Introduction to Genetics

Life Science

Traits survey

Why causes these differences?

Genetics - The study of heredity (how traits are passed on)

x =or

or

The study of heredity started with the work of Gregor Mendel and his

pea plant garden

Mendel was an Austrian Monk that lived in the mid 1800’s

Mendel noted that the size of pea plants varied. He cross-bred these pea plants to find some surprising results.

Mendel’s cross between tall pea plants yielded all tall pea plants.

X =

His cross between small pea plants yielded all small pea plants.

X =

Mendels’ cross between tall pea plants and small pea plants yielded all tall pea plants.

x =

Mendel then crossed these second generation tall pea plants and ended up with 1 out 4 being small.

x =

Mendel’s work led him to the understanding that traits such as plant height are carried in pairs of information not by single sets of information.

Hereditary information (DNA) is carried in Chromosomes!

DNA

DNA is found in all living cells– It controls all functions

inside a cell– DNA stores all the genetic

information for a living organism

– Single cell like an amoeba– Multi cell like a human

GeneticsSmall sections of DNA are responsible for a

“trait”. These small sections are called “Genes”.– Gene - A segment of DNA that codes for

a specific trait– Trait - A characteristic an organism

can pass on to it’s offspring through DNA

Gene

Genes

There are two main kinds of genes:– Dominant - A gene that is ALWAYS

expressed and hides others– Recessive - A gene that is only expressed

when a dominant gene isn’t present (hidden)

Dominant and Recessive Genes• A dominant gene will hide a

recessive gene!Example:• A “widows peak” is dominant, not

having a widows peak is recessive.• If one parent contributes a

gene for a widows peak, and the other parent doesn’t, the off- spring will have a widows peak.

Widows Peak

Alleles

All organisms have two copies of each gene (one from female, one from male)

Homozygous- Two copies of the same geneHeterozygous - Two different genes

Example:A Widows Peak, dominant trait, would be

symbolized with a capital “W”, while no widows peak, recessive trait, would be symbolized with a

lower case “w”.

Father - No Widows Peak - w

Mother - Has a Widows Peak - W

Example:For the widows peak:

WW - has a widows peak Homozygous dominantWw - has a widows peak Heterozygousww - no widows peak Homozygous recessive

Genotype vs Phenotype

• Genotype- Genes you haveEx: WW, Ww, or ww• Phenotype- physical features SHOWNEx: WW Widows peakWw Widows peakww NO widows peak

Example:Since Herman has no widows peak, he must

be ______, since Lilly has a widows peak she could be EITHER ______ or ______.

ww WW or Ww

Punnett SquarePunnett Square - A tool we use for predicting

the traits of an offspring– Letters used as symbols to represent genes– Capital letters= dominant genes– Lower case letters= recessive genes– Genes always exist in pairs

Punnett SquaresWe can use a “Punnet Square” to determine

what pairs of genes Lilly has

Ww ww

Ww www

w

W w

Assume Lilly is heterozygous

Assume Herman is homozygous recessive ww

• A Punnet Square begins with a box 2 x 2

• One gene is called an “allele”

• One parents pair is split into alleles on top, the other along the side

• Each allele is crossed with the other allele to predict the traits of the offspring

Ww

Punnett SquaresNotice that when Lilly is crossed with Herman,

we would predict that half the offspring would be “Ww”, the other half would be “ww”

Half “Ww”, Heterozygous, and will have a widows peak

Half “ww”, Homozygous, and will not have a widows peak

Ww ww

Ww www

w

W w

Try it yourself!Create a punnett square assuming Lilly is

homozygous dominant (WW)

w

w

W WWhat is the chance of the offspring having a widows peak?

Try it yourself!All of the offspring will have a widows peak!

Ww

Www

w

W W

Ww

Ww

100%

GeneticsRecall that Herman and Lilly had another

offspring, Marylin. She had NO widows peak.

Genetics

So which is true? Is Lilly homozygous dominant (WW) or is she heterozygous (Ww)?

Ww

Www

w

W W

Ww

Ww

Ww ww

Ww www

w

W w

Genetics

Ww

Www

w

W W

Ww

Ww

Ww ww

Ww www

w

W w

If Lilly were heterozygous, then 1/2 of their offspring should have a widows peak, 1/2 shouldn’t

If Lilly were homozygous, all of their children will have a widows peak