Transcript Unit 2: Cytology - Union Academy Charter School

UNIT 7: MENDELIAN GENETICS
Union Academy Charter School
Traits

Your characteristics:
 Can
be PHYSICAL – how your
body LOOKS
 eye
color & hair color
 Can
be PHYSIOLOGICAL – how
your body WORKS
 cystic
fibrosis, sickle cell anemia,
diabetes
Alleles
G

Different forms of a gene
g
R
Different DNA sequences
 Alleles for eye color
 Blue, brown, green, hazel, etc.



S
s
t
T
Think of the word OPTIONS!!
Alleles are represented by letters (ex. B,b).
alleles of a
gene at a
gene locus
Alleles


For each trait, you get one allele (letter) from Mom and
one allele from Dad.
These alleles can be represented by capital or lowercase
letters.
 Capital
letters are dominant – winner!
 Lowercase letters are recessive – loser!
Dominant
The allele that SHOWS
 Written with a CAPITAL
letter

 In
pea plants, tall is
DOMINANT over short
T
= Tall
 t = Short
Recessive
The allele that is HIDDEN
unless you get it from
BOTH parents (bb).
 Written with a lowercase
letter

 Blue
eye allele = b
Homozygous vs. Heterozygous

Homo means SAME
 When
both alleles are the SAME
 Homozygous
dominant - TT
 Homozygous recessive – tt
 Aka “pure bred”

Hetero means DIFFERENT
 When
both alleles are DIFFERENT
 Heterozygous
 Aka
“hybrid”
– Tt
Out of the three,
which two look
the same and
why?
Phenotype

PHenotype – PHysical features

Use descriptive words ONLY!!
 Tall,
short, yellow, green, etc.
TT = tall
 tt = short
 Tt = tall

Genotype
G
g
R

Genetic make-up for a trait

‘Type-o-Genes’

To figure out genotype, ask 2 questions:
 Homozygous
or Heterozygous?
 Dominant or Recessive?
 TT
– Homozygous Dominant
 Tt – Heterozygous
 tt – Homozygous Recessive
S
s
t
T
alleles of a
gene at a
gene locus
Genotype determines phenotype
Who discovered all of this?

Gregor Mendel is the “Father of Genetics.”
 Austrian
monk – grew pea plants in the garden.
 Studied traits like height, color, seed shape etc.
 Used mathematical probability to calculate results.
Mendel’s laws

Proposed 3 principles/laws:

Principle of dominance – some traits are dominant over others.

Law of segregation – each parent donates half of
the genotype, chromosomes segregate independently.

Law of independent assortment – alleles segregate independently
(crossing over).

If genes are very close together on chromosomes they are considered
linked.
Punnett Square
A tool used to PREDICT offspring outcome
 Shows the cross between two parents


If a heterozygous tall plant is crossed with a short plant (Tall is dominant),
what kind of offspring would be possible?
Tt
x
t
t
tt
T
t
Tt
Tt
Genotype
Phenotype
tt
2/4 Heterozygous
2/4 Tall
tt
2/4 Hom Recessive
2/4 Short
Punnett Square
T
T
T
TT
TT
t
Tt
Tt
The letters on the OUTSIDE of
the Punnett Square represent
the SPERM and EGG of the
parents
P generation
The letters on the INSIDE of
the Punnett Square represent
the OFFSPRING (Babies)
F1 generation
Practice Problem

In rabbits Brown fur is dominant to White fur. What would the offspring
be when a Homozygous brown rabbit mates with a white rabbit?
B = brown
BB
x
B
b = white
bb
B
Genotype
b
Bb
Bb
b
Bb
Bb
4/4 Heterozygous
Phenotype
4/4 Brown
Practice Problem

In Pea plants, tall plants are dominant over short
plants. What would the offspring be if two hybrid
pea plants pollinated?
Practice Problem

In rose bushes, straight thorns are dominant over
curved thorns. If a homozygous dominant
straight-thorned rose bush pollinates a
homozygous recessive curved-thorned rose bush
what kind of thorns would their offspring have?
Practice Problem

In rabbits, brown fur is dominant to white fur. If
two brown rabbits mate and have offspring with
white hair, what do we know about both parents?
Show the Punnett square to prove your answer.
Practice Problem

In beagles, white snouts are recessive to brown
snouts. What would the offspring look like if a
white snout beagle mates with a hybrid brown
snout beagle?
Non-Mendelian genetics

Mendel’s knowledge only went so far – there were
exceptions that he did not study.
 Incomplete
dominance – mixing of traits in a heterozygote
(red + white = pink).
 Codominance
– both traits are expressed
(black + white = black and white spots).
 Multiple
alleles – some traits have more than two options
(blood type).
Incomplete dominance - mix

Heterozygotes are a mix of two traits.
Ex. Wavy hair is a mix of straight and curly.
 Ex. Brachydactyly (short digits) – mix of digit or no digit.
 Ex. Pink flowers are a mix of red and white.
 Ex. Grey rabbits are a mix of black and white rabbits.


Not represented as capital and lowercase!
 Ex.
R = red and W = white
Practice a Punnett square!
RR X WW
RR
RW
WW
RW X RW
Codominance – both expressed

Both traits are expressed in a heterozygote.
 Ex.
Red flower + white flower = flower with red and white
pattern.
 Ex. Black rabbit + white rabbit = white & black spots

Not represented as capital and lowercase!
Codominance – the roan cow

A roan cow is a classic codominance example
 Roan

= red and white spots
What color will the offspring be if you cross two
roan cows? (R = red, W = white)
Multiple alleles – more than 2 options


Some traits have more than 2 alleles
One example is blood type.
The A and B represent
antigens
(carb/proteins that
decorate blood cells).
B A
A
B
Blood type
A
B
AB
O
Genotype(s)
AA (IAIA)
AO (IAi)
BB (IBIB)
BO (IBi)
AB (IAIB)
OO (ii)
Can donate to:
A, AB
B, AB
AB
A, B, AB, O
Can receive from
A, O
B, O
A, B, AB, O
O
Blood type practice problem
Cross a heterozygous Type A female with a Type O male.
What are all the possible blood types of the offspring?
Parent 1 – IAi , Parent 2 – ii
IA
i
i
IAi
ii
i
IAi
ii
Polygenic inheritance

Polygenic inheritance: one trait can be affected by
multiple genes.
 Results
 Skin
in a bell curve of results.
color, height, weight etc.