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Introduction to Genetics
Why do children look like their parents?
• The answer is … genetics
• Genetics is the scientific study of heredity
• Heredity is the passing down of traits from parents to offspring (by genes)
Chromosomes, DNA, and Genes• Chromosomes are made of DNA
• Each strand of DNA contains genes
• Each gene contains the code, or instructions, for one trait you will display
• A trait is a specific characteristic (such as having dimples, or being able to roll your tongue)
• So, chromosomes = DNA = genes = code = traits
Gregor Mendel• Australian monk in the 1860s
• Studied different traits of garden pea plants
• Discovered the basic laws of genetics
• Called the “Father of Genetics”
Gregor Mendel• Used scientific method and
mathematics to systematically study many generations of pea plants (over 29,000!)
• 1st person to successfully predict how traits are passed from one generation to the next
• Trait – a characteristic (what an organism looks like or how an organism behaves)
Pea Plants• Pea plants worked well because they grow quickly
• Mendel identified several traits of pea plants
Dominant vs. Recessive• Mendel discovered that one trait
would be dominant over the other. Dominant trait = stronger.
• The other trait would be recessive. Recessive trait = weaker.
• If a pea plant had the genes for both traits, then the plant would display only the dominant trait
• Example: one trait for yellow seeds (dominant), one trait for green seeds (recessive)…the pea plant would have yellow seeds
Dominant vs. Recessive
• The only time a pea plant would display a recessive trait, is when both of its genes coded for the recessive trait
• Example: both genes code for green seeds (recessive)…the pea plant will have green seeds
Genes vs. Alleles• GENE: section of DNA that
determines a trait
• ALLELE: particular form of a trait• Represented by letters • Dominant: capital letter• Recessive: lowercase letter
Example: yellow seeds vs. green seedsDominant: yellow seeds (Y)Recessive: green seeds (y)
Y
y
Combinations of Alleles• For each trait, we have two alleles
• We get one allele from each parent
• 3 possibilities• YY – homozygous dominant – yellow seeds• Yy – heterozygous – yellow seeds• yy – homozygous recessive – green seeds
• Homo = same
• Hetero = different/other
Y
y
Genotype & Phenotype• Genotype – the specific combination
of genes that an organism has • YY• Yy• yy
• Phenotype – the physical appearance that the organism will display
• yellow seeds• green seeds
Y
y
Let’s Practice!
• Height: • Dominant: Tall (T)• Recessive: Short (t)
• Alleles: T, t
Genotype PhenotypeHomozygous dominantHeterozygousHomozygous recessive
• Height: • Dominant: Tall (T)• Recessive: Short (t)
• Alleles: T, t
Genotype PhenotypeHomozygous dominant TT tallHeterozygous Tt tallHomozygous recessive tt short
• Seed shape: • Round is dominant (R) • Wrinkled is recessive (r)
• Alleles: R, r
Genotype PhenotypeHomozygous dominantHeterozygousHomozygous recessive
• Seed shape: • Round is dominant (R) • Wrinkled is recessive (r)
• Alleles: R, r
Genotype PhenotypeHomozygous dominant RR roundHeterozygous Rr roundHomozygous recessive rr wrinkled
• Seed Coat Color: • Green is dominant (G)• White is recessive (g)
• Alleles: G, g
Genotype PhenotypeHomozygous dominantHeterozygousHomozygous recessive
• Seed Coat Color: • Green is dominant (G)• White is recessive (g)
• Alleles: G, g
Genotype PhenotypeHomozygous dominant GG greenHeterozygous Gg greenHomozygous recessive gg white
“Hidden” Genes• Genes can “skip” generations if the
parents carry the allele for the recessive gene
• Example: Both parents are Yy(heterozygous) for seed color.
• Yellow is dominant, so both parents will be yellow
• But both parents are also carrying the recessive allele for green (y)
• If both parents pass down the yallele, the offspring will be yy and have green seeds
y
Y
You Try!How can two tall (T) pea plants have a short (t) offspring?
• What genotype will the offspring be?
• What phenotype will the offspring be?
• What genotype must BOTH parents be?
• What phenotype will they be?
t
T
You Try!How can two tall (T) pea plants have a short (t) offspring?
• What genotype will the offspring be?• tt
• What phenotype will the offspring be?• short
• What genotype must BOTH parents be?• Tt (they are tall but each one needs a short
allele to pass to offspring)
• What phenotype will they be?• tall
t
T
You Try!G = green pod, g = yellow pod.
• If an offspring has a yellow pod, what genotype does it have?
• What TWO possible genotypes can each parent have?
• What color(s) could the parents be?
G
g
You Try!G = green pod, g = yellow pod.
• If an offspring has a yellow pod, what genotype does it have?
• gg
• What TWO possible genotypes can each parent have?
• Gg or gg (as long as each parent has at least ONE g to give)
• What color(s) could the parents be?
• Green (Gg) or yellow (gg)
G
g
You Try!R= round seeds, r = wrinkled seeds. • An offspring has round seeds.
• Mother is rr. What is her phenotype?
• What genotype will the offspring have?
• What phenotype MUST the father be?
• What 2 genotypes can the father be?
R
r
You Try!R= round seeds, r = wrinkled seeds. • An offspring has round seeds.
• Mother is rr. What is her phenotype?• wrinkled
• What genotype will the offspring have?• Rr (because mother can only give an r, but
offspring needs an R to be round)
• What phenotype MUST the father be?• Round (must have at least one R)
• What 2 genotypes can the father be?• RR or Rr
R
r
Great Job! • Make sure you know the following:• Relationship between chromosomes, DNA, genes, and
traits• Who Gregor Mendel was and why he’s so important• Vocabulary:
• Trait• Gene• Allele• Dominant• Recessive• Genotype• Phenotype• Homozygous dominant• Heterozygous• Homozygous recessive
• How recessive traits can be “hidden” and skip generations
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