Transcript Document

Genetics and Heredity
Genetics
• Genetics is the study of heredity
• It looks at understanding the
biological properties that are
transmitted from parent to
offspring
DNA
• DNA (deoxyribonucleic acid) is the
genetic code for almost every living
organism
• DNA is often called a double helix
because of the way it coils
– Some ‘organisms’ like mitochondria use
RNA (ribonucleic acid) instead of DNA
• There are 4 parts to DNA called
nucleotides:
 A (adenine)
 T (thymine)
 C (cytosine)
 G (guanine)
• A long sequence of nucleotides makes
up a gene:
– A gene is a portion of the DNA responsible
for certain traits or characteristics
– Ex) body part structure, hormones,
proteins, eye color, hair color etc . . .
• Each cell in our body has exactly the
same DNA, but only certain genes are
‘turned on’ at a time
– Ex) the genes that determine hair color
are only turned on in our hair follicles, and
the genes that determine our height are
only ‘turned on’ in our bone and muscle
cells
• The DNA in our cells is very long and
would stretch over a meter if uncoiled
• Since it is so long, it is divided into
chromosomes
Chromosomes
• Chromosomes are small, rod shaped
bodies in the nucleus of a cell that
contain the DNA
• Humans have 23 pairs of chromosomes
– There are 46 chromosomes in total – you
get 23 from your mother, and 23 from your
father
DNA Coiling
Dominant and Recessive Traits
• Most of the time, our traits are
controlled by more than one gene;
sometimes though they are controlled
by a single gene
• When a single gene controls a trait it
can be dominant or recessive
• Dominant: ‘stronger’ trait
• Recessive: ‘weaker’ trait
• Ex) If you get a dominant trait from either
of your parents it will override or mask a
recessive trait
– Brown eyed genes are dominant over blue
eyed genes
• Even though a recessive trait is not
physically shown, you still carry the
trait, and can pass it on to your
offspring
Genotype
• A genotype is the combination of
genes inherited from your parents
– B = brown eyes (dominant genes are
always written with capital letters)
– b = blue eyes (recessive genes are always
written with small letters)
• A person with brown eyes could have
either BB or Bb as their genotype
• A person with blue eyes could only
have bb as their genotype
What do you think would happen if one parent
had BB brown eyes? What color eyes would their
offspring have?
• Homozygous: if both alleles (individual
genes) are the same
– Ex) BB or bb
• Heterozygous: if the alleles are
different
– Ex) Bb
Phenotype
• The phenotype is the visible expression
of genes:
– Ex) The person has brown eyes, or the
person has blue eyes
• The color is the phenotype in this
example
Some traits controlled by a single gene
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Tongue Rolling
Widow's Peak
L/R interlocking finger
Attached earlobes
Hitchhiker Thumb (r)
Chin fissure
Darwin tubercle
Polydactyl
Dimples
Long eyelashes
Short big toe
Punnett’s Square
• A Punnett’s Square is a table used to
figure out the probability of offspring
having a certain trait
• They allow us to easily cross the genes
of the mother and father to determine
the possible genotypes and
phenotypes
• A Punnett’s Square
takes the alleles
from each parent
and examines the
outcomes
• The father’s genes
usually go on the
top and the
mother’s genes go
on the side
Phenotypes:
• 75% will be brown eyed
• 25% will be blue eyed
Genotypes:
• 25 % will be homozygous dominant (BB)
• 50 % will be heterozygous dominant (Bb)
• 25 % will be homozygous recessive (bb)
• Example 1: Curly hair (C) is dominant
over straight hair (c). If a homozygous
recessive man (cc) and a
heterozygous dominant (Cc) woman
have a child, what will the genotypes
and phenotypes be? Use a Punnett’s
Square to find the answer.
Step 1: Fill in the alleles for the
mother and father
c
C
c
c
Step 2: Cross the alleles to fill in
the table (the dominant gene is
always written first)
C
c
c
Cc
cc
c
Cc
cc
Step 3: from the table, figure out
the genotypes and phenotypes
C
c
c
Cc
cc
Genotypes:
• 50% heterozygous dominant
• 50% homozygous recessive
c
Cc
cc
Phenotypes:
• 50% curly hair
• 50% straight hair
Try This . . .
• Example 2: Draw the Punnett’s square
for a heterozygous red corsage (Rr)
matched with a heterozygous red
corsage (Rr). Give the genotypes and
phenotypes.
R
r
R
RR
Rr
r
Rr
rr
Genotypes:
• 25% homozygous dominant
• 50% heterozygous dominant
• 25% homozygous recessive
Phenotypes:
• 75% red corsages
• 25% white corsages
• Example 3: Your father is homozygous
dominant for tongue rolling, while your
mother is heterozygous dominant for
tongue rolling. Using a Punnett’s
Square to determine the genotypes
and phenotypes.
T
t
T
TT
Tt
T
TT
Tt
Genotypes:
• 50% homozygous dominant
• 50% heterozygous dominant
Phenotype:
• 100% tongue rolling
Incomplete Dominance
• Some traits are controlled by more
than one gene
• When there isn’t a dominant trait it
is called incomplete dominance
• The most common examples are
with plants:
– Red flowers and white flowers are
mixed, but the result is a pink flower, or
a red and white striped flower; neither
color dominates
Sex-linked Inheritance
• X and Y chromosomes determine sex
(gender)
• Females: XX
Males: XY
• The X and Y chromosomes also carry
the genes for certain characteristics
including hormone production and
development of sex organs
• The X chromosome carries more traits
than the Y as it is larger
• Sex linked traits tend to impact men
more than woman because the men
only get one X chromosome
Sex linked Examples
• Colour blindness is carried on the X. A
person needs to have all of their X
chromosomes affected to have the
disorder
– This means that in order for a female to
have the trait, both her X chromosomes
need to have the gene
• Hemophelia is
an X-linked
recessive trait.
Your blood
does not clot
properly.
Xh
Y
XH
XHXh
Carrier
XHY
Affected
Xh
XhXh
XhY
Unaffected
Unaffected
Karyotyping
• Karyotyping is a process where we
photograph chromosomes of a cell
• Chromosomes are studied and paired
up and analysis is done to identify
chromosomal abnormalities
• This is usually done before a baby is
born
Diseases and Defects
• There are at least 4000 hereditary
diseases
• Examples include muscular dystrophy
(cannot control muscles), Huntington’s
chorea (attacks nerve cells in the
brain), and cystic fibrosis (affects the
pancreas and the bronchioles in the
lungs)
Try this . . .
• Example 4: If a homozygous dominant
woman and a colour blind man have
kids, what percentage will be colour
blind?