Transcript Heredity Mendel and His Peas

Chapter 5
 Heredity: the passing of traits from parents to
offspring.
 Example: you might have curly hair, while both
of your parents have straight hair or you might
have blue eyes even though both of your parents
have brown eyes. HOW IS THIS?
 Gregor Mendel:
 Born in 1822
 Austria
 Grew up on farm and learned all about flowers
and fruit trees
Mendel continued….
 Entered a monastery when he was 21,
where monks taught science and
performed experiments.
 He did well in school but he did not
pass his final exam; therefore, he went
back to the monastery.
 While at the monastery he discovered
the principles of heredity.
 Mendel studied plant inheritance and
tried to understand it,
 he realized that some traits would show
up in the first generation and then not in
the second generation.
 When developing an experiment Mendel
decided to study only one organism.
 The organism Mendel used was the pea
plant because he had studied it before.
Pea Plants
 Mendel used the pea plant for several
reasons.
1. Grow quickly
2. Many different kinds available
3. Able to self-pollinate
4. Able to cross-pollinate
 Mendel only studied one characteristic
at a time. (characteristic: a feature that
has different forms in a population).
 The different forms are called traits
 The characteristics that Mendel
studied were
1. Seed shape
2. Plant height
3. Flower color
First Experiment
 Mendel crossed pea plants to study seven
different characteristics.
 He crossed plants that had purple flowers with
plants that had white flowers (figure 5 p 117).
 The offspring, first generation, had all purple
flowers. (first generation- self-pollination)
 Mendel wondered why all the flowers came out
purple when he started out with a white flower
and a purple flower. What happened to the
traits of the white flower?
 Mendel discovered that all the traits
that were expressed (shown) were
called dominant traits and the traits
that did not show up were called
recessive traits.
 Recede means “to go away or back off”
 Mendel theorized that only certain
traits would be expressed when
combined with other traits.
Second Experiment
 Mendel did the same experiment on each of
the seven characteristics.
 In second generation plants, some expressed
the recessive traits.
 Mendel decided to count the number of
plants with each trait that turned up in the
second generation. (Table 1 p 118)
 The dominant trait showed up more than
the recessive trait.
Ratios
 Mendel then figured out the ratios of
dominant traits to recessive traits.
(ratio-a relationship between 2
different numbers that is often
expressed as a fraction).
 Math practice: p 118
Traits and Inheritance
 After Mendel’s experiments he concluded that there
are two sets of instructions for each characteristic.
 Scientists call these instructions for an inherited trait
genes.
 Each parent gives one set of genes to the offspring
giving the offspring two forms of the same gene fro
every characteristic.
 The different forms (dominant and recessive) of gene
are known as alleles.
 Dominant-capital letter, Recessive-lowercase letter
 An organism’s appearance is known as its
phenotype, what it looks like.
 For example: the pea plants could either
have a purple flowers or white flowers that
would be the phenotype.
 Albinism prevents hair, skin, and eyes from
having normal coloring; therefore affecting
the appearance of a person.
 The inherited alleles from each parent come
together to form an organism’s genotype.
 Remember that dominant traits are capital
letters and recessive traits are lowercase
letters. So in the pea plants the allele for
purple flowers is (P)-dominant, and the allele
for white flowers is (p)-recessive.
 So if the pea plant has two dominant alleles
then its genotype would be (PP), if it had two
recessive alleles then its phenotype would be
(pp).
Homozygous vs. Heterozygous
 If an organism has the genotype (PP) or (pp)
it is said to be homozygous because it has
the same two alleles.
 If an organism has the genotype (Pp) it is
said to be heterozygous because it has two
different alleles. When this happens the
dominant trait is expressed and the
recessive trait is masked by the dominant
allele.
Punnett Square
 A punnett square is used to organize all possible
combinations of offspring from particular parents.
P
P
p
p
Pp
Pp
Pp
Pp
 All offspring will be heterozygous dominant for purple
 In Mendel’s experiments the first generation
was self-pollinated making all offspring
(Pp).
 In the second generation he crossed a (Pp)
and a (Pp)
 Make the punnett square for the outcome of
the offspring.
 What would the ratio of purple flowers to
white flowers be?
Probability
 The mathematical chance that something
will happen is known as probability.
 It is usually written as either a fraction or
percentage.
 Example: when you toss a coin there is a 50
% chance that is will land on heads and a 50
% chance it will land on tails.
 So when you cross a (Pp) and (Pp) the
probability will be …___%_ purple and
___%_white
Incomplete Dominance
 Scientists have discovered that some traits
are not completely dominant over another
trait. Therefore each trait influences that
outcome of the offspring. This is called
incomplete dominance.
 When crossing snapdragons( figure 5 p 124)
one being white and one being red the
offspring outcome is 4 pink snapdragons
instead of 3 red and 1 white.
Genes and Traits
 Sometimes one gene can code for many traits. For
instance the tiger in figure 6 appears to be white,
but it also has blue eyes. One gene controlled
those two traits.
 Other times many genes can code for the same
trait. At least two genes determine human eye
color. That’s why many shades of a single color are
possible.
 The environment can also effect the outcomes. A
guinea pig could have genes for long hair but its
fur could be cut. Also height can be controlled by a
healthy diet.
Meiosis
 Sexual reproduction- two parents
 Homologous chromosomes- same genes and
same structure.
 Meiosis- forming of sex cells, producing
cells with half the number of chromosomes.
(23-mom, 23-dad = 46)
 Male-sperm, Female-egg
 Genes are located on chromosomes
Sex Chromosome
 Female-XX
 Male-XY
X
X
X
Y
XX
XY
XX
XY
 So what is the probability of the offspring being a girl?
Boy?
 What is the genotype for a boy? girl?
 Read Sex-Linked Disorders p 131
Sex-Linked Disorders
 Colorblindness -males
 Hemophilia- prevents blood from clotting.
 Genetic disorders can be traced through a family tree.
Counselors use a diagram called a pedigree. This helps
to see the outcomes of future generations.
 Some diseases that are traced:
 Cystic fibrosis-lung problems (recessive trait)