NICE GENES!UNIT 3 INTRODUCTION

Think about this…• We share 99% of our

DNA with this chimpanzee…

• And 60% of the DNA in the banana he is eating!!

Nature Theory• Search for "behavioral"

genes - source of debate

• arguments used to excuse criminal acts

• Twin studies - twins raised apart have shown same interests and behaviour

Nurture Theory• Nurture theorists

believe genetics ultimately don't matter - our behaviours originate from our upbringing

Nurture Theory“Give me a dozen healthy infants, well-formed,

and my own specified world to bring them up in and I'll guarantee to take any one at random and train him to become any type of specialist I might select...regardless of his talents, penchants, tendencies, abilities, vocations and race of his ancestors.”

-Behavioural Psychologist John Watson-

Nature Vs. Nurture• Researchers agree that the link between a gene

and a behavior is not the same as cause and effect. • A gene may increase the likelihood that you'll

behave in a particular way, it does not make your actions

What is a Chromosome?• A human body cell

contains 46 chromosomes which are paired up to make 23 pairs.

• These cells are Diploid.• 1 of the pair comes

from mom, 1 from dad!

What is a Gene?• Each chromosome is

one molecule of DNA.• The smaller sections of

DNA, which code for certain features, are called Genes.

• Eg. Blue eye pigment, height, chin shape, etc.

Homologous Chromosomes• Both chromosomes contain the same genes, BUT they are

not identical. • For example: EYE COLOUR

The mother’s chromosome could have the coding for blue pigment and the father’s could have coding for brown.

MITOSIS

The Role of Mitosis• Two Stages

-Divide nucleus & DNA

-Divide cell• Purpose: to produce 2

identical cells for…

-Growth

-Repair of tissue

-Replace dead cells

-Asexual Reproduction

Terms to know!• DIPLOID (2n):

Full complement of chromosomes.

• In humans 2n = 46• HAPLOID (n):

Number of unique chromosomes

• In humans n = 23

Diploid or Haploid? In a cabbage cell the Diploid number is

2n = 18

What is the Haploid number?

n = 9

How many homologous pairs?

9

Sister Chromatids• A condensed molecule of

DNA (chromosome) is called a Chromatid.

• A sister Chromatid is an exact replica of the original!

• Chromatids are held together by a centromere

Stages of Mitosis - Interphase

• This is the parent cell• Rapid growth• Cell doing its job• DNA replication

(chromatin)• Prepares for division

Stages of Mitosis - Prophase• P = PREPARING• DNA condenses into

chromatids• Nuclear membrane

disappears• Spindle fibers form

from centrioles and attach to centromeres.

Stages of Mitosis - Metaphase• M = MIDDLE• Chromatids line up

down the middle.• Pulled into place by

spindle fibres.

Stages of Mitosis - Anaphase• A = APART• Chromatids are pulled

apart (by spindle fibres)

Stages of Mitosis - Telophase• T = TEARING• Nuclear membrane

reforms• cell divides• 2 identical daughter

cells (DIPLOID – 2n)

Mitosis in Plant Cells• No centrioles• A cell plate forms,

then cell wall.

MEIOSISMitosis for our sex cells

Meiosis and Variation• Meiosis will not

produce clones each time!

• Variation is key for species survival, allows organisms to adapt!

• During Meiosis, two events occur which increase variation…

Independent (Random) Assortment

• When tetrads line up at Metaphase I, the paternal and maternal chromosomes line up randomly on the left and right.

• 223 = 8 388 608 different combinations!

Crossing Over• In the tetrad, pieces of

homologous chromatids can change places, creating different chromosomes.

• This is desirable and occurs frequently

• If one piece gets misplaced, a mutation occurs (genes are missing)

Errors during Meiosis• Errors usually occur during Anaphase I, due to

NONDISJUNCTION. • The homologous dyads in a tetrad do not separate. • The resulting gametes can have too many or too

few chromosomes.

Chromosomal AbnormalitiesDown Syndrome-Trisomy 21

-1 in 700 births

-An extra chromosome #21

-Abnormal facial features, development

-Probability increases with age of mother

Chromosomal AbnormalitiesKlinefelter Syndrome (XXY)

-1 in 800

-Extra ‘X’ from mother

-Sterile ‘male’, long arms

Chromosomal AbnormalitiesSuper male XYY

-Extra ‘Y’ from father

-Tends to produce violent males

Chromosomal AbnormalitiesTurner’s Syndrome XO

-1 in 10 000

-One missing sex chromosome

-Girl is usually short and sterile

Junk DNAThe Wheat from the Chaff • Less than 2% of the genome codes for proteins. • Repeated sequences that do not code for proteins

("junk DNA") make up at least 50% of the human genome.

