Transcript chapter-6-mendel-heredity

CHAPTER
6: MENDEL &
HEREDITY
Mendel’s Breeding Experiments
• The garden pea is a good
subject for studying heredity
for several reasons:
1. Several traits of the garden
pea exist in two clearly
different forms.
2. The male and female
reproductive parts of garden
peas are enclosed within the
same flower. This allows you to
easily control mating.
3. The garden pea is small, grows
easily, matures quickly, and
produces many offspring.
Mendel carried out his experiments in three steps:
Modern terms for Mendel's findings
• Dominant alleles are indicated by writing the first letter of
the trait as a capital letter.
• Recessive alleles are also indicated by writing the first letter
of the dominant trait, but the letter is lowercase.
• If the two alleles of a particular gene present in an
individual are the same, the individual is said to be
homozygous.
• If the alleles of a particular gene present in an individual
are different, the individual is heterozygous.
• In heterozygous individuals, only the dominant allele is
expressed; the recessive allele is present but unexpressed.
• The set of alleles that an individual has is called its
genotype.
• The physical appearance of a trait is called a phenotype.
Black colored fur in mice is a dominant trait over white fur. If
a pure black mouse mated with a pure white mouse, what
would be the outcome?
Punnett Squares
• A Punnett square is a diagram that predicts the outcome of
a genetic cross by considering all possible combinations of
gametes in the cross.
• The possible gametes that one parent can produce are
written along the top of the square.
• The possible gametes that the other parent can produce
are written along the left side of the square.
• Each box inside the square is filled in with two letters
obtained by combining the allele along the top of the box
with the allele along the side of the box.
• Punnett squares can be used to predict the outcome of a
monohybrid cross (a cross that considers one pair of
contrasting traits between two individuals).
• Punnett squares allow direct and simple predictions to be
made about the outcomes of genetic crosses.
Monohybrid Cross: Homozygous Plants
Monohybrid Cross: Heterozygous Plants
Red colored flowers of a certain plant are dominant
over yellow colored flowers.
The diagram below shows the possible outcomes of
the offspring of two hybrid flowers.
More examples
1. The allele for dimples (D) is dominant to the allele
for no dimples (d). A man heterozygous for dimples
marries a woman who is also heterozygous for
dimples.
a) What is the man’s genotype and the woman’s
genotype?
b) What is the man’s phenotype and the woman’s
phenotype?
c) Do a cross to determine all potential dimple
genotypes and phenotypes for the offspring of this
man and woman.
2. The allele for hitchhiker’s thumb (h) is recessive
to straight thumb (H). If a man and his wife are
both homozygous recessive, will any of their
offspring potentially have hitchhikers thumb?
a) What is the man’s genotype and the woman’s
genotype?
b) What is the man’s phenotype and the woman’s
phenotype?
c) What genotype(s) must the offspring have in
order to have the phenotypic trait of hitchhiker’s
thumb?
d) Do a cross to determine all potential hitchhiker’s
thumb genotypes and phenotypes for the
offspring of this man and woman. Is it possible
for any offspring of the F1 generation to have
hitchhiker’s thumb?
3. In humans, free earlobes are dominant over
attached earlobes. If one parent is homozygous for
free earlobes, while the other has attached
earlobes can they produce any children with
attached earlobes?
4. In humans widow’s peak is dominant over straight
hairline. A heterozygous man for this trait marries a
woman who is also heterozygous.
a) List possible genotypes of their offspring.
b) List the phenotypic ratio for their children.
5. In a certain breed of dogs, a gene (L) codes for hair
length. The dominant trait is short hair and the
recessive is long hair. Suppose a heterozygous female
dog and a homozygous recessive male dog mate.
a) What is the male dog’s genotype and the female
dog’s genotype?
b) What is the male dog’s phenotype and the female
dog’s phenotype?
c) What will be the genotypic ratio of the F1
generation?
d) What will be the phenotypic ration of the F1
generation?
6. In fruit flies, the allele for normal wings (V) is
dominant to the allele for short wings (v). Suppose
two fruit flies heterozygous for the trait are mated.
a) What is the male fruit fly’s genotype and the
female fruit fly’s genotype?
b) What is the male fruit fly’s phenotype and the
female fruit fly’s phenotype?
c) What will be the genotypic ratio of the F1
generation?
d) What will be the phenotypic ration of the F1
generation?
Quick check!
The diagram on the right shows the expected
results of a cross between two pea plants. T
and t represent the alleles for the tall and
dwarf traits, respectively. Use the figure
below to answer questions 1–3.
1. What are the genotypes of the plants that
were crossed?
A. tt on the top; tt along the side
B. Tt on the top; tt along the side
C. Tt on the top; Tt along the side
3. If this cross produced 240
D. TT on the top; TT along the side
offspring, how many of the
2. What genotypic ratio is expected in the
offspring would be
offspring of this cross?
expected to have the
F. 1 Tt : 1 tt
dwarf trait?
G. 3 Tt : 1 tt
A. 0
H. 1 Tt : 3 tt
B. 60
C. 120
J. 1 TT : 1 tt
D. 180
Inheritance of Traits
• Geneticists often
prepare a pedigree, a
family history that
shows how a trait is
inherited over several
generations.
• Pedigrees are
particularly helpful if
the trait is a genetic
disorder and the family
members want to
know if they are
carriers or if their
children might get the
disorder.
Determining Unknown Genotypes
• Animal breeders often need to know whether an organism
with a dominant phenotype is heterozygous or homozygous
for a trait.
• In a test cross, an individual whose phenotype is dominant,
but whose genotype is not known, is crossed with a
homozygous recessive individual
• Like Punnett squares, probability calculations can be used
to predict the results of genetic crosses.
• Probability is the likelihood that a specific event will occur.
number of one kind of possible outcome
Probability 
total number of all possible outcomes
Scientists can determine several pieces of genetic
information from a pedigree:
Autosomal or Sex-Linked?
• If a trait is autosomal, it will appear in both sexes equally.
• If a trait is sex-linked, it is usually seen in males.
• A sex-linked trait is a trait whose allele is located on the X
chromosome.
• Examples: Red-green colorblindness, Male Pattern
Baldness, Hemophilia
Dominant or Recessive?
• If the trait is autosomal dominant, every individual with the
trait will have a parent with the trait.
• If the trait is recessive, an individual with the trait can have
one, two, or neither parent exhibit the trait.
Variation
Variation within a
species is the way
that two animals of
the same species
display different
characteristics
and/or behavior.
There are two types
of variation; genetic
and environmental.
Some genetic disorders