Transcript Genetics PPT - Ms. George`s Science Class

Genetics:
the Study of Heredity
Look at these three girls
What physical traits do
you think they
inherited from their
parents?
• ______________
• ______________
• ______________
• ______________
Heredity
• Living organisms receive traits from
their parents.
• Traits are “characteristics that can be
used to identify or describe an
organism.”
• This passing on of traits from parents
to their offspring is called “heredity.”
• Think of 3 physical traits you received
from your parents: ____________,
_____________, _____________.
How does this happen?
• Traits are passed from parents to their
offspring.
• If 2 Dalmation dogs have puppies, what
will the puppies look like?
_________________________
• Today we will look at how traits are
passed from one generation to the
next.
• What is a trait?
http://learn.genetics.utah.edu/content/inheri
tance/traits/
• Each person begins life as a tiny zygote…
a single cell created from a female sex
cell (egg) and a male sex cell (sperm).
• Each of these sex cells contains 23
chromosomes. Why?
• Human cells contain 46 chromosomes
(23 pairs—half from the mother and half
from the father)
• What is Heredity?
http://learn.genetics.utah.edu/content/i
nheritance/intro/
Karyotype
What are chromosomes made of?
DNA
Proteins and
deoxyribonucleic acid
DNA carries information in
a code.
DNA looks like a twisted
ladder.
This shape is called a
“double helix.”
Chromosomes carry genes
• Genes contain the
instructions that
determine what traits
you will have.
• Hair color, eye color,
and height are
examples of traits
determined by your
genes.
• Genes are found in
specific places on
chromosomes.
How are traits (genes) passed on?
• You received a gene for eye color from
your mother and a gene for eye color
from your father. What determines
which color gene will “show” in you?
• Back in the 1860’s, an Austrian monk
named Gregor Mendel did a lot of
research and discovered patterns of
inheritance.
Mendel is known as the Father of Genetics
Gregor Mendel did
years and years
of research
experimenting
with pea plants.
Why pea plants?
Why Pea Plants???
Peas are:
• Easy to grow
• Reproduce quickly
• Produce many offspring
• Easy to “cross” (breed)
through pollination
• Have several traits with 2
easy-to-see forms, such
as purple or white flowers,
yellow or green pods,
smooth or wrinkled seeds.
Mendel observed that when he crossed a
purebred short pea plant with another
purebred short pea plant…
The offspring were
always short.
When purebred
plants were bred,
their offspring
looked just like
themselves.
Parents
Offspring
Next, he crossed short with
• The first generation of
offspring were all tall
• BUT…
• When he crossed the
those plants together,
look what happened…
• A short one showed up
in the next offspring
generation!
tall
Parents
Offspring
• How could a trait (shortness) that
disappeared in the first generation
show up in the second generation?
• Well, Mendel repeated this experiment
many times, then came up with a
hypothesis…
Mendel’s hypothesis:
• Mendel stated that if there is a PAIR of
factors that determine a trait, then
• A tall offspring pea plant could have an
tall factor that shows but still have a
short factor that “hides.”
• The hidden shortness factor could get
passed on to the next generation.
Let’s look at an example
P
F1
F2
• The top 2 seeds are
the parents, or P
generation
• The next row is their
offspring, the F1
generation
• Look at F2
• Which trait was
hidden in F1?
________________
Mendel’s Conclusions
• Each parent must contribute one
factor to each offspring.
• These factors may be strong or
weak.
• When a strong and weak factor
are paired, only the strong one
shows.
• A weak factor will be expressed
ONLY if two weak factors are
paired.
Dominant and Recessive Traits
• The stronger trait is called the DOMINANT
trait.
• The weaker trait (the one that seems to
disappear) is called the recessive trait.
• A professor of genetics at Cambridge
University in England named Reginald
Punnett developed a simple method for
figuring out the probability that a trait will
show up in offspring.
REGINALD PUNNETT (1875-1967)
Punnett Square
A Punnett Square is divided into 4 boxes
We will take a look at a pea plant cross-breeding
experiment using a Punnett square
First, we will cross a purebred green pea plant
(PP genotype) with a purebred yellow pea plant
(pp genotype) PP x pp
Note: each letter is called an “allele”
P
P
p
Pp
Pp
p
Pp
Pp
Describe the offspring from the PP x pp cross:
• What is their phenotype (what do they
all LOOK like)? __________________
• What is their genotype (pair of alleles)?
___ ___
Next, we cross two Pp genotype
pea plants: Pp x Pp
P
p
P
PP
Pp
p
Pp
pp
Describe the offspring from the Pp x Pp cross:
• Do all the offspring look the same? In
other words, do they all have the same
phenotype? ______
• What different genotypes (pairs of
alleles) do the offspring have?
___ ___ , ___ ___ , ___ ___
Let’s say we want to cross two
guinea pigs with different traits for
fur color to see what their offspring
will look like…
Must-have information:
• First, we need to know the genotype
of each parent.
• Dominant alleles are symbolized by a
capital letter (T for tan hair)
• Recessive alleles are symbolized by a
lower-case letter (t for white hair)
• Each parent has 2 alleles that make up
their genotype.
For example,
• Tan hair is a dominant trait in guinea pigs.
• A mother guinea pig has tan hair and her
genotype is TT (homozygous)
• White hair is recessive.
• The father guinea pig is white, and white is
recessive, so his genotype is tt
(homozygous)
• Now we can set up our Punnett Square…
The cross is TT x tt
The mother’s alleles are written on the top
The father’s alleles are written along the side.
Mom
T
T
t
Tt
Tt
t
Tt
Tt
Dad
What will the Tt offspring look like? _________________
Describe the next generation
T
t
T
TT
Tt
t
Tt
tt