observing patterns in inherited traits -...
TRANSCRIPT
© Cengage Learning 2015
Biology Concepts and Applications | 9e
Starr | Evers | Starr
© Cengage Learning 2015
Chapter 13
Observing Patterns in
Inherited Traits
After completing today’s activities,
students should be able to:
1. Describe Mendel’s experiments that led to our understanding of genetics.
2. Define and distinguish between self-fertilization, cross-fertilization, true-breeding organisms, hybrids, the P generation, the F1 generation, and the F2 generation.
3. Define and distinguish between the following pairs of terms: heterozygous versus homozygous, dominant allele versus recessive allele, genotype versus phenotype.
4. Define the law of segregation and explain how it applies to reproduction.
5. Define locus.
• People have selected and mated dogs with
preferred traits for more than 15,000 years.
• Over thousands of years, such genetic tinkering
has led to the incredible variety of body types
and behaviors in dogs today.
• The biological principles underlying genetics
have only recently been understood.
Biology and Society: Our Longest-Running Genetic Experiment: Dogs
© 2013 Pearson Education, Inc.
6
7
• Heredity is the transmission of traits from one
generation to the next.
• Genetics is the scientific study of heredity.
• Gregor Mendel
– worked in the 1860s,
– was the first person to analyze patterns of
inheritance, and
– deduced the fundamental principles of genetics.
HERITABLE VARIATION AND PATTERNS OF INHERITANCE
© 2013 Pearson Education, Inc.
Mendel studied
garden peas because
they
-were easy to grow
-came in many readily
distinguishable varieties
-are easily manipulated
-can self-fertilize
Figure 9.4
Dominant Recessive
White
Pod shape Inflated Constricted
Flower position
Pod Color
Dominant Recessive
Flower color Purple
Axial Terminal
Green Yellow
Tall Dwarf
Seed shape Round Wrinkled
Seed color Yellow Green Stem length
Traits of Mendel’s Pea Plants
• Mendel studied
garden peas
Self-Fertilization
– These plant are
easily
manipulated
– These plants
can self-fertilize
Figure 9.2
Stamen
Carpel
Cross-Fertilization
Figure 9.3
Removed stamens from purple flower
1
2
3
Transferred pollen from stamens of white flower to carpel of purple flower
4 Planted seeds from pod
Pollinated carpel matured into pod
White
Stamens
Carpel
Parents (P)
Purple
Offspring (F1)
• When offspring inherit a pair of identical
alleles for a trait in many generations
• Mendel had “True-breeding” pea plants for
white flowers and purple flowers
All plants have purple flowers
Purple flowers
Purple flowers
True Breeding
Mendel’s Observation: Some of my pea plants always produce purple flowers while
some always produce white flowers.
More Terminology
• Hybrid
– The offspring of two different true-breeding
organisms
– The offspring of a “genetic cross”
• P Generation
– The parents in the genetic cross
• F1 generation
– The hybrid offspring of the genetic cross
• F2 generation
– Offspring of two F1 organisms
• Mendel frames a question around his
observation:
– What will happen if I cross a true-breeding
purple-flower plant with a true-breeding white-
flower plant?
• Mendel Hypothesizes:
– Let’s make some hypotheses to answer the
question
• Mendel makes predictions based on his
hypotheses:
• Mendel carries out experiment
• A cross between plants that differ in only one trait
Monohybrid Crosses
Figure 9.5
P Generation (true-breeding parents)
All plants have purple flowers
Purple flowers
White flowers
F1 Generation
So, which hypothesis was correct?
Mendel’s Next Question
• Was the gene for white flowers lost?
• Hypothesis: ____________________
• Prediction: _____________________
• Mendel crosses the F1 plants to each
other to find out.
• What type of
cross is this?
Figure 9.5
P Generation (true-breeding parents)
All plants have purple flowers
Fertilization among F1 plants (F1 × F1)
F2 Generation
3/4 of plants have purple flowers
1/4 of plants have white flowers
Purple flowers
White flowers
F1 Generation
Hypothesis 1
• There are alternative forms of genes called alleles.
– Flower color gene has 2 alleles: purple and white
Hypothesis 2 • For each trait, an organism has two alleles -
One from the egg; one from the sperm
• Homozygous
– An organism that has two identical alleles for a
gene
• Heterozygous
– An organism that has two different alleles for a gene
Hypothesis 3
• If the two alleles of a pair differ, then one allele
determines the organism’s appearance and the other
has no effect on the organism
• Dominant Alleles
– An allele is dominant when its effect on a trait masks the
effect of a recessive allele that is paired with it.
– Indicated by a capital letter (e.g. P for purple)
• Recessive Alleles
– An allele is recessive when its effect on a trait is masked by
the presence of a dominant allele that is paired with it
– Indicated by a lower case letter (e.g. p for white)
Hypothesis 4 • Now known as The Law of Segregation
– A sperm or egg carries only one allele for
each trait. Fertilization creates allele pairs
again.
Figure 9.6
Genotype: The genetic alleles that an
individual carries
Ex: PP, Pp, or pp
Phenotype: An individual’s physical
traits.
Ex: Purple or white flowers
Punnett Square: A tool used to predict
the outcome of a genetic cross
Figure 13-3 p207
Genotype vs. Phenotype • Genotype = genetic makeup
• Phenotype = appearance or observable traits
© 2016 Pearson Education, Inc.
Figure 9.7
Homologous
chromosomes
Gene loci Dominant
allele
P a B
P a b Recessive
allele
Genotype: PP aa Bb
Homozygous for the dominant allele
Homozygous for the recessive allele
Heterozygous with one dominant and one recessive allele
Genetic Alleles and Homologous Chromosomes
