Chapter 11 Genetics
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Transcript of Chapter 11 Genetics
Gregor Mendel
• Used pure breed pea plants to understand inheritance
• Used self pollination to cross breed– Pollen from the male
part of the pea plant fertilizes the female egg of the same flower
Cross Pollination
• Cross Pollination– Mendel cut away the male parts of the
plant and then dusted pollen from another plant on the flower
– Seed that come from cross-pollination inherit all of the characteristics from two parents
Genes and Dominance
• A trait is a specific characteristic that varies from one individual to another
• The chemical factors that determine traits are called genes
• The different forms of a gene are called alleles
• Video Clip!!
Mendel’s Work
• Mendel crossed two pure breed plants with different forms of the same trait– One plant was tall and one plant was short
(trait: plant height)
• The offspring plants were called hybrids– Hybrids are the offspring of crosses between
parents with different forms of a trait
Principle of Dominance
• The principle of dominance states that some alleles are dominant and others are recessive– An organism with a dominant allele for
a specific trait will always show that form of the trait.
– An organism with a recessive allele for a specific trait will only show that form only when the dominant allele for that trait is not present.
Segregation
• Segregation is the separation of alleles during gamete formation
• Gametes are specialized cells involved in sexual reproduction
Probability and Punnet Squares
• Principle of probability can be used to predict the outcomes of genetic crosses.
• Probability is the likelihood that a particular event will occur.– Probability can predict the outcome of
genetic crosses because alleles segregate randomly.
Punnet Squares
• These can be used to predict and compare the genetic variations that will result from a cross.
TT
50%
TT
50%
Tt
50%
Tt
50%
T T
T
t
Punnet Square, cont.
• Homozygous – TT or tt, organisms that have two identical alleles for a particular trait.
• Heterozygous – Tt, organisms that have different alleles for the same trait
• Phenotype – physical characteristics, – TT = tall, Tt = tall, tt = short
• Genotype – genetic makeup, TT, Tt, tt.
TT
(tall)
Tt
(tall)
Tt
(tall)
tt
(short)
T t
T
t
Ratios:
Phenotype- 3 Tall: 1 Short
Genotype- 1 TT: 2Tt: 1tt
Probabilities Predict Averages
• Probabilities predict the average outcome of a large number of events.
• Probability cannot predict the precise outcome of an individual event
Mendelian Genetics
• Mendel found that the gene controlling seed shape did not affect the gene controlling seed color.
• Principle of Independent Assortment– Genes for different traits can segregate
independently during the formation of gametes
Independent Assortment
• Independent Assortment accounts for the many genetic variations observed in plants, animals, and other organisms
Beyond Dominant and Recessive Alleles
• Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes.
Incomplete Dominance
• One allele is not completely dominant over the other.
• The heterozygous phenotype is somewhere between two homozygous phenotypes
Codominance• Both alleles contribute to the
phenotype.• For Example: In chickens, the allele
for black feathers is codominant with the allele for white feathers. These chickens will have both black and white feathers.
Multiple Alleles/Polygenic Traits
• Multiple alleles are when there is more than two possible alleles that exist for each trait in a population.
Example: blood type (A, B, AB, O)• Polygenic traits are traits that are
controlled by 2 or more genes. Example: variations in human skin color; height
Meiosis
• The daughter cells (haploid) produced contain only HALF of the number of chromosomes as the original parent cell (diploid).
• How does this happen? Meiosis is a process of reduction division—that is, the number of chromosomes is cut in half. No replication occurs.
Meiosis
Crossing-over• During meiosis I,
chromosomes pair up with their corresponding homologous pair chromosomes. They form a tetrad.
• These tetrads exchange alleles and form new allele combinations in a process known as crossing-over.
• Video Clip!!