Transcript Slide 1

Chapter 10
 For a long time, people have observed that
offspring look like their parents.
Even before we knew about genes, people were
breeding livestock to get certain traits in the
offspring.
They knew that something caused babies to look
like their parents.
Most thought that traits of parents were blended
in the offspring.
Chapter 10
 Gregor Mendel – “Father of Modern
Genetics”
 Austrian Monk
 Work occurred in 1850s
 His work provides framework of what we
know today
 Traits are distinguishing characteristics that
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are inherited.
Mendel Laid the groundwork for genetics.
Genetics is the study of biological inheritance
patterns and variation.
Gregor Mendel showed that traits are
inherited as discrete units.
Many in Mendel’s day thought traits were
blended.
 Mendel studied pea plants
 Why study pea plants?
 Reproduces quickly (14 days!)
 Self-pollinating or sexually reproduce
 Many varying traits
 Makes lots of offspring (100+!)
 Easy to grow
 Understood basic plant reproduction
 Pollen ♂ & egg ♀ fused when fertilized
 One plant has ♀&♂ structures
 Most plants are self-fertilizing (purebred)
 Studied the traits of the plants
 Traits = characteristics
 Mendel cross fertilized plants with contrasting
traits (tall and short)
 If allowed to self-pollinate, true-breeding plants
produce offspring identical to themselves.
 Self-pollination – pollen and ovum come from the
same plant.
 True-breeding – always produce offspring with the
same trait.
 He wondered what plants would look like if male
sex cells in the pollen of one plant fertilize the egg
cells on another plant = cross-pollination.
 Cross-pollination - using the pollen of one plant to
fertilize the ovum of another plant
 He systematically cross-pollinated two
different true-breeding plants called
the Parent generation (P).
 Mendel systematically cross-pollinated two different true-
breeding plants called the Parent generation (P).
 His data revealed patterns of inheritance.
 Mendel made three key decisions in his experiments.
 use of purebred plants
 control over breeding
 observation of seven
“either-or” traits
 Cross Fertilize = taking pollen from one plant and
fertilizing the egg of another
• Offspring is called a Hybrid
• This caused odd outcomes
• Tall plant + short plant = tall plant
• Mendel used pollen to fertilize selected pea plants.
• P generation crossed to produce F1 generation
Mendel controlled the
fertilization of his pea plants
by removing the male parts,
or stamens.
He then fertilized the female
part, or pistil, with pollen from
a different pea plant.
• Mendel allowed the resulting plants to self-pollinate.
– Among the F1 generation, all plants had purple flowers
– F1 plants are all heterozygous
– Among the F2 generation, some plants had purple flowers
and some had white
• Mendel observed patterns in the first and second generations of his
crosses.
• What patterns do you observe?
• Mendel drew three important conclusions.
– Traits are inherited as discrete units.
– Organisms inherit two copies of each gene, one from each
parent.
– The two copies segregate
during gamete formation.
– The last two conclusions are
called the law of segregation.
purple
white
 Genes encode proteins that produce a diverse range of
traits.
 The same gene can have many versions.
 A gene is a piece of DNA that directs a cell to make a
certain protein.
 Each gene has a locus, a
specific position on a pair of
homologous chromosomes.
 An allele is any alternative form of a gene occurring at
a specific locus on a chromosome.
– Each parent donates
one allele for every
gene.
– Homozygous describes
two alleles that are the
same at a specific
locus.
– Heterozygous
describes two alleles
that are different at a
specific locus.
 Genes influence the development of traits.
 All of an organism’s genetic material is called the
genome.
• A genotype refers to the makeup of a specific set of
genes.
• A phenotype is the physical expression of a trait.
• Alleles can be represented using letters.
– A dominant allele is
expressed as a phenotype
when at least one allele is
dominant.
– A recessive allele is
expressed as a phenotype
only when two copies are
present.
– Dominant alleles are
represented by uppercase
letters; recessive alleles by
lowercase letters.
• Both homozygous dominant and heterozygous genotypes
yield a dominant phenotype.
• Most traits occur in a
range and do not follow
simple dominantrecessive patterns.
 Punnett squares illustrate genetic crosses.
 The Punnett square is a grid system for predicting all
possible genotypes resulting from a cross.
 The axes represent
the possible gametes
of each parent.
 The boxes show the
possible genotypes
of the offspring.
•The Punnett square yields
the ratio of possible
genotypes and
phenotypes.
The inheritance of traits follows the rules of probability.
