Download How are genes inherited?

Patterns of Inheritance
Sec 3.3 p. 50-54
Learning Outcomes
• I can explain how heritable
characteristics are transmitted from
parent to offspring
• I can identify and explain the role
that dominant and recessive genes
play in the inheritance of
characteristics
Hereditary
• Remember when we talked
about Hereditary Traits
• Hereditary: is the passing of
characteristics from parents
to offspring
- Discrete Traits: You have or you don’t (i.e. blue
eyes, brown hair, skin complexion, etc..)
– Continuous Traits: Traits that take a range of
forms (Don’t have an exact criteria i.e. height,
there is no actual height to where someone is
considered tall or short, it goes in a range)
Why do we only display certain types
of traits from our parents?
• -On our DNA molecules are
segments of certain combinations
of bases that make up what we
call genes.
• The DNA molecules are wound
tightly into chromosomes that
come in pairs
• Therefore our genes come in pairs
– Creating Alleles: which is a possible
form of a gene
Why Certain Genes?
• This question has puzzled certain scientists
long before genes were ever discovered
• “The Father of Genetics” Gregory Mendel, a
Austrian Monk
– In 1866 while studying pea plants discovered
Three important principles
• The inheritance of each trait is a result of a “unit”
(genes) that are passed on the decedents unchanged
• That an individual inherits one such unit from each
parent for each trait
• That trait may not show up in an individual but can
still be passed on to the next generation
Mendel’s Experiment
Dominant vs Recessive
• Mendel called tallness a dominant trait
because it seemed to dominate the short trait.
(TT)
• He called shortness a recessive trait because it
seemed to disappear. (tt)
• Offspring of tall vs short crossing were called
hybrids because they carry a gene for tallness
and a gene for shortness. (Tt)
What happened when two hybrids
were crossed?
Shortness appeared again.
Recording Inherited Traits
• Geneticists use letters to represent the genotype
or combination of genes.
• Capital letters represent dominant genes. The
letter T represents the tall gene.
• Lower case letter represent recessive genes. The
letter t represents the recessive gene for short
plants.
TT, Tt, tt
• (TT) is a tall plant because both (T) genes are
dominant.
• (tt) is a short plant because there are no (T)
genes.
• (Tt) is tall because (T) is dominant over (t).
Punnett Squares
• Punnett Squares are an easy way to explain
genetic experiments.
Tt
Tt
tt
tt
Punnett Squares
Give it a try: See what type of offspring occur when you mate two Hybrids
together (Tt) vs. Two Purebreds (TT & tt)
How are genes inherited?
The parents
Homer has straight hair
Marge has curly hair
The families
Is hair curliness controlled by genes or by the environment?
Boy or Girl?
“Allele”
X
Y
Girl
28/09/2015
XX
X
XY
Boy
X
Homer
• Homer has straight hair
• Every cell in Homer has two sets of genes
• Homer’s has two straight hair genes
• The straight hair gene is dominant
• We can show the straight hair gene with
a capital S
• Homer’s genes: SS
Marge
• Marge has curly hair
• Every cell in Marge has two sets of genes
• Marge has two curly hair genes
• The curly hair gene is recessive
• We can show the curly hair gene with a
small s
• Marge’s Allele’s: ss
Making a new Simpson
• Homer makes sperm gamete cells
• Each sperm has one gene for hair type
• Homer only has straight hair genes
• So all of his Sperm have straight hair
genes
Homer’s Allele’s
S
S
Making a new Simpson
• Marge makes eggs (ova gametes)
• Each egg has one gene for hair type
• Marge only has curly hair genes
• So all of her eggs have curly hair genes
Marge’s Allele’s
s
s
Making a new Simpson
s
S
S
Ss
Ss
s
Ss
Ss
Making a new Simpson
Ss
Ss
Ss
Ss
• All of the Simpson babies have one gene for curly hair and one
gene for straight hair
• So what type of hair do they have?
Making a new Simpson
Ss
THEY ALL HAVE STRAIGHT HAIR!
• The straight hair gene is DOMINANT
• Just one copy of this gene will give you that characteristic
• The curly hair gene is RECESSIVE
• You need two copies of the gene to have that characteristic
The next generation?
• Lisa has straight hair
• Every cell in Lisa has two sets of genes
• Lisa has one gene for straight hair, which
is dominant.
• She also has one gene for curly hair,
which is recessive.
• Lisa’s genes: Ss
The next generation?
• Milhouse has straight hair
• Every cell in Milhouse has two sets of
genes
• Milhouse has one gene for straight hair,
which is dominant.
• He also has one gene for curly hair, which
is recessive.
• Milhouse’s genes: Ss
The next generation?
• Milhouse’s genes: Ss
S
s
• Lisa’s genes: Ss
S
s
The next generation?
S
S
Ss
s
Ss
s
Ss
ss
The next generation
SS
Ss
Ss
ss
• Straight hair is dominant
• What is the % they will have a child with straight Hair?
• What is the % the child will have curly hair?
¼ = 25%
¾ = 75%
The next generation
Lisa’s
characteristic
Lisa’s genes
Milhouse’s
characteristic
Milhouse’s genes
Likes School
Ss
Like School
Ss
Good eyesight
EE
Bad eyesight
ee
Brave
Bb
Not brave
Bb
Yellow Hair
hh
Blue Hair
Hh
Musically Talented
Mm
Not Musically Talented
mm
Leadership Qualities
LL
Not a leader
Ll
Female
XX
Male
XY
Pedigrees
• A pedigree is a diagram that shows the
history of a trait from generation to
generation.
Purebreds
• Purebred: Mating an organism whose
ancestors have only produced the trait
(characteristic) that is wanted.
• Using the example of cat fur: black and
white.
Allele’s BB
• A breeder that wants cats with only white
fur should select purebred parents that have
produced white fur in their families for
generations. This is termed “true-breeding”.
• Preferably BOTH parents will come from
purebred families.
Allele’s
bb
HYBRID
• The offspring of two purebred parents for
different traits.
• Imagine a black purebred male cat reproduced
with a white purebred female cat. What would
you expect the offspring to look like?
INCOMPLETE DOMINANCE
• The dominant-recessive pattern doesn’t always
happen.
• Sometimes BOTH alleles are expressed by the
organism.
• For example, if you cross a purebred white
snapdragon flower with a purebred red
snapdragon, the offspring will be pink.
What if this
appeared with
the cat example?
When the offspring is unlike either
parent…
• Sometimes blue eyed parents produce a brown
eyed child.
• Sometimes brown haired parents create a redheaded child.
• This is because one gene is not controlling these
traits but MANY genes are controlling these
traits. Therefore, MANY traits are possible.
• Inheritance in Humans is too complex to solely
be explained by Dominant vs. Recessive gene
patterns, or incomplete dominance
ENVIRONMENTAL FACTORS
• We have already learned that environment
can affect variation.
• For example, nutrition can influence how tall
you grow to be.
• Similarly, environment can affect how genes
are expressed.
• For example, the presence of alcohol while a
embryo is developing can affect the
development of the brain and facial features
although the embryo's DNA is normal.