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Biology
Chapter 11
Genetics
Ch 11-1
Genetics
• The scientific study of heredity
Fertilization
• The joining of male and female reproductive cells
Gregor Mendel
• Austrian monk
• “Father of Genetics”
• Worked with garden peas
• Studied only one trait at a time
• Peas he started with were true-breeding
o If allowed to self-pollinate, they would produce offspring identical to
themselves
• Wanted to produce seeds by mating two different plants
• Cut off the male parts from one plant and used it to pollinate another plant
o Called cross-pollination – produced seeds from two different plants
Trait
• A specific characteristic that varies from one individual to another
• Called the original plants the P (parental generation)
• Called the offspring F1 or first filial generation
Hybrid
• Offspring of crosses between parents with different traits
• In Mendel’s time people thought if two organisms with 2 different traits
mated then the offspring would show an intermediate between the 2 traits
• In each cross he noticed the plants displayed traits of only 1 plant i.e. tall,
yellow pea
Mendel drew two conclusions
• 1st – biological inheritance is determined by factors that are passed from one
generation to the next
o genes – chemical factors that determine traits
o different forms of a gene are called alleles
• 2nd The Principle of Dominance
o States that some alleles are dominant and others are recessive
o An organism with a dominant allele for a particular form of a trait
will always exhibit that form of the trait
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o An organism with a recessive allele for a trait will exhibit that trait
only when the dominant allele for that trait is not present
Segregation
o Mendel allowed all seven kinds of F1 hybrid plants to produce an F2
generation by self-pollination
ƒ The recessive allele reappeared in the F2 generation in about ¼
of the plants
ƒ Mendel suggested that the alleles for tallness and shortness in
the F1 plants segregated from each other during the formation
of gametes – sex cells
o When each F1 plant flowers and produces gametes, the two alleles
segregate from each other so that each gamete carries only a single
copy of each gene
Chapter 11-2
Probabilities and Punnett Squares
Probabilities
• The likelihood that a particular event will occur
• The principles of probability can be used to predict the outcomes of genetic
crosses
• Predict the average outcome of a large number of events
• Cannot predict the precise outcome of an event
o Therefore, the larger the number of offspring the closer the numbers
will be to the expected values
o Single trait heterozygous cross – 3:1 ratio
• Punnett Squares
o Used to predict and compare the genetic variations that will result
from a cross
• Organisms that have two identical alleles for a particular trait are considered
homozygous – TT homozygous dominant, tt homozygous recessive
• Organisms that have two different alleles for a particular trait are
heterozygous – Tt
• Homozygous individuals are true breeding for a particular trait
• Heterozygous individuals are hybrid for a particular trait
• Genotype – the genetic make-up of an organism
• Phenotype – the outward expression of a genotype
Independent Assortment
• Mendel wanted to know if seed color had anything to do with a pea being
wrinkled or smooth
• He performed a two factor cross
• 1st he crossed true-breeding plants that produced only round yellow peas
with plants that produced only green wrinkled peas
• The plants produced only round, yellow peas
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He wanted to know if the 2 dominant alleles would stay together or if they
would “segregate independently”
F2 plants produced 556 peas
315 round, yellow
32 wrinkled, green
209 had a combination of the phenotypes
The results were close to a 9:3:3:1 ratio
The Principle of Independent Assortment states that – genes for different
traits can segregate independently during the formation of gametes
This is where we get genetic variation
Beyond Dominant and Recessive Alleles
• Not all genes show the simple pattern of dominant and recessive alleles
• The majority of genes have more than 2 alleles
• Many important traits are controlled by more than one gene
• 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 another allele
• Mirabilis – 4 o’clock RR+WW = RW (pink)
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The heterogeneous phenotype is somewhere between the two homozygous
phenotypes
Codominance
• Both alleles contribute to the phenotype
• In chickens white and black feathers are codominant
• They produce feathers with white and black sections independent of one
another, ex. Barred Rock
Multiple Alleles
• Genes with more than 2 alleles
• Does not mean an individual can have more than 2 alleles but that more than
2 alleles for a trait exist in a population
• Rabbits page 273
Polygenetic Traits
• Traits that are produced by the intersection of several genes
• Traits that are controlled by 2 or more genes
• Skin color in humans due partly to 4 different genes that control this trait
Applying Mendel’s Principles
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Beginning of 1900’s American geneticist Thomas Morgan Hunt looked for a
model organism to advance the study of genetics
Wanted an animal that was small and easy to keep in a lab
Decided to use Drosophila melanogaster, the common fruit fly
A single pair could produce 100 offspring
Genetics and the Environment
• Characteristics of an organism are not solely determined by genes
• Environmental factors can play a large part
Ch 11-4
Meiosis
Chromosome Number
• Homologous
o Each chromosome from 1 parent has a corresponding chromosome
from the other parent
• Diploid
o A cell that contains both sets of homologous chromosomes
o Represented by 2n
• Haploid
o A cell that contains only 1 set of chromosomes
o A gamete
o Represented by N
Phases of Meiosis
• A process of reduction division in which the number of chromosomes per
cell is cut in half through the separation of homologous chromosomes in a
diploid cell
• Involves two distinct divisions – Meiosis I and Meiosis II
• By the end of meiosis II a diploid cell has been split into 4 haploid cells
Gamete Formation
• In male animals haploid gametes produced by meiosis are called sperm, in
plants pollen grains contain sperm
• Female gametes are called eggs in animals and egg cells in plants
• In female animals, cell division at the end of meiosis I and II are uneven, so a
single cell becomes an egg and receives most of the cytoplasm
• The three other cells are called polar bodies and usually do not participate in
reproduction
Meiosis I
• Interphase I
o DNA is replicated, duplicate chromosomes are formed
• Prophase I
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o Chromosomes pair with their homologous chromosomes and
forms a tetrad
ƒ 4 chromatids in a tetrad
ƒ At this stage crossing-over occurs
ƒ New combinations of alleles are formed
Metaphase
o Spindle fibers attach to the new chromosomes
Anaphase, Telophase, and Cytokinesis are like mitosis
Cells that are formed from meiosis I have different alleles from each
other and from the original diploid cell
Cells that form from Meiosis I have different chemistry and alleles from
each other and from the original diploid cell
Meiosis II
o Cells do not replicate chromosomes
o In the end 4 haploid cells are produced- 2 chromosomes each