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