PP or Pppurple flowerwhite flowerpp

The Punnett SquareIn three steps, its an easy way to determine the probability of offspring:

Step 1: Make a 2 X 2 Square gridStep 2: Put the alleles of each parent on the outside of the squareStep 3: Combine alleles to make potential offspring in the middle of the square

Sample Punnett problemThe gene for free ear-lobes is dominant (E), and the gene for attached earlobes is recessive (e). If a heterozygous male mates with a homozygous recessive female, what is the probability that their child will have attached earlobes?

Solution:MaleFemaleEe

eEeee

eEeee

50% chance heterozygous (Ee free lobes)

50% homozygous recessive (ee attached lobes)