Transcript Unit 7 - genetics

Unit 7
Genetics
Gregor Mendel
• Genetics – the study of how characteristics are
transmitted from parents to offspring.
• Gregor Mendel was an Austrian monk
– Chore: Gardening
– Observed plants and then started experimenting
with them
He observed 7 characteristics
Plant height – short vs. long stem •
Flower position – Axial vs. terminal
Pod color – (green vs. yellow)
Pod appearance (inflated vs.
constricted)
• Seed texture (smooth vs. wrinkled)
• Seed color (yellow vs. green)
•
• Flower color (purple vs. white)
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•
These 7
characteristics are
called traits. Traits
are genetically
determined
variants of a
characteristic.
Each
Characteristic
occurs in two
contrasting traits
Mendel and Cross Pollination
• Mendel used cross-pollination in order to study
his plants
– Cross pollination – pollination in which pollen is
transferred between flowers of two different plants
– Plants were bred for several generations that were
true-breeding for specific traits (like pure bred dogs)
and called those the “P generation)
• Offspring of the P generation were called F1
• Offspring of the F1 generation were called F2.
Recessive vs. Dominant Traits
• Mendel concluded that inherited characteristics
are controlled by factors that occur in pairs
• He found that on factor in the pair masked the
other.
– Dominant – the trait that could be observed
– Recessive – the trait that was masked
Laws
• Law of segregation
– A pair of factors is segregated, or separated, during
the formation of gametes.
• Law of Independent Assortment
– Factors for individual characteristics are distributed
to gametes independent of one another
– This only happens for genes that are located on
separate chromosomes or are far apart on the same
chromosome
Mendel and his traits
• We no know that the factors (yellow vs. green
pea color, for example) that Mendel studied are
called alleles
– Alleles are alternative forms of the same gene
– One allele for each trait is passed from each parent
to the offspring
Genotype vs. phenotype
• Genotype
– Genetic makeup of an
organism (think: genes)
– It will be symbolized by a
pair of letters
• TT, Tt, tt …
• Phenotype
– The physical appearance
of the organism
– Which trait it has
• Tall vs. short
Probability
• The likelihood that a specific event will occur.
• A probability may be expressed as a decimal,
percentage, or fraction.
• Probability = # times and event is expected to happen
# of opportunities for an event to happen
• Punnett squares are used to predict the outcome of
genetic crosses
– monohybrid cross - cross in which one characteristic is
tracked
– Test cross – an individual of unknown genotype X
homozygous recessive individual
• It is used to determine the genotype of the unknown individual
• Examples of genotypes that you will find in a Punnett’s
square
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Pure breeding
Hybrid
Heterozygous
homozygous
Types of dominance
• Complete dominance
– When a heterozygous individuals and dominant
homozygous individuals are indistinguishable in
phenotype
• Ex. Gg (heterozygous genotype) = black (phenotype)
GG (dominant homozygous genotype) = black (phenotype)
• Incomplete dominance
– Two or more alleles influence the phenotype and
results in a phenotype intermediate (between) the
dominant and the recessive trait
• Ex. When red four o’clock flowers are crossed with white
four o’clock flowers, the offspring produced are pink four
o’clock flowers
• Codominance
– When both alleles are for a gene are expressed in
heterozygous offspring
• Ex. Blood typing
Inheritance
• Polygenic inheritance
– When a characteristic, such as eye color, is controlled by two
or more genes
• Complex characteristics
– Polygenic traits that are influenced by both genes and the
environment
• Marking of a siamese cat
• Multiple alleles
– Characteristics, such as ABO blood groups, that are
controlled by three or more alleles (forms) of a single gene
Genes and sex
• Genes reside on chromosomes
• Sex chromosomes vs. autosome
– Sex chromosomes contain genes that determine an
organism’s gender (2 of them).
– The rest of the chromosomes that are not involved in
determining the sex of an individual are autosomes (44 of
them).
• Sex determination
– XX female
– XY male
Genes and linkage
• Linked genes
– Pairs of genes that tend to be inherited together
• Chromosome mapping
– The farther apart two genes are located in
chromosome, the more likely a cross-over will occur
– Researchers use recombinant percentages to
construct chromosome maps that show relative
gene positions
Sex-Linked Genes
• Sex linked
– Genes found on the X chromosome are “X-linked
genes”.
– A sex-linked trait is a trait whose allele is located on
the sex chromosome.
– Males have only 1 X chromosome. So, a male who
carries a recessive allele on the x chromosome will
exhibit the sex linked trait.
• Ex. colorblindness
Sex-Influenced Trait
• A trait that is expressed differently in men than
in women, even if the gene is on an autosome
and both sexes have the same genotype
– Ex. Male pattern baldness
Pedigree
• A diagram the reveals inheritance patterns of
genes
• Geneticists use pedigrees to trace diseases or
traits through families
Hemophilia – Offspring of Dueen
Victoria
Genetic screening and counseling
• Genetic screening examines a person’s genetic
makeup and potential risks of passing disorders
to their offspring
• Genetic counseling informs the screened
individuals about problems that might affect
their children