Section 11-1The Work of Gregor Mendel

Gregor Mendel

Genetics – the scientific study of heredity

Gregor Mendel – used garden peas to study heredity, first to discover the theory and 3 laws of inheritance

Genotype & Phenotype Mendel bred (“crossed”) plants with 7 different traits,

each trait could turn out 2 different ways.

P generation – original pair of plants F1 generation – “first generation,” P generation’s

offspring• F2 generation – “second generation,” F1 generation’s offspring

Genotype & Phenotype

Each individual has a phenotype = observable trait or combinations of traits Examples: “Short stem,” “axial flowers & green peas,”

“blue eyes and type A blood,” “diabetes”• Mendel knew families tended to have similar phenotypes…

and he wanted to know why.

Mendel’s Experiments He crossed peas with the same phenotypes, and

peas with different phenotypes, then counted how many of their offspring shared their parents’ traits

Mendel made four conclusions:

1. Inheritance is determined by physical factors passed on from one generation to the next. After his death, other scientists discovered:

• Genes – pieces of DNA that determine a trait

• Alleles – different forms of genes

• “Allele is expressed” = “Allele gives the bearer its version of a trait”

Mendel made four conclusions:

Example: All peas inherit a gene for the “seed color” trait.

There are two versions (alleles) of the gene: green and yellow.

Practice: All humans inherit a gene for blood type.

What are the different alleles for that gene?

Mendel made four conclusions:

2. Mendel’s Law of Dominance: Not all alleles are automatically expressed.

• Dominant allele – always expressed if it’s present • Usually represented by a capital letter

• Recessive allele – only expressed if a dominant allele is not present• Usually represented by a lower case letter

Mendel made four conclusions:

Genotype = An individual’s combination of alleles.Rules for writing: Keep the two alleles for a gene next to each other. If one of the alleles is dominant, put it first.

• Gene & allele examples: “The gene for eye color has the alleles B and b”

• Genotype examples: “Aa,” “TtYY,” XAXa,” “PPiiUuH1H2”

• Mendel learned that genotype determines phenotype

Practice

If G is the allele for pointy ears and g is the allele for floppy ears, what will be the ear shape phenotypes of the puppies with these genotypes?

The dominant allele is _ for the trait ___________

The recessive allele is _ for the trait ___________

Genotype GG = Phenotype __________

Genotype Gg = Phenotype __________

Genotype gg = Phenotype ___________

Practice

G is for pointy ears and g is for floppy ears. Also, H is for a pink nose and h is for a black nose.

Genotype GGHH = Phenotype ______ and ______

Genotype GgHh = Phenotype ______ and ______

Genotype gghh = Phenotype ______ and _______

Genotype GGhh = Phenotype ______ and _______

Genotype Gghh = Phenotype ______ and _______

Genotype ggHH = Phenotype ______ and _______

Practice

Which of these are traits and which are phenotypes?

1. Finger length 2. Blue eyes 3. Long hair 4. Number of leaves 5. Shape of tentacles 6. Warbling song

Practice

Which of these are alleles and which are traits? 1. Eye color 2. Bone integrity 3. i 4. Insulin shape 5. B 6. Na

Practice

Which of these are phenotypes and which are genotypes?

1. Curly hair 2. Jj 3. PP 4. Arthritic knees 5. Type B blood 6. Spotted fur and a pink nose 7. HHGg 8. Purple leaves and spiny stem

Summary Example

There is a gene for the hair curliness trait. This trait can express as one of two phenotypes: an individual has curly hair or straight hair.

The allele for curly hair is C, the allele for straight hair is c.

If your genotype is CC, your phenotype will be curly hair.

Summary Example

In an alien species, there is a gene for the ____________ trait. This trait can express as one of two phenotypes: an individual has __________ or __________.

The allele for __________ is A, the allele for __________ is a.

If your genotype is Aa, your phenotype will be __________.

Mendel made four conclusions: Mendel found that if he crossed peas with different

phenotypes for a trait, all the F1 plants would have the same dominant phenotype. He wondered if the recessive allele had disappeared… so he let the F1 plants self-pollinate, creating the F2 generation.

The recessive phenotype reappeared in 1/4th of the F2 generation!

Mendel made four conclusions:3. Mendel’s Law of Segregation: Although an individual has two

alleles for every gene, s/he passes only one of them on to each offspring.

• i.e., each gamete carries only a single copy of a gene, because

gametes are haploid.

Practicing the Law of SegregationIf an individual’s genotype is YY, all of that individual’s gametes

will have just ONE Y allele.

If an individual’s genotype is Tt, that individual’s gametes will have EITHER the T allele OR the t allele, not both.

(Some gametes are written with more than one letter. If Dad’s genotype is LTLt, he will make a sperm that has the LT allele or a sperm that has the Lt allele.)

Genotype YY makes gamete Y or gamete Y.

Genotype Tt makes gamete T or gamete t.

Genotype bb makes gamete _ or gamete _

Genotype Ii makes gamete _ or gamete _

Genotype K1K2 makes gamete __ or gamete __

Mendel made four conclusions:4. We’ll learn Mendel’s final conclusion on a later date. Stay

tuned!

Overview Suppose there’s a gene for eye color, with the

alleles B for brown eyes or b for blue eyes. A woman has the genotype Bb, which gives

her the phenotype brown eyes. Meiosis produces her gametes…

bb

bbb

b

BB

BBB

B

She can make gametes that are EITHER B or b.Half of her gametes will be one, half will be the other.

bB

S P

hase

1st C

ytok

ines

is

2nd C

ytok

ines

is

Normal cell in G1

Four Gametes