 Monohybrid crosses examine the inheritance of only one
specific trait.
 homozygous dominant-homozygous recessive: all
heterozygous, all dominant
– heterozygous-heterozygous
– 1:2:1 homozygous dominant:
heterozygous:homozygous recessive
– 3:1 dominant:recessive
• heterozygous-homozygous recessive
• 1:1 heterozygous:homozygous recessive
• 1:1 dominant:recessive
• A testcross is a cross between an organism with an unknown
genotype and an organism with the recessive phenotype.
 Heredity patterns can be calculated with probability.
 Probability is the likelihood that something will happen.
 Probability predicts an average number of occurrences, not an exact
number of occurrences.
• Probability =
number of ways a specific event can occur
number of total possible outcomes
• Probability applies to
random events such as
meiosis and fertilization.
What is the
probability
that a baby will
be a girl?
A boy?
 The larger the number of individuals, the closer the
resulting offspring numbers will get to expected values.
 Probabilities do not guarantee
outcomes!!!!
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Probability =
number of ways a specific event can occur
number of total possible outcomes
Tall: ¾ =75%
Short: ¼ =25%
Step by step how to guide
 Alleles represented by letters
 Capital letters = dominant (T)
 Lowercase letters = recessive (t)
 Dominant letter goes before the recessive letter within Punnett square
 Two letters combined = trait
 TT, Tt, tt
 One from mom and one from dad
 Homozygous = both letters same
 TT or tt
 Heterozygous = both letters differ
 Tt
 A diagram that shows all possible outcomes of a
genetic cross
 Can be used to predict probabilities
 Phenotype – an observable trait
 Tall or short
 Genotype – genetic make-up or combination of alleles
 TT
Tt
tt
1. Identify the Parents being used in the cross
 Homozygous or heterozygous
 TT, Tt, tt?
 Most important step!
2. Segregate the alleles in each set of genes for each
parent
 Show meiosis creating the haploid cells
Parent: (P)
Heterozygous Tall
crossed with
Heterozygous Tall
Segregation
of alleles:
Draw the
Square:
Place the
parents:
What’s next?
Ratio of allele
combinations
Ratio of traits
1.
Tt x tt
-be sure to show both types of ratios
2.
Cross a homozygous tall plant with a short plant
-be sure to show both types of ratios
 A dyhybrid cross involves two traits.
 Mendel’s dihybrid crosses with heterozygous plants yielded a
9:3:3:1 phenotypic ratio.
• Mendel’s dihybrid crosses
led to his second law,
the law of independent
assortment.
• The law of independent
assortment states that
allele pairs separate
independently of each
other during meiosis.
1. Identify the Parents being used in the cross and figure
out the combo of both traits.
 Homozygous or heterozygous
 TTGG, TTGg, TTgg, TtGG, TtGg, Ttgg, ttGG, ttGg, ttgg?
 Most important step!
2. Segregate the alleles in each set of genes for each
parent
 Show meiosis creating the haploid cells
Heterozygous Tall, Heterozygous Green plant crossed
with Heterozygous Tall, Heterozygous Green plant
Step 3 – Set up and complete the
Punnett Square
Phenotypic Ratio:
1.
Cross a heterozygous tall, yellow plant with a
homozygous tall, heterozygous green plant.
2.
Cross a heterozygous tall, yellow plant with a short,
heterozygous green plant.
 Traits are determined by genes which are passed
from parent to offspring.
 Some forms of a genes may be dominant and some
recessive for a given trait.
 Most sexually reproducing organisms have 2 alleles
for a gene that separate when eggs and sperm are
formed.
 Alleles for different genes can segregate
independently of one another.
Chapter 10
 KEY CONCEPT: Genes can be mapped to specific locations
on chromosomes.
 Gene linkage was explained through fruit flies.
 Morgan found that linked traits are on the same
chromosome.
 Chromosomes, not genes, assort independently during
meiosis.
Wild type
Mutant
 Linked genes are not inherited together every time.
 Chromosomes exchange homologous genes during
meiosis.
 Genes located close
together on a
chromosome are likely
to be inherited
together.
http://www.csun.edu/~cmalone/pdf360/Ch061chi%202pt.pdf
 Linkage maps estimate distances between genes.
 The closer together two genes are, the more likely they
will be inherited together.
 Cross-over frequencies are related to distances between
genes.
 Linkage maps show the relative locations of genes.
 Cross-over frequencies can be converted into map
units.
– gene A and gene B cross over 6.0 percent of
the time
– gene B and gene C cross over
12.5 percent of the time
– gene A and gene C cross over 18.5
percent of the time