Genetics

Study of Heredity

• Why do cats have kittens and not puppies? Why do humans give birth to other humans? What controls what your child will look like?

• Genetics: the study of heredity

• Heredity: passing of traits from parents to offspring.

Imagine a time before we knew of DNA and genes. These things

were a great mystery.

Original Ideas in Genetics

• Blended Inheritance:– Mother’s traits and

Father’s traits are blended to make the child’s traits.

• A strong man and a weak woman would have a child with average strength.

• We know this is not true today, because of Gregor Mendel.

Gregor Mendel

• Born in Austria in 1822

• Became a monk• Worked in the

monastery garden• Was interested in

science• Began to experiment

with peas.

Mendel cross-fertilized peas

• Why peas?– Reproduce really quickly– Could control the matings of pea plants– Peas are Hermaphrodites– Grows easily and produces MANY offspring

What did he do?• Self-pollinated two

different plants

• This made each plant true-breeding.

• Finally, he had two separate true-breeding plants that he could cross-pollinate.

Vocabulary• Self-pollination-letting the male part of one plant

fertilize the female part of the same plant. (Hermaphroditic part) AKA:inbreeding

• True-breeding-a plant that produces offspring that display only one form of a trait. (Ex- purple plant makes purple plants)

• Cross-pollination-letting the male part of one plant fertilize the female part of a different plant (more like dating)

What traits did Mendel look at?

• Seed shape

• Seed color

• Pea Pod shape

• Pea Pod color

• Flower color

• Flower position

• Plant size

Mendel had specific names for each generation of plants.

• P generation– Parental generation, the first two individuals

that are crossed in a breeding experiment.

• F1 generation

– First filial generation, the offspring of the P generation

• F2 generation

– Second filial generation, the offspring F1

generation

As Mendel worked, he discovered…

• Blending inheritance was not correct!

• Wrinkled + smooth peas =– No half-wrinkled peas!!!

– Only fully wrinkled or smooth peas.

– With continued work, he discovered…..

….that “lost” traits reappear.

Mendel’s Four Hypotheses• Mendelian Theory of Heredity

– 1. When gametes are formed, only one allele for each gene goes into each gamete. This happens during Meiosis II.

• There’s only one allele in each sperm and only one allele in each egg.

– 2. An individual gets two genes for each trait (one from each parent)

• Each pea plant gets one gene for pod color (one from each parent plant)

– 3. These two genes are called Alleles and there are different versions of alleles

• For the pea pod color trait: yellow or green or white, etc.

– 4. Of these different versions of alleles, one is dominant and one is recessive

• For the pea pod trait : Green alleles are dominant, yellow alleles are recessive

What does this mean? Example:• You have two genes (alleles) for

each trait. 1. Your Gametes have half of your chromosomes (only 1 of the pair of allele for each trait).

• 2. At fertilization, the embryo gets one allele from each parent. In this case, you get Mom’s allele for wrinkles and Dad’s allele for smoothness (3. There are different versions of alleles).

• 4. Of the different versions of alleles, one is dominant and one is recessive.

• Dominant = smoothness Recessive = wrinkled

What do you mean by dominant and recessive?

• Example:• 1 allele for wrinkles

(recessive)• 1 allele for smoothness

(dominant)• You have 1 dominant and 1

recessive allele. The dominant shows through, masking the recessive allele.

• You appear smooth!

• The dominant allele always shows through, masking the recessive allele.

• The recessive allele only shows through when there is no dominant allele present.

We represent alleles with letters.• Example: • Trait: Pea pod color• Alleles: 1 wrinkled, 1 smooth• Smooth = dominant• Wrinkled = Recessive• **Dominant alleles are written with capital

letters and Recessive alleles are written with lower-case letters.

• Ss• Will this pea appear smooth or wrinkled

and how do you know?• Smooth, because the dominant allele will

mask the recessive allele.

• Another One:• Trait: Eye color• Alleles: 1 blue, 1

brown• Brown = dominant• Blue = Recessive• Bb• What color eyes will

this person have?• Brown

A couple more….

• Trait: Eye color• Alleles: 2 blue• Brown = dominant• Blue = Recessive• bb• What color eyes will

this person have?• Blue!

• Trait: Freckles• Alleles: 1 freckles, 1

no freckles• No freckles =

dominant• Freckles = Recessive• Ff• Does this person have

freckles?• Yes

Some vocabulary

• Try identifying the following combinations of alleles:

• Tt heterozygous• jj Homo. Recess.• Ll heterozygous• MM Homo. Dominant

• Homozygous Dominant– Both alleles are dominant, BB

(both are capital letters)

• Homozygous Recessive– Both alleles are recessive, bb

(both are small letters)

• Heterozygous– One allele is recessive, the other is

dominant

– Bb (one capital, one brown small)

– **Always write the dominant/capital letter first.

A few last terms!

• Genotype– Set of alleles that an individual has (letters)– Bb

• Phenotype– Physical appearance of the individual (what

you see)– Brown eyes

Determine the genotype of the following individuals.

• An alien has 1 allele for wings and 1 allele for no wings. (Wings are dominant)

• Ww• An alien has 2 alleles for striped fur. (Striped fur

is recessive)• nn• An alien has three eyes. (Three eyes is a recessive

trait)• ff

Determine the phenotype of the following individuals.

• An alien has 1 allele for wings and 1 allele for no wings. (Wings are dominant)

• It has wings.• An alien has 2 alleles for striped fur. (Striped fur is recessive)• It has striped fur.• An alien has a genotype tt. (t = allele for three eyes, which is

a recessive trait)• It has three eyes.• An alien with the genotype Rr. (R=red eyes, r=purple eyes)• It has red eyes.

Understanding how traits are inherited is important!

• Traits like eye color and hair color, but also heritable diseases like Cancer.

• So scientists want to know the probability of passing on these diseases. To calculate probabilities, geneticists use Punnett squares.

Let’s say that you and your husband go to a geneticist and discover that you both are

heterozygous for Huntington’s Disease! What are the chances of your children inheriting the

disease?Mom’s gametes

Dad’s gametes

Dad’s genotype: Rr

Mom’s genotype: Rr

Let’s analyze this Punnett Square!

• 1 out of 4 will be homozygous dominant (RR)• 2 out of 4 will be heterozygous (Rr)• 1 out of 4 will be homozygous recessive (rr)• Which genotype will result in Huntington’s Disease?

– The “rr” genotype only!

• So your chances of having a child with Huntington’s Disease is ¼ or 25%.

Try one on your own!

Cross a mother with the genotype Dd and a father with the genotype DD. What are the

chances that their child will have the genotype DD, like Daddy?