Transcript Document

The Case of The Pink Nose
A young Mr. and Mrs. Potato Head are ready to
have their first little spud, a baby tater tot. As
excited as they are to have a new bundle of
joy to care for, they are extremely worried
about one thing. What will tater tot’s nose
look like? Mr. PH’s nose is most appealing.
Unfortunately, Mrs. PH’s pink nose, in their
opinion is rather unflattering. It would be their
little spud’s best interest to have the nose of
Mr. PH. But, what are the chances? That is
why they have come to you. They want you
to give them as much information as possible
so they can make the right decision.
Back to the basics…..

Genetics – the study of inherited traits

Traits are passed down from one
generation to the next when genes are
passed from parent to offspring.

Traits are passed on structures called
chromosomes
 Consists of 40% DNA and 60% protein

Sections of DNA code for genes.
 Each chromosome carries hundreds of
genes

Humans have 46 chromosomes
 22 pairs of autosomes
 1 pair of sex chromosomes
○ Female has two X chromosomes
○ Male has one X and one Y chromosome
Karyotype
23rd pair of chromosomes, the sex
chromosomes, identify an individual as
male or female

A karyotype is used to
show similar
chromosomes paired up
next to each other based
on size and shape.

The similar pairs of
chromosomes are called
homologous
chromosomes, or
homologs.
Homologous chromosomes carry the same
genes, but may contain different versions of the
same gene.
 Different versions of the same gene are known
as ALLELES.

Meiosis
Organisms pass their genes from parent to
offspring via the cells that are produced from a
type of cell division called meiosis.
In humans, meiosis occurs in the ovaries in
females and the testis in male, producing eggs
and sperm.
Cells produced in meiosis are called gametes and
contain ½ the number of chromosomes and
genes.
When a gamete from a female fuses with a
gamete from a male, genetic information from
each partner is passed (and present) in the
offspring.
Dominant and Recessive Alleles

Some genes that are passed from the
parents are dominant, or expressed,
when one allele is present.

Some alleles are recessive and will only
be expressed of the same allele is
passed down from both parents
Back to Mr and Mrs PH

What do we know?
 In the world of potato heads
○ Dominant = red nose (R)
○ Recessive = pink nose (r)
 Mrs. PH has a pink nose
○ Received the recessive trait from both parents
 Genotype = rr
 Phenotype = pink nose
○ She is homozygous recessive for nose color
“homo” – same
“zygous” – join together
What do we know about Mr PH?


Phenotype – red nose
So what did he receive from his parents?
Possibility #1
 Dominant trait from both parents
 Genotype = (RR)

He may be homozygous dominant
“homo” – same
“zygous” – join together
OR…
Possibility #2

Mr PH received the dominant form of the
trait from one parent and the recessive trait
from the other parent.
 Phenotype – red nose
 Genotype – Rr

Mr PH may be heterozygous for nose color.
“hetero” – different
“zygous “- join together
What does this mean?
Mrs PH will pass down (r) and there is a
chance, if Mr PH is heterozygous for nose
color, he could pass down the recessive form
of the trait (r) or the dominant form of the trait
(R).
Or
 Mrs PH will pass down (r) and, if Mr PH is
homozygous for nose color, he could pass
down just the (R) form of the trait.

As long as the dominant trait is passed down by one parent,
the dominant allele will be the phenotype
Warm up # ______
What is Mrs PH’s phenotype and
gentoype?
 How do you know what her genotype
and phentype is?
 Answer the same two questions for Mr
PH.
Be ready to share your answer!

Punnett Square

Used to determine the probability of
tater tot having a pink nose
Step 1
 What do we know?
○ Red nose = dominant
○ Pink nose = recessive
Step 2

Substitute the trait, nose color, with a
letter. We can use any letter, but
oftentimes, we use the first letter of the
dominant trait, which would be R (for
red)
Red nose – dominant – R
Pink nose – recessive - r
Step 3

What are we crossing?
 We are crossing the genotypes of Mr and
Mrs PH.
rr X RR
Mrs PH
Mr PH as homozygous
Possibility #1
Mr. PH was happy to
inform us his parents
were both homozygous
dominant (RR) for nose
color!
Step 4

Construct a punnett square with Mr and
Mrs PH’s genotypes
r
R
R
r
What’s in the box? Combine the genotypes
(a)
r
(c)
R
Rr
(d)
R
Rr (a x d)
(a x c)
(b)
r
Rr (b x c)
Rr (d x b)
Recommend to put
the dominant allele 1st
Step 5
This box represents all
of the possible allele
combinations Tater Tot
may receive from Mr
and Mrs PH.
Rr
Rr
Rr
Rr
Do you see all boxes are the same?
There is a 4 out of 4 (4:4, 4/4, 100% )probability
that:
Genotype = Rr heterozygous for nose color
Phenotype = Red nose