heredity
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TRANSCRIPT
HEREDITY
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HEREDITY
Passing of traits from parents to offspring
Trait: distinguishing quality
Why then don’t you look exactly like your parents?
About 150 years ago Gregor Mendel performed some experiments that began to help us find the answers
GREGOR MENDEL
Born 1822 in Austria
Grew up on his family’s farm where he learned to grow flowers and fruit trees
Went to a monastery where he worked in the garden
Interested in how traits are passed from parents to offspring
GREGOR MENDEL
Sometimes a trait that appeared in one generation did not show up in any of the offspring in the next generation.
In the 3rd generation, the trait showed back up. Mendel noticed similar patterns in people, plants, and many other living things.
He chose to study garden pea plants.
SELF-POLLINATING PLANTS
Garden peas were a good choice because
They grow quickly
Usually self-pollinating
Come in many varieties
Self-pollinating: contains both male and female reproductive structures
TRUE-BREEDING PLANTS
Mendel chose to study only one characteristic, like plant height or pea color, at a time
He made sure to use true-breeding plants
When true-breeding plants self-pollinate they always produce offspring with the same trait the parent plant has.
CROSS-POLLINATION
Mendel wanted to see what would happen if he crossed two plants that had different forms of a trait
To do this, he used cross-pollination. This method removes the male reproductive parts of a plant so that it can’t self-pollinate.
MENDEL’S FIRST EXPERIMENT
Performed crosses to study 7 different characteristics
Each cross was between the two traits of each characteristic
MENDEL’S FIRST EXPERIMENT
•Mendel got similar results for each of the crosses that he made.
•One trait always appeared and the other trait seemed to vanish.
TYPES OF TRAITS
Dominant trait: observable trait when at least one dominant allele for a characteristic is inherited
Recessive trait: trait that is observable only when two recessive alleles for the characteristic are inherited
SCIENTIFIC METHOD IN MENDEL’S WORK
Ask a Question: How are traits inherited?
Form a Hypothesis: Inheritance has a pattern.
Test the Hypothesis: Cross true-breeding plants and offspring.
Analyze the Results: Identify patterns in inherited traits.
Draw Conclusions: Traits are inherited in predictable patterns.
MENDEL’S SECOND EXPERIMENT
MENDEL’S SECOND EXPERIMENT
Mendel allowed the 1st generation to self-pollinate.
This time the plant with the dominant trait for purple flowers was allowed to self-pollinate. As you can see, the recessive trait for white flowers show up again.
MENDEL’S SECOND EXPERIMENT
Mendel decided calculate the ratios of dominant to recessive traits in each characteristic. Calculate the dominant-to-recessive ratio for each characteristic.
Characteristic Dominant Recessive Ratio
Flower color 705 purple 224 white 3.15:1
Seed color 6,002 yellow 2,001 green ___________
Seed shape 5,474 round 1,850 wrinkled ___________
Pod color 428 green 158 yellow ___________
Pod shape 882 smooth 299 bumpy ___________
Flower position 651 along stem 207 at tip ___________
Plant height 787 tall 277 short ___________
GENES AND ALLELES
Mendel realized that his results could be explained only if each plant had two sets of instructions for each characteristic
Each parent donates one set of instructions, genes, to the offspring.
The fertilized egg would then have 2 forms of the same gene for every characteristic- one from each parent.
The two forms of a gene are known as alleles.
PUNNETT SQUARES
Used to visualize all possible combinations of alleles from parents.
To make a Punnett Square, draw a square and then divide it into 4 sections.
Next, write the letters that represent alleles from one parent along the top of the box.
Write the letters that represent alleles from the other parent along the side of the box.
PUNNETT SQUARES
R R
r
r
Make a Punnett Square for a cross between a purebreed round seed plant(R) and a purebreed wrinkled seed plant(r).
R
R
R
R
rr
r r
GENOTYPES AND PHENOTYPES
Dominant alleles are symbolized with capital letters. Recessive alleles are symbolized with lowercase letters.
Genotype: Offspring’s inherited combination of alleles
Phenotype: Organism’s appearance
An organism’s phenotype is due to its genotype
ALLELES
Heterozygous: having two alleles that are different for a given gene
Homozygous: having identical alleles for a given gene