• Repetitive sequences are thought to have no direct functions, but they shed light on chromosome structure and dynamics. Over time, these repeats reshape the genome by rearranging it, creating entirely new genes, and modifying and reshuffling existing genes.

How is DNA Arranged?• Genes appear to be concentrated in random areas

along the genome, with vast expanses of non-coding DNA between.

• Stretches of up to 30,000 C and G bases repeating over and over often occur adjacent to gene-rich areas, forming a barrier between the genes and the "junk DNA." These C-G islands are believed to help regulate gene activity.

• Chromosome 1 has the most genes (2968), and the Y chromosome has the fewest (231).

Variations and Mutations • Scientists have identified about 1.4 million locations

where single-base DNA differences (SNPs) occur in humans. This information promises to revolutionize the processes of finding chromosomal locations for disease-associated sequences and tracing human history.

• The ratio of sperm to egg cell mutations is 2:1 in males vs females. Researchers point to several reasons for the higher mutation rate in the male, including the greater number of cell divisions required for sperm formation than for eggs.

http://www.ornl.gov/sci/techresources/Human_Genome/home.shtml

Genetics…A Little History

Gregor Mendel• Austrian monk

• Grew and tested pea plants

• Started with two pure plants (tall plants which bore smooth yellow seeds)

• Saw that first generation (F1) was all the same, but F2 had some short plants with green, wrinkled seeds….what’s up?

Mendel (cont)• Came up with Law of Heredity (how traits are

passed on through generations)

• Mendel rap!

Predicting Probability• The Punnett Square is used to predict the

genotypes and phenotypes of possible offspring!

• Expressed as a ratio, % or a fraction

• This is not an outcome…just the likelihood of the outcome!

Monohybrid Cross• In a monohybrid cross we

observe 1 pair of alleles for 1 gene.

• Example: Colour of flower• Alleles:

B = purple

b = white

Monohybrid Cross

Results:

75% chance of purple flowers

25% chance of white flowers

Example 1• Brown eye colour (E) is dominant to blue

eyes (e). What would be the eye colour of offspring of a heterozygous mother and homozygous recessive father?

Example 1

Mom →

↓ Dad

B b

b

b

Example 1

Mom →

↓ Dad

B b

b

Bb

(Brown)

bb

(Blue)

b

Bb

(Brown)

bb

(Blue)

Example 2• Long tails (T) in rats is dominant to short-

tailed rats (t). What is the ratio of phenotypes between 2 heterozygous rat parents?

Example 2Momma rat →

↓ Rat Daddy

T t

T

t

Example 2Momma rat →

↓ Rat Daddy

T t

T TT

(Long)

Tt

(Long)

t Tt

(Long)

tt

(short)

Dihybrid Cross• We observe 2 pair of alleles for 2 different genes.• Note: The 2 genes are not linked…they must be on

2 separate chromosomes!• Example…Mendel’s Peas• Gene 1 (on chromosome A): Colour of seed

Alleles: Y = Yellow, y = Green• Gene 2 (on chromosome B): Shape of seed

Alleles: R = Round, r = wrinkled

Parental GenerationRRYY rryy

Plant with round, X Plant with

Yellow seeds wrinkled, green

seeds

Alleles:

RY ry

F1 Generation• All plants produce

round, yellow seeds RY

ry RrYy

100%

Cross the F1 Generation• RrYy X RrYy

• Gametes:

• RY, Ry, ry, rY RY, Ry, ry, rY

The Results!• 9 Yellow Round

• 3 Yellow Wrinkled

• 3 Green Round

• 1 Green Wrinkled

• Try These: Practice Problem• Pg. 166 a-c

The Test Cross• Also called a “back cross”

• Purpose: To determine the genotype of an individual showing the dominant phenotype!

• Question: Is the genotype Tt or TT?

• Answer: Cross the individual with a homozygous recessive individual

Outcomes of the Test Cross

• If the genotype of the parent in question is TT…

• Then 100% of the offspring from the test cross will show the dominant trait!

T T

t Tt Tt

t Tt Tt

Outcomes of the Test Cross

• If genotype of parent in question is Tt…

• Then 50% of offspring from test cross will be recessive for the trait!

• Appearance of only one recessive individual means parent must be heterozygous!

T t

t Tt tt

t Tt tt

Try These!• P.167 #2,8,9

• P. 233 #22

• P. 184 #2,3,4,11