GENETICS

Beyond Mendel

Gregor Mendel(1822-1884)

• Worked with pea plants in monastery garden– This was lucky, because peas have pretty simple genetics, as do

many genes in most organisms.– His observations of dominant & recessive patterns of heredity

allowed him to develop 2 important “laws”:

• LAW OF SEGREGATION– 2 alleles for a trait separate (segregate) when gametes

are formed [It’s random: Zz may pass on either Z or z]

• LAW OF INDEPENDENT ASSORTMENT– Alleles of different genes separate independently of

one another during gamete formation • [“alleles for gene Z are not linked to alleles for gene W”]

Basic Punnett Squares• Single trait cross (only 1 letter of alphabet) = 2x2• In basic Mendelian genetics, heterozygous shows

only the dominant trait.• Can predict probability of offspring genotypes

R R r r

R R

r r

Rr Rr

rr rr

RR RR

Rr Rr

Dihybrid Cross Punnett Square• Looks at 2 traits at once = 4X4 (16 boxes)

• Step 1: find parents’ genotypes (example: XXgg, XYGg)• Step 2: find the 4 different possible gamete combinations for each parent

(You will randomly get 2, FOIL looks at all possibilities)

Dihybrid Cross Punnett Square• Looks at 2 traits at once = 4X4 (16 boxes)

• Step 1: find parents’ genotypes (example: XXgg, XYGg)• Step 2: find the 4 different possible gamete combinations for each parent

(You will randomly get 2, FOIL looks at all possibilities)

for ex: (mom: Xg Xg Xg Xg) (dad: XG Xg YG, Yg)

Mom» Xg Xg Xg Xg

XG

Xg

Dad YG

Yg

Dihybrid Cross Punnett Square• Looks at 2 traits at once = 4X4 (16 boxes)

• Step 1: find parents genotypes (example: XXgg, XYGg)• Step 2: find the 4 different possible gamete combinations for each parent

(You will randomly get 2, FOIL looks at all possibilities)

for ex: (mom: Xg Xg Xg Xg) (dad: XG Xg YG, Yg)• Step 3: Fill in squares (should have 4 letters in each box)

» Xg Xg Xg Xg

XG

Xg

YG

Yg

XXGg XXGg XXGg XXGg

XXgg XXgg XXgg XXgg

XYGg XYGg XYGg XYGg

XYgg XYgg XYgg XYgg

How many heterozygous girls?

Life is messy.

There are exceptions to most rules.

Mendel’s genetics only work for simple traits.

Exceptions...Some traits are controlled by more than 1 gene.

=Polygenic Traits [poly = many](Ex: Height, pigmentation [eye color, hair color, skin color]…)

Some traits are neither dominant, nor recessive.=Co-dominance or incomplete dominance

Some traits are sex-linked. (on the X chromosome)

Some traits have more than 2 possible choices (= multiple alleles)

Some traits are NOT actually independent.

Some traits change with the environment!

Sex Linked Traits• Occur on the X chromosome (pair #23)

• Usually show up in males more than females, because males only have 1 X.– Females can be carriers, Males cannot.

– EXAMPLES: Albinism (sometimes), Hemophilia, Colorblindness, Duchenne Muscular Dystrophy, Menkes…

Pedigrees• A pedigree is a family history, a genetic family tree• Can show an inheritance pattern

– Very helpful for showing genetic disorders and sex-linked traits– Lines show mating and offspring

• Horizontal (--) lines show mates, vertical ( l ) lines show offspring

– Each row is a generation (Roman numerals may # generations)

Usually means female

Usually means male

or Usually indicates an affected individual May have different color for carriers (heterozygous)

Sex-linked Punnett Square• The trait is written as a superscript on the X because it is

attached to the X chromosome

Example: – If N=normal pigment/ A= albino (lack of pigment)

– Mom= carrier (XNXA) XN XA

– Dad= normal (XNY) XN

– How many albino kids?

