Transcript Genetics

Genetics
Chapter 12
Gregor Mendel
• Gregor Mendel was a monastery priest who
carried out the first important studies of
heredity
– Heredity – the passing on of characteristics from
parents to offspring
– Genetics is the branch of biology that studies the
patterns of inheritance and variations in organisms
• Mendel was the first to succeed in
predicting how traits would transfer
from one generation to the next.
Mendel’s Experiment
• Mendel chose to study pea plants
– They reproduce sexually – meaning they
have two distinct sex cells (called gametes)
• The female gamete is the ovule (located on the pistil)
• The male gamete is in the pollen grain
– The structure of the pea plants allowed Mendel to control
their fertilization
• The transfer of male pollen grains to the pistil is called pollination
• Fertilization is when the male and female gametes unite
• Mendel studied one trait at a time and analyzed the
results mathematically
Monohybrid Crosses
• Mendel crossed short pea
plants with tall pea plants
• Mendel called this cross a
hybrid – a cross between two
parents that show different
forms of a trait
• Mendel’s first experiments were
monohybrid since the parents
differed only in one trait.
Monohybrid Crosses
• The first generation (P1) are the
initial cross organisms
– In this case, a short pea plant and a
tall pea plant
• The second generation (F1)
includes the offspring from the
initial cross
– For Mendel, this resulted in all tall
plants
• The third generation (F2) is the
result of crossing the offspring
from the F1 generation
– For Mendel, this resulted in 75%
tall plants and 25% short
Mendel’s Conclusions
• Mendel concluded that each
organism had two factors that
controlled each trait.
• One factor is passed down
from each parent
• The factors are located on the
chromosomes
• The different forms of the
genes are called alleles
Rule of Dominance
• Even though the F1 generation plants
had a tall allele from one parent and a
short allele from the other, they all
appeared tall
• Mendel concluded that one trait was
dominant over the other trait which he
called recessive
• Plants with two alleles for tallness (TT)
were tall.
• Plants with two alleles for shortness
were short (tt)
• Plants with one allele for tallness and
one for short (Tt) were tall
Law of Segregation
• If an organism has two different alleles for a trait, that
organism can make two different types of gametes.
– Tt plant can produce T gametes and t gametes
• Fertilization from a Tt + Tt cross will result in random
pairs of the available gametes
(four possible combinations)
–
–
–
–
T from male + T from female = TT
T from male + t from female = Tt
t from male + T from female = Tt
t from male + t from female = tt
Describing Offspring
• There are two ways to describe the
results of a pairing
– An offspring’s genotype
– An offspring’s phenotype
• Genotype – the gene combination of an organism
– A pea plant with two alleles for tallness has the genotype,
TT
– A pea plant with one allele for tallness and one allele for
shortness has the genotype, Tt
• Phenotype – the physical appearance of the organism
– An pea plant with two alleles for tallness has the
phenotype, tall
– An pea plant with one allele for tallness and one allele for
shortness has the phenotype, tall
Describing Offspring
• If an organism has the same two alleles
for a trait, the organism is homozygous
– TT
– tt
TT
tt
• If an organism has two different alleles
for a trait, the organism is heterozygous
– Tt
• Recessive traits must be homozygous (tt) to be
expressed
• Dominant traits will display if they are homozygous
(TT) or heterozygous (Tt)
Tt
Predicting Offspring
• Mendel devised a way to predict the possible
outcome of a cross. This method is called a
Punnett Square (or Punnett Chart)
• Punnett Square’s take into account that
fertilization of gametes is random
• To use a Punnett Square, you need to know the
genotypes of the parent generation.
• Punnett Squares can be used to predict offspring
from a monohybrid or a dihybrid cross
Monohybrid Punnett Square
• Put one of each type of possible gamete from one parent on top
of the square
• Put one of each type of possible
gamete from the other parent on
the side of the square
• Fill each box with the gamete of that
box’s row and column
• The possible offspring combinations
can be seen
TT
Tt
• The ratio of offspring phenotypes
after a heterozygous monohybrid
cross is 3:1
– In this case we see 3 Tall and 1 Short
Tt
tt
Monohybrid Crosses
Homozygous tall crosses with
heterozygous tall
T
T
T
TT
TT
t
Tt
Tt
Homozygous short crosses
with heterozygous tall
t
t
T
Tt
Tt
t
tt
tt
Genotype Ratio: 2TT : 2Tt
Genotype Ratio: 2Tt : 2tt
Phenotype Ratio: All Tall
Phenotype Ratio: 2 Tall : 2 Short
Incomplete Dominance
• Incomplete dominance is a condition in which one
allele is not completely dominant over another.
• The phenotype expressed is somewhere between the
two possible parent phenotypes.
• In snapdragons, flower color is controlled by
incomplete dominance. The two
alleles are red (R) and white (r).
The heterozygous genotype (Rr)
is expressed as pink.
Dihybrid Crosses
• Mendel also performed
dihybrid crosses –
involving two traits
• He wondered if the two
traits would be inherited
together.
