Download Genetics and Mendel

GENETICS
AND
MENDEL
DEFINITIONS

Heredity
-the passing of traits
from parents to
offspring
When something is said
to be hereditary, is it
also genetic?

Genes
-located on
chromosomes, genes
produce or influence a
specific trait in the
offspring
When something is
said to be genetic, is it
also hereditary?
GREGOR MENDEL
-key player in genetics
and evolution
 -used garden peas
because they could
cross fertilize, and
they had several
characteristics that
could be displayed in
only one of two ways

ONE TRAIT WAS ALWAYS
DOMINANT…
Mendel would cross
pollinate peas, and
found that one form of
a trait would always
win over another, no
matter how many
times he repeated the
experiment
 For example, round
peas always won over
wrinkled peas, and
produced offspring
that were round!

DOMINANT GENE VS. RECESSIVE GENE
Dominant gene
-determine the
expression of the genetic
trait in offspring, and
can be passed on to
offspring

Recessive gene
-overruled by the
dominant gene, but are
still present in the
DNA, and can be
passed on to offspring

3 LAWS OF HEREDITY
1. Inherited characteristics are controlled by
genes, which occur in pairs.
 2. One gene masks the effect of another. This is
the principle of dominance.
 3. A pair of genes separate during the formation
of sex cells. This is the law of segregation.

DOMINANT AND RECESSIVE GENES


The dominant genotype is always expressed as
RR. This means you got an “R” from your mom
and an “R” from your dad.
The recessive genotype is always expressed as rr.
This means you got an “r” from your mom and an
“r” from your dad.
Round peas were RR
Wrinkled peas were rr
HOMOZYGOUS GENOTYPES

If a trait is RR, we say its genotype is homozygous
dominant.
-homozygous-both genes of a pair are identical
-dominant-they are both dominant genes
If a trait is rr, we say its genotype is
homozygous recessive

-homozygous-both genes of a pair are identical
-recessive-they are both recessive genes
HETEROZYGOUS GENOTYPES

What if a person gets “R” from their mom, and “r”
from their dad?
-Their genotype is Rr.
-We say that their genotype is heterozygous.
-Heterozygous-a genotype in which the genes
of a pair are different
PHENOTYPE
After you examine a genotype, you can predict
what an individual will look like.
 What they look like is their PHENOTYPE!

A pea that is RR will be round.
A pea that is rr will be wrinkled
A pea that is Rr will be...?
ROUND!
In heterozygous
genotypes, the
phenotype expressed
will always be the
dominant gene.
ALLELES



When there are two or more alternate forms of a
gene, we call them alleles
There were two alleles for pea smoothness-round
or wrinkled.
How many alleles are their for eye color?
IT GETS COMPLICATED!!!
FROM WWW.SEWANEE.EDU


Right now there are three known gene pairs that control
eye color. The bey 2 gene on chromosome 15 contains a
brown and blue allele. Also on chromosome 15, the bey 1
gene is the central brown gene. On chromosome pair 19 the
gey gene contains a green allele and a blue allele.
A green allele is dominant over a blue allele, and a brown
allele is dominant over both green and blue alleles. For the
bey 2 gene if a person has a brown allele then they will
have brown eyes. In the gey gene the green allele is
dominant over the blue allele, but it is still recessive next
to a brown allele. For example if a person has a brown
allele on chromosome 15, but all the other alleles are blue
or green, they will have brown eyes. A green eyed person
would have a green allele on chromosome 19 and all or
some other blue alleles. Blue eyes are produced only with
two blue eye genes. All four alleles must be blue to produce
a blue eyed person.
PUNNETT SQUARE

We use punnett
squares to predict
one’s phenotype and
genotype. You must
know the parent’s
genotype in order to
make accurate
predictions.
GENOTYPE AND PHENOTYPE

In the previous slide, the genotype is
RR-25%
 Rr-50%
 Rr-25%


In the previous slide, the phenotype is
Round-75%
 Wrinkled-25%

You can predict how many percent of the offspring will
be wrinkled and how many will be round!
SHEEP EXAMPLE





In sheep, A is
dominant and is white
wool.
a is recessive and is
black wool
What is the phenotype
of the parents?
What percent of the
offspring are white?
What percent of the
offspring are black?
GET RID OF THE BLACK SHEEP IN THE
FAMILY!

A farmer has an award winning, white,
homozygous dominant male sheep that he wants
to breed. He buys a bunch of white female sheep,
but is unsure if they are homozygous dominant or
heterozygous. He wants to breed his sheep, but
wants to make sure he’s not going to get any
black sheep. Black sheep produce wool that he
can’t sell, and he already has 3 living on his farm.
He decides to perform a test cross so that he can
find a homozygous female to breed with his
award winning male. What should he do?
ANSWER



He should take his black male who is “aa” and
cross it within an unknown female.
If all of the offspring are white, that means the
unknown female had to be “AA”.
If half of the offspring are white, and half of the
offspring are black, that means the unknown
female had to be “Aa”.
MULTIPLE ALLELES


If a gene has multiple forms of an allele, you
must use a number subscript to describe the form
you are using in your punnett square.
Fruit flies have 4 different alleles for eye color
Phenotype
Genotype
Wild type
E1E1,
E1E2,
Apricot
E2E2,
E2E3,
Honey
E3E3,
E3E4
White
E4E4
Dominant to
E1E3,
E2E4
E1E4
Apricot, honey, white
Honey, white
White
Nothing
COMPLETE A PUNNETT SQUARE FOR
MULTIPLE ALLELES

Perform a punnett square for two flies
The female fly is E1E3
 The male fly is E2E4


What are the phenotypes of the offspring?
INTERMEDIATE INHERITANCE
When two genes are equally dominant, they
interact to produce a new phenotype.
 This is called INTERMEDIATE INHERITANCE
 An example is snap dragons…red + white=pink

IF YOU CROSS RED SNAP DRAGONS
(RR) WITH WHITE SNAP DRAGONS
(WW), YOU GET PINK SNAP DRAGONS
100% OF THE TIME. WHAT PERCENT
OF RED, WHITE, AND PINK
SNAPDRAGONS WOULD YOU GET IF
YOU CROSSED TWO PINK
SNAPDRAGONS?
25% RED
50% PINK
25% WHITE
CODOMINANCE
Both genes are expressed at the same time!
 Example-a red bull crossed with a white cow,
produces roan offspring. It has intermingled
white and red hair.
 When two roan cows reproduce, you should be
able to predict the probability of the phenotypes
of the offspring…

RED (R)
WHITE (W)
RR
RW
RW
WW
RED (R)
WHITE (W)
CAN YOU
THINK OF AN
EXAMPLE OF
CODOMINANCE
IN HUMANS?
BLOOD TYPE
Parent Alleles
A
B
O
A
AA
(A)
AB
(AB)
AO
(A)
B
AB
(AB)
BB
(B)
BO
(B)
O
AO
(A)
BO
(B)
OO
(O)
DIHYBRID CROSSES


When you want to predict the phenotype of 2
different traits, you can perform a dyhybrid cross.
For example:
-RR-round peas
-Rr-round peas
-rr-wrinkled peas
-YY-yellow pea
-Yy-yellow pea
-yy-green pea