YXNXN XNXA

XNY XAY

Colorblindness Pedigree

Colorblindness is sex-linked.

Is it possible for a female to be colorblind?

I.

II.

III.

QuickTime™ and a decompressor

are needed to see this picture.

= male

= female

Shaded= affected

Incomplete Dominance• Heterozygous Phenotype is a blend/in-between,

sort of a compromise, with no true dom. or rec.

• Example: pink snapdragons are heterozygous, and can produce red, pink, or white flowers when two pink flowers are crossed!

Note: 1 letter used:

• Another example:

Curly hair + straight hair

= wavy hair!

Multiple Alleles• Some traits are controlled by

3 or more alleles.• Example: Human Blood Type

– There are 3 possible alleles: A, B, or O

Polygenic Traits

Most traits are controlled by more than 1 gene, so you get a spectrum of possibleappearances between the parents’ traits

.Examples: Height, Weight, Skin & Hair color Also: Diabetes, schizophrenia, and many other conditions

Co-Dominance• Neither allele is dominant nor recessive• Both traits are shown at the same time

• Example: Human Blood Type– There are 3 possible alleles: A, B, or O, but there

are 4 possible bloodtypes: A, B, AB, or O– A & B are codominant, o is the only recessive

Genotype: Phenotype:

AA Type A blood

Ao Type A blood

BB Type B blood

Bo Type B blood

AB Type AB blood

oo Type O blood

Blood Type Punnett SquareI= Dominant bloodtype allele (A or B)

written IA or IB to show difference

i= recessive bloodtype allele (o)

What is your blood type?

Ask your parents if they know their blood types, and maybe you can see how you got yours.

Bloody Facts:• Blood Banks always need Type O because

it is a “universal donor”.– People with type A, B, AB, or O can be given type O

blood without the body rejecting it.– People with blood type O can ONLY accept type O,

and will violently reject any other blood type.– Giving a Type A person a Type B blood transfusion,

or vice versa, will probably kill them as it will be rejected and attacked by their immune system

• Type AB is a “universal recipient”– Their body will accept any blood type, since their

immune systems are used to both A and B forms, and the recessive O is non-threatening.

Environmental Influence• Some phenotypes depend on the environment.

• Example: Hydrangea flower color– Blue flowers appear on plants in acidic soil

– Pink flowers appear on plants in basic soil

– If you replant a pink plant in acid soil, the flowers will be blue next year!

– Other example:

Artic fox coat is reddish brown in summer, but turns white in winter as the pigment gene is temporarily “turned off” to camouflage itself as the landscape changes

Genetic Disorders• There are 4 ways to get a genetic disorder:

– Autosomal Dominant (from a dominant gene on a chromosome in pairs 1-22, not on X [#23])

Achondroplasia, Marfan Syndrome, Huntington’s ….

– Autosomal Recessive (not on X chromosome)Sickle Cell Anemia, Tay-Sachs, Cystic Fibrosis …

– Sex-linked (on X chromosome) RecessiveHemophilia, Colorblindness, Albinism…

– Mutation: An error in copying DNACan usually be seen in a karyotype; Down’s Syndrome, Cri du chat…Remember: Autosomes= any chromosome other than X or Y

Autosomal disorders occur equally among males and females.

Interesting Fact: In the U.S., many cases of Huntington’s

disease can be traced back to two brothers who left England in the 1600’s because they were accused of witchcraft.

The charges were based on strange behavior including constant dance-like movements, which are a symptom of Huntington’s disease.

You’ll learn about this and many other conditions in your upcoming

GENETIC DISORDER PROJECT!

Genetic Disorder Project:

Your Mission:Work with your partner, focus on assignment.

(up to 2 people can sign up for each topic)

Research your assigned disorder(use at least 3 good online sources)

Create & deliver a PowerPoint presentation. Write your own Reflective Report

This will be at least 10% of your 3rd Qtr grade!Do it well!