• His P1 generation crossed round yellow peas (RRYY)
with wrinkled green peas (rryy)
• The F1 generation was all round and yellow
• The F2 generation had 9 round yellow, 3 round green,
3 wrinkled yellow and 1 wrinkled green
Law of Independent Assortment
• The fact that traits for the color of the pea
and the shape of the pea were passed on
independently of each other led to the
Law of Independent Assortment
• When a pea plant with the genotype RrYy
produces gamete, the alleles R and r will
separate from each other (Law of
Segregation) as well as the from the Y and
y alleles (Law of Independent Assortment)
• Alleles will sort independently unless they
are “linked”. This usually occurs when they
are so close to each other on the
chromosome that they one is rarely passed
on without the other.
Dihybrid Punnett Square
• Put one of each type of possible gamete combination from one
parent on top of the square
• Put one of each type of possible
gamete combination from the other
parent on the side of the square
• Fill each box with the gamete of that
box’s row and column
• The possible offspring combinations
can be seen
• The ratio of offspring phenotypes
after a heterozygous dihybrid
cross is 9:3:3:1
– In this case we see 9 round yellow,
3 round green, 3 wrinkled yellow and
1 wrinkled green
Dihybrid Punnett Square
• Homozygous
round and
yellow with a
heterozygous
yellow and
round
• RRYY with
RrYy
• Offspring are all
round and
yellow
RY
RY
RY
RY
RY
RRYY
RRYY
RRYY
RRYY
Ry
RRYy
RRYy
RRYy
RRYy
rY
RrYY
RrYY
RrYY
RrYY
ry
RrYy
RrYy
RrYy
RrYy
Dihybrid Punnett Square
• Homozygous
wrinkled and
green with a
heterozygous
yellow and round
• rryy with RrYy
• 4 round yellow,
4 round green,
4 wrinkled yellow,
4 wrinkled, green
ry
ry
ry
ry
RY
RrYy
RrYy
RrYy
RrYy
Ry
Rryy
Rryy
Rryy
Rryy
rY
rrYy
rrYy
rrYy
rrYy
ry
rryy
rryy
rryy
rryy
Sex Determination
• Remember that humans have 22 pairs of
autosomes and 1 pair of sex chromosomes
• These sex chromosomes determine the gender
of the offspring
– XX is a female
– XY is a male
• Each offspring gets an X from the
mother and either an
X or a Y from the father
Sex Determination
• Predicting the sex of the offspring can be done
using a Punnett Square
• Each time a male gamete fertilizes a female
gamete, there is a 50% chance for either sex
Sex-Linked Traits
• Traits controlled by genes carried on the X or Y
chromosomes are called sex-linked traits
• Most of these types of traits are carried on the
X chromosome
• The alleles for different forms of the sex-linked
traits are shown as superscripts on the X
– XR XR
– XRY
XR Xr
X rX r
XrY
• Because the Y does not carry an allele, a male
could not be heterozygous for a sex-linked trait
Predicting Sex-Linked Traits
• The chances that an offspring will have a sex-linked
trait can be predicted using a Punnett Square
Sex-Linked Crosses
Colorblindness is a recessive sex-linked trait. Use XN for the normal allele and
Xn for the colorblind allele
Heterozygous Normal mother
Colorblind mother and Normal father
and Colorblind father
Xn
Xn
XN
Y
XNXn
X nY
XN Xn
X nY
Xn
Y
XN
XNXn
XN Y
Xn
X nX n
X nY
Normal daughters (carriers)
50% Normal daughters and sons
Colorblind sons
50% colorblind daughters and sons
Blood Type
• Human blood types demonstrate multiple alleles (more
than two alleles of the gene)
– A, B, O
• Human blood types also demonstrate codominance –
where heterozygous alleles can be expressed equally
– A and B are codominant
– O is recessive
• These alleles are written as IA, IB, and i
–
–
–
–
IAIA or IAi will have type A blood
IBIB or IBi will have type B blood
IAIB will have type AB blood
ii will have type O blood
Blood Type Crosses
Type O mother and Type AB father
i
i
Homozygous Type A mother and
Homozygous Type B father
IA
IB
IAi
IB i
IA
IAi
IB i
IA
50% blood type A
50% blood type B
IB
IB
IAIB
IAIB
IAIB
IAIB
100% type AB
Polygenic Traits
• Polygenic traits are
traits that are
controlled by two or
more genes. These
traits often show a
great variety of
phenotypes, e.g. skin
color.
Pedigrees
• A pedigree is a chart constructed to show an
inheritance pattern (trait, disease, disorder) within a
family through multiple generations.
• Through the use of a pedigree chart and key, the
genotype and phenotype of the family members and
the genetic characteristics (dominant/recessive,
sex-linked) of the trait can be
tracked.
• An example of a pedigree key:
Pedigrees
• Family with a dominant autosomal genetic
trait
Pedigrees
• Family with a recessive sex-linked genetic
trait