Transcript Mendel/Punnett Squares PPT

Mendel’s Laws of Heredity
What is genetics:
Genetics – the branch of biology
that studies heredity.
Heredity – the passing on of traits
from parents to offspring.
Traits – Characteristics that
are inherited.
• The work of an Austrian monk
named Gregor Mendel was
particularly important in the
understanding of inheritance.
• Mendel was born in 1822 in what
is now the Czech republic.
• He studied to be a priest and
spent several years studying science
and math at the University of
Vienna.
• Mendel spent the next 14 years of
his life working in the local
monastery and teaching high
school.
• At the monastery, Mendel was in
charge of gardening and spent
most of his time tending to and
studying the plants around him.
• Mendel paid close attention to the
common garden pea plants that
grew at the monastery.
• These pea plants reproduced
sexually, meaning they had both
male and female sex cells.
Gametes – male or female sex cells.
Mendel observed seven traits that are
easily recognized and apparently only
occur in one of two forms:
1. flower color is purple or white
2.
seed color is yellow or green
3.
flower position is axil or terminal
4.
pod shape is inflated or constricted
5.
stem length is long or short
6.
pod color is yellow or green
7.
seed shape is round or wrinkled
• He preformed experiments with
these pea plants by forcing plants
with different traits to pollinate
each other.
• These experiments are called
genetic crosses.
Mendel’s Monohybrid Crosses.
• In his first experiment,
Mendel decided to cross
pollinate two plants that
had different seed color.
• One of the plants had all
green seeds and one of the
plants had all yellow
seeds.
• The crossing of one
trait is called a monohybrid cross.
• Mendel expected the first
generation of new plants to have a
mixture of green and yellow seeds.
• These first generation plants
however all had yellow seeds.
• Mendel then decided to cross two
of the offspring in the first
generation and see what type of
seeds he would find in the 2nd
generation.
• To his amazement Mendel found
that in the second generation, the
plants had both green and yellow
seeds, and the ratio of the seeds per
plant was three yellow seeds to
every one green (3:1).
• After careful mathematical
calculations and a lot of guesswork,
Mendel made the largest
breakthrough in modern science.
• Mendel concluded that each trait
for seed color was represented by
something that the plant carried.
• Because every plant had a “mom”
and “dad”, Mendel concluded that
each trait had two parts, and that it
is the combination of these parts
that give the organism it’s traits.
• He called these parts Alleles.
Alleles – Gene form, Y or y,
for each variation of a trait
of an organism.
Dominant – visible, observable trait
of an organism that masks a recessive
form of a trait.
Recessive – A hidden trait of an
organism that is masked by a
dominant trait.
Law of Segregation
• After noticing that the allele for
green seeds reappeared in the
second generation, Mendel
concluded that the two alleles for
each trait must separate when
gametes are formed.
Law of Segregation – In other words, A
parent only passes one form of a gene
or allele (i.e. either A or a but not both)
at random to each offspring.
Genotype and Phenotype
Phenotype – the way an
organism looks and behaves.
Physical characteristics of an
organism.
Genotype – the gene combination
of an organism.
Homozygous – when an
organisms two alleles for a
trait are the same.
Heterozygous – when an
organisms two alleles for
a trait are not the same.
Mendel’s Dihybrid Crosses
Studying Two Traits at once:
Mendel also did a type of cross where
two traits were followed at one time - a
Dihybrid Cross.
AA or Aa = purple; aa = white
BB or Bb = tall; bb = short
Law of Independent Assortment –
Genes of different traits are inherited
independently from each other.
• Segregation and Independent
Assortment of Alleles when the
alleles are on separate chromosomes.
• In other words, green peas don’t
always come from tall plants, they
may be yellow and tall.
Punnett Squares:
in 1905, Reginald Punnett, an
English biologist devised a short
way to show the expected
proportions of possible genotypes
in the offspring of a cross.
Your turn! Try these on your own
paper.
Problem 1: (a) A man with a widow's
peak (WW) marries a woman with a
continuous hairline (ww). A widow's
peak is dominant over a continuous
hairline. What kind of hairline will
their children have?
Answer:
Genotype: Ww (all children will be
heterozygous)
Phenotype: Widow's peak
(phenotype of all children)
w
w
W
Ww
Ww
W
Ww
Ww
(b) Suppose one of their children (Ww)
marries someone who is also
heterozygous (Ww). What type of
hairline will their children have?
Problem 2: A man and a woman are
heterozygous for freckles. Freckles
(F) are dominant over no freckles
(f). What are the chances that their
children will have freckles?
Problem 3: A woman is homozygous
dominant for short fingers (SS). She
marries a man who is heterozygous for
short fingers (Ss). Will any of their
children have long fingers (ss)? yes / no
Problem 4: Jane and John are
expecting a baby and know that they
are both carriers (ie heterozygous) of
cystic fibrosis (Cc). What is the
probability that their child will have
cystic fibrosis (cc)? What is the
probability that their child will be a
carrier of cystic fibrosis?