Download Introduction to Genetics

Introduction to Genetics
How are traits passed on?
S
Gregor Mendel
S Austrian monk who studied pea plants
S He chose these plants to study because they were:
S Easy to grow
S Mature quickly
S Have sharply contrasting traits
Traits of
Pea Plants
Pea Plants
S Naturally, pea plants self-pollinate.
S Mendel observed that some plants, when they were allowed
to self-pollinate, always “bred” true.
S This means that these plants were “true-breeding” (Ex. Tall
plants always produced tall plants.)
S Mendel cross-pollinated some plants to observe the results.
S When he crossed true-breeding tall plants and true breeding
short plants, he found that he always got tall plants
S Then, he crossed the resulting tall plants, and found that the
short plant reappeared.
S P = Parental Generation
S F1 = first filial generation
S F2 = Second filial generation
Why did Mendel get these
results?
S All living things have genes, or sections of chromosomes
that have instructions for making different characteristics
S There are always two versions of these genes, one from each
parent
S
Called Alleles
S
Represented by letters
Alleles
S Alleles, or the two forms of each gene, can be
S Dominant –
S
this trait will always show up if the dominant gene is present
S
Represented by capital letters (B)
S Recessive –
S
this trait will only show up in the absence of the dominant gene
S
Represented by lowercase letters (b)
S Examples of
S Dominant traits – tongue rollers, brown eyes, dark hair, left
thumb hand clasping
S Recessive traits – non-tongue rollers, light eyes, light hair, right
thumb hand clasping
Allele Combinations
(Genotype)
S Depending on the two alleles present, an organism can be
considered homozygous or heterozygous for a trait
S Homozygous – two alleles are the same, either both dominant or
both recessive
S Homozygous Dominant (HD) – two dominant alleles (TT)
S Homozygous Recessive (HR) – two recessive alleles (tt)
S Heterozygous – two alleles are different, one dominant and one
recessive (Tt)
S The different arrangements of alleles is called an organism’s
genotype
Phenotype
S An organism’s genotype will determine its phenotype, or
physical characteristic
S For example:
S A plant with the alleles TT is homozygous dominant & one
with the alleles Tt is heterozygous
S Both plants have the dominant gene for tallness (T) and will
therefore have a tall phenotype
Back to Mendel’s Plants
S Why did the trait for short plants disappear in the F1
generation?
S Why did it reappear in the F2 generation?
S Punnett squares can be used to demonstrate the different
possible combinations of alleles…..
S Brown eyes are dominant over blue eyes. A man with
brown eyes, whose entire family has only ever had brown
eyes, marries a woman with blue eyes. Give the genotype
and phenotype ratios possible for their future children.
S A brown-eyed man, whose mother had blue eyes, marries a
blue-eyed woman. Give the genotype and phenotype ratios
possible for their future children.
S Right-handedness is dominant over left-handedness. Give
the genotype and phenotype ratios possible for the future
children of a heterozygous couple.
S In certain goats, there is a fainting gene that is recessive if the goats
are startled. Give the genotype and phenotype ratios possible for the
offspring of two heterozygous goats. Can they have offspring that
will faint?
S In fruit flies, normal wings are dominant over vestigial wings.
A purebred normal-winged male is mated with a vestigialwinged female. Show the results for the F1 and F2
generations. Include all work and ratios.
Punnett Square
Competition
S
S A left-handed woman, whose mother was right-handed,
marries a right-handed man, whose father was left-handed.
What are the genotypes and phenotypes possible for their
future children? Show all work and ratios.
S In seals, long whiskers are dominant over short whiskers.
Show the genotype and phenotypes possible for the offspring
of two heterozygous seals. Include all work and ratios.
S In pea plants, tall stems are dominant over short stems. A
short plant is crossed with a pure tall plant. Show the results
for the F1 and F2 generations. Include all work and ratios.
S Normal skin pigment is dominant over the albino allele, which
causes a lack of pigment in the skin. An albino man marries a
normally-pigmented woman, whose mother was albino. What
are the possible genotype and phenotypes for their future
children. Include all work and ratios.
S In dogs, wire hair is dominant to smooth hair. A purebred
wire-haired female is mated with a smooth-haired male. Show
the results for the F1 and F2 generations. Include all work and
ratios.
S Tongue rolling in humans is a dominant trait. A man, who
cannot roll his tongue marries a woman who can roll her
tongue. Their first child is not able to roll her tongue. What
are the chances for the other possible future children as to
whether or not they can roll their tongue? Show all work and
genotype and phenotype